Geologic Evidence for Isostasy
Surveying the Andes and HimalayasThe concept of isostasy was first discovered by surveyors that tried to map large mountain ranges, such as the Andes (by the French Scientists Pierre Bouguer , La Condamine and Louis Godi) or Himalayas (under the leadership of Sir George Everest). It was noted that different surveying techniques yielded results that differed by more than experimental errors could account for. It was soon found out that the additional mass of a large mountain range attracts a plumb bob towards it and deflects it from the true vertical. As a result trigonometric surveying techniqies that relied on levels and plumb bobs yielded results that were different from astronomical surveys.
Careful calculations, however, turned out that these mountain ranges were actually lighter than expected, leading in 1855 to the theories of Airy and Pratt, stating that mountains are composed of less dense material floating on a denser substrate. (Neither Airy, nor Pratt coined the term isostasy, it was introduced in 1889 by C.E. Dutton)
Isostatic ReboundIf our concept of a floating crust is correct one would assume that large topographic changes, such as the removal of a mountain range (by erosion), or the deposition (and subsequent melting) of a large ice sheet lead to isostatic adjustments of the Earth's crust. Removal of a llarge load should cause the crust to float up higher, while loading of crustal rocks should make the crust sink deeper. Isostatic rebound has been observed for many formerly glaciated locations. The little animated GIF to the right (courtesy of Tom Rothhamel, Univ. of Montana) illustrates the effects of an icesheet on the underlying rocks.
Today,
with the glaciers gone the land surface is still rising in some parts of the
world. One of the best examples comes from scandinavia, which is still rebounding
from the last ice age. As a result of isostatic rebound it is often
possible to see old shorelines that formed at the end of the last glaciation,
when the land surface was still depressed, and are now many meters above
present sea level. Early on many geologists recognized thick stacks of sediments that were deposited in very shallow water. Problem was, that many of these stacks were several kilometers thick. Geological evidence, however, showed that sediments deposited at the bottom of the stack were deposited in approximately the same water depth as sediments from the top of the stack. Isostasy offers an explanation for this observation. As you will see in one of the later exercises, a stack of sediments, if thick enough, can and and will sink under its own weight, leaving the top of the sediments very close to the water surface.
