140 THE EARTH AS MAN'S ABODE China owes its color to the loess which it picks up as it carves its path through extensive plains of loess, all of which was probably blown from the Gobi desert. Inasmuch as about three fourths of the earth's surface is covered by the ocean, it is to be surmised that tremendous quantities of land have been carried by the winds over the oceans, where it settled. In the drier sections of the world the wind carries away the finer particles and leaves the sand, blowing it up into dunes. The wind is constantly changing the shape and size of the sand dunes, moving them from place to place, and overwhelming everything in their paths. Fig. 37. Half Dome in Yosemite National Park, an example of exfoliation. (Courtesy of the U. S. Geol. Survey.) Temperature Changes Bring About Disintegration of Rocks. Inasmuch as rocks are poor conductors of heat, they heat unevenly and set up strains which result in cracks, just as glassware cracks when it is placed under such a strain. Water which gathers in the cracks of rocks expands when it freezes and thus splits the rocks apart. "Exfoliation" is the name given to the process in which sharp edges and corners of rocks are rounded off and layer after layer peels off to form rounded domes. Exfoliation is thought to be caused by chemical action, weathering, and expansion due to the decreased pressure resulting from removal of heavy overlying loads.
THE GRADATION OF THE EARTH 141 Gravity Acts as a Transporting Agent. Gravity constantly moves disintegrated portions of rocks to leave fresh surfaces to be acted upon by all the forces of erosion. Loosened stones are disturbed by the wind, animals, plant growth, snow, and water and fall to the base of a cliff or mountain to form talus slopes. These rocks, which are thus constantly exposed, weather more rapidly than those which are protected by layers of disintegrated and decomposed rock in the valleys. When the soil is loosened by rains, landslides often occur, sometimes with very disastrous results. Plants Are Disintegrating Agents. The seeds of plants may lodge in small crevices in rocks, whence they send out roots whose gradual growth in diameter exerts pressures of hundreds of pounds per square inch. Many large rocks are broken by this means. Many Chemical Changes Bring About Decomposition of Rocks. Water, charged with carbon dioxide gas, dissolves limestone to form caves. Water may dissolve many other substances, such as sulfur dioxide, and carry them to otherwise insoluble rocks, which it is thus able to decompose through the action of these dissolved substances on the rock to produce soluble substances, which are then leached out. Underground gases and gases from volcanoes thus render underground waters very active, especially where high pressures cause very large quantities of these gases to dissolve in water. On the surface, the ever constant films of water contain oxygen and carbon dioxide dissolved from the atmosphere. In the presence of water, a great many rocks are acted upon by these two substances to form new, more soluble, or mechanically weaker materials. Water itself is an important decomposing agent; it combines with many rock materials, causing them to swell and decrease in strength or to become more soluble. Running Water Is the Most Active Eroding Agent. The erosive power of running water depends upon its velocity; it increased about sixty-four times for each time the velocity is doubled. The tremendous power of the water to plow out debris and transport it can be seen after a cloudburst has occurred on a freshly plowed hillside. Such dry spots as Death Valley seldom have rain, so that there is little vegetation to prevent the ravages of the infrequent cloudbursts, which wash out deep ravines and obliterate roadbeds is