Thursday, October 11, 2007

The importance of numbers in sedimentary geology

A few years ago, Chris Paola published a paper in Sedimentology on "Quantitative models of sedimentary basin filling". I was skimming through it today, and found these thoughts about the role and status of quantitative reasoning in sedimentary geology:
...what is needed is researchers who are skilled in the field but at the same time understand what quantitative modelling is about: why and how people make approximations, why approaches to modelling can and must differ, and, above all, what the mathematics in the models mean physically. Just as there is no substitute for experience in learning to work in the field, there is no substitute for experience in developing physical insight. And there is no shortcut: we need researchers who are good at at both traditional, descriptive geology and quantitative geology. 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.

Some 30 years after the initial 'physics scare' associated with bedforms and sedimentary structures, a set of basic principles from fluid and sediment mechanics now appears routinely in introductory sedimentology textbooks. Popular items include settling velocity and Stoke's Law, the Reynolds and Froude numbers, and the basic force balance for steady, uniform channel flow. This material is typically presented somewhere near the beginning of the book and then is largely ignored. (...) There remains a striking contrast between the role of fluid and sediment physics in sedimentary geology and that of thermodynamics in igneous and metamorphic geology. In 'ig-met' texts the underlying thermodynamic principles are introduced and then applied repeatedly. Whereas in hard-rock petrology, thermodynamics permeates the discipline, in sedimentary geology, sediment mechanics still seems a little like taking vitamins: it is surely good for you, but most people cannot say exactly why. There are several reasons for this. In current practice, process-based interpretation is often applied in a piecemeal, descriptive way, to no apparent end beyond providing the interpreter with one more adjective. In addition, the quantitative material that is traditionally taught more often not the most important. For instance, a real appreciation of the implications of the sediment-continuity equation as the governing relation for physical sedimentation is far more useful than the details of sediment-transport formulae or even the definition of the Reynolds number.
Although I still have a lot to learn myself, I couldn't agree more.

ps. Check out what Lord Kelvin had to say about the importance of numbers in science.


Anonymous said...

That is a great paper...I pull it out as reference often.

There is still a gap between the "observationalists" and the "theorists" but it is closing. People like me need to get into the classroom and tank labs and those people need to get out in the field and see just how messy nature can be.

Unknown said...

Kind of late for me to get into the classroom :). I am an "observationalist" (despite the fact that I like to play with numbers), and I still need to think more about scales and rates and relative importance of processes than I usually do.

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