S. WARREN CAREY, PART 2

                     VISCO ELASTICITY MODEL

Did you ever wonder why it was so hard for the geological community of the early and middle parts of the 19^th century to accept the concept of continental drift?  Probably not, but now’s your chance.  After all, Wegener, du Toit and others had presented a varied body of evidence that it HAD occurred which, nowadays, appears totally conclusive.  So, why did so many prominent geologists disagree?

Well, human cussedness obviously played a part.  If you grew up instinctively harboring  fixist ideas, and those ideas had served you well throughout your career, damned if you were going to let a bunch of  Johnny-come-latelies turn everything upside down!

However, if you rejected drift you needed some scientific argument to back you up.  And such, you thought, existed – from the field of seismology!  Continental drift couldn’t be true because it was impossible!

Early drifters had at best a bunch of crude ideas about HOW drift occurred, and all of them required some degree of mobility in, at least. the upper mantle.  Nay-sayers could point to seismic evidence that indicated that the mantle, far from being the least bit soupy, was more rigid than steel!  Hence, no drift.  It was all an illusion.  Here is how that works. 

It transpires that some physical properties of the interior of the earth can be deduced by what happens to earthquake waves as they pass along their paths.  In particular, how rigid a material happens to be is reflected in how rapidly the energy contained in a seismic wave is dissipated – converted to heat.  Precise seismic measurements showed conclusively that the mantle was extremely rigid.  Voila!  The mantle had the properties of hard, cold steel – and continental drift, Q.E.D,  was out of the question.

Well, S. Warren Carey blew that particular ship out of the water with an important paper that nobody reads any more:

Carey, S. W., 1953, The rheid concept in geotectonics, , Journal  of the Geological Society of Australia, Volume 1, Issues 1, 2.

In this paper, the old Tasmanian Devil points out the fact – that should have been obvious – that the response of a material to an applied force depends in large part on for how long the force is applied.  Look at the model at the top of the page.  If a force is applied for only an instant,  the spring will show significant (elastic) displacement but the piston will not have time to respond very much.  Hence the material will appear to be totally elastic.  However, apply the force for a geological significant time and the “dashpot” (think a cylinder filled with goop) will deform so much that the elastic contribution can be totally ignored.  In other words, on a tectonic time scale, the mantle behaves very much like a soupy, viscous fluid!  Among other things, this makes thermal convection not only possible, but likely.

If you have trouble with this concept, try a thought experiment.  Mentally strap on your crampons, grab an ice ax, and find a nice steep glacier to climb.  On the time scale of your ascent you can treat the glacial ice as a fine, trustworthy solid – even though, as a good geologist, you know that, on a time scale of weeks or even days, it is flowing steadily  downhill!