VI. The lithosphere

The main solid body of the earth is composed of rocks and soil, and is called the lithosphere. Partly surrounding this solid body is the hydrosphere, composed largely of the oceans, which keep some three-fourths of the solid earth from contact with the atmosphere, the layer of gas that surrounds this entire terrestrial sphere. The major features of the lithosphere are the continents and ocean basins. If we include the shallowly submerged continental shelves with the continents, then we may say that about one-third of the earth's surface is occupied by the continental elevations and the remainder, by the ocean basins. An idea of the differences in altitude between the ocean bottoms and the land surfaces may be had from the fact that the greatest oceanic depths are over 35,000 ft., or about 6 mi., and the highest mountains are nearly that high above sea level.

VII. Environments

1. The major physical features of the earth are pretty well known to everyone. When we consider the chemical composition of the lithosphere, however, we find that common knowledge is much poorer.

As a first approach to the subject, let us consider two rather broad types of environment that may be recognized in the lithosphere. At the surface the pressure and temperature are comparatively low. Beneath the surface of the earth, however, and particularly at depths of 10 or more miles, the temperatures and pressures are considerably higher because they increase with increasing depth. The kinds of chemical compounds that tend to occur in the lithosphere depend in large measure (to a great extent) on the environment. We usually speak of the surface environment on the one hand, and deep-seated environment on the other hand, meaning by the latter an environment beneath the surface at a depth great enough to involve a considerable increase in temperature and pressure.

2. Now it happens that many rocks are formed deep beneath the surface of the earth, and as long as they are in their original environment of high temperatures and pressures they remain unchanged. The deep seated environments tend toward the production of complicated chemical compounds, whereas the surface environment is more suitable to simpler compounds. The rocks formed deep beneath the surface may be brought nearer to the surface through the operation of geological changes, and thus they pass from a deep-seated environment to the surface environment. Obviously, of course, our direct observation is confined pretty well to surface conditions, but in some areas we may find rocks from far below which may still be largely unaltered by surface conditions. It is from such rocks that we reconstruct and interpret the conditions of deep-seated environments, those great laboratories in which the rocks of the earth are largely made.

VIII. Igneous rocks

We already know that the lava cools and hardens into solid rocks. In all cases it has come up from within the lithosphere, and there are reasons for thinking that it may¹ have come up a long way, perhaps several scores of kilometers. Molten rock does not always rise far enough to reach the earth's surface, and may consolidate between walls of older rocks. Any rock which forms by consolidation from a molten condition is called an igneous rock. These rocks which become solid after ejection upon the earth's surface, either on land or below water, are extrusive, and those which harden from molten material injected below the earth's surface are intrusive.

Molten rock is usually called magma when it is well² below the earth's surface, and lava when it is just below or upon the earth's surface.

Notes

¹ may = perhaps

² well — значительно