The Geoscientists’ Blind Spot

One advantage of looking back at the history of earth science is to recognize patterns that suggest certain biases.

Consider, for instance, continental drift.  Now this is often portrayed as Wegener right, others stupid dunderheads, but obviously that is too simple. First off, Wegener had a mix of good and bad observations.  Aside from fitting continents (a somewhat old parlor game by then), he noted common terrestrial species, ice deposits far from the pole, and the fundamental division between continental crust and oceanic crust.  But he also put a lot of weight on his own grossly inaccurate geodetic surveys and so concluded that Pleistocene deposits on either side of the Atlantic predated the separation of the continents.  But the big objection to continental drift was simply: how would it occur?

Here’s the funny thing: this is common to any number of ideas based off of observation in earth science.  If you want to bet, bet in favor of observation-based occurrences having occurred and against objections based on an incomplete explanation of how it works.

How common is this?  A few examples follow:

Consider isostasy, the notion that elevated areas are supported by a mass deficit at depth much as ships remain above water so long as their interior has a lower density than water. The basic observations go back to the Indian survey in the 1840s where reconciliation of astronomical and triangulation-based measurements required that the Himalaya have something light under them. That in and of itself didn’t reveal much about how this might occur, but G.K. Gilbert’s famous studies of the shoreline of Pleistocene Lake Bonneville put some serious bounds on what might be happening.

Provo shorelines of Lake Bonneville, from Gilbert, USGS PP 1, 1890.

Provo shorelines of Lake Bonneville, from Gilbert, USGS PP 1, 1890.

Gilbert lucidly explained that the best explanation was that the crust in the center of the lake had risen up in response to the removal of the load of water.  Beyond that, he clearly articulated that this was likely a reflection of flow at greater depths in the earth and made a crude estimate that such flow might be occurring about 32 miles down. The explanation includes a simple summary of what we now call elastic plate theory. At about the same time his tome was published, he gave a talk at the first meeting of the Geological Society of America where he clearly separated the rigid support of smaller features like individual mountains from isostatic support or large features (a term only just invented by Clarence Dutton to describe the relations Airy and Pratt had understood from the Indian survey). [This presentation predated recognition of isostasy as the mechanism behind the rebound from ice sheets; Gilbert was asked about this and thought it worth examination]. Present in the audience was Andrew Lawson, who would soon join the faculty at the University of California.  Lawson and Gilbert were to be fairly close colleagues in the years ahead, yet Lawson would not accept isostasy until about 1920.  Why?  Here is what he wrote (published in 1948, but the manuscript was kicking around long before that):

Isostasy, as a fundamental principle of geology which explains the uplift and support of mountains, was discovered by Airy, named by Dutton, and rationalized by Barrell. Until the appearance of Barrell’s papers (1914-1915) on the strength of the earth’s crust it had been logically impossible to accept or to apply the doctrine propounded by Airy. Geologists, familiar as they were with the structure of mountains, were ready enough to believe that orogenesis involved downthrust as well as uplift, but the sinking of the root of a great range into heavier rock, to secure support by flotation, implies a disposal of the heavier rock displaced. How is the displaced rock disposed of? No one was able to answer that question. Geological opinion was committed to the view that the rocks of the lithosphere become increasingly stronger with depth.

Despite Gilbert’s truly insightful presentation and the lack of any hostility between the two men, Lawson would not accept isostasy until somebody could tell him how it worked. (It is unclear how he could reconcile his beliefs with gravity studies and evidence like Gilbert’s). Once this barrier was cleared, though, he became an evangelist for isostasy.

This is hardly the only other example.  For instance, large overthrusts were known to geologists in the Alps back in the 19th century, but geophysicists said that such faults could not possibly be active at the low angles geologists observed. The simple argument was that if you calculate the force needed to push a kilometer-thick block of rock across other rock, you would shatter the kilometer-thick block before it would move.  So since there was no mechanism for this to work, it couldn’t really have happened.  It turns out that there are a couple of adjustments to the simple physical model that make it all work: the first was the notion that high fluid pressure on the overthrust would reduce friction considerably (Hubbert and Rubey getting the credit here), the second was the recognition that a wedge shape instead of a block would allow the stresses to be reduced as the load was reduced (Bill Chapple and then Dahlen and Suppe and Davis). Large low-angle normal faults and “Snowball Earth” have yielded some similar arguments.

Basically it seems that the thing that will generate the greatest objection to a new concept is when it collides with some perception of how the world works that is incomplete.  Such opposition can be quite helpful: continental drift without subduction and seafloor spreading makes little sense; the physical models for overthrusts can now help in understanding many features of thrust belts that appeared too complex without the model. But blind opposition on that basis, a refusal to materially engage in the attempt to understand the observations, isn’t helpful.  And so this is something to keep in mind in being critical about some new idea: is the idea observationally flawed? Or is it that it asks us to reform our view on how the earth works?

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2 responses to “The Geoscientists’ Blind Spot”

  1. Miksha says :

    Reblogged this on The Mountain Mystery and commented:
    The Grumpy Geophysicist’s blog has an insightful piece today. He relates the slow acceptance of continental drift (later morphed into plate tectonics) to an inherent nature in the way humans look at new ideas. It’s a good read…

    Like

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