Geology: The Science of the Earth's Crust

by William J. Miller

Shore lines of the great ancestor of Great Salt Lake also show warping of the earth’s crust, some parts of a definite shore line being several hundred feet higher than others.

Very significant evidence pointing to profound crustal movements consist in the finding of fossil remains of marine animals in the strata high above sea level, very commonly from one to three miles, in many parts of the world, especially in the high mountains. In Wyoming, nearly horizontal strata of the Mesozoic Age carrying marine fossils lie two miles or more above sea level. The fact that given formations, carrying marine fossils representing certain definite portions of geologic time, are found at various altitudes up to several miles in many parts of the world, shows that the land in those places has really risen relative to sea level.

It should not be presumed from the above discussion that the sea level itself has never changed. Thus, the vast areas of thick ice sheets in both North America and Europe during the Great Ice Age represented sufficient water withdrawn from the sea to very appreciably lower its level. All land-derived materials, carried into the sea mainly by rivers, displace sea water, with consequent rise of its level. If all existing lands were worn down and carried into the sea, its level would be raised some hundreds of feet. Subsidence of any part of the ocean bottom would cause a lowering of sea level. There is a strong reason to believe that some such shiftings of sea level have occurred during the vast lapse of geologic time. During certain periods erosion of the land predominated, and during other periods building up of the land predominated, as pointed out in the chapters on geologic history. It is not thought that shifting of sea level has ever amounted to more than a few hundred feet, at least not during the millions of years of the more clearly recorded earth history.

We have thus far considered slow upward and downward movements of the earth’s crust without notable structural changes in the rocks. Another type of crustal disturbance causes more or less profound changes in the structures of the rocks themselves. Just how the earth originated is a matter of uncertainty, but we can be sure that for many millions of years it has been a shrinking body. The outer, or crustal, portion of the earth, in adjusting itself to the contracting interior, has had many pressures, stresses, and strains set up within it. As results of such forces the rocks at and near the earth’s surface have in various places, and at various times, been broken (faulted) and subjected to sudden movements (see [discussion beyond]), while those well within the crustal portion, that is to say a few miles or more down, have, in many cases, been bent (folded), or even crumpled. For these reasons the surface and near-surface crustal portions are called the “zone of fracture,” while the more deeply buried portions comprise the “zone of flowage.” In the zone of flowage the rocks, where subjected to great lateral pressure, act like plastic materials and therefore bend rather than break, because of the great weight of overlying materials. Laboratory experiments have confirmed the findings of geologists in this regard. Small masses of rocks properly inclosed in nickel-steel cylinders have been subjected to slow differential pressures equivalent to those which obtain twenty to forty miles within the earth. Under such conditions rocks have been made to change shape very notably without fracturing. Both geological observations and experiments have led us to conclude that not even small fractures or crevices can remain open at a depth greater than ten or twelve miles even in the hardest rocks.

From time to time, during the long history of the earth, forces of lateral pressure have been slowly exerted along more or less localized zones or belts within the earth’s crust, and the rocks have been deformed chiefly by bending or folding, especially in those regions where mountains of the folded type have developed. Movements of this type are considered beyond in the chapter on mountains. Rock folds vary in size from microscopic to miles across, and they exhibit many shapes. [Plate 7] will give the reader a good idea of actual rock folds of common sizes and shapes in various places. Folded structures are most clearly discernible in sedimentary rocks, because of their stratified (layered) arrangement. Since folds in hard rocks rarely, if ever, develop except at a depth of some miles within the earth, they show at the surface only where great thicknesses of overlying materials have been stripped off by erosion.

Fig. 8.—An outcrop of stratified crystalline limestone (or marble) exhibiting two small sharp folds—a syncline on the left and an anticline on the right—near Lenox, Mass, These folds developed during the great mountain-making disturbance at the end of the Ordovician period fully 20,000,000 years ago. (After Dale, U. S. Geological Survey.)

From the standpoint of our consideration of slow earth-crust movements, it is important to bear in mind that lateral pressure in the zone of flowage has not only notably deformed rocks, but that, as a result of the buckling forces, given rock masses have, in many cases, been notably shifted downward or upward—mainly upward—from their original positions. Folded strata carrying shells of sea animals are commonly found thousands of feet above sea level in many of the great mountain ranges of the world. During the process of folding on a large scale the crust of the earth is very appreciably shortened at right angles to the direction of applied pressure, due to squeezing or bending of the strata. In the case of the Appalachian mountains of Pennsylvania it has been estimated that such shortening amounts to about twenty-six miles or, in other words, that the strata originally spread out horizontally across an area whose width was about 100 miles have been squeezed or folded into an area whose width is twenty-six miles less.