The interior of Earth is not subject to direct investigation,
but its properties must be indirectly deduced from the study of
earthquake waves that propagate through the interior rocks. From an
earthquake near the surface, both pressure (compression) waves and
transverse (side‐to‐side) waves move outwards in all directions. Wave
energy moving into the interior, however, has its path slowly changed by
refraction as the wave moves through regions of slowly changing
properties. These waves reach the surface after a time that depends on
the length of the path and the velocity of propagation at each point
along that path. Careful analysis at seismographic stations of the time
of arrival of earthquake waves over the surface of Earth yields
information about the densities, temperatures, and pressures of Earth's
interior. A thin crust (at its thickest only 30 kilometers deep), which
contains the continental masses and the ocean floors, overlies a denser
outer mantle. The uppermost layer of the mantle acts as solid material, a lithosphere no more than about 80 kilometers deep. Most of the mantle slowly flows under pressure and acts as a plastic, or malleable, asthenosphere.
In an annulus about the surface of Earth, opposite an earthquake, exists the shadow zone,
in which you cannot observe pressure waves. The path of pressure waves
is significantly affected by a sharp refraction that astronomers
interpret as the point of transition between the mantle and an interior core
that is substantially different from the outer part of the planet. The
shadow zone for transverse waves, however, covers the whole of Earth
opposite the earthquake source. No transverse wave energy apparently
passes through the core, indicating that its physical state, in the
outer regions at least, must be liquid. The innermost core, however,
though at higher temperatures, is likely solid because of an even higher
pressure there. As the center of Earth continues to slowly cool over
time, this inner core must be slowly growing in size at the expense of
the liquid outer core. Evidence also shows that this inner core is
rotating faster than the rest of the planet, completing one full turn in
two‐thirds of a second less time than at the surface. Applying other
physical principles together with laboratory study of the nature of
different materials under high temperature and pressure suggests the
characterization of Earth's interior as shown in Table 1. (See Figure 1
for a diagram of Earth's interior.)
Figure 1 The interior of Earth.
Seismographic study of moonquakes has shown that the lunar structure
is the same as the crust‐mantle‐core structure of Earth, with the
significant differences being that the lunar mantle is primarily solid
(the lunar lithosphere is about 800 kilometers deep and overlies only a
shallow plastic asthenosphere), and the small iron core is frozen solid
(see Figure 2). As the Moon's mantle and core continue to slowly cool,
their materials shrink at different rates, producing stress at the
core‐mantle interface; moonquakes thus occur in a deep spherical shell
marking this interface. Because the Moon's outer mantle is frozen,
unlike that of Earth, there is no interior convection, no surface plate
tectonics, and no crustal quakes, other than an occasional tremor
produced by the impact of a small meteor. In terms of interior
structure, Earth and the Moon may be contrasted according to the
information in Table 2.
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