Tuesday, January 22, 2008

Geologic misconceptions: 2D vs. 3D

[This is a contribution to Accretionary Wedge #5]

One of the problems that a geologist is often faced with is the difficulty of reconstructing a complex three-dimensional geometry and history from limited information that is often one-dimensional (e.g., well data, cores) or two-dimensional (outcrops, 2D seismic sections). Humans in general, and geologists in particular tend to look for evidence where the light is better, and we are tempted to think that the beautiful core we have described, the one good outcrop face we have, the one textbook-quality seismic line on our wall is a good representation of the geology and stratigraphy of a much broader area, and that one can build a coherent story without knowing much about the third dimension.

That, of course, may well be true of ‘layercake’ stratigraphy: after all, a single thickness value can be used to fully characterize the geometry of a layer that has the same thickness over a large area. But, as Brian points out, ‘layercake stratigraphy’ should be considered an oxymoron: every sedimentary layer shows some thickness variations if traced for a long enough distance, even if some layers change their thickness more slowly than others. Stratigraphy is only layercake-like for human observers; subtle but persistent variations in thickness and relief can become striking geometries with some vertical exaggeration. Again, if this variation only occurred in one direction, a two-dimensional section along the same direction would summarize very well the whole story.

However, complex three-dimensionality is the rule rather than the exception in geology. Take for example a meandering river: its geometry is complex enough as it is, a single snapshot of a snaky morphology in time. But try imagining what happens as point bars and levees are deposited and cutbanks are cut; the channel changes its position over time and, over thousands and hundreds of thousands of years, it leaves behind an extremely complicated stack of deposits that would probably be difficult to fully understand even if you somehow could see and describe everything at the greatest detail in 3D. Obviously, a nice outcrop or a number of cores through such a deposit can provide a wealth of information, but we would be fooling ourselves if we thought that a single fining-upward sequence with some cross-bedding (that is, the classic point-bar facies model) was enough to understand a fluvial system.

But strong three-dimensionality is not restricted to fluvial deposits; look at any present-day depositional system in Google Earth and you will find that alluvial fans, deltas, barrier islands and tidal inlets, wind-blown dune fields are all intricate patterns, usually with lines running in more than one direction. Yet many of the classic facies and stratigraphic models are either one- or two-dimensional. Maybe, probably, these are necessary and useful simplifications and conceptual models, but they can only be useful if one is also aware how far they are from capturing the full 3D complexity of nature.

That being said, I have to add that 3D is not always better than 2D. Nowadays, some of the best three-dimensional geological datasets are 3D seismic surveys, and, with the increasing availability of such gold-mines of stratigraphic beauty (there are other uses as well, but let’s focus on one thing for now :) ) it is easy to fall victim to the temptations of colorful three-dimensional displays. Despite claims like ‘3D interpretation and visualization are the future’, the truth is that a good set of old-fashioned maps and cross sections are more valuable in the long term than some glossy presentation slides with no exact spatial location.

Unless, of course, you can visualize and share your data relying on an easy-to-use and truly three-dimensional viewer. Like Google Earth. Even William Smith would be excited about that.

Detail from "Geological view and section through Dorsetshire and Somersetshire to Taunton, on the road through Yeovil toWimborn[e] Minster, &c.", by William Smith, 1819. Source: Oxford Digital Library

Sunday, January 13, 2008

Geo-highlights from Hindered Settling 2007

It is kind of late to do this 2007 retrospective, but what the heck. As pointed out by Ron, 2007 has been the year when a real geology blogger community started to develop. The evolution of Hindered Settling from an eclectic mix of notes about science, geology, skepticisim, atheism, technology, etc., written in Hungarian and in English (or Hunglish?), to a much more geoscience-oriented, English-only site is in part the result of this trend.

So here are a few posts from 2007 that I think should be on this list:

Photos from Brazos Bend State Park - if you live in Houston, Brazos Bend State Park is one of the best places to get away from the city and see some wildlife & nature. No mountains, of course, but at least you can look at oxbow lakes and learn about photography. For some reason, the photos I have taken there over the years have become fairly popular.

On the Great Unconformity, James Hutton, and Geologic Time

Photos and impressions from a stunning glacial lake and delta in the Canadian Rockies, with some sedimentology mixed in

On flame structures

Sedimentology on Mars
- wet or dry gravity flows?

Thoughts about the Black Sea flood and its potential link to the spread of agriculture in Europe

Saturday, January 12, 2008

Absurd catastrophism

If there was an icon for 'blogging on non-peer-reviewed non-research', in the style of 'blogging on peer-reviewed research', this post would qualify for it. Although it is advertised as publishing "cutting-edge, peer-reviewed, creationist research papers", Answers Research Journal (published by Answers in Genesis) is definitely not cutting-edge, not peer-reviewed, and is clearly not research. The evidence: the first few materials that are available online. There is a paper on "catastrophic granite formation"; here is a passage that gives you a flavor:
"Thus the formation of granite intrusions in the middle to upper crust involves four discrete processes — partial melting, melt segregation, magma ascent, and magma emplacement. According to conventional geologists (Petford et al. 2000), the rate-limiting step in this series of processes in granite magmatism is the timescale of partial melting (Harris, Vance, and Ayres 2000; Petford, Clemens, and Vigneresse 1997), but “the follow-on stages of segregation, ascent, and emplacement can be geologically extremely rapid - perhaps even catastrophic.” However, as suggested by Woodmorappe (2001), the required timescale for partial melting is not incompatible with the 6,000–7,000 year biblical framework for earth history because a very large reservoir of granitic melts could have been generated in the lower crust in the 1,650 years between Creation and the Flood, particularly due to residual heat from an episode of accelerated nuclear decay during the first three days of the Creation Week (Humphreys 2000; Vardiman, Snelling, and Chaffin 2005). This very large reservoir of granitic melts would then have been mobilized and progressively intruded into the upper crust during the global, year-long Flood when the rates of these granite magmatism processes would have been greatly accelerated with so many other geologic processes due to another episode of accelerated nuclear decay (Humphreys, 2000; Vardiman, Snelling, and Chaffin 2005) and catastrophic plate tectonics (Austin et al.1994), the likely driving mechanism of the Flood event."
Here is what I honestly do not understand. Let's accept for a moment the idea that granites can form relatively fast, and pretend that radioactive dating has some major issues, as these people claim (it doesn't, of course), so that all the granites on Earth fit the 6000-year timeframe. But what about the stuff that the granites were generated from? That must be older, right? And if the whole crust is less than a few thousand years old, what about the mantle? And, if one can speculate about "accelerated nuclear decay during the first three days of the Creation Week" or "catastrophic plate tectonics", why not just say that granites were created on the second day, after the mantle was ready to start convection by the end of the first day? Or, even better and simpler (Occam's razor!), why not just come up with something like:
And God said, Let there be granite: and there was granite. And God saw the granite, that it was good: and God divided the crust from the mantle.
I am looking forward to the time when somebody realizes that the Universe was created yesterday, and it is only an illusion that we have been around for a bit longer than that. Imagine all the wonderful research opportunities that such a revolutionary working hypothesis would generate. I can already see papers and headlines like:
Updated relativity theory shows that time is shorter than you think

Plate tectonic hit-and-run: after hitting North America yesterday with several microcontinents, the Pacific Plate continued to subduct as if nothing happened

Fossil record from 4:15 pm yesterday shows that lightning-fast giant snails were abundant on Earth for more than 7 minutes

Accelerated ice flow during the last few minutes of Creation Hour is likely responsible for death of Ötzi the iceman

Scuba diver killed by massive rain of pelagic forams

Catastrophic hair growth in early humans
Additional research ideas are welcome; 'Answers Research Journal' is calling for papers now.

UPDATE: In my rush to publish the above results, I forgot to mention some previous work on similar subjects: Afarensis - an expert in points to the Precambrian archaeology -ist, who suggested long ago that the Cambrian explosion was caused by a bacteria trying to form a synthesis with a mitochondria and that peanut butter is a leftover from this explosion. I would only add that there is new evidence suggesting that the Precambrian started at 1 am yesterday, after Creation Hour ended, and it probably lasted for several hours, until the bacteria committed adultery.

Saturday, January 05, 2008

Three photos from the Canadian Rockies in winter

More here.

Upper Falls, Johnston Canyon

Lake Louise

Elk near Bow Valley Parkway
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