The earth as a whole appears to stand between steel and glass in rigidity. It is a matter of common observation that rocks stand high in this respect and in the consequent difficulty with which they can be bent without breaking. Because of the earth's contraction the crust endures a constant strain, which must gradually become enormous. This strain is increased by the fact that sediment is transferred from the lands to the borders of the sea and there forms areas of thick accumulation. From this has arisen the doctrine of isostasy, or of the equalization of crustal pressure. An important illustration of
this is the oceanward and equatorial creep which has been described in Chapter XI. There we saw that when the lands have once been raised to high levels or when a shortening of the earth's axis by contraction has increased the oceanic bulge at the equator, or when the reverse has happened because of tidal retardation, the outer part of the earth appears to creep slowly back toward a position of perfect isostatic adjustment. If the sun had no influence upon the earth, either direct or indirect, isostasy and other terrestrial processes might flex the earth's crust so gradually that changes in the form and height of the lands would always take place slowly, even from the geological point of view. Thus erosion would usually be able to remove the rocks as rapidly as they were domed above the general level. If this happened, mountains would be rare or unknown, and hence climatic contrasts would be far less marked than is actually the case on our earth where crustal movements have repeatedly been rapid enough to produce mountains.
Nature's methods rarely allow so gradual an adjustment to the forces of isostasy. While the crust is under a strain, not only because of contraction, but because of changes in its load through the transference of sediments and the slow increase or decrease in the bulge at the equator, the atmosphere more or less persistently carries on the tapping process. The violence of that process varies greatly, and the variations depend largely on the severity of the climatic contrasts. If the main outlines of the cyclonic hypothesis are reliable, one of the first effects of a disturbance of the sun's atmosphere is increased storminess upon the earth. This is accompanied by increased intensity in almost every meteorological process. The most important effect, however, so far as the earth's crust is concerned would apparently be the rapid and
intense changes of atmospheric pressure which would arise from the swift passage of one severe storm after another. Each storm would be a little tap on the tensely strained crust. Any single tap might be of little consequence, even though it involved a change of a billion tons in the pressure on an area no larger than the state of Rhode Island. Yet a rapid and irregular succession of such taps might possibly cause the crust to crack, and finally to collapse in response to stresses arising from the shrinkage of the earth.
Another and perhaps more important effect of variations in storminess and especially in the location of the stormy areas would be an acceleration of erosion in some places and a retardation elsewhere. A great increase in rainfall may almost denude the slopes of soil, while a diminution to the point where much of the vegetation dies off has a similar effect. If such changes should take place rapidly, great thicknesses of sediment might be concentrated in certain areas in a short time, thus disturbing the isostatic adjustment of the earth's crust. This might set up a state of strain which would ultimately have to be relieved, thus perhaps initiating profound crustal movements. Changes in the load of the earth's crust due to erosion and the deposition of sediment, no matter how rapid they may be from the geological standpoint, are slow compared with those due to changes in barometric pressure. A drop of an inch in barometric pressure is equivalent to the removal of about five inches of solid rock. Even under the most favorable circumstances, the removal of an average depth of five inches of rock or its equivalent in soil over millions of square miles would probably take several hundred years, while the removal of a similar load of air might occur in half a day or even a few hours. Thus the erosion and deposition
due to climatic variations presumably play their part in crustal deformation chiefly by producing crustal stresses, while the storms, as it were, strike sharp, sudden blows.
Suppose now that a prolonged period of world-wide mild climate, such as is described in Chapter X, should permit an enormous accumulation of stresses due to contraction and tidal retardation. Suppose that then a sudden change of climate should produce a rapid shifting of the deep soil that had accumulated on the lands, with a corresponding localization and increase in strains. Suppose also that frequent and severe storms play their part, whether great or small, by producing an intensive tapping of the crust. In such a case the ultimate collapse would be correspondingly great, as would be evident in the succeeding geological epoch. The sea floor might sink lower, the continents might be elevated, and mountain ranges might be shoved up along lines of special weakness. This is the story of the geological period as known to historical geology. The force that causes such movements would be the pull of gravity upon the crust surrounding the earth's shrinking interior. Nevertheless climatic changes might occasionally set the date when the gravitative pull would finally overcome inertia, and thus usher in the crustal movements that close old geologic periods and inaugurate new ones. This, however, could occur only if the crust were under sufficient strain. As Lawson[143] says in his discussion of the "elastic rebound theory," the sudden shifts of the crust which seem to be the underlying cause of earthquakes "can occur only after the accumulation of strain to a limit and ... this accumulation involves a slow creep of the region affected.
In the long periods between great earthquakes the energy necessary for such shocks is being stored up in the rocks as elastic compression."
If a period of intense storminess should occur when the earth as a whole was in such a state of strain, the sudden release of the strains might lead to terrestrial changes which would alter the climate still further, making it more extreme, and perhaps permitting the storminess due to the solar disturbances to bring about glaciation. At the same time if volcanic activity should increase it would add its quota to the tendency toward glaciation. Nevertheless, it might easily happen that a very considerable amount of crustal movement would take place without causing a continental ice sheet or even a marked alpine ice sheet. Or again, if the strains in the earth's crust had already been largely released through other agencies before the stormy period began, the climate might become severe enough to cause glaciation in high latitudes without leading to any very marked movements of the earth's crust, as apparently happened in the Mid-Silurian period.