Page 57 - Petrophysics

REVIEW OF COMPOSITION OF THE GLOBE 31

In the crustal plates, the P-wave velocity ranges from about 6.4 to
7 km/s. At the Moho discontinuity, where the P waves enter the mantle,
the velocity increases to about 8 km/s. The velocity ranges from 9 to
10 km/s in the upper mantle, 12 to 13 in the middle mantle, and peaks
at 13.7 km/s at 2,800 km depth. When the P and S waves encounter the
liquid core, the P-wave velocity decreases sharply to about 8 km/s and
the S waves disappear, because a liquid cannot support a shear wave.
At the inner solid core of the earth, the P-wave velocity increases once
more to about 1 1.3 km/s.

CRUST

Crust is the term that originated for the outer solid shell of the earth
when it was generally believed that the interior was completely molten,
and it is still used to designate the outer shell, which has different
properties than the underlying mantle. The crust varies in thickness
and composition. The continental masses are composed of a veneer of
sediments over a layer of light-colored granitic rocks. The granite-type
layer is called the SIAL layer because its most abundant components are
silicon and aluminum, with an average density of 2.7 g/cm3. Below the
SIAL layer, there is a layer of dark rocks resembling basalt and gabbro,
which is known as the SIMA layer because its principal constituents are
silicon and magnesium. The density of SIMA is slightly higher than the
SIAL layer, about 2.9 g/cm3. Under the oceans, the SIMA layer is covered
only by a thin layer of soft sediments (Figure 2.1).
The mantle is a shell, which is apparently a plastic-like solid, that
extends about 2,900 km deep from the Moho discontinuity to the outer
liquid core. The movement of crustal plates and continents on top of the
mantle is partially explained by the theory of convective currents within

Figure 2.1. Cross-section of the must at a continental shelf showing the relationship
between the SUL (granite rocks) and SIMA (basalt) layers under the continents and
oceans [2].