The earth
consists of four concentric shells, which are separated by three
discontinuities. At these boundaries, earthquake waves changes in speed and
direction. These four shells of the earth are called the crust, the mantle, the
outer core, and the inner core. They differ in state or condition, in composition,
in density, and in elasticity. The degree of elasticity is factor that
determines the speed of earthquake.
The
crust extends down to the Mohorovieie (M-) discontinuity (or Moho) and includes
the continents (which consist mostly of granite) and the ocean basins which
consist mostly of basalt. Only the outer part of crust is visible; deeper rocks
may become exposed by uplift, erosion, or may be brought to the
surface by volcanism
The
continent and ocean basins are more or less in balance with another. This
tendency toward equilibrium is termed isostasy. The continents stand higher
than the ocean basins because they are lighter. Both the heavy and the light
columns of rock, however, are in balance at an unknown depth called the level
of compensation.
Below
the crust is the mantle. It consists of two or three zones of rock that are
under considerable pressure, which keeps the rock from the melting at the high
temperature that exists there. The bottom of the mantle is marked by the
Wiechert-Gutenberg discontinuity, 1,800 mile down.
The outer core
is 1,360 mile thick. In spite of increased pressure at this depth, the
temperature is high enough to keep this zone in a molten state. The composition
is believed to be largely iron, perhaps like that of iron-nickel meteorites.
The inner core
extends 790 miles to the center of the earth, where its specific gravity may be
more than 17. The enormous pressure (perhaps 3 million times that at the
surface) keeps this zone solid in spite of the high temperature.
Zoning
of the earth is one of the major that must be accounted for by any acceptable
hypothesis of the origin of the earth (see Chap. 1).
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