tag:blogger.com,1999:blog-63774102024-03-12T23:51:29.430-05:00Hindered Settlingrandom notes of a skeptical geologistAnonymoushttp://www.blogger.com/profile/02317685391212630354noreply@blogger.comBlogger217125tag:blogger.com,1999:blog-6377410.post-43104722383162421782012-06-07T20:20:00.000-05:002012-06-07T20:20:02.932-05:00Moving to Hinderedsettling.comIf you haven't already noticed, I have moved my blog over to a Wordpress site, to the the <a href="http://Hinderedsettling.com/">Hinderedsettling.com</a> domain. New blog posts (if any - I didn't have a whole lot of time lately to blog) will appear over there. Please update your links and feeds!Anonymoushttp://www.blogger.com/profile/02317685391212630354noreply@blogger.com0tag:blogger.com,1999:blog-6377410.post-88739744280431808972011-10-16T08:13:00.000-05:002011-10-17T06:40:15.193-05:00The case for scales, rates, and numbers in geologyA long time ago I used to study geology in a nice city called Cluj, in the middle of that interesting part of Romania known as Transylvania. This was a place and time where and when I learned about quartz, feldspar, species of coral and foraminifera in great detail, heard about sequence stratigraphy and turbidites for the first time, and went on some great geological field trips. Not to mention the half-liter bottles of beer that would be significant components of any decent geological trip or spontaneous philosophical discussion in the evening. The less pleasant part was that many of the classes I took involved brute-force memorization of fossils, minerals, chronostratigraphic names, and formations. Although the geological vocabulary that I picked up was pretty broad and proved useful as a good set of words, terms, and definitions to play with, I forgot many of the details by now. If you asked me what was the difference between granite and granodiorite, I would have to <a href="http://en.wikipedia.org/wiki/Granodiorite">check</a>. I don't remember at all what fossils are characteristic of the <a href="http://en.wikipedia.org/wiki/Late_Jurassic">Late Jurassic</a>. And, despite doing some fieldwork myself over there, I cannot remember the stratigraphic nomenclature in Transylvanian Basin; I would have to look it up (probably <a href="http://www.sciencedirect.com/science/article/pii/S0264817206000316">in this paper</a>).<br />
<br />
After college, it took me about one year to realize with convincing clarity that there was a lot left to learn. I went on to grad school on the other side of the planet, at <a href="http://www.stanford.edu/">a well-known university</a>. Many of the classes I took over there were - unsurprisingly - quite different; a lot more focus on laws, processes and the connections between geological things than the 'things' themselves. It was also there that I started to see the links between geology and physics and math. I picked up quite a bit of math and physics during high school, but then quickly relegated them to the status of "stuff that is rarely used in geology". At grad school, it dawned on me that numbers and mathematical laws are not only useful in geology, but are in fact necessary for doing good earth science. Maybe I am stating the obvious, but here it goes anyway: geology deals with enormous variations in scale, both in space and time; and it is not enough to say that the river was deep (how deep?), the tectonic deformation was fast (how fast?), the sea-level highstand lasted long (how long?), or the sediment gravity flows were high-energy flows (I am not even sure what that means). One of the most important things I learned was an appreciation for physical and quantitative insight in geology, that is, having at least an idea, a feel for what are the scales and rates involved in the formation of the rocks you are looking at. I cannot say it better than <a href="http://www.geo.umn.edu/people/profs/PAOLA.html">Chris Paola</a>, one of the important and influential advocates of moving sedimentary geology closer to physics and math:<br />
<blockquote>
"For the 'modal' sedimentary-geology student, it is not sophisticated computational skills or training in advanced calculus that is lacking, but rather the routine application of basic quantitative reasoning. This means things like estimating scales and rates for key processes, knowing the magnitudes of basic physical properties, and being able to estimate the relative importance of various processes in a particular setting. Understanding scales, rates and relative magnitudes is to quantitative science what recognizing quartz and feldspar is to field geology. Neither requires years of sophisticated training, but both require repetition until they become habitual."</blockquote>
Developing these skills is a lot easier <a href="http://www.agilegeoscience.com/journal/2011/9/28/learn-to-program.html">if one is not afraid of tinkering with simple computer programs</a>. Want to really understand what Stokes' law is about? There is no better way than typing the equation into an Excel spreadsheet or a Matlab m-file and see how the plot of settling velocity against grain size looks like. What about settling in a fluid with different viscosity? Change the variable, and compare the result with the previous curve. High-level programming languages like Matlab or Python* are a lot easier to learn than languages closer to 'computerese' and farther from English, and they are great tools for these kinds of exercises and experiments. As somebody interested in stratigraphic architecture, I have become especially fond of creating surfaces that vaguely resemble real-world landscapes and then see how the evolution of these surfaces through time - deposition over here, erosion over there - creates stratigraphy. Complex three dimensional geometry is a lot easier to grasp if you can visualize and dissect it on the computer screen.<br />
<br />
Of course, numbers, diagrams and images that come from computer programs are only useful if they demonstrably say something about the real world. Data collection in the field and the laboratory are equally important. But nowadays we often have more data than we wished for, and quantitative skills come handy for visualizing and analyzing large datasets - and comparing them to models.<br />
<br />
<div class="separator" style="clear: both; text-align: center;">
<a href="http://3.bp.blogspot.com/-ZbK2Cr3l3-U/Tpnc8xtaoYI/AAAAAAAAAg0/rqb3ZhbSXhk/s1600/SediTilte_right_new.jpg" imageanchor="1" style="clear: left; float: left; margin-bottom: 1em; margin-right: 1em;"><img border="0" src="http://3.bp.blogspot.com/-ZbK2Cr3l3-U/Tpnc8xtaoYI/AAAAAAAAAg0/rqb3ZhbSXhk/s1600/SediTilte_right_new.jpg" /></a></div>
Not everyone is excited about the growing number of earth scientists who tend to see equations 'in the rocks'. The logo of the <a href="http://www.geo.umn.edu/orgs/seds/">Sedimentology Research Group at the University of Minnesota</a> features the <a href="http://en.wikipedia.org/wiki/Exner_equation">Exner equation</a> carved into a pebble, allegedly as a response to the exclamation "I haven't seen yet an equation written on the rocks!" There is some concern that many geology graduates nowadays do not get to see, to map and to sample enough real rocks and sediments in the field. Although I think this unease is not entirely unsubstantiated, I wouldn't want to sound as pessimistic as Emiliano Mutti - one of the founding fathers of deepwater sedimentology - does in the last phrase of a <a href="http://onlinelibrary.wiley.com/doi/10.1111/j.1365-3091.2008.01019.x/abstract">review article</a>:<br />
<blockquote>
"This approach raises a problem, and not a small one: in connection with data collection in the field, how many field geologists are being produced in these times of increasingly computerized geology; and how good are they?"</blockquote>
As far as I know, geological field work is still an important part of the curriculum in many departments of geology - as it clearly should be. The number one reason I have become a geologist was that I loved mountains, hiking, and being outdoors in general, way before I started formally studying geology. And I still take every opportunity to go to the field. But I cannot see the growth of "computerized geology" - and of quantitative geology in general - as a bad thing. Does dry quantification take away the beauty and poetry of geology? I don't think so. <a href="http://www.amazon.com/Unweaving-Rainbow-Science-Delusion-Appetite/dp/0618056734">Unweaving the rainbow</a>, unfolding a mountain, and reconstructing a turbidity current only add to our appreciation of the scale and grandeur of geology.<br />
<br />
* I will let you know later whether this is true about Python...<br />
<br />
** I have started writing this post for Accretionary Wedge #38, mostly because I found the <a href="http://all-geo.org/highlyallochthonous/2011/09/call-for-posts-accretionary-wedge-38-back-to-school/">call for posts</a> quite inspiring, but haven't finished it in time. Read <a href="http://all-geo.org/highlyallochthonous/2011/10/wisdom-from-the-geoblogosphere-school-of-learning-doing-accretionary-wedge-38/">all the good stuff at Highly Allochthonous</a>.Anonymoushttp://www.blogger.com/profile/02317685391212630354noreply@blogger.com3tag:blogger.com,1999:blog-6377410.post-19927876057687698172011-07-16T20:36:00.011-05:002011-07-17T11:13:37.786-05:00Salt and sediment: A brief history of ideas<b>Salty weirdness</b><br />
Salt is a weird kind of rock. At first sight, it behaves like most other rocks: if you pick up a piece, it is hard, it is heavy, and it breaks if hit with a hammer. But put it under stress for thousands of years, and salt will behave like a fluid: relatively small forces can cause it to flow toward less stressful surroundings. This often means it will try to find its way to the surface.<br />
<br />
When deposited, sand and mud have lots of pore space filled with water and have relatively low density. However, as they get buried by more sediment, much pore space is lost, both through compaction and cementation. Sediments turn into sedimentary rocks, become harder, and their density increases. In contrast, salt doesn't have much pore space to begin with; its density will stay the same, regardless of depth of burial. As both salt and sediment are buried to greater depths, an unstable condition develops: lighter salt lying under denser material. In addition, the location of the salt layer in the sediment column is not entirely random: it is in the nature of sedimentary basins to initially place salt at the bottom of the sediment pile. Extensive salt layers usually form early in a basin's lifetime, when seawaters invade for the first time shallow depressions on a continent that is about to split into two along a rift zone. The <a href="http://en.wikipedia.org/wiki/Dead_Sea">Dead Sea</a> is an obvious example that comes to mind.<br />
<br />
Layering salt and sediment in this unstable order is a recipe for a spectacular geological show. As salt is trying to find its way to the surface, it forms drop-shaped blobs called diapirs; but also ridges, walls, and salt sheets. Several sheets can connect laterally into a huge salt canopy, a new salt layer that is entirely out-of-place or <a href="http://all-geo.org/highlyallochthonous/2011/06/the-far-travelled-ground/">allochtonous</a>. Salt can also act as a lubricating layer at the base of a thick sequence of sedimentary rocks. But I am rushing ahead a little bit; salt tectonics is such a new - but rapidly growing - science that salt canopies, despite their widespread presence in the subsurface Gulf of Mexico, were not recognized and described until the 1980s.<br />
<br />
<b>Tectonics vs. buoyancy, Europe vs. America</b><br />
Before the beginning of the twentieth century, even with <a href="http://www.amazon.com/Salt-World-History-Mark-Kurlansky/dp/0142001619">the role that salt played in human history</a>, little was known about how salt domes formed. This was an age of rampant speculation; surface data was scarce because salt does not last very long after exposed as it quickly gets dissolved and washed away by precipitation. Many geologists thought that formation of salt domes didn't require any significant salt deformation or displacement. But things have changed dramatically in 1901, with the discovery of the Spindletop oil field on top of a salt dome in southeastern Texas. The recognition that oil is often found on top of and around salt domes created a much stronger interest in understanding how exactly salt formations are put in place.<br />
<br />
European geologists thought that the main driving force was compression, the force that causes folding and thrusting and builds mountains. In Romania, where the Eastern Carpathians take a sharp turn toward the southwest, salt was found in the cores of oil-bearing anticlines. The contacts with the surrounding rocks were clearly discordant. These are the structures that prompted Ludovic Mrazec, professor of geology at University of Bucharest, to coin the term "diapir" in 1907.<br />
<br />
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="http://2.bp.blogspot.com/-m8S5Ib8wRrY/TiHuwVB6CFI/AAAAAAAAAfc/bhE1DN51_xM/s1600/Screen+shot+2011-07-16+at+2.32.11+PM.png" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="347" src="http://2.bp.blogspot.com/-m8S5Ib8wRrY/TiHuwVB6CFI/AAAAAAAAAfc/bhE1DN51_xM/s400/Screen+shot+2011-07-16+at+2.32.11+PM.png" width="400" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">Mrazec's explanation of how salt diapirs form. From Barton (1925).</td></tr>
</tbody></table>
<br />
Salt in Germany and Poland also seemed to occur invariably in a compressional setting, in the cores of folds, next to folds that had no salt associated. It seemed obvious that salt was 'pushed up' by tectonic forces, and it appeared unlikely that the rise of salt itself was causing the folding.<br />
<br />
But the discovery of a multitude of salt diapirs in the Gulf of Mexico made it clear that they can occur far away from any mountains and compressive tectonic forces. The much simpler setting and relative lack of deformation in the Gulf proved informative. "The Roumanian salt-dome geologist possibly may have more to learn from the American salt domes than the American salt-dome geologist has to learn from the Roumanian domes. The occurrence of the American domes in a region of tectonic quiescence suggests that tectonic thrust cannot have the importance postulated by Mrazec" - wrote Donald Barton in 1925.<br />
<br />
This was also the time when the density difference between salt and sediment came into discussion. Gravity measurements in the Gulf of Mexico showed anomalies above salt domes that were due to the lower density of salt. It was increasingly recognized that density inversion must play an important role in diapirism, especially where compressive tectonic forces were absent. In addition, by the 1930s geologists have reached a consensus that salt diapirs must somehow punch through the overlying sediment. They seemed to ignore the fact that, as Wade (1931) put it, you cannot drive a putty nail through a wooden board. As mentioned before, salt does behave like a fluid over geological time scales. But how can it penetrate thick layers of hardened sedimentary rock?<br />
<br />
<b>A brilliant idea: downbuilding</b><br />
The solution to this problem came in 1933, from the same Donald Barton who was discussing the differences between European and American salt domes in 1925. He suggested that diapirs can form without much piercement of the sediment above. Instead, once a small dome is initiated, it simply can stay in place, always at or close to the surface, while sediment is deposited around it and the source salt layer subsides: "it is the sediments which move, and not the salt core. The energy requirement (...) is very much less than if there were actual upward movement of the salt."<br />
<br />
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="http://1.bp.blogspot.com/-D74p7IbthyA/TiIK5LYqY8I/AAAAAAAAAfg/eG5WBBs4lOo/s1600/Screen+shot+2011-07-16+at+5.04.00+PM.png" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="280" src="http://1.bp.blogspot.com/-D74p7IbthyA/TiIK5LYqY8I/AAAAAAAAAfg/eG5WBBs4lOo/s400/Screen+shot+2011-07-16+at+5.04.00+PM.png" width="400" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">The evolution of salt diapirs through 'downbuilding'. Salt domes are always close to the surface and diapirism goes hand-in-hand with sedimentation. From Barton (1933).</td></tr>
</tbody></table>
<br />
This was a key insight: it got rid of the "room problem", the need for moving huge volumes of hard rock out of the way of the rising salt. It also highlighted that salt movement can happen at the same time with sedimentation, a fact that became abundantly obvious later as high-quality seismic data became available. But the concept of 'downbuilding' was ignored for the next fifty years.<br />
<br />
<br />
<center><iframe height="160" src="http://dl.dropbox.com/u/25694950/diapir_evolution_1.swf" width="345"><p><p><p><p><p><p><p><p><p><p><p><br /> <p>Your browser does not support iframes.</p></p></p></p></p></p></p></p></p></p></p></p></iframe></center><br />
<div style="text-align: center;">
<span class="Apple-style-span" style="font-size: 13px;">Animation showing how downbuilding works. Blue represents salt, yellow is sediment. To mimic mass balance for salt (-- what is lost from the source layer must go into the salt dome), the blue area is kept constant through the animation. Right-click image for animation controls.</span></div>
<br />
<br />
<b>The beauty of instabilities</b><br />
The main reason for conveniently forgetting Barton's idea was that density inversion between two fluids could be nicely studied in the lab and described with elegant equations. In one of the papers that kicked off this fascination with Rayleigh-Taylor instabilities, Nettleton (1934) used corn syrup and less dense crude oil to visualize diapir-like blobs of fluid in a transparent cylinder and to show that gravity alone, without any help from contractional forces, was enough to generate structures similar to salt domes.<br />
<br />
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="http://3.bp.blogspot.com/-8Yad4Z_cXEM/TiIrvIHBgSI/AAAAAAAAAfk/vhHdqzrjxms/s1600/nettleton+salt+experiment.png" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="640" src="http://3.bp.blogspot.com/-8Yad4Z_cXEM/TiIrvIHBgSI/AAAAAAAAAfk/vhHdqzrjxms/s640/nettleton+salt+experiment.png" width="308" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">Less dense crude oil (black) forming diapir-like blobs as rising through higher-density corn syrup (yellow). Redrawn from Nettleton (1934).</td></tr>
</tbody></table>
One problem with this approach was that oil and syrup can be photographed during deformation, but the transient structures could not be carefully dissected and analyzed later. Materials of higher viscosity were needed for that; however, increasing the viscosity resulted in a density difference too small to get the fluids moving in the first place. The trick was to place the whole experiment in a centrifuge and use the centrifugal force to imitate a larger-than-normal gravitational force. This approach formed the basis of a productive line of research on gravity tectonics in the laboratory of the Norwegian-Swedish geologist Hans Ramberg. The results are probably more relevant to what is happening deeper in the Earth, at higher temperatures and pressures, where most rocks become more similar in behavior to salt.<br />
<br />
<b>Modern salt tectonics</b><br />
By the late 1980s it has become quite obvious that kilometer-thick piles of sedimentary rock cannot be treated as fluids and salt-sediment interaction is more similar to placing and deforming slabs of brittle material on top of a viscous fluid. Seismic from salt-bearing sedimentary basins suggested that the history of salt movement and sedimentation were highly interconnected and Barton's downbuilding concept was strongly relevant.<br />
<br />
Three-dimensional seismic data also showed the variety and complexity of allochtonous salt bodies in salt-rich sedimentary basins. Sandbox experiments with more realistic material properties and ongoing sedimentation during deformation were performed and <a href="http://www.beg.utexas.edu/indassoc/agl/animations/">the results beautifully visualized</a>. The behavior of turbidity currents flowing over complex salt-related submarine topography was investigated. Hundreds of scientific papers were written on salt tectonics, both by industry geoscientists and researchers in the academia.<br />
<br />
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="http://1.bp.blogspot.com/--Mn_IH_Iq2c/TiI41_yLbjI/AAAAAAAAAfo/aRKNBoxvaoM/s1600/Screen+shot+2011-07-16+at+8.19.49+PM.png" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" src="http://1.bp.blogspot.com/--Mn_IH_Iq2c/TiI41_yLbjI/AAAAAAAAAfo/aRKNBoxvaoM/s400/Screen+shot+2011-07-16+at+8.19.49+PM.png" width="500" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span class="Apple-style-span" style="font-size: small;"><span class="Apple-style-span" style="font-size: small;"><span class="Apple-style-span" style="font-size: 13px;">N-S cross section in the Gulf of Mexico. Large volumes of the Jurassic Louann salt have been displaced and squeezed into a salt canopy surrounded by much younger sediments. From Pilcher et al., 2011</span></span></span></td></tr>
</tbody></table>
And there is quite a bit left to explore and understand.<br />
<br />
<b>References and further reading</b><br />
Barton, D. C. (1926) The American Salt-Dome Problems in the Light of the Roumanian and German Salt Domes, AAPG Bulletin, v. 9, p. 1227–1268.<br />
<br />
Barton, D. C. (1933) Mechanics of Formation of Salt Domes with Special Reference to Gulf Coast Salt Domes of Texas and Louisiana, AAPG Bulletin, v. 17, 1025–1083.<br />
<br />
Hudec, M., & Jackson, M. (2007) Terra infirma: Understanding salt tectonics. Earth Science Reviews, 82(1-2), 1–28.<br />
<br />
Jackson, M. (1996) Retrospective salt tectonics, in M.P.A. Jackson, D.G. Roberts, and S. Snelson, eds., Salt tectonics: a global perspective: AAPG Memoir 65, p. 1–28. [great summary of the history of salt tectonics]<br />
<br />
Mrazec, L. (1907) Despre cute cu sȋmbure de străpungere [On folds with piercing cores]: Bul. Soc. Stiint., Romania, v. 16, p. 6–8.<br />
<br />
Nettleton, L. L. (1934) Fluid Mechanics of Salt Domes, AAPG Bulletin, v. 18, p. 1–30.<br />
<br />
Pilcher, R. S., Kilsdonk, B., & Trude, J. (2011) Primary basins and their boundaries in the deep-water northern Gulf of Mexico: Origin, trap types, and petroleum system implications. AAPG Bulletin, v. 95(2), p. 219–240.<br />
<br />
Wade, A. (1931) Intrusive salt bodies in coastal Asir, south western Arabia: Institute of Petroleum Technologists Journal, v. 17, p. 321–330, 357–361.<br />
<br />Anonymoushttp://www.blogger.com/profile/02317685391212630354noreply@blogger.com0tag:blogger.com,1999:blog-6377410.post-44928730037810756952011-07-08T09:00:00.000-05:002011-07-08T09:00:06.045-05:00Where on Google Earth? WoGE #296Hindered Settling hasn't hosted a Where-on-Google-Earth <a href="http://zsylvester.blogspot.com/2008/07/where-on-google-earth-138.html">in a long time</a>, but <a href="http://andiwhere.wordpress.com/2011/06/29/where-on-googleearth-woge-295/">WoGE #295 (hosted at Andiwhere's)</a> had such a range of colors and geological features that I couldn't refrain from looking for it and, once found it, had to post the solution. So, after a short break in the game (busy week!) here is WoGE #296 -- the rules of the game are nicely described over <a href="http://woge-felix.blogspot.com/p/rules-of-geological-whereongoogleearth.html">here</a>. I invoke the <a href="http://lablemminglounge.blogspot.com/2007/07/where-on-google-earth-33.html?showComment=1184537160000#c7649395639509891078">Schott rule</a>. Posting time is<b> July 8, 2011, 14:00 UTC</b>.<br />
<br />
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="http://2.bp.blogspot.com/-VzIvbwq6umE/ThcJkj2Fo_I/AAAAAAAAAe8/motTST4D9PA/s1600/WOGE296.png" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" src="http://2.bp.blogspot.com/-VzIvbwq6umE/ThcJkj2Fo_I/AAAAAAAAAe8/motTST4D9PA/s640/WOGE296.png" width="500" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span class="Apple-style-span" style="font-size: small;"><span class="Apple-style-span" style="font-size: small;"><span class="Apple-style-span" style="font-size: 13px;">click on image for larger view</span></span></span></td></tr>
</tbody></table>
Anonymoushttp://www.blogger.com/profile/02317685391212630354noreply@blogger.com5tag:blogger.com,1999:blog-6377410.post-9840520470669062702011-04-17T21:22:00.001-05:002011-04-17T21:48:00.884-05:00Stretching the truth: vertical exaggeration of seismic data<span style="float: left; padding-bottom: 5px; padding-left: 5px; padding-right: 5px; padding-top: 5px;"><a href="http://www.researchblogging.org/"><img alt="ResearchBlogging.org" src="http://www.researchblogging.org/public/citation_icons/rb2_large_gray.png" style="border: 0;" /></a></span>If someone showed a photograph of the famous Cuernos massif (Torres del Paine National Park, Chile) like the one below, it would be - probably, hopefully - obvious to everybody that something is wrong with the picture. Our eyes and brains have seen enough mountain scenery that we intuitively know how steep is 'steep' in alpine landscapes. The peaks in this photograph just look too extreme, too high if one takes into account their lateral extent.<br />
<br />
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="http://1.bp.blogspot.com/-SNujmsggaks/TatY0_G6PaI/AAAAAAAAAeo/1QKFWpyGLiQ/s1600/DSC_3529_vert_exag.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" src="http://1.bp.blogspot.com/-SNujmsggaks/TatY0_G6PaI/AAAAAAAAAeo/1QKFWpyGLiQ/s640/DSC_3529_vert_exag.jpg" width="500" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">The Cuernos in Torres del Paine National Park, Chile, vertically exaggerated by a factor of two.</td></tr>
</tbody></table>
For comparison, here is the original shot:<br />
<br />
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="http://1.bp.blogspot.com/-unAWiaq_ejE/TataPnr_VfI/AAAAAAAAAes/EV5EPkA4MIE/s1600/DSC_3529.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" src="http://1.bp.blogspot.com/-unAWiaq_ejE/TataPnr_VfI/AAAAAAAAAes/EV5EPkA4MIE/s640/DSC_3529.jpg" width="500" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">The Cuernos, beautiful, without exaggeration.</td></tr>
</tbody></table>
Yet this kind of vertical stretching of images is the rule rather than the exception when displaying seismic sections, both on computer screens and in scientific papers. There are two main reasons for this: first, subsurface geometries are often more obvious when vertically stretched and slopes are larger than in real life. Second, more often than not, we have no precise knowledge of the actual vertical scale. Raw seismic reflection data is recorded in time: we are measuring how long it takes before a seismic wave propagates down to a discontinuity and comes back to the surface. This time measure is called 'two-way traveltime'; in order to convert it to depth, knowledge of the velocity of sound through the rocks is needed:<br />
<br />
<div style="text-align: center;">
depth = seismic_velocity * two-way_traveltime / 2</div>
<div style="text-align: left;">
<br /></div>
<div style="text-align: left;">
The problem is that the velocity of the wave varies as it goes deeper (usually increases with depth as rocks become 'harder'); and, unless we are looking at perfect layercake stratigraphy (not that common!), it also changes laterally. So, if we want to look at the actual geological structures, without distortions due to varying velocities, we need to do a depth conversion and we need a 'velocity model' that roughly describes the spatial distribution of velocities. Precise velocity measurements often come from wells where depth is well known; less precise estimates can be backed out from the seismic recordings themselves, but the solution is often non-unique and multiple iterations are necessary to build a good velocity- and depth model. As a result, seismic reflection data is often interpreted with two-way traveltime on the vertical axis, without depth conversion; and not knowing the true vertical scale makes it easier to use vertical exaggeration with vengeance. </div>
<br />
A recent paper, published in Marine and Petroleum Geology, shows that vertical exaggeration of seismic data is indeed very common. Simon Stewart of Heriot-Watt University has looked through 1437 papers published between 2006-2010 and found that 75% of the papers show seismic displays with vertical exaggeration of a factor larger than 2. Only 12% are shown with roughly equal horizontal and vertical scales.<br />
<br />
<br />
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="http://1.bp.blogspot.com/-bvvXKnW2z0g/TatyDuDcOGI/AAAAAAAAAew/TU8lEfoFx4E/s1600/vertical+exaggeration+data.png" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="306" src="http://1.bp.blogspot.com/-bvvXKnW2z0g/TatyDuDcOGI/AAAAAAAAAew/TU8lEfoFx4E/s400/vertical+exaggeration+data.png" width="400" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">Histogram of vertical exaggerations in 1437 papers. From Stewart (2011).</td></tr>
</tbody></table>
One of the effects of vertical exaggeration is the strong steepening of dips. A 10 degree slope at a vertical exaggeration of 10 becomes an almost vertical drop of 60 degrees; it is hard not to think of these exaggerated slopes as steep slopes, even though they are not that abrupt in reality. Depositional geometries often have very small dips and significant vertical exaggeration is needed to illustrate the overall shapes.<br />
<br />
The paper suggests that published seismic sections should be labeled with an estimate of the vertical exaggeration, in addition to the usual horizontal and vertical scales [I am guilty myself of not doing this as it should be done]. Even better, one can go further and create several versions of the figure with different vertical exaggerations. The cross section of a submarine lobe deposit below is a fine example of such a display. Showing only the version that was exaggerated vertically 25 times would suggest that this is a deposit at the base of a steep slope; the 1:1 figure at the top brings us back to reality and clearly shows that this morphology and stratigraphy are both extremely flat.<br />
<br />
<br />
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="http://3.bp.blogspot.com/-2y9vYnE3qkU/Tatz6ejeClI/AAAAAAAAAe4/-yUMyenJYbE/s1600/deptuck+example.png" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" src="http://3.bp.blogspot.com/-2y9vYnE3qkU/Tatz6ejeClI/AAAAAAAAAe4/-yUMyenJYbE/s400/deptuck+example.png" width="500" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span class="Apple-style-span" style="font-size: small;"><span class="Apple-style-span" style="font-size: small;"><span class="Apple-style-span" style="font-size: 13px;">Dip section of a submarine lobe deposit, offshore Corsica. From Deptuck et al. (2008).</span></span></span></td></tr>
</tbody></table>
To see more about scales and vertical exaggeration in geology, check out <a href="http://all-geo.org/highlyallochthonous/2011/04/not-to-scale/">this recent post at Highly Allochthonous</a>; and the <a href="http://www.agilegeoscience.com/journal/2011/4/11/scales-of-sea-level-change.html">nice</a> <a href="http://www.agilegeoscience.com/journal/2011/4/7/the-scales-of-geoscience.html">illustrations</a> that Matt has put together over at Agile*.<br />
<br />
<b>References</b><br />
<br />
<span class="Z3988" title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.jtitle=Sedimentology&rft_id=info%3Adoi%2F10.1111%2Fj.1365-3091.2007.00926.x&rfr_id=info%3Asid%2Fresearchblogging.org&rft.atitle=Dimensions+and+architecture+of+late+Pleistocene+submarine+lobes+off+the+northern+margin+of+East+Corsica&rft.issn=00370746&rft.date=2008&rft.volume=55&rft.issue=4&rft.spage=869&rft.epage=898&rft.artnum=http%3A%2F%2Fdoi.wiley.com%2F10.1111%2Fj.1365-3091.2007.00926.x&rft.au=Deptuck%2C+M.&rft.au=Piper%2C+D.&rft.au=Savoye%2C+B.&rft.au=Gervais%2C+A.&rfe_dat=bpr3.included=1;bpr3.tags=Geosciences%2CGeology%2C+Oceanography">Deptuck, M., Piper, D., Savoye, B., & Gervais, A. (2008). Dimensions and architecture of late Pleistocene submarine lobes off the northern margin of East Corsica <span style="font-style: italic;">Sedimentology, 55</span> (4), 869-898 DOI: <a href="http://dx.doi.org/10.1111/j.1365-3091.2007.00926.x" rev="review">10.1111/j.1365-3091.2007.00926.x</a></span>
<br />
<br />
<span class="Z3988" title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.jtitle=Marine+and+Petroleum+Geology&rft_id=info%3Adoi%2F10.1016%2Fj.marpetgeo.2010.10.003&rfr_id=info%3Asid%2Fresearchblogging.org&rft.atitle=Vertical+exaggeration+of+reflection+seismic+data+in+geoscience+publications+2006%E2%80%932010&rft.issn=02648172&rft.date=2011&rft.volume=28&rft.issue=5&rft.spage=959&rft.epage=965&rft.artnum=http%3A%2F%2Flinkinghub.elsevier.com%2Fretrieve%2Fpii%2FS0264817210001935&rft.au=Stewart%2C+S.&rfe_dat=bpr3.included=1;bpr3.tags=Geosciences">Stewart, S. (2011). Vertical exaggeration of reflection seismic data in geoscience publications 2006–2010 <span style="font-style: italic;">Marine and Petroleum Geology, 28</span> (5), 959-965 DOI: <a href="http://dx.doi.org/10.1016/j.marpetgeo.2010.10.003" rev="review">10.1016/j.marpetgeo.2010.10.003</a></span><br />
<br />Anonymoushttp://www.blogger.com/profile/02317685391212630354noreply@blogger.com9tag:blogger.com,1999:blog-6377410.post-20169903385579582302011-01-15T22:41:00.003-06:002011-01-16T00:16:07.740-06:00Snorkeling and geology in Kealakekua Bay, Big Island, Hawaii<br />
For a long time, I didn't think it was worth spending more than an hour on a beach, even the most beautiful ones, unless there were some nice cliffs nearby showing some interesting geology. My views in this regard have changed dramatically about three years ago, when I spent a week on The Big Island of Hawaii, and the hotel where we were staying offered free rental of snorkeling gear. I put on the mask and the fins, trying to remember how this was supposed to work (I did a bit of snorkeling in Baja California many years before that), and put my face into the not-too-interesting-looking waters in the front of the hotel.
<br />
<br />
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="http://1.bp.blogspot.com/_zWNW_P1SBcA/TSjAV_3XrBI/AAAAAAAAAdo/RbuWca1h5zM/s1600/Kealakekua%2BGoogle%2BEarth.png" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" height="450" src="http://1.bp.blogspot.com/_zWNW_P1SBcA/TSjAV_3XrBI/AAAAAAAAAdo/RbuWca1h5zM/s400/Kealakekua%2BGoogle%2BEarth.png" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">Kealakekua Bay in Google Earth, with some explanations added</td></tr>
</tbody></table>
<br />
I was in for a surprise. The water was far from crystal clear, but I could still see fantastic coral creations lined up along the bay and lots of fish of so many colors and patterns that it felt unreal. Until then I thought that this kind of scenery was hard to see unless you were a filmmaker working for Discovery Channel or a marine biologist specializing in tropical biodiversity. The next day I spotted a couple of green turtles frolicking in the water, clearly not bothered by the nearby snorkelers, and I already knew that I needed to look into the possibility of buying a simple underwater camera.
<br />
<br />
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="http://skeptic.smugmug.com/Nature/Hawaii-2011/IMG1870/1147445983_mnXbE-XL.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" src="http://skeptic.smugmug.com/Nature/Hawaii-2011/IMG1870/1147445983_mnXbE-XL.jpg" width="500" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">Lots of coral, mostly belonging to the genera Lobites (lobe coral) and Pocillopora (cauliflower coral)</td></tr>
</tbody></table>
<br />
Three years later I went back to the Big Island with more excitement about tropical beaches, plus bigger plans and a bit more knowledge about snorkeling. After going through a few well-known snorkeling sites on the west coast, like Kahalu'u Beach in Kona and Two Step near Pu'uhonua o Honaunau park, we got on a nice boat (run by a company called <a href="http://www.fair-wind.com/">Fair Wind</a> - strongly recommended!) and did some snorkeling in Kealakekua Bay.<br />
<br />
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="http://skeptic.smugmug.com/Nature/Hawaii-2011/IMG1853/1147441866_c45cB-XL.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" src="http://skeptic.smugmug.com/Nature/Hawaii-2011/IMG1853/1147441866_c45cB-XL.jpg" width="500" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">Visibility in Kealakekua Bay is usually very good</td></tr>
</tbody></table>
<br />
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="http://skeptic.smugmug.com/Nature/Hawaii-2011/IMG2030/1147435553_dmZqd-XL.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" src="http://skeptic.smugmug.com/Nature/Hawaii-2011/IMG2030/1147435553_dmZqd-XL.jpg" width="500" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">Old wrinkles of pahoehoe lava getting encrusted by algae and corals and chewed up by sea urchins</td></tr>
</tbody></table>
<br />
Kealakekua Bay is difficult to reach; there is no road and no parking lot nearby. You either have to hike in, paddle through the bay in a kayak, or take a boat. I have heard before that this was the best snorkeling spot in Hawai'i, but I think that is an understatement. Unlike all the other spots we tried during the last few years in Hawaii (and that includes several beaches on Kauai and Hanauma Bay on Oahu, the presidential snorkeling site), the water at Kealakekua Bay was calm and very clear, with fantastic visibility.<br />
<br />
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="http://skeptic.smugmug.com/Nature/Hawaii-2011/IMG2034/1147436466_QV7py-XL.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" src="http://skeptic.smugmug.com/Nature/Hawaii-2011/IMG2034/1147436466_QV7py-XL.jpg" width="500" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">Heads of cauliflower coral, with yellow tangs for scale</td></tr>
</tbody></table>
<br />
I will not attempt to describe this whole new world; instead I will let the photographs speak for themselves (as always, more photos at <a href="http://skeptic.smugmug.com/Nature/Hawaii-2011/15319256_y4Fny">Smugmug</a>). Even better, if you go to the Big Island, make sure that you visit this place with some snorkeling gear.<br />
<br />
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="http://skeptic.smugmug.com/Nature/Hawaii-2011/IMG2054/1147438369_kzwkv-XL.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" src="http://skeptic.smugmug.com/Nature/Hawaii-2011/IMG2054/1147438369_kzwkv-XL.jpg" width="500" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">Yellow tangs (Zebrasoma flavescens) often congregate in large schools and it is difficult to stop taking pictures of them</td></tr>
</tbody></table>
<br />
When I was at Kealakekua Bay, I didn't know much about the local geology. The big cliff bordering the bay toward the northwest, called Pali Kapu o Keoua (see image above), shows a number of layered lava flows that belong to the western flank of Mauna Loa; and I suspected that this must have been a large fault scarp, but that was the end of my geological insight. A couple of hours worth of research after I got home revealed that Pali Kapu o Keoua was a fault indeed: it is called the Kealakekua Fault and it has been mapped, along with the associated submarine geomorphological features, in the 1970s and 1980s by U.S. Geological Survey geoscientists. It turns out that one of the shipboard scientists and key contributors to these studies was <a href="http://soundwaves.usgs.gov/2008/06/staff.html">Bill Normark</a> (see also a <a href="http://www.wired.com/wiredscience/2010/04/geological-heroes-marine-geologist-bill-normark/">post about Bill at Clastic Detritus</a>). While in California in the late 1990s, I was lucky to get to know Bill and have some truly inspiring discussions with him about turbidites, geology, and wine, so this was a doubly valuable little discovery to me.<br />
<br />
So what is the origin of the Kealakekua Fault? The Hawaiian Islands are far away from any tectonic plate boundaries, so there is not a lot of opportunity here for inverse or strike-slip faults to develop. However, the Hawaiian volcanoes are humongous mountains and their underwater slopes are extremely steep by submarine slope standards: gradients of 15-10˚ are common. [This is in contrast by the way with the relatively gentle slopes of 3-8˚ the subaerial flanks of the volcanoes, a difference that - it just occurred to me - has to do something with the different thermal conductivities of water and air. Water is ~24 times more efficient at cooling lavas, or anything for that matter, than air, so once a volcano sticks its head out of the water, basaltic lava flows are pretty efficient at carrying volcanic material far away from the crater, thus building gently sloping shield volcanoes. The same flows are promptly solidified and stopped by the cool ocean waters as soon as they reach the coast.] Slopes that are this steep are also unstable; the underwater parts of these volcanoes tend to fail from time to time and large volumes of rock rapidly move to deeper waters as giant submarine landslides. Seafloor mapping around the islands revealed that the underwater topography is far from smooth; instead, in many places it consists of huge slide and slump blocks.
<br />
<br />
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="http://1.bp.blogspot.com/_zWNW_P1SBcA/TTJR_s2yihI/AAAAAAAAAd0/5mg3ar4L8LY/s1600/big_island_morphology.png" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" src="http://1.bp.blogspot.com/_zWNW_P1SBcA/TTJR_s2yihI/AAAAAAAAAd0/5mg3ar4L8LY/s1600/big_island_morphology.png" width="500" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">Topographic map of the Big Island. Note the location of Kealakekua Fault and the rugged seafloor to the southwest of it, marking the area affected by slides and slumps. This is a map based on higher-resolution bathymetric data collected during a collaborative effort led by JAMSTEC (Japan Marine Science and Technology Center). Source: <a href="http://geopubs.wr.usgs.gov/i-map/i2809/">U.S. Geological Survey Geologic Investigations Series I-2809</a> </td></tr>
</tbody></table>
<br />
Kealakekua Fault is probably part of the head scarp of one such giant landslide, called the Alika landslide. This explains the steep slopes in the bay itself: after a narrow wave-cut platform, a spectacular wall covered with coral - the continuation of the cliff that you can see onshore - dives into the deep blue of the ocean as you float away from the shore. In contrast with submarine landslides that involve well stratified sediments failing along bedding surfaces and forming relatively thin but extensive slide deposits, the Hawaiian failures affect thick stacks of poorly layered volcanic rock and, as a result, both their volumes and morphologic relief are larger (see the paper by Lipman et al, 1988). The entire volume of the Alika slide is estimated to be 1500-2000 cubic kilometers. That is about a hundred times larger than all the sediment carried by the world's rivers to the ocean in one year! The slides have moved at highway speeds and generated tsunamis. There is evidence on Lanai island for a wave that carried marine debris to 325 meters above sea level; this tsunami was likely put in motion by the Alika landslide*.<br />
<br />
You don't want to be snorkeling in Kealakekua Bay when something like that happens. And it will happen again, it is a matter of (geological) time. <a href="http://walrus.wr.usgs.gov/posters/underlandslides.html">Giant underwater landslides</a> are part of the normal life of these mid-ocean, hotspot-related volcanoes.
<br />
<br />
<b>Reference</b><br />
Lipman, P., Normark, W., Moore, J., Wilson, J., Gutmacher, C., 1988, The giant submarine Alika debris slide, Mauna Loa, Hawaii. Journal of Geophysical Research, vol. 93, p. 4279-4299.<br />
<br />
<span class="Apple-style-span" style="font-size: x-small;">*tsunamis generated by landslides is a whole new exciting subject that we have no time now to dive or snorkel into.</span>Anonymoushttp://www.blogger.com/profile/02317685391212630354noreply@blogger.com2tag:blogger.com,1999:blog-6377410.post-13794198491856885662010-11-25T00:27:00.003-06:002010-11-25T07:37:07.169-06:00Morphology of a forced regression<br />
'Forced regression' is an important concept in sequence stratigraphy - it occurs when <i>relative</i> sea level falls and the shoreline shifts in a seaward direction, regardless of how much sediment is delivered to the sea. This is in contrast with 'normal' regressions, which take place when relative sea level doesn't change or it is rising, but rivers bring lots of sediment to the coast and are able to push the shoreline seaward. These concepts are commonly illustrated with simple cartoons (<a href="http://sepmstrata.org/terminology/forced-reg.html">like the ones on the SEPM sequence stratigraphy website</a>), showing how beach deposits stack in a dip direction, and how their tops are eroded by rivers as sea level continues to fall.<br />
<br />
Unless you live in a horizontally challenged flatland (vertical-land? 2D seismic-land?), real regressions happen in three dimensions, and their morphology is much more complicated, more interesting, and more beautiful than what one can dream up with a few lines in a single cross section. The example below is an airborne lidar image from Finland. The original data has a horizontal resolution of 2 meters and a vertical resolution of 30 centimeters.
<br />
<br />
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="http://3.bp.blogspot.com/_zWNW_P1SBcA/TO3vMNwLKaI/AAAAAAAAAc4/U4TXwAGOsUU/s1600/4513371053_24ba962440_b.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" src="http://3.bp.blogspot.com/_zWNW_P1SBcA/TO3vMNwLKaI/AAAAAAAAAc4/U4TXwAGOsUU/s400/4513371053_24ba962440_b.jpg" width="500" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">Airborne lidar image of uplifted coastal plain in Finland<br />
Image courtesy of Jouko Vanne, Geological Survey of Finland</td></tr>
</tbody></table>
The two dominant morphologies and deposit types clearly visible in the image are (1) ancient coastlines, formed as sand brought to the sea by rivers was reworked by waves into beach ridges; and (2) an incised river valley that cuts through these shoreline deposits. Note how the river seems to be incising and migrating laterally at the same time, generating a scalloped valley edge. The reason for this forced regression during a time of global sea-level rise is the isostatic rebound of the Scandinavian Peninsula after the retreat of the ice sheet.
<br />
<br />
Looking at this crystal-clear morphology, it is tempting to think that this area must look very interesting in Google Earth as well. It turns out that it doesn't; this is actually a pretty heavily vegetated land, not too spectacular on conventional satellite imagery (see figure below). The laser rays of the lidar are able to see through the non-geomorphological 'noise' and show stunning geomorphological detail.
<br />
<br />
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="http://1.bp.blogspot.com/_zWNW_P1SBcA/TO38pj0_BlI/AAAAAAAAAdA/yrzq0NuhL0c/s1600/finland_valley.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" src="http://1.bp.blogspot.com/_zWNW_P1SBcA/TO38pj0_BlI/AAAAAAAAAdA/yrzq0NuhL0c/s400/finland_valley.jpg" width="500" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">Comparison of satellite image from Google Earth with detail of lidar topography</td></tr>
</tbody></table>
To explore a higher resolution version of this image, and for additional lidar visualizations of similar beauty, check out <a href="http://www.flickr.com/photos/65726513@N00/sets/72157622857163259/">Jouko Vanne's Flickr site</a>. The National Land Survey of Finland has started collecting this kind of data in 2008 and they are planning to cover the whole country with high-resolution DEMs within a few years.
<br />
<br />
A great way to spend taxpayer money, as far as I am concerned.
<br />
<br />Anonymoushttp://www.blogger.com/profile/02317685391212630354noreply@blogger.com3tag:blogger.com,1999:blog-6377410.post-45878963146723137952010-11-20T20:10:00.003-06:002010-11-26T11:22:33.988-06:00Einstein, tea leaves, meandering rivers, and beer<br />
<span style="float: left; padding-bottom: 5px; padding-left: 5px; padding-right: 5px; padding-top: 5px;"><a href="http://www.researchblogging.org/"><img alt="ResearchBlogging.org" src="http://www.researchblogging.org/public/citation_icons/rb2_large_gray.png" style="border: 0;" /></a></span> If you make your tea the old-fashioned way, ending up with a few tea leaves at the bottom of the teacup, and you start stirring the tea, you would expect the leaves to move outward, due to the push of the centrifugal force. Instead the leaves follow a spiral trajectory toward the center the cup. The physical processes that result in this 'tea leaf paradox' are essentially the same as the ones responsible for building point bars in meandering rivers. It turns out that the first scientist to make this connection and analogy was none other than Albert Einstein.
<br />
<br />
In a <a href="http://dx.doi.org/10.1007%2FBF01510300">paper published in 1926</a> (English translation <a href="http://www.ucalgary.ca/~kmuldrew/river.html">here</a>), Einstein first explains how the velocity of the <strike>fluid</strike> tea flow is smaller at the bottom of the cup than higher up, due to friction at the wall. [The velocity has to decrease to zero at the wall, a constraint called 'no-slip condition' in fluid mechanics.] To Einstein it is obvious that "the result of this will be a circular movement of the liquid" in the vertical plane, with the liquid moving toward the center at the bottom of the cup and outward at the surface (see the figure below). For us, it is probably useful to think things out in a bit more detail.
<br />
<br />
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="http://4.bp.blogspot.com/_zWNW_P1SBcA/TOhjLXSdBzI/AAAAAAAAAb8/Q_fmOUjvaMA/s1600/Screen+shot+2010-11-20+at+6.06.37+PM.png" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" src="http://4.bp.blogspot.com/_zWNW_P1SBcA/TOhjLXSdBzI/AAAAAAAAAb8/Q_fmOUjvaMA/s320/Screen+shot+2010-11-20+at+6.06.37+PM.png" width="220" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;"><span class="Apple-style-span" style="font-size: small;"><span class="Apple-style-span" style="font-size: small;"><span class="Apple-style-span" style="font-size: 13px;">Einstein's illustration of secondary flow in a teacup</span></span></span></td></tr>
</tbody></table>
A smaller velocity at the bottom means a reduced centrifugal force as well. But overall, the tea is being pushed toward the sidewalls of the cup, and this results in the water surface being higher at the sidewalls than at the center. The pressure gradient that is created this way is constant throughout the whole <strike>water</strike> tea column, and overall it balances the centrifugal force (unless you stir so hard that the tea spills over the lips). This means that the centrifugal force wins at the top, creating a velocity component that points outward, but loses at the bottom, creating a so-called secondary flow that is pointing inward. The overall movement of the liquid has a helical pattern; in fact, those components of the velocity that act in a direction perpendicular to the main rotational direction are usually an order of magnitude smaller than the primary flow.
<br />
<br />
Einstein goes on to suggest that the "same sort of thing happens with a curving stream". He also points out that, even if the river is straight, the strength of the Coriolis force resulting from the rotation of the Earth will be different at the bottom and at the surface, and this induces a helical flow pattern similar to that observed in meandering rivers. [This force and its effects on sedimentation and erosion are much smaller than the 'normal' helical flow in rivers.] In addition, the largest velocities will develop toward the outer bank of the river, where "erosion is necessarily stronger" than on the inner bank.
<br />
<br />
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="http://1.bp.blogspot.com/_zWNW_P1SBcA/TOh4ELeNdmI/AAAAAAAAAc0/aAji0enSICw/s1600/sketch+of+secondary+circulation-01.jpg" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" src="http://1.bp.blogspot.com/_zWNW_P1SBcA/TOh4ELeNdmI/AAAAAAAAAc0/aAji0enSICw/s320/sketch+of+secondary+circulation-01.jpg" width="520" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">Secondary flow in a river, the result of reduced centrifugal forces at the bottom</td></tr>
</tbody></table>
<br />
I find the tea-leaf analogy an excellent way to explain the development of river meanders and point bars; just like tea leaves gather in the middle of the cup, sand grains are most likely to be left behind on the inner bank of a river bend. Yet Einstein's paper is usually not mentioned in papers discussing river meandering -- an interesting omission since a reference to Einstein always lends more weight and importance to one's paper (or blog post).<br />
<br />
A recent and very interesting exception is a paper published in Sedimentology. It is titled "<a href="http://onlinelibrary.wiley.com/doi/10.1111/j.1365-3091.2009.01109.x/abstract">Fluvial and submarine morphodynamics of laminar and near-laminar flows: a synthesis</a>" and points out how laminar flows can generate a wide range of depositional forms and structures, like channels, ripples, dunes, antidunes, alternate bars, multiple-row bars, meandering and braiding, forms that are often considered unequivocal signs of turbulent flow. [This issue of Sedimentology is open access, so do click on the link and check out the paper!]. As they start discussing meandering rivers and point bars, Lajeunesse et al. suggest that Einstein's teacup is extremely different dynamically from the Mississippi River, yet it can teach us about how point bars form:
<br />
<blockquote>
A flow in a teacup with a Reynolds number of the order of 10<sup>2</sup> cannot possibly satisfy Reynolds similarity with the flow in the bend of, for example, the Mississippi River, for which the Reynolds number is of the order of 10<sup>7</sup>. Can teacups then be used to infer river morphodynamics? </blockquote>
<blockquote>
The answer is affirmative. When dynamical similarity is rigorously satisfied, the physics of the two flows are identical. However, even when dynamical similarity is not satisfied, it is possible for a pair of flows to be simply two different manifestations of the same phenomenon, both of which are described by a shared physical framework. Any given analogy must not be overplayed because the lack of complete dynamic similarity implies that different features of a phenomenon may be manifested with different relative strengths. This shared framework nevertheless allows laminar-flow morphodynamics to shed useful light on turbulent-flow analogues.</blockquote>
Apart from helping understand river meandering, the tea leaf paradox <a href="http://bmf.aip.org/resource/1/biomgb/v1/i1/p014103_s1">has inspired a gadget that separates red blood cells from blood plasma</a>; and helps getting rid of <i>trub</i> (sediment remaining after fermentation) from beer.
<br />
<br />
That explains the 'beer' part of the title. And it is time to have one.
<br />
<br />
<b>References</b>
<br />
<br />
<span class="Z3988" title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.jtitle=Die+Naturwissenschaften&rft_id=info%3Adoi%2F10.1007%2FBF01510300&rfr_id=info%3Asid%2Fresearchblogging.org&rft.atitle=Die+Ursache+der+M%EF%BF%BDanderbildung+der+Flu%EF%BF%BDl%EF%BF%BDufe+und+des+sogenannten+Baerschen+Gesetzes&rft.issn=0028-1042&rft.date=1926&rft.volume=14&rft.issue=11&rft.spage=223&rft.epage=224&rft.artnum=http%3A%2F%2Fwww.springerlink.com%2Findex%2F10.1007%2FBF01510300&rft.au=Einstein%2C+A.&rfe_dat=bpr3.included=1;bpr3.tags=Geosciences%2CPhysics">Einstein, A. (1926). Die Ursache der Meanderbildung der Flusslaufe und des sogenannten Baerschen Gesetzes <span style="font-style: italic;">Die Naturwissenschaften, 14</span> (11), 223-224 DOI: <a href="http://dx.doi.org/10.1007/BF01510300" rev="review">10.1007/BF01510300</a></span>
<br />
<br />
<span class="Z3988" title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.jtitle=Sedimentology&rft_id=info%3Adoi%2F10.1111%2Fj.1365-3091.2009.01109.x&rfr_id=info%3Asid%2Fresearchblogging.org&rft.atitle=Fluvial+and+submarine+morphodynamics+of+laminar+and+near-laminar+flows%3A+a+synthesis&rft.issn=00370746&rft.date=2010&rft.volume=57&rft.issue=1&rft.spage=1&rft.epage=26&rft.artnum=http%3A%2F%2Fdoi.wiley.com%2F10.1111%2Fj.1365-3091.2009.01109.x&rft.au=LAJEUNESSE%2C+E.&rft.au=MALVERTI%2C+L.&rft.au=LANCIEN%2C+P.&rft.au=ARMSTRONG%2C+L.&rft.au=M%C3%83%C2%89TIVIER%2C+F.&rft.au=COLEMAN%2C+S.&rft.au=SMITH%2C+C.&rft.au=DAVIES%2C+T.&rft.au=CANTELLI%2C+A.&rft.au=PARKER%2C+G.&rfe_dat=bpr3.included=1;bpr3.tags=Geosciences%2CPhysics">Lajeunesse, E., Malverti, L., Lancien, P., Armstrong, L., Metivier, F., Coleman, S., Smith, C., Davies, T., Cantelli, A., & Parker, G. (2010). Fluvial and submarine morphodynamics of laminar and near-laminar flows: a synthesis <span style="font-style: italic;">Sedimentology, 57</span> (1), 1-26 DOI: <a href="http://dx.doi.org/10.1111/j.1365-3091.2009.01109.x" rev="review">10.1111/j.1365-3091.2009.01109.x</a></span>
<br />
<br />Anonymoushttp://www.blogger.com/profile/02317685391212630354noreply@blogger.com0tag:blogger.com,1999:blog-6377410.post-29052815460897850942010-09-12T00:43:00.005-05:002010-11-25T00:54:37.957-06:00Wave ripples on an eroding beach<br />
I shot these photos in 2003, at <a href="http://www.tpwd.state.tx.us/spdest/findadest/parks/sea_rim/">Sea Rim State Park</a> in east Texas, close to the border with Louisiana, a relatively remote and beautiful state park along the Gulf coast that suffered a lot of damage during both Hurricane Rita in 2005 and Hurricane Ike in 2008. On that chilly November day the light was great and the variety of shapes and patterns created by wave ripples and exposed during low tide was amazing.<br />
<div style="text-align: left;">
<br /></div>
<div style="text-align: center;">
<a href="http://skeptic.smugmug.com/Landscapes/Sea-Rim-State-Park-Texas-2003/DSC03046/36405212_Xd4Pp-XL-1.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" src="http://skeptic.smugmug.com/Landscapes/Sea-Rim-State-Park-Texas-2003/DSC03046/36405212_Xd4Pp-XL-1.jpg" width="500" /></a></div>
<div class="separator" style="clear: both; text-align: center;">
<br /></div>
<div class="separator" style="clear: both; text-align: left;">
Wave ripples are more symmetric than current ripples. Needless to say, wave ripples originate thanks to the back-and-forth movement of sand by waves, whereas current ripples form under unidirectional flows (like rivers and turbidity currents). Wave ripples are also more regular than current ripples, extend for much longer distances laterally, and often terminate - or continue - in 'Y'-shaped junctions. For the same wavelength, they are also taller; the L/H ratio of most wave ripples is between 4 and 10, in contrast with current ripples that have an L/H value of ~20.</div>
<div class="separator" style="clear: both; text-align: center;">
<br /></div>
<div class="separator" style="clear: both; text-align: center;">
<a href="http://skeptic.smugmug.com/Landscapes/Sea-Rim-State-Park-Texas-2003/DSC03049/36405271_6dECr-XL-1.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" src="http://skeptic.smugmug.com/Landscapes/Sea-Rim-State-Park-Texas-2003/DSC03049/36405271_6dECr-XL-1.jpg" width="500" /></a></div>
<div class="separator" style="clear: both; text-align: center;">
<br /></div>
<div class="separator" style="clear: both; text-align: left;">
Perfectly symmetrical ripples form under bidirectional currents that are perfectly symmetrical themselves; but this tends to be the exception rather than the rule, as shoaling waves create a net shore-directed movement of the water. The resulting ripples are asymmetric, with the steeper side facing the coast, but still more symmetric and more regular than pure unidirectional ripples. Weak tidal currents can cause the asymmetry as well. The photo below shows wave ripples with a significant asymmetry that makes them difficult (if not impossible) to distinguish from current ripples.</div>
<div class="separator" style="clear: both; text-align: center;">
<br /></div>
<div class="separator" style="clear: both; text-align: center;">
<a href="http://skeptic.smugmug.com/Landscapes/Sea-Rim-State-Park-Texas-2003/DSC03057/36405475_aXBK6-XL-1.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" src="http://skeptic.smugmug.com/Landscapes/Sea-Rim-State-Park-Texas-2003/DSC03057/36405475_aXBK6-XL-1.jpg" width="500" /></a></div>
<div class="separator" style="clear: both; text-align: center;">
<br /></div>
<div class="separator" style="clear: both; text-align: left;">
This set is also asymmetric, but to a lesser degree:</div>
<div class="separator" style="clear: both; text-align: left;">
<br /></div>
<div class="separator" style="clear: both; text-align: center;">
<a href="http://skeptic.smugmug.com/Landscapes/Sea-Rim-State-Park-Texas-2003/DSC03059/36405556_jrSjh-XL-1.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" src="http://skeptic.smugmug.com/Landscapes/Sea-Rim-State-Park-Texas-2003/DSC03059/36405556_jrSjh-XL-1.jpg" width="500" /></a></div>
<div class="separator" style="clear: both; text-align: center;">
<br /></div>
<div class="separator" style="clear: both; text-align: left;">
Sea Rim State Park is at a location along the Gulf coast where the beach is eroding and the coastline is retreating; it is a typical example of a transgressive coast. The erosion is the results from both sea-level rise and from lack of longshore sediment transport strong enough to nourish the beach with sand. The evidence for the transgression is quite obvious: banks of well-consolidated muds that were originally deposited in the lagoon behind the coastal barrier are being eroded by the advancing waves:</div>
<br />
<div class="separator" style="clear: both; text-align: center;">
<a href="http://skeptic.smugmug.com/Landscapes/Sea-Rim-State-Park-Texas-2003/DSC03033/36404791_krs3J-XL-1.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" src="http://skeptic.smugmug.com/Landscapes/Sea-Rim-State-Park-Texas-2003/DSC03033/36404791_krs3J-XL-1.jpg" width="500" /></a></div>
<br />
More on wave ripples at <a href="http://my.opera.com/nielsol/blog/symmetrical-wave-ripple-marks">Olelog</a>.Anonymoushttp://www.blogger.com/profile/02317685391212630354noreply@blogger.com1tag:blogger.com,1999:blog-6377410.post-5764471373265975362010-08-15T15:29:00.005-05:002010-11-25T00:55:24.449-06:00The complexity of sinuous channel deposits in three dimensions<br /><span style="float: left; padding-bottom: 5px; padding-left: 5px; padding-right: 5px; padding-top: 5px;"><a href="http://www.researchblogging.org/"><img alt="ResearchBlogging.org" src="http://www.researchblogging.org/public/citation_icons/rb2_large_gray.png" style="border: 0;" /></a></span> The beauty of the shapes and patterns created by meandering rivers has long attracted the attention of many geomorphologists, civil engineers, and sedimentologists. Unless they are fairly steep or have highly stable and unerodible banks, rivers do not like to follow a straight course and tend to develop a sinuous plan-view pattern. The description and mathematical modeling of these curves is a fascinating subject, but that is not what I want to talk about here and now. It is hard enough to understand the plan-view evolution of rivers, especially if one is interested in the long-term results - when cutoffs become important -, but things get really complicated when it comes to the three-dimensional structure of the deposits that meandering rivers leave behind. The same can be said about sinuous channels on the seafloor, created and maintained by dirty mixtures of water and sediment (called turbidity currents). An ever-increasing number of seafloor and seismic images show that highly sinuous submarine channels are almost as common as their subaerial counterparts, but much remains to be learned about the geometries of their deposits that accumulate through geological time.<br />
<br />
Using simple modeling of how channel surfaces migrate through time, two recent papers attempt to illustrate the three-dimensional structure of sinuous fluvial and submarine channel deposits. In the Journal of Sedimentary Research, <a href="http://jsedres.sepmonline.org/cgi/content/abstract/80/5/440">Willis and Tang (2010)</a> show how slightly different patterns of fluvial meander migration result in different deposit geometries and different distribution of grain size, porosity and permeability. [These properties are important because they determine how fluids flow - or don't flow - through the pores of the sediment.] River meanders can either grow in a direction perpendicular to the overall downslope orientation, or they can keep the same width and migrate downstream through translation. In the latter case - which is often characteristic of rivers incising into older sediments -, deposits forming on the downstream, concave bank of point bars will be preferentially preserved. These deposits tend to be finer grained than the typical convex-bank point bar sediments. In addition to building a range of models and analyzing their geometries, Willis and Tang also ran simulations of how would oil be displaced by water in them. One of their findings is that sinuous rivers that keep adding sediment in the same area over time (in other words, rivers that aggrade) tend to form better connected sand bodies than rivers which keep snaking around roughly in the same horizontal plane, without aggradation.<br /><br />
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="http://3.bp.blogspot.com/_zWNW_P1SBcA/TGgKT-Ke2wI/AAAAAAAAAZ0/zL9qLP5pisI/s1600/Willis+and+Tang+2010+1.png" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" src="http://3.bp.blogspot.com/_zWNW_P1SBcA/TGgKT-Ke2wI/AAAAAAAAAZ0/zL9qLP5pisI/s400/Willis+and+Tang+2010+1.png" width="450" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">Map of deposits forming as river meanders grow (from Willis and Tang, 2010).</td></tr>
</tbody></table><table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="http://1.bp.blogspot.com/_zWNW_P1SBcA/TGgK5hHYI-I/AAAAAAAAAZ4/kmjcho8CSqI/s1600/Willis+and+Tang+2010+2.png" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" src="http://1.bp.blogspot.com/_zWNW_P1SBcA/TGgK5hHYI-I/AAAAAAAAAZ4/kmjcho8CSqI/s400/Willis+and+Tang+2010+2.png" width="450" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">Cross sections through the deposits of two meander bends (locations shown in figure above). Colors represent permeability, red being highly permeable and blue impermeable sediment. From Willis and Tang, 2010.</td></tr>
</tbody></table>Check out the paper itself for more images like these, plus discussions of concave-bank deposition, cutoff formation, and filling of abandoned channels.<br />
<div class="separator" style="clear: both; text-align: center;"></div><br />
The second paper (<a href="http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V9Y-5070DN7-2&_user=4808056&_coverDate=06%2F02%2F2010&_rdoc=1&_fmt=high&_orig=search&_sort=d&_docanchor=&view=c&_acct=C000050785&_version=1&_urlVersion=0&_userid=4808056&md5=a43c32e4de1b2b908638b41ca2330bfa">Sylvester, Pirmez, and Cantelli, 2010</a>; and yes, one of the authors is also the author of this blog post, so don't expect any constructive criticism here) focuses on submarine channels and their overbank deposits, but the starting point and the modeling techniques are similar: take a bunch of sinuous channel centerlines and generate surfaces around them that reflect the topography of the system at every time step. However, we know much less about submarine channels than fluvial ones, because it is much more difficult to collect data at and from the bottom of the ocean than it is from the river in your backyard. The result is that some of the simplifications in our model are controversial; to many sedimentary geologists, submarine channels and their deposits are fundamentally different from rivers and point bars, and there is not much use in even comparing the two. Part of the problem is that not all submarine channels are made equal, and, when looking at an outcrop, it is not easy - or outright impossible - to tell what kind of geomorphology produced the stratigraphy. In fact, the number of exposures that represent highly sinuous submarine channels, as observed on the seafloor and numerous seismic images, is probably fairly limited. One thing is quite clear, however: many submarine channels show plan-view migration patterns that are very similar to those of rivers, and this large-scale structure imposes some significant constraints on the geometry of the deposits as well.<br />
<br />
That being said, nobody denies that there are plenty of significant differences between real and submarine 'rivers' [note quotation marks]. A very important one is the amount of overbank - or levee - deposition: turbidity currents often overflow their channel banks as thick muddy clouds and form much thicker deposits than the overbank sediment layers typical of rivers. When these high rates of levee deposition combine with the strong three-dimensionality of channel migration, complex geometries result that are quite tricky to understand just by looking at a single cross section.<br /><br />
<div class="separator" style="clear: both; text-align: center;"></div><table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="http://1.bp.blogspot.com/_zWNW_P1SBcA/TGg_XuoB_JI/AAAAAAAAAZ8/WFqVB7EuYu4/s1600/Sylvester+et+al+2010+1.png" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" src="http://1.bp.blogspot.com/_zWNW_P1SBcA/TGg_XuoB_JI/AAAAAAAAAZ8/WFqVB7EuYu4/s400/Sylvester+et+al+2010+1.png" width="500" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">Cross section and chronostratigraphic diagram through a submarine channel system with inner and outer levees (from Sylvester et al., 2010).</td></tr>
</tbody></table>One of the consequences of the channel migration is the formation of erosional surfaces that develop through a relatively long time and do not correspond to a geomorphologic surface at all (see the red erosional zones in the Wheeler diagram above). This difference between stratigraphic and geomorphologic surfaces is essential, yet often downplayed or even ignored in stratigraphy. In terms of geomorphology, the combination of channel movement in both horizontal and vertical directions and the extensive levee deposition results in a wide valley with scalloped margins and numerous terraces inside:<br />
<br />
<table align="center" cellpadding="0" cellspacing="0" class="tr-caption-container" style="margin-left: auto; margin-right: auto; text-align: center;"><tbody>
<tr><td style="text-align: center;"><a href="http://1.bp.blogspot.com/_zWNW_P1SBcA/TGhIoPLD58I/AAAAAAAAAaA/xUuCtmu9ojc/s1600/Sylvester+et+al+2010+2.png" imageanchor="1" style="margin-left: auto; margin-right: auto;"><img border="0" src="http://1.bp.blogspot.com/_zWNW_P1SBcA/TGhIoPLD58I/AAAAAAAAAaA/xUuCtmu9ojc/s400/Sylvester+et+al+2010+2.png" width="500" /></a></td></tr>
<tr><td class="tr-caption" style="text-align: center;">Three-dimensional view of an incising channel-levee system (from Sylvester et al., 2010).</td></tr>
</tbody></table>This second paper is part of a nice collection focusing on submarine sedimentary systems that is going to be published as a special issue of Marine and Petroleum Geology, a collection that originated from a <a href="http://www.ucalgary.ca/shubbard/node/65">great conference</a> held in 2009 in <a href="http://zsylvester.blogspot.com/2009/03/three-photos-from-chilean-patagonia.html">Torres del Paine National Park</a>, Southern Chile.<br />
<br />
PS. As I am typing this, I see that Brian over at Clastic Detritus is <a href="http://clasticdetritus.com/2010/08/15/sea-floor-sunday-70-black-sea-submarine-channel-system/">also thinking about submarine channels and subaerial rivers</a>... Those channels formed by saline density currents on the slope of the Black Sea are fascinating.<br />
<br />
<span class="Z3988" title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.jtitle=Journal+of+Sedimentary+Research&rft_id=info%3Adoi%2F10.2110%2Fjsr.2010.046&rfr_id=info%3Asid%2Fresearchblogging.org&rft.atitle=Three-Dimensional+Connectivity+of+Point-Bar+Deposits&rft.issn=1527-1404&rft.date=2010&rft.volume=80&rft.issue=5&rft.spage=440&rft.epage=454&rft.artnum=http%3A%2F%2Fjsedres.sepmonline.org%2Fcgi%2Fdoi%2F10.2110%2Fjsr.2010.046&rft.au=Willis%2C+B.&rft.au=Tang%2C+H.&rfe_dat=bpr3.included=1;bpr3.tags=Geosciences%2CGeology%2C+Energy">Willis, B., & Tang, H. (2010). Three-Dimensional Connectivity of Point-Bar Deposits <span style="font-style: italic;">Journal of Sedimentary Research, 80</span> (5), 440-454 DOI: <a href="http://dx.doi.org/10.2110/jsr.2010.046" rev="review">10.2110/jsr.2010.046</a></span><br />
<br />
<span class="Z3988" title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.jtitle=Marine+and+Petroleum+Geology&rft_id=info%3Adoi%2F10.1016%2Fj.marpetgeo.2010.05.012&rfr_id=info%3Asid%2Fresearchblogging.org&rft.atitle=A+model+of+submarine+channel-levee+evolution+based+on+channel+trajectories%3A+Implications+for+stratigraphic+architecture&rft.issn=02648172&rft.date=2010&rft.volume=&rft.issue=&rft.spage=&rft.epage=&rft.artnum=http%3A%2F%2Flinkinghub.elsevier.com%2Fretrieve%2Fpii%2FS0264817210001157&rft.au=Sylvester%2C+Z.&rft.au=Pirmez%2C+C.&rft.au=Cantelli%2C+A.&rfe_dat=bpr3.included=1;bpr3.tags=Geosciences%2CGeology">Sylvester, Z., Pirmez, C., & Cantelli, A. (2010). A model of submarine channel-levee evolution based on channel trajectories: Implications for stratigraphic architecture <span style="font-style: italic;">Marine and Petroleum Geology</span> DOI: <a href="http://dx.doi.org/10.1016/j.marpetgeo.2010.05.012" rev="review">10.1016/j.marpetgeo.2010.05.012</a></span>Anonymoushttp://www.blogger.com/profile/02317685391212630354noreply@blogger.com0tag:blogger.com,1999:blog-6377410.post-80526027377124628842010-07-10T19:45:00.004-05:002010-11-25T00:57:20.376-06:00A hike in the Bucegi Mountains, Romania<br />
Recently I had a chance to revisit a fantastic hiking trail in the Bucegi Mountains, located in the Romanian Carpathians (or Transylvanian Alps, for those who prefer a more exotic name). The Bucegi are among the most spectacular hiking and climbing places in Eastern Europe, with some of the tallest cliffs in the region. Back in the good old days when I used to live closer to some significant topographic relief (as opposed to a living on a %^$#@ flat passive margin), this hike was one of our favorites. The main attraction is a steep climb along a valley floor that usually has some snow even during the summer months. In the steepest sections there is no proper trail and usually there is nobody else around; this is the perfect place if you want some outstanding scenery without the crowds.
<br />
<br />
The predominant rock type in these mountains is the Bucegi Conglomerate, a Cretaceous formation with lots of limestone clasts. The limestone pebbles, cobbles and boulders were eroded from Jurassic carbonates that outcrop in the western parts of the Bucegi. This is one of the thickest conglomerate accumulations I have seen and I know of; its thickness reaches 2000 meters in places. It was probably deposited as fan deltas along a rocky coast, with rivers that were directly depositing coarse-grained sediment onto a submarine slope. There is evidence for deposition by sediment gravity flows: many conglomerate layers show no obvious stratification (which one would expect in a river deposit) and normal grading is common. Toward the top, there is one spectacular layer, likely deposited by a single flow, with limestone blocks tens of meters across. [These blocks are often called <i>olistoliths</i>.]
<div class="separator" style="clear: both; text-align: center;">
<br /></div>
<div class="separator" style="clear: both; text-align: center;">
<a href="http://skeptic.smugmug.com/Friends/Oldies-But-Goldies-I/bucegi4/44582029_7Nxxm-XL-1.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" src="http://skeptic.smugmug.com/Friends/Oldies-But-Goldies-I/bucegi4/44582029_7Nxxm-XL-1.jpg" width="400" /></a></div>
<div style="text-align: center;">
Typical Bucegi Conglomerate (photo taken in 1995)<br />
<br /></div>
<div style="text-align: center;">
<div style="text-align: left;">
Originally these rocks were described as 'molasse' (one of those terms that probably were invented only to hide our ignorance about the relationships between mountain building and sedimentation), likely reflecting deposition in shallow marine environments. In the late seventies, when the idea that thick piles of coarse sediment could be of deep-marine origin was still big news in geology, the Bucegi Conglomerate actually <a href="http://www.nature.com/nature/journal/v276/n5683/abs/276060a0.html">made it onto the pages of Nature</a>.</div>
</div>
<br />
<iframe frameborder="0" height="400" marginheight="0" marginwidth="0" scrolling="no" src="http://maps.google.com/maps/ms?ie=UTF8&hl=en&msa=0&msid=105339637352396792358.0004897bc14eab05b635f&ll=45.431265,25.49273&spn=0.029012,0.080263&t=h&output=embed" width="510"></iframe><br />
<small>View <a href="http://maps.google.com/maps/ms?ie=UTF8&hl=en&msa=0&msid=105339637352396792358.0004897bc14eab05b635f&ll=45.431265,25.49273&spn=0.029012,0.080263&t=h&source=embed" style="color: blue; text-align: left;">Bucegi Hike</a> in a larger map</small><br />
<br />
<div style="text-align: left;">
In any case, our hike in June was long and strenuous (see the map above), but the weather was outstanding and we had the whole mountain to ourselves: apart from the meteorologists at the Omu Peak, we haven't seen a human being while hiking.<br />
<br />
Here are a few more photographs (<a href="http://skeptic.smugmug.com/Nature/Bucegi-Mountains-2010/12499728_XhhPk">see the rest at Smugmug</a>):</div>
<div style="text-align: left;">
<br />
<div class="separator" style="clear: both; text-align: center;">
<a href="http://skeptic.smugmug.com/Nature/Bucegi-Mountains-2010/DSC8874/896205450_NgCGe-XL.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" src="http://skeptic.smugmug.com/Nature/Bucegi-Mountains-2010/DSC8874/896205450_NgCGe-XL.jpg" width="500" /></a></div>
<div style="text-align: center;">
There is still plenty of snow in the 'Valea Alba' ('white valley') in June</div>
<div style="text-align: center;">
<br /></div>
<div class="separator" style="clear: both; text-align: center;">
<a href="http://skeptic.smugmug.com/Nature/Bucegi-Mountains-2010/DSC8934/896217780_wQCob-XL.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" src="http://skeptic.smugmug.com/Nature/Bucegi-Mountains-2010/DSC8934/896217780_wQCob-XL.jpg" width="500" /></a></div>
<div style="text-align: center;">
The artist previously known as erosion</div>
<div style="text-align: center;">
<br /></div>
<div class="separator" style="clear: both; text-align: center;">
<a href="http://skeptic.smugmug.com/Nature/Bucegi-Mountains-2010/DSC8986/907882192_mBJZU-XL.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" src="http://skeptic.smugmug.com/Nature/Bucegi-Mountains-2010/DSC8986/907882192_mBJZU-XL.jpg" width="500" /></a></div>
<div style="text-align: center;">
That's all conglomerate</div>
<div style="text-align: center;">
<br /></div>
</div>
<div style="text-align: left;">
<div class="separator" style="clear: both; text-align: center;">
<a href="http://4.bp.blogspot.com/_zWNW_P1SBcA/TDkSp5K4SeI/AAAAAAAAAZg/qbUng85C1-c/s1600/DSC_8952.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" src="http://4.bp.blogspot.com/_zWNW_P1SBcA/TDkSp5K4SeI/AAAAAAAAAZg/qbUng85C1-c/s400/DSC_8952.jpg" width="400" /></a></div>
<div style="text-align: center;">
When you clearly need a log-scale for grain size (note the two limestone 'grains' and the normal grading above the lower one)<br />
<br />
<div class="separator" style="clear: both; text-align: center;">
<a href="http://2.bp.blogspot.com/_zWNW_P1SBcA/TDkXBhQ0HvI/AAAAAAAAAZs/JdZXDzZzE54/s1600/DSC_8948.jpg" imageanchor="1" style="margin-left: 1em; margin-right: 1em;"><img border="0" src="http://2.bp.blogspot.com/_zWNW_P1SBcA/TDkXBhQ0HvI/AAAAAAAAAZs/JdZXDzZzE54/s400/DSC_8948.jpg" width="500" /></a></div>
This picture gives an idea how big the limestone blocks in the previous photo are: note the two guys in the front for scale</div>
</div>Anonymoushttp://www.blogger.com/profile/02317685391212630354noreply@blogger.com2tag:blogger.com,1999:blog-6377410.post-46680064679441977702010-05-27T06:44:00.017-05:002010-11-25T00:57:45.695-06:00Deep-sea landscapes from the ice age<br />The <a href="http://scienceblogs.com/highlyallochthonous/2010/05/accretionary_wedge_call_for_po.php">upcoming edition of Accretionary Wedge</a> is going to focus on geo-images. I was always fascinated by the beauty of landscapes and landforms, natural patterns and textures, as <a href="http://zsylvester.blogspot.com/2009/12/lunar-crater-volcanic-field-nevada.html">many</a> <a href="http://zsylvester.blogspot.com/2009/11/three-photos-from-kauai.html">of</a> <a href="http://zsylvester.blogspot.com/2008/11/images-from-south-africa-landscapes.html">the</a> <a href="http://zsylvester.blogspot.com/2009/03/three-photos-from-chilean-patagonia.html">posts</a> <a href="http://zsylvester.blogspot.com/2009/01/earth-water-wind-and-fire-lava-viewing.html">on</a> <a href="http://zsylvester.blogspot.com/2008/11/images-from-south-africa-patterns_30.html">this </a><a href="http://zsylvester.blogspot.com/2008/11/liesegang-bands-in-sandstone.html">blog</a> can testify; that is one of the reasons why I became a geologist.<br /><br />However, this time I want to show a different kind of geo-image. These are not usual photographs; they are pictures of landscapes that existed thousands or millions of years ago. The 'photographer' uses acoustic waves instead of light. Once the data is recorded, a whole lot of processing and editing is required to get a reasonable result. Most often it is not trivial to make sure that the final image indeed comes close to capturing one geological moment in time, and part of it is not hundreds of thousands or millions of years older than the rest. It is a bit like stacking vertically pictures that come from time-lapse photography, but parts of the older images are erased later and get replaced with pixels that belong to more recent shots.<br /><br />I am talking about maps that come from three-dimensional seismic surveys, especially their shallower sections located near the seafloor. Using this kind of data, it is possible to reconstruct ancient landscapes through careful mapping. The result is never going to be perfect, or even comparable to present-day satellite imagery, on one hand due to the limited lateral and vertical resolution, and on the other hand due to the removal of significant parts of the stratigraphic record through erosion.<br /><br />Still, it is amazing that it is possible to reconstruct for example how the Gulf of Mexico looked like during a glacial period. The images below come form the continental slope of the Gulf, and are buried a few hundred feet below the seafloor. This morphology most likely formed during a glacial period when rivers were crossing the exposed shelf and delivering sediment directly onto the upper slope. <br /><br /><a onblur="try {parent.deselectBloggerImageGracefully();} catch(e) {}" href="http://2.bp.blogspot.com/_zWNW_P1SBcA/S_-axI-QmxI/AAAAAAAAAY4/-RjG02h1mz8/s1600/Fuji_Einstein_1_with_logo_small2.jpg"><img style="display:block; margin:0px auto 10px; text-align:center;cursor:pointer; cursor:hand;width: 350px;" src="http://2.bp.blogspot.com/_zWNW_P1SBcA/S_-axI-QmxI/AAAAAAAAAY4/-RjG02h1mz8/s400/Fuji_Einstein_1_with_logo_small2.jpg" border="0" alt=""id="BLOGGER_PHOTO_ID_5476265840918698770" /></a><center>source: <a href="http://see-atlas.leeds.ac.uk:8080/homePages/generic.jsp?resourceId=0900006480015f8d">Virtual Seismic Atlas</a></center><br /><br />Two submarine channels are visible, both of them directly linked to a delta that was deposited at the shelf edge. Colors correspond to thickness: red is thick, blue is thin. The next image shows the surface underlying the channels; in this case, the topographic surface is draped with seismic amplitude:<br /><br /><a onblur="try {parent.deselectBloggerImageGracefully();} catch(e) {}" href="http://4.bp.blogspot.com/_zWNW_P1SBcA/S_-oj9O_i4I/AAAAAAAAAZA/TLY_lcshXXw/s1600/Fuji_Einstein_4_with_logo_small2.jpg"><img style="display:block; margin:0px auto 10px; text-align:center;cursor:pointer; cursor:hand;width: 400px" src="http://4.bp.blogspot.com/_zWNW_P1SBcA/S_-oj9O_i4I/AAAAAAAAAZA/TLY_lcshXXw/s400/Fuji_Einstein_4_with_logo_small2.jpg" border="0" alt=""id="BLOGGER_PHOTO_ID_5476281007592147842" /></a> <center>source: <a href="http://see-atlas.leeds.ac.uk:8080/homePages/generic.jsp?resourceId=0900006480015fb7">Virtual Seismic Atlas</a></center><br /><br />There are more images from this ancient landscape available at the <a href="http://see-atlas.leeds.ac.uk:8080/home.jsp">Virtual Seismic Atlas</a>, a great resource for geo-imagery in general (see <a href="http://clasticdetritus.com/2008/09/14/virtual-seismic-atlas/">this post</a> at Clastic Detritus for more detail). It is best to view these 'photographs' at larger resolution (which is pretty big in this case!) -- you can do that if you go to the <a href="http://www.seismicatlas.org/">VSA website</a>.Anonymoushttp://www.blogger.com/profile/02317685391212630354noreply@blogger.com4tag:blogger.com,1999:blog-6377410.post-15311239493972999802010-05-15T13:12:00.016-05:002012-03-30T04:16:09.057-05:00Garmin Forerunner 110 GPS watch - a review<br />
A couple of years ago I decided to take running a bit more seriously and to try to keep track of when, how much, and how fast I run. As a dedicated Apple-afficionado and beginner runner, the obvious choice was the Nike+ sensor (which you place in the sole of your shoe), coupled with an iPod Nano. I have been using this setup for about two years now, and I was fairly happy with it. It was easy to start using it, it definitely helped me run more and faster than before, and GPS units were just too big or too nerdy (even for me) to carry around on a Saturday morning run in the park.<br />
<br />
However, it has always bugged me that the precision and accuracy of the Nike+ system was far from perfect, and I knew that GPS watches could do much better, not to mention that you can also put your run on a map. I caved in to the temptation a few days ago and ordered a <a href="https://buy.garmin.com/shop/shop.do?cID=142&pID=63511">Garmin Forerunner 110 GPS watch</a>; here are some initial observations.<br />
<br />
<a href="http://3.bp.blogspot.com/_zWNW_P1SBcA/S-7z27dzGsI/AAAAAAAAAYY/GOICQq0zqSM/s1600/31RxLN1Qe6L._SL500_AA300_.jpg"><img alt="" border="0" id="BLOGGER_PHOTO_ID_5471578722302892738" src="http://3.bp.blogspot.com/_zWNW_P1SBcA/S-7z27dzGsI/AAAAAAAAAYY/GOICQq0zqSM/s320/31RxLN1Qe6L._SL500_AA300_.jpg" style="cursor: hand; cursor: pointer; display: block; height: 300px; margin: 0px auto 10px; text-align: center; width: 300px;" /></a><br />
The Forerunner 110 is designed to be relatively small and simple, with limited functionality. In other words, it is targeting people like me: mostly outdoor runners (it is not very good for biking and useless for indoor running) who don't need all kinds of functionalities that most other Garmin GPS watches have. It gives you basic information like pace, time, distance, and heart rate (if you are using it with a heart rate monitor), and that's about it. The relatively small size and reasonably good (=minimalistic) look means that you can wear this gadget on your wrist pretty much every day, without looking like a total nerd.<br />
<br />
In terms of usability, the Forerunner 110 does pretty well. It doesn't rely on the touch interface that is built into the latest and greatest Garmin sports watches; instead, it has four large buttons that are easy to push when you want to -- or not to push inadvertently when you don't want to. This can be important in the middle of a sweaty run when you are not really in the mood for the subtleties of dealing with a sensitive touch interface. For example, I often have problems with the touch-wheel of the iPod nano. Recording a run basically comes down to (1) waiting until the watch gets a GPS fix; and (2) pushing the 'start/stop' button. In my limited experience, getting a GPS fix works pretty well and relatively fast, although it did take about 5 minutes the first couple of times. That is too much for a runner. Yesterday and today however it was much better, it locked on to the satellites in less than a minute.<br />
<br />
So far so good. The one major issue I ran into was that, after a first recorded run, when I wanted to upload the data to the <a href="http://connect.garmin.com/">Garmin Connect website</a>, I couldn't get the watch to talk to my MacBook. It took lots of trial-and-error and one-and-a-half hours on the phone with the Garmin help desk to figure out that the charging clip that's supposed to attach to the four exposed contacts on the back of the watch was not exactly where it should have been, despite the fact that the watch was charging (or it looked like it was charging anyway). This might be just a reflection of my limited intellectual capabilities, but I doubt that I am the only one who will run into this problem.<br />
<br />
When it comes to uploading your workout data to a website for visualization and analysis, the Garmin ecosystem definitely leaves the Nike+ setup in the dust. The obvious advantage is the visualization of your runs in Google Maps. This is a major plus for a map-lover; but in addition to that, the Garmin Connect website makes it very easy to export the data and visualize it with Google Earth or any other software that can handle geospatial data. No export options exist for the runs you have recorded with the Nike+ sensor. In addition, the quality and usability of the Garmin graphs showing pace/speed through time is way better than the flashy but largely useless attempt that Nike has put together. Compare these two graphs (representing the same run):<br />
<br />
<a href="http://1.bp.blogspot.com/_zWNW_P1SBcA/S-7yBDkEsnI/AAAAAAAAAYI/C_OBJdEXo-Q/s1600/Screen+shot+2010-05-15+at+11.26.08+AM.png"><img alt="" border="0" id="BLOGGER_PHOTO_ID_5471576697252131442" src="http://1.bp.blogspot.com/_zWNW_P1SBcA/S-7yBDkEsnI/AAAAAAAAAYI/C_OBJdEXo-Q/s320/Screen+shot+2010-05-15+at+11.26.08+AM.png" style="cursor: hand; cursor: pointer; display: block; margin: 0px auto 10px; text-align: center; width: 450px;" /></a><br />
<br />
<center>Nike+ website</center><br />
<br />
<a href="http://3.bp.blogspot.com/_zWNW_P1SBcA/S-7yfGi2jNI/AAAAAAAAAYQ/e8lkQnAvEzw/s1600/Screen+shot+2010-05-15+at+11.28.04+AM.png"><img alt="" border="0" id="BLOGGER_PHOTO_ID_5471577213448391890" src="http://3.bp.blogspot.com/_zWNW_P1SBcA/S-7yfGi2jNI/AAAAAAAAAYQ/e8lkQnAvEzw/s320/Screen+shot+2010-05-15+at+11.28.04+AM.png" style="cursor: hand; cursor: pointer; display: block; margin: 0px auto 10px; text-align: center; width: 450px;" /></a><br />
<br />
<center>Garmin Connect</center><br />
<br />
The Nike+ graph is pretty close to useless, whereas the one from Garmin Connect looks like a plot based on real data and it shows real trends (e.g., that I was running significantly slower during the last half of the run). And this is not a reflection of poor data quality coming from the iPod software; it turns out that the resolution of that data is much better than what Nike shows you. In general, Garmin treats the workout data in a <a href="http://connect.garmin.com/activity/33322668">much more scientific yet simple manner</a>, also giving you the options of taking the data elsewhere, whereas the Nike website is colorful and animated, but has limited and closed information that has been dumbed down too much for my taste.<br />
<br />
To wrap it up, despite a few - hopefully short-lived - annoyances, I am fairly happy with this new gadget. I will try to find out later how well it can be used for geotagging photographs while hiking or doing field work, something I still don't have a simple solution for.<br />
<br />
<span style="font-weight: bold;">Update (6/20/2010):</span> I have been using this watch for more than a month now. It works pretty well for running, although I did have a problem today: it froze at one point, and I couldn't record any new data. It was very hot and humid, and I guess the contacts on the back side of the watch couldn't handle the amount of salty sweat I was producing. Now it works again. Also, it is a good idea to turn on the GPS reception a few minutes before you start the run because sometimes it still takes 2-3 minutes to get the coordinates.<br />
<br />
In terms of using it for hiking and geotagging photographs: I did a hike using both this watch and an older Garmin unit, and noticed that the accuracy of theForerunner 110 is better than that of the Garmin eTrex Vista Cx. The watch worked much better in the forest and in a deep, narrow valley, where the GPS signal must have been weak. The problem is that the battery of the Forerunner 110 doesn't last long enough for a full-day hike; after about 5 hours of constant GPS recording, I couldn't use it any more.<br />
<br />
<b>Update (2/4/2011):</b> It looks like this watch (certainly the one that I am using) has a <b>major flaw</b>: when connected to a computer, the USB connection is easily broken because of the questionable design of the contacts on the back of the watch and the clip. The watch freezes and the only way I could get it back to life was to do a hard reset. This means that any data you have on the watch is lost. I have lost running data due to this issue several times; the last time it was especially annoying since it successfully got rid of the GPS record of my first marathon. Thanks, Garmin!<br />
<br />
<b>Update (3/30/2012):</b> The problem I mentioned in the update above hasn't occurred since I did a <a href="http://www8.garmin.com/support/download_details.jsp?id=4985">software update</a>. However, a few months ago (about one and a half years after I bought the watch) the strap broke and I don't think there is an easy way to replace it. Also, often (but not always) it takes 20-30 minutes to get a GPS lock. It is time to get a new watch, and I think I will stay away from Garmin for now.Anonymoushttp://www.blogger.com/profile/02317685391212630354noreply@blogger.com20tag:blogger.com,1999:blog-6377410.post-85736915627600083642010-05-08T13:46:00.012-05:002010-11-25T00:58:55.657-06:00Evapor-art from the Permian Castile Formation, west Texas<br />The Late Permian Castile Formation is a ~500 m thick accumulation of evaporites in west Texas and south-eastern New Mexico. Its most striking feature is the vast number of alternating thin layers of lighter- and darker-colored deposits, layers that seem to be continuous across most of the Delaware Basin. The white laminae are mostly gypsum and anhydrite; the darker layers consist of calcite and organic matter.<br />
<br />
<a href="http://skeptic.smugmug.com/Nature/Guadalupe-Mountains-National/DSC8577/860169270_2xW5f-X2-1.jpg" onblur="try {parent.deselectBloggerImageGracefully();} catch(e) {}"><img alt="" border="0" src="http://skeptic.smugmug.com/Nature/Guadalupe-Mountains-National/DSC8577/860169270_2xW5f-X2-1.jpg" style="cursor: hand; cursor: pointer; display: block; margin: 0px auto 10px; text-align: center; width: 500px;" /></a><br />
Most experts agree that these laminations reflect seasonal changes; that is, a pair of white and dark layers corresponds to one year. The thicker gypsum layer was deposited during the dry season; the thinner calcite layer with the organic material formed during the humid season when algae were more abundant and only carbonates could precipitate from the lower-salinity water [for more details, see <a href="http://www3.interscience.wiley.com/journal/118835043/abstract">this paper</a>].<br />
<br />
<a href="http://3.bp.blogspot.com/_zWNW_P1SBcA/S-c3yb1QiuI/AAAAAAAAAXk/ChvJgxhSoNg/s1600/Screen+shot+2010-05-09+at+5.29.24+PM.png" onblur="try {parent.deselectBloggerImageGracefully();} catch(e) {}"><img alt="" border="0" id="BLOGGER_PHOTO_ID_5469401612068883170" src="http://3.bp.blogspot.com/_zWNW_P1SBcA/S-c3yb1QiuI/AAAAAAAAAXk/ChvJgxhSoNg/s320/Screen+shot+2010-05-09+at+5.29.24+PM.png" style="cursor: hand; cursor: pointer; display: block; height: 320px; margin: 0px auto 10px; text-align: center; width: 297px;" /></a><br />
The image above gives an idea how laterally persistent these laminations can be; the two photographs come from cores that are 24 km (~15 miles) apart (source: Kirkland, D.W., 2003, <a href="http://www3.interscience.wiley.com/journal/118835043/abstract">An explanation for the varves of the Castile evaporites (Upper Permian), Texas and New Mexico, USA</a>, Sedimentology 50, p. 899-920).<br />
<br />
These evaporites are often affected by small-scale faulting and folding; the resulting patterns are quite variable and aesthetically pleasing (well, at least according to me). I shot these photos on a recent geological trip to Guadalupe Mountains National Park, in a roadcut near highway 180; more pictures <a href="http://skeptic.smugmug.com/Nature/Guadalupe-Mountains-National/12103695_6B4v3">here</a>.<br />
<br />
<a href="http://skeptic.smugmug.com/Nature/Guadalupe-Mountains-National/DSC8587/860172358_gcNzx-X2-1.jpg" onblur="try {parent.deselectBloggerImageGracefully();} catch(e) {}"><img alt="" border="0" src="http://skeptic.smugmug.com/Nature/Guadalupe-Mountains-National/DSC8587/860172358_gcNzx-X2-1.jpg" style="cursor: hand; cursor: pointer; display: block; margin: 0px auto 10px; text-align: center; width: 500px;" /></a><br />
<br />
<a href="http://skeptic.smugmug.com/Nature/Guadalupe-Mountains-National/DSC8579/860170232_xJoGi-X2-1.jpg" onblur="try {parent.deselectBloggerImageGracefully();} catch(e) {}"><img alt="" border="0" src="http://skeptic.smugmug.com/Nature/Guadalupe-Mountains-National/DSC8579/860170232_xJoGi-X2-1.jpg" style="cursor: hand; cursor: pointer; display: block; margin: 0px auto 10px; text-align: center; width: 500px;" /></a><br />
<br />
<a href="http://skeptic.smugmug.com/Nature/Guadalupe-Mountains-National/DSC8578/860169757_xSX4G-X2-1.jpg" onblur="try {parent.deselectBloggerImageGracefully();} catch(e) {}"><img alt="" border="0" src="http://skeptic.smugmug.com/Nature/Guadalupe-Mountains-National/DSC8578/860169757_xSX4G-X2-1.jpg" style="cursor: hand; cursor: pointer; display: block; margin: 0px auto 10px; text-align: center; width: 500px;" /></a>Anonymoushttp://www.blogger.com/profile/02317685391212630354noreply@blogger.com4tag:blogger.com,1999:blog-6377410.post-27399752351183877242010-03-28T21:40:00.010-05:002010-11-25T06:18:52.533-06:00Texas wildflowers<br />
The weather has been awesome around here lately (yes, in Houston!, the weather!, awesome!), and otherwise uninteresting roadside places are starting to be flooded with colors. Here are a few shots; more <a href="http://skeptic.smugmug.com/Nature/Texas-Wildflowers-2008/4563434_gQLBE">over at Smugmug</a>.<br />
<br />
<a href="http://skeptic.smugmug.com/Nature/Texas-Wildflowers-2008/DSC8017/821968380_oonvc-L.jpg" onblur="try {parent.deselectBloggerImageGracefully();} catch(e) {}"><img alt="" border="0" src="http://skeptic.smugmug.com/Nature/Texas-Wildflowers-2008/DSC8017/821968380_oonvc-L.jpg" style="cursor: hand; cursor: pointer; display: block; margin: 0px auto 10px; text-align: center; width: 500px;" /></a><br />
<br />
This one is actually from 2008:
<br />
<br />
<a href="http://skeptic.smugmug.com/Nature/Texas-Wildflowers-2008/DSC0116/272695889_KAiJ7-L.jpg" onblur="try {parent.deselectBloggerImageGracefully();} catch(e) {}"><img alt="" border="0" src="http://skeptic.smugmug.com/Nature/Texas-Wildflowers-2008/DSC0116/272695889_KAiJ7-L.jpg" style="cursor: hand; cursor: pointer; display: block; margin: 0px auto 10px; text-align: center; width: 500px;" /></a><br />
<br />
One of the issues with photographing flowers is that parts of the pictures are often out of focus, as it is obvious in this shot:<br />
<br />
<a href="http://skeptic.smugmug.com/Nature/Texas-Wildflowers-2008/DSC8042/821973701_BPBEF-L.jpg" onblur="try {parent.deselectBloggerImageGracefully();} catch(e) {}"><img alt="" border="0" src="http://skeptic.smugmug.com/Nature/Texas-Wildflowers-2008/DSC8042/821973701_BPBEF-L.jpg" style="cursor: hand; cursor: pointer; display: block; margin: 0px auto 10px; text-align: center; width: 500px;" /></a><br />
<br />
And a way to deal with that is to take a number of pictures that are focused at different distances and then merge them in Photoshop (as described in <a href="http://tv.adobe.com/watch/photoshopcafe-tv/merge-depth-of-field-with-photoshop-cs4">this video</a>). This image was put together from four different photographs, and is a somewhat better version of the previous scene (the fact that the wind was pretty strong didn't help):<br />
<br />
<a href="http://skeptic.smugmug.com/Nature/Texas-Wildflowers-2008/Texas-wildflowers-2010-photo-1/821973059_fGYEo-L.jpg" onblur="try {parent.deselectBloggerImageGracefully();} catch(e) {}"><br /></a><br />
<a href="http://skeptic.smugmug.com/Nature/Texas-Wildflowers-2008/Texas-wildflowers-2010-photo-1/821973059_fGYEo-L.jpg" onblur="try {parent.deselectBloggerImageGracefully();} catch(e) {}"><img alt="" border="0" src="http://skeptic.smugmug.com/Nature/Texas-Wildflowers-2008/Texas-wildflowers-2010-photo-1/821973059_fGYEo-L.jpg" style="cursor: hand; cursor: pointer; display: block; margin: 0px auto 10px; text-align: center; width: 500px;" /></a><br />
<br />Anonymoushttp://www.blogger.com/profile/02317685391212630354noreply@blogger.com2tag:blogger.com,1999:blog-6377410.post-86827472954218069132009-12-27T18:43:00.007-06:002010-11-25T06:19:07.384-06:00Three photos from California Academy of Sciences, San Francisco<br />
<a href="http://www.calacademy.org/">This</a> is a fantastic place! Clearly worth a visit if you have a few hours to spend in San Francisco.<br />
<br />
More photographs <a href="http://skeptic.smugmug.com/Nature/California-2009/10770524_k8bcM">here</a>.<br />
<br />
<a href="http://skeptic.smugmug.com/Nature/California-2009/DSC7718/750736899_WfD2U-XL.jpg" onblur="try {parent.deselectBloggerImageGracefully();} catch(e) {}"><img alt="" border="0" src="http://skeptic.smugmug.com/Nature/California-2009/DSC7718/750736899_WfD2U-XL.jpg" style="cursor: hand; cursor: pointer; display: block; margin: 0px auto 10px; text-align: center; width: 500px;" /></a>
<center>Madagascar giant day gecko</center><br />
<br />
<a href="http://skeptic.smugmug.com/Nature/California-2009/DSC7694/750736450_xgm6c-XL.jpg" onblur="try {parent.deselectBloggerImageGracefully();} catch(e) {}"><img alt="" border="0" src="http://skeptic.smugmug.com/Nature/California-2009/DSC7694/750736450_xgm6c-XL.jpg" style="cursor: hand; cursor: pointer; display: block; margin: 0px auto 10px; text-align: center; width: 500px;" /></a>
<center>Coral reef from the Philippines</center><br />
<br />
<a href="http://skeptic.smugmug.com/Nature/California-2009/DSC7642/750734695_nWUq6-XL.jpg" onblur="try {parent.deselectBloggerImageGracefully();} catch(e) {}"><img alt="" border="0" src="http://skeptic.smugmug.com/Nature/California-2009/DSC7642/750734695_nWUq6-XL.jpg" style="cursor: hand; cursor: pointer; display: block; margin: 0px auto 10px; text-align: center; width: 500px;" /></a>
<center>Jellyfish</center><br />
<br />Anonymoushttp://www.blogger.com/profile/02317685391212630354noreply@blogger.com0tag:blogger.com,1999:blog-6377410.post-57465020156742286342009-12-23T16:37:00.014-06:002010-11-25T06:19:56.866-06:00Lunar Crater volcanic field, Nevada<br /><br />
<a onblur="try {parent.deselectBloggerImageGracefully();} catch(e) {}" href="http://skeptic.smugmug.com/Nature/California-2009/DSC7559/750729773_PSgab-XL-1.jpg"><img style="display:block; margin:0px auto 10px; text-align:center;cursor:pointer; cursor:hand;width: 500px; " src="http://skeptic.smugmug.com/Nature/California-2009/DSC7559/750729773_PSgab-XL-1.jpg" border="0" alt="" /></a><br />I was on my way to San Francisco / AGU last week when I saw these volcanoes and shot these pictures through the airplane window. It turns out that this is the <a href="http://vulcan.wr.usgs.gov/Volcanoes/Nevada/description_nevada_volcanics.html#lunar_crater">Lunar Crater volcanic field in Nevada</a>, named after the largest crater that is more than 1000 meters across and about 130 m deep. There are 95 vents that are 4.2 million to 15,000 years old. Lunar Crater is the largest feature in the image below; it is a <a href="http://en.wikipedia.org/wiki/Maar">maar</a>; most of the other vents formed <a href="http://en.wikipedia.org/wiki/Cinder_cone">cinder cones</a>.<br /><br /><a onblur="try {parent.deselectBloggerImageGracefully();} catch(e) {}" href="http://skeptic.smugmug.com/Nature/California-2009/DSC7561/750734487_mdkk6-XL.jpg"><img style="display:block; margin:0px auto 10px; text-align:center;cursor:pointer; cursor:hand;width: 500px; " src="http://skeptic.smugmug.com/Nature/California-2009/DSC7561/750734487_mdkk6-XL.jpg" border="0" alt="" /></a><br /><br /><a onblur="try {parent.deselectBloggerImageGracefully();} catch(e) {}" href="http://3.bp.blogspot.com/_zWNW_P1SBcA/Szf52mq_54I/AAAAAAAAAWA/8SBdONODZY0/s1600-h/lunar_crater.001.png"><img style="display:block; margin:0px auto 10px; text-align:center;cursor:pointer; cursor:hand;width: 500px;" src="http://3.bp.blogspot.com/_zWNW_P1SBcA/Szf52mq_54I/AAAAAAAAAWA/8SBdONODZY0/s320/lunar_crater.001.png" border="0" alt=""id="BLOGGER_PHOTO_ID_5420075393052829570" /></a><br />Here is a map showing the Houston - San Francisco flight track and the location of the volcanic field:<br /><br /><iframe width="500" height="400" frameborder="0" scrolling="no" marginheight="0" marginwidth="0" src="http://maps.google.com/maps/ms?ie=UTF8&hl=en&msa=0&ll=38.381498,-116.07193&spn=0.293888,0.613861&t=h&msid=105339637352396792358.00047bbe5aed3095cc1cd&output=embed"></iframe><br /><small>View <a href="http://maps.google.com/maps/ms?ie=UTF8&hl=en&msa=0&ll=34.156006,-108.826095&spn=37.956457,78.574219&t=h&msid=105339637352396792358.00047bbe5aed3095cc1cd&source=embed" style="color:#0000FF;text-align:center">Lunar Crater volcanic field</a> in a larger map</small><br />
<br />Anonymoushttp://www.blogger.com/profile/02317685391212630354noreply@blogger.com1tag:blogger.com,1999:blog-6377410.post-15584645136689328652009-11-27T22:51:00.007-06:002011-01-16T00:15:15.759-06:00Three photos from Kauai<br />
The rest are <a href="http://skeptic.smugmug.com/Nature/Hawaii-2009/9635959_sUxhY">here</a>.<br /><br /><a onblur="try {parent.deselectBloggerImageGracefully();} catch(e) {}" href="http://skeptic.smugmug.com/Nature/Hawaii-2009/DSC6864/649859596_QmwzU-XL.jpg"><img style="display:block; margin:0px auto 10px; text-align:center;cursor:pointer; cursor:hand;width: 500px;" src="http://skeptic.smugmug.com/Nature/Hawaii-2009/DSC6864/649859596_QmwzU-XL.jpg" border="0" alt="" /></a><center>Colors in Alakai swamp</center><br /><br /><a onblur="try {parent.deselectBloggerImageGracefully();} catch(e) {}" href="http://skeptic.smugmug.com/Nature/Hawaii-2009/IMG0622/649853693_9bLam-XL.jpg"><img style="display:block; margin:0px auto 10px; text-align:center;cursor:pointer; cursor:hand;width: 500px;" src="http://skeptic.smugmug.com/Nature/Hawaii-2009/IMG0622/649853693_9bLam-XL.jpg" border="0" alt="" /></a><center>Green turtle at Tunnels Beach</center><br /><br /><a onblur="try {parent.deselectBloggerImageGracefully();} catch(e) {}" href="http://skeptic.smugmug.com/Nature/Hawaii-2009/DSC6666/649772585_xbMWZ-XL-1.jpg"><img style="display:block; margin:0px auto 10px; text-align:center;cursor:pointer; cursor:hand;width: 500px;" src="http://skeptic.smugmug.com/Nature/Hawaii-2009/DSC6666/649772585_xbMWZ-XL-1.jpg" border="0" alt="" /></a><center>Na Pali Coast</center><br />
<br />Anonymoushttp://www.blogger.com/profile/02317685391212630354noreply@blogger.com4tag:blogger.com,1999:blog-6377410.post-6489158118081154182009-11-26T07:46:00.030-06:002010-11-25T06:20:49.987-06:00Hillslope diffusion<br />
Modeling erosion and deposition of sediment using the diffusion equation is among the important subjects that are usually omitted from sedimentary geology textbooks. Part of the reason for this is that ‘conventional’ sedimentary geology tended to only pay lip service to earth surface processes and was more interested in describing the stratigraphic record than figuring how it relates to geomorphology. Nowadays, a good discussion of stratigraphy and sedimentology cannot ignore anymore what geomorphologists have learned about landscape evolution. (One textbook that clearly recognizes this is <a href="http://www.amazon.com/gp/product/0521857805">this one</a>.) <br />
<br />
But let's get back to the subject of this post. Hillslope evolution can be modeled with the diffusion equation, one of the most common differential equations in science, applied for example to describe how differences in temperature are eliminated through heat conduction. In the case of heat, the heat flux is proportional to the rate of spatial temperature change; on hillslopes, the sediment flux is proportional to the spatial rate of change in elevation. This last quantity of course is the slope itself. In other words, <br />
<br />
<center>q = -k*dh/dx,</center><br />
or<br />
<br />
<center>q = -k*slope,</center><br />
where q is the volumetric sediment flux per unit length, k is a constant called diffusivity, h is the elevation, and x is the horizontal coordinate.<br />
<br />
We also know that sediment does not disappear into thin air: considering a small area of the hillslope, the amount of sediment entering and leaving this area will determine how large the change in elevation will be:<br />
<br />
<br />
<center>dh/dt = -dq/dx,</center><br />
in other words, deposition or erosion at any location is determined by the change in sediment flux.<br />
<br />
Combining this equation with the previous one, we arrive to the diffusion equation: <br />
<br />
<center>dh/dt = k*d<sup>2</sup>h/dx<sup>2</sup>.</center><br />
Note that the quantity on the right side is the second derivative (or curvature) of the slope profile. Large negative curvatures result in rapid erosion; places with large positive curvature have high rates of deposition. Through time, the bumps and troughs of the hillslope are smoothed out through erosion and deposition.<br />
<br />
The simplest possible case is the diffusion of a fault scarp. The movie below illustrates how a 1 m high fault scarp gets smoothed out through time; the evolution of slope and curvature are also shown. The dashed line indicates the original topography, at time 0. [The plots were generated using <a href="http://activetectonics.asu.edu/diffuse/">Ramon Arrowsmith's Matlab code</a>; right-click the flash animation for playback controls].<br />
<br />
<object align="" classid="clsid:D27CDB6E-AE6D-11cf-96B8-444553540000" codebase="http://download.macromedia.com/pub/shockwave/cabs/flash/swflash.cab#version=6,0,0,0" id="diffusion_2.swf" height="582" width="520"><param NAME=movie VALUE="diffusion_1.swf">
<param NAME=quality VALUE=high>
<param NAME=bgcolor VALUE=#E8E8E8>
<embed src="http://homepage.mac.com/zoltan.sylvester/diffusion_2.swf" quality=high bgcolor=#E8E8E8 WIDTH="520" NAME="Yourfilename" ALIGN="" TYPE="application/x-shockwave-flash" PLUGINSPAGE="http://get.adobe.com/flashplayer/"></EMBED> </OBJECT> <br />
<br />
More complicated slope profiles can be modeled as well; here is an example with two fault scarps:<br />
<br />
<object align="" classid="clsid:D27CDB6E-AE6D-11cf-96B8-444553540000" codebase="http://download.macromedia.com/pub/shockwave/cabs/flash/swflash.cab#version=6,0,0,0" id="diffusion_1.swf" height="210" width="520"><param NAME=movie VALUE="diffusion_1.swf">
<param NAME=quality VALUE=high>
<param NAME=bgcolor VALUE=#E8E8E8>
<embed src="http://homepage.mac.com/zoltan.sylvester/diffusion_1.swf" quality=high bgcolor=#E8E8E8 WIDTH="520" NAME="Yourfilename" ALIGN="" TYPE="application/x-shockwave-flash" PLUGINSPAGE="http://get.adobe.com/flashplayer/"></EMBED> </OBJECT> <br />
<br />
Note how both erosion and deposition get much slower as the gradients become more uniform.<br />
<br />
The simplicity of the diffusion equation makes it an attractive tool in modeling landscape evolution. In addition to hillslopes and fault scarps, it has been successfully applied in modeling - for example - river terraces, deltaic clinoforms, cinder cones, fluvial systems, and foreland basin stratigraphy. However, it is important to know when and where the assumptions behind it become invalid. For example, steep slopes often have a non-linear relationship between sediment flux and slope as mass movements dramatically increase sediment flux above a critical slope value. Also, the models shown here would fail to reproduce the topography of a system where not all sediment is deposited at the toe of the steeper slope, but a significant part is carried away by a river. And that brings us closer to advection; a subject that I might take notes about at another time.<br />
<br />
Further reading: 1) The book <a href="http://www.amazon.com/Quantitative-Modeling-Earth-Surface-Processes/dp/0521855977/ref=sr_1_1?ie=UTF8&s=books&qid=1259354349&sr=8-1">"Quantitative Modeling of Earth Surface Processes" by Jon Pelletier</a> has a chapter with lots of details about the diffusion equation. 2) <a href="http://serc.carleton.edu/quantskills/activities/hancock.html">Analog and numerical modeling of hillslope diffusion</a> - a nice lab exercise.Anonymoushttp://www.blogger.com/profile/02317685391212630354noreply@blogger.com6tag:blogger.com,1999:blog-6377410.post-5459757792049811432009-10-24T18:14:00.007-05:002010-11-25T06:21:55.810-06:00Upcoming conference on seismic geomorphology<br />
Ever since geoscientists and engineers started using seismic waves to figure out what lies under our feet, seismic reflection technology kept improving and today most oil-rich sedimentary basins have a wide coverage of high-quality three-dimensional datasets. In addition to finding structures and locations in the subsurface that are likely to be filled with hydrocarbons, these huge data volumes can also be used to reconstruct landscapes that are long gone from the Earth's surface. After all, stratigraphy is what is left behind from an ever-changing topography, and it is a mistake to think that stratigraphy can be understood without knowing a few things about geomorphology and landscape evolution.<br /><br />Seismic data confirms that the past, indeed, is not that different from the present: if you peek (that is, listen) into these volumes of rock, you see ancient meandering rivers and submarine channels, deltas, barrier islands, mouth bars, and estuaries. However, in addition to and beyond the excitement of seeing another beautiful example of a sinuous channel or other depositional and erosional features, a lot remains to be learned from the true and large-scale three-dimensionality of these datasets. For a geologist, there is no other data type that offers such a degree of three-dimensionality. Even the largest outcrops offer only random two-dimensional sections through a system; the temptation is strong to convince ourselves that we can extrapolate to get an idea about the third dimension, but more often than not we are probably wrong, at least in the details of our extrapolation. This is actually worse than the case of "<a href="http://www.sln.org.uk/storyboard/stories/h11.htm">The Blind Men and the Elephant</a>"; it is more like the blind men and a random cut through the elephant (but I will stop this chain of analogies right there).<br /><br />Long story short, I (re)started to blog about this subject because there is going to be <a href="http://www.sepm.org/activities/researchconferences/deep_water/deep_waterhome.htm">an SEPM research conference in Houston</a>, a conference that focuses on using three-dimensional seismic data to better understand how sediment moves or gets deposited on continental slopes. It should be an interesting collection of talks and papers.<br /><br /><br /><a onblur="try {parent.deselectBloggerImageGracefully();} catch(e) {}" href="http://4.bp.blogspot.com/_zWNW_P1SBcA/SuOZyBBeZUI/AAAAAAAAAUo/S0TcsPPET8s/s1600-h/DWSeismicImageWeb.jpg"><img style="display:block; margin:0px auto 10px; text-align:center;cursor:pointer; cursor:hand;width: 320px; height: 245px;" src="http://4.bp.blogspot.com/_zWNW_P1SBcA/SuOZyBBeZUI/AAAAAAAAAUo/S0TcsPPET8s/s320/DWSeismicImageWeb.jpg" border="0" alt=""id="BLOGGER_PHOTO_ID_5396325863067116866" /></a><br />The image above is from the conference website and announcement; it happens to come from a paper that I am going to present, on a shelf-edge delta and its related slope channels in the Gulf of Mexico (the higher-resolution version is coming soon...). Colors represent thickness (red is thick, blue means thin). There are two leveed channels taking sediment from the shelf-edge delta into the deep ocean.Anonymoushttp://www.blogger.com/profile/02317685391212630354noreply@blogger.com2tag:blogger.com,1999:blog-6377410.post-74043252781809828592009-07-19T17:29:00.006-05:002010-11-25T06:22:11.501-06:00Two gigapans from Cliffs of Moher, Ireland<br />I shot these gigapans recently, while we were visiting some deep-water rocks in County Clare, Ireland (see <a href="http://strata.geol.sc.edu/Deepwater/Clare-Basin-Clastics.html">more detail on these rocks</a> and <a href="http://skeptic.smugmug.com/gallery/8770511_4r3et">a few photos from the trip</a>). One afternoon we took some time off from the turbidites to do a bit of geo-tourism at the Cliffs of Moher, a series of spectacular escarpments along an 8 km long stretch of the western coast of Ireland. They are 702 feet (214 meters) high at the highest point and expose Late Carboniferous (Namurian) sandstones and shales that were mostly deposited as deltaic and fluvial sediments of the Tullig and Kilkee cyclothems.<br /><br />This place is one of the most visited tourist attractions in Ireland, and for a good reason: the combination of the cliffs, the landscape, and abundant wildlife is, indeed, spectacular.<br /><br />This is a view to the south (<a href="http://www.gigapan.org/viewGigapanFullscreen.php?id=27345">launch full screen viewer</a>):<br /><iframe src="http://api.gigapan.org/beta/gigapans/27345/snapshots/87490,83479,83403,83402/iframe/flash.html" frameborder="0" height="400" scrolling="no" width="100%"></iframe><br /><br />And this is a view to the north (from O'Brian's Tower; <a href="http://www.gigapan.org/viewGigapanFullscreen.php?id=27340">launch full-screen viewer</a>):<br /><iframe src="http://api.gigapan.org/beta/gigapans/27340/snapshots/83397,83395,83394,83392,83391/iframe/flash.html" frameborder="0" height="400" scrolling="no" width="100%"></iframe><br /><br />Unfortunately, these stamp-sized windows do not do justice to the panoramas; it is a good idea to click on the "Launch full-screen viewer" links.Anonymoushttp://www.blogger.com/profile/02317685391212630354noreply@blogger.com0tag:blogger.com,1999:blog-6377410.post-79154233841227197152009-05-09T11:18:00.035-05:002010-11-25T06:23:09.078-06:00More reasons to conclude that coastal 'chevrons' are not related to mega-tsunamis<br />
<span style="float: left; padding-bottom: 5px; padding-left: 5px; padding-right: 5px; padding-top: 5px;"><a href="http://www.researchblogging.org/"><img alt="ResearchBlogging.org" src="http://www.researchblogging.org/public/citation_icons/rb2_large_gray.png" style="border: 0;" /></a></span> If there was any doubt left that coastal sand accumulations called 'chevrons' are *not* related to gigantic tsunamis (see previous thoughts on the subject <a href="http://zsylvester.blogspot.com/2008/03/some-questions-about-megatsunami.html">here</a> and <a href="http://zsylvester.blogspot.com/2008/05/some-questions-about-megatsunami.html">here</a>; Ole also has a <a href="http://my.opera.com/nielsol/blog/2009/04/29/chevrons">recent blog post</a>, and see a news release <a href="http://www.geologytimes.com/research/Contrary_to_recent_hypothesis_chevrons_are_not_evidence_of_megatsunamis.asp">here</a>), the <a href="http://geology.gsapubs.org/cgi/content/abstract/37/5/403">May issue of Geology</a> provides additional arguments to show that this is the case. <a href="http://earthweb.ess.washington.edu/dwp/people/profile.php?name=bourgeois">Joanne Bourgeois</a> of University of Washington and <a href="http://geoweb.tamu.edu/profile/RWeiss">Robert Weiss</a> of Texas A&M University, both experts in the sedimentology of tsunami deposits, present two lines of arguments. First they show that the orientation of the Madagascar chevrons is significantly different from what is predicted through modeling the tsunami. While the tsunami wave tends to hit the coast with an overall perpendicular orientation, due to wave refraction, the 'chevrons' are oriented at various angles to the coast, angles that are more consistent with predominant wind directions. Second, they look at the sediment transport conditions and suggest that even coarse sand must have been in suspension in flows deep enough to create the chevrons. However, dune-like bedforms cannot develop without sediment being transported as bedload; therefore, the bedforms must have a different origin than mega-tsunamis. The obvious alternative is parabolic dunes; these well-known bedforms show up when vegetation partially covers the dune's tails and slows down sediment transport. The authors don't hesitate to draw the conclusion that <br />
<blockquote>
The extraordinary claim of "chevron” genesis by mega-tsunamis cannot withstand simple but rigorous testing.</blockquote>
I am far from being a tsunami expert, but I find this subject fascinating. The issue of suspended load vs. bedload and stratified or laminated vs. graded bedding is equally important for deposition from tsunami waves and turbidity currents. It is worth spending a bit of time and blogspace to explore the kind of analysis of sediment transport conditions that this paper presents.<br />
<br />
Although I see no reasons to disagree with the paper's conclusions (as it could be predicted from my previous posts on the subject), at first reading I didn't fully understand the line of reasoning about suspended load vs. bedload. So here goes my attempt to understand it.<br />
<br />
The argument goes as follows. The Rouse number is the ratio between the settling velocity of a certain grain size and the shear velocity of the flow, multiplied by von Karman's constant (which is ~0.4): Ro = ws/k*u_shear. For a grain of a given size, if the Rouse number is larger than 2.5, the grain's settling velocity is much larger than the upward-directed component of the turbulence, and the grain tends to stay close to the bottom, in the bedload. [This is equivalent to saying that the settling velocity has to be larger or equal to the shear velocity, a condition also known as the suspension criterion]. If the Rouse number is less than 0.8, the flow is turbulent enough to keep the grain fully suspended. In between these values, there is a zone of transitional behavior. For the flows that might have deposited the chevrons, the Rouse number is always less than 2.5, regardless of how the other parameters like the Froude number, grain diameter, and roughness length are varied. Although the authors state that the flows must have been deeper than 8 m (because most chevrons are higher than 4 m, and the flow must be at least twice as high as the bedform), there seems to be no other constraint on tsunami behavior [note that I did not have access - yet - to the supplementary web material].<br />
<br />
So the question is: doesn't this reasoning apply to other types of flows as well? For example, the Mississippi River is certainly deeper than 8 m in many places -- does this mean that it is able to suspend very coarse (2 mm diameter) sand? In other words, what is the difference between flow in a tsunami run-up and the Mississippi River? The answers might be obvious to many, but they are certainly not obvious to me.<br />
<br />
One thing we can do is to create a different kind of plot: instead of plotting the Rouse number against flow depth, let's plot velocity vs. depth. I have a better feeling for what are reasonable velocities for different kind of flows than I do for Rouse numbers. The Rouse number would form the third dimension of the plot; one can visualize that as a contour map of Rouse numbers as a function of flow depth and velocity:<br />
<br />
<a href="http://3.bp.blogspot.com/_zWNW_P1SBcA/SgX6AXE6cJI/AAAAAAAAATo/cEMuhAuuTZU/s1600-h/Picture+1.png" onblur="try {parent.deselectBloggerImageGracefully();} catch(e) {}"><img alt="" border="0" id="BLOGGER_PHOTO_ID_5333944217791787154" src="http://3.bp.blogspot.com/_zWNW_P1SBcA/SgX6AXE6cJI/AAAAAAAAATo/cEMuhAuuTZU/s400/Picture+1.png" style="cursor: hand; cursor: pointer; display: block; margin: 0px auto 10px; text-align: center; width: 500px;" /></a><br />
The Rouse numbers shown in this plot are valid for a grain diameter of 2 mm and roughness length of 1 m (using the same equations for settling velocity and shear velocity as in Bourgeois & Weiss 2009). Anything coarser than this cannot be called sand any more. So if this grain size doesn't stay in the bedload, there is no chance for finer sediment either. It is obvious from the plot that, for flows deeper than 8-10 m, very coarse sand will be part of the <span style="font-style: italic;">bedload</span> unless flow velocity is larger than ~5 m/s. The Mississippi River at New Orleans has velocities on the scale 1.5 m/s, so 1-2 mm sand should definitely stay close to the bottom, and in fact it does.<br />
<br />
We know however that tsunamis are not exactly tranquil flows like the big old Mississippi at New Orleans. The larger ones are fast and furious and Google Earth might need massive updates after they rearrange entire coastal landscapes. [Don't get me wrong, I am not trying to diminish the power and destructive force of the Mississippi.] In other words, the Froude number of a tsunami run-up is larger than the Froude number of the Mississippi River. The Mississippi is relatively slow and deep; the tsunami is fast and relatively shallow. The Froude number is the ratio between velocity and the square root of gravity multiplied by flow depth:<br />
<br />
<br />
<center>Fr = u/√(g*d)</center><br />
This number for the Mississippi is much less than one (these flows are called subcritical flows). On the other hand, tsunamis are waves of very large wavelengths, and they behave even in the open ocean as shallow water waves (wavelength 20 times larger than water depth). For these kinds of waves, the velocity is solely a function of water depth:<br />
<br />
<br />
<center>u = √(g*d)</center><br />
If we assume that the tsunami run-up has a comparable velocity to that of the tsunami wave in the nearshore zone, we find that the Froude number of the run-up must be around 1. This is obviously a very back-of-the-envelope argument, but the point is that these flows must have in general relatively large Froude numbers. If we plot the lines for Fr = 1 and Fr =1.5 on the depth-velocity diagram (see above), we can see how different likely tsunami behavior is from that of large rivers. It also becomes evident that even coarse sand would not be part of the bedload in these flows, especially not in flows deep enough to build the 'chevrons'. Which means that sandy tsunami deposits are likely to be largely unstructured or poorly structured sand sheets rather than several m thick accumulations of cross-bedded sand.<br />
<br />
And that ends my Saturday exercise in Fluid Mechanics 101.<br />
<br />
<span style="font-weight: bold;">Reference</span><br />
<span class="Z3988" title="ctx_ver=Z39.88-2004&rft_val_fmt=info%3Aofi%2Ffmt%3Akev%3Amtx%3Ajournal&rft.jtitle=Geology&rft_id=info%3Adoi%2F10.1130%2FG25246A.1&rfr_id=info%3Asid%2Fresearchblogging.org&rft.atitle=%22Chevrons%22+are+not+mega-tsunami+deposits--A+sedimentologic+assessment&rft.issn=0091-7613&rft.date=2009&rft.volume=37&rft.issue=5&rft.spage=403&rft.epage=406&rft.artnum=http%3A%2F%2Fgeology.gsapubs.org%2Fcgi%2Fdoi%2F10.1130%2FG25246A.1&rft.au=Bourgeois%2C+J.&rft.au=Weiss%2C+R.&rfe_dat=bpr3.included=1;bpr3.tags=Geosciences">Bourgeois, J., & Weiss, R. (2009). "Chevrons" are not mega-tsunami deposits--A sedimentologic assessment <span style="font-style: italic;">Geology, 37</span> (5), 403-406 DOI: <a href="http://dx.doi.org/10.1130/G25246A.1" rev="review">10.1130/G25246A.1</a></span><br />
<br />
Links to this post:<br />
<a href="http://network.nature.com/people/primatediaries/blog/2009/05/18/scientia-pro-publica-4-in-memory-of-stephen-jay-gould">Scientia Pro Publica #4</a>Anonymoushttp://www.blogger.com/profile/02317685391212630354noreply@blogger.com5tag:blogger.com,1999:blog-6377410.post-22086205871918772362009-03-28T13:42:00.014-05:002010-11-25T06:23:27.283-06:00Normal grading<br />
In sedimentology, the word 'grading' has nothing to do with exams and assignments. Instead, it refers to a regularly decreasing or increasing grain size within one sedimentary layer. Because it is much more common than the other alternative, upward decreasing grain size is called 'normal grading'. Grains that consistently increase in size toward the top of the bed are responsible for 'inverse grading'. Upward fining and coarsening are related terms that are often used to describe grain-size trends in not one, but multiple beds.<br /><br /><a onblur="try {parent.deselectBloggerImageGracefully();} catch(e) {}" href="http://skeptic.smugmug.com/photos/204185916_cf2hk-L.jpg"><img style="display:block; margin:0px auto 10px; text-align:center;cursor:pointer; cursor:hand;width: 412px; height: 600px;" src="http://skeptic.smugmug.com/photos/204185916_cf2hk-L.jpg" border="0" alt="" /></a><center>Normal grading in a turbidite from the Talara Basin, Peru</center><br /><br />The simplest way to generate normal grading is to put some poorly sorted sand and water in a container, shake it up, and then let it settle. The larger grains will settle faster than the smaller ones (as <a href="http://en.wikipedia.org/wiki/Stokes'_law">Stokes' law</a> tells us) and most of the large grains will end up at the bottom of the deposit. [Note that some fine grains will be at the bottom as well - the ones that were already close to the bottom at the beginning of sedimentation.] This kind of static suspension settling is <span style="font-style:italic;">not</span> how most sediment is deposited on a river bed or a beach; even if a grain is part of the <a href="http://en.wikipedia.org/wiki/Suspended_load">suspended load</a>, it usually goes through a phase of bedload transport, that is, a phase of jumping and rolling and bouncing on the bed, before it comes to rest. The resulting deposit usually has lots of thin layers, laminations, and there are no obvious and gradual upward changes in grain size.<br /><br />What is needed is a sediment-rich flow that suddenly slows down <span style="font-style:italic;">or</span> spreads out and looses its power to carry most of its sediment load. Grains are getting to the bottom so fast that there is not much time for the flow to keep them rolling and bouncing around; instead they quickly get buried by the other grains that are ready to take a geological break. While this is still quite different from static suspension settling (because the flow did not come to a full stop), it can be thought of as a modified version of static settling: all is needed is a horizontal velocity component, in addition to the vertical one. Of course, the segregation of the coarser grains to the bottom of the flow may have started much earlier. Typically, they never made it to the top in the first place.<br /><br /><br /><a onblur="try {parent.deselectBloggerImageGracefully();} catch(e) {}" href="http://skeptic.smugmug.com/photos/483361006_L5X8P-X2-1.jpg"><img style="display:block; margin:0px auto 10px; text-align:center;cursor:pointer; cursor:hand;width: 500px;" src="http://skeptic.smugmug.com/photos/483361006_L5X8P-X2-1.jpg" border="0" alt="" /></a><center>Conglomerate bed in the Cretaceous Cerro Toro Formation, Torres del Paine National Park, Southern Chile. There is some inverse grading at the base of this bed, before the size of the clasts starts decreasing</center><br /><br />Such large, sediment-laden flows are not very common, certainly not on a human timescale. When they do occur, they tend to show up in the news, especially if human artifacts, or humans themselves, become part of the normally graded deposits. Deposits of snow avalanches, volcanic ash-laden pyroclastic flows, subaerial debris flows, <a href="http://walrus.wr.usgs.gov/tsunami/itst.html">tsunamis</a>, submarine turbidity currents can all show normal grading. The images shown here all come from deposits of large submarine gravity flows. Some of them (like the one below) have a muddy matrix, but the grading is still obvious (the two large clasts at the top of the bed have lower densities).<br /><br /><a onblur="try {parent.deselectBloggerImageGracefully();} catch(e) {}" href="http://skeptic.smugmug.com/photos/206742562_a46Jg-L.jpg"><img style="display:block; margin:0px auto 10px; text-align:center;cursor:pointer; cursor:hand;width: 415px; height: 600px;" src="http://skeptic.smugmug.com/photos/206742562_a46Jg-L.jpg" border="0" alt="" /></a><center>Normally graded conglomerate layer with a muddy matrix, Cerro Toro Formation, Chile</center><br /><br />In recent years, some questions have been raised about the common presence of normal grading, especially in turbidites. The fact is that normal grading is often seen in rocks of all ages, and, in a simple view, it is a reflection of larger grains getting quickly to the bottom.<br /><br />Normal grading is normal, after all.Anonymoushttp://www.blogger.com/profile/02317685391212630354noreply@blogger.com0tag:blogger.com,1999:blog-6377410.post-19366431619361732612009-03-08T12:30:00.008-05:002010-11-25T06:23:56.008-06:00Description does not suffice for an explanation<br />
On February 3, 1967, J. R. L. Allen gave the fifth "Geologists' Association Special Lecture", entitled "Some Recent Advances in the Physics of Sedimentation". This is from the introduction:<br /><br /><blockquote>"Two stages can generally be recognised in the historical growth of a reasonably advanced scientific discipline. There is an early, descriptive stage in which with little guide from theory, an attempt is made to collect, define and analyse phenomena. In the later, explanatory stage we see that efforts are concentrated on the production of generalisations and on the explanation of the reduced phenomena in terms of general laws. Of course, there is never a single point in time at which there is change over the entire scope of a discipline from the descriptive to the explanatory stage. The change is, rather, uneven, taking place earlier in some branches than in others, and more gradually in one branch than in another. <br /><br />Sedimentology stands today in a period of transition. Its subject matter is sedimentary deposits, and its goal the origin and meaning of these in the context of planetary studies in general. But it is apparent, except to adherents of geological phenomenalism, that sedimentary deposits cannot be explained in terms of themselves. Already we are in possession of major generalisations about these deposits, and our chief task for some years should be to explore and ratify them in terms of general laws in order that our understanding of the sedimentary record can be made sharper. In those parts of the field where major generalisations have already been established, the provision of further descriptive data is of little value, except in so far as light is shed on the problems of particular deposits. These are validly a part of the subject, leading to a refinement of certain planetary laws. But the other and no less important laws in terms of which we should seek to frame our understanding are those of general chemistry, physics and biology. In order to achieve this framework in the case of detrital sediments, it will be necessary to set aside for a while the problems of particular deposits. This will, of course, be unacceptable to those who claim that geology, or sedimentology, is only to do with rocks as conceived in a historico-geographical manner. But they will be proved wrong, provided we keep our major goals in mind, for it is a mistake to suppose that a description will suffice for an explanation. Most of our explanations will probably turn out to be no better than qualitative, so complex are most sedimentary systems, but we should nevertheless attempt them and try to frame them as exactly as possible."</blockquote><br />Forty years after publication of the paper, this seems as timely as ever.<br /><br />Reference:<br /><a href="http://www.sciencedirect.com/science/article/B94SW-4V53XGV-1/2/f865e1f51b32828da9ae07a22aebf0a6">Allen, J. R. L., 1969. Some Recent Advances in the Physics of Sedimentation. Proceedings of the Geologists' Association 80:1-42.</a>Anonymoushttp://www.blogger.com/profile/02317685391212630354noreply@blogger.com0tag:blogger.com,1999:blog-6377410.post-83136701536690753472009-03-05T15:21:00.020-06:002010-11-25T06:24:39.213-06:00Three photos from Chilean Patagonia<br />
I was lucky to attend a few days ago <a href="http://www.sepm.org/activities/researchconferences/chile/chile_home.htm">a field conference</a> in southern Chile, looking at deep-water rocks in an area that includes Torres del Paine National Park. It was good to be back in this place of unbloggable beauty. The conference was well organized (of course! - <a href="http://clasticdetritus.com/2009/02/28/conference-is-over-time-to-relax/">Brian was one of the conveners</a>) and we were extremely lucky with the weather: no rain at all on the outcrops, beautiful sunshine most of the time. Although I have been to Chilean Patagonia three times before on various geological field trips and even did some field work there, I realized during this conference that it doesn't matter how many times you have seen some rocks, there is always a chance to rethink what you thought you have already settled in your mind (see blog title). It was also good to see that these field conferences are increasingly not just about the local geology: many if not most presentations and spontaneous discussions compare the local outcrop data with sedimentary systems from other basins, and try to think about how the always-too-small outcrops would look like in seismic sections and volumes.<br />
<br />
Brian did not have time to take a lot of photos, so here are three shots (more <a href="http://skeptic.smugmug.com/gallery/7490281_WpCjq">here</a>). As if anybody needed more shots of the Paine Grande and the Cuernos. <br />
<br />
<a href="http://skeptic.smugmug.com/photos/483356728_68mLB-XL.jpg" onblur="try {parent.deselectBloggerImageGracefully();} catch(e) {}"><img alt="" border="0" src="http://skeptic.smugmug.com/photos/483356728_68mLB-XL.jpg" style="cursor: hand; cursor: pointer; display: block; margin: 0px auto 10px; text-align: center; width: 500px;" /></a>
<center>Conference participants examine the turbidites of the Punta Barrosa Formation</center><br />
<br />
<br />
<a href="http://skeptic.smugmug.com/photos/483363041_UnxsZ-XL.jpg" onblur="try {parent.deselectBloggerImageGracefully();} catch(e) {}"><img alt="" border="0" src="http://skeptic.smugmug.com/photos/483363041_UnxsZ-XL.jpg" style="cursor: hand; cursor: pointer; display: block; margin: 0px auto 10px; text-align: center; width: 500px;" /></a>
<center>The Paine massif (Paine Grande and Cuernos), with Rio Serrano in the foreground</center><br />
<br />
<br />
<a href="http://skeptic.smugmug.com/photos/483361465_zowqz-XL.jpg" onblur="try {parent.deselectBloggerImageGracefully();} catch(e) {}"><img alt="" border="0" src="http://skeptic.smugmug.com/photos/483361465_zowqz-XL.jpg" style="cursor: hand; cursor: pointer; display: block; margin: 0px auto 10px; text-align: center; width: 500px;" /></a>
<center>Strong winds on Paine Grande</center><br />
<br />
Update - here is a Gigapan:<br />
<script>
<br /> function FlashProxy() {}<br /> FlashProxy.callJS = function() {}<br />
</script><br />
<br />
<br />
<center><embed height="300" src="http://gigapan.org/viewer/PanoramaViewer.swf?url=http://share.gigapan.org/gigapans0/18452/tiles/&suffix=.jpg&startHideControls=0&width=42349&height=11090&nlevels=9&cleft=0&ctop=0&cright=42349.0&cbottom=11090.0" type="application/x-shockwave-flash" width="500"></embed></center><center><a href="http://gigapan.org/viewGigapanFullscreen.php?id=18452">Launch full screen viewer</a></center><br />
<br />
<br />
<center>[it is strongly recommended that you do launch the full screen viewer if you want to do justice to the Gigapan]</center>Anonymoushttp://www.blogger.com/profile/02317685391212630354noreply@blogger.com3