Page 34 - Global Tectonics

THE INTERIOR OF THE EARTH 21

longer exists and, where applicable, it is preferable to
410
use the terms upper and lower crust. Unlike the 660
Moho, the Conrad discontinuity is not always present Vp
within the continental crust, although the seismic V s
2000
velocity generally increases with depth. Lower
mantle
In some regions the velocity structure of continen-
tal crust suggests a natural division into three layers. Depth (km) Outer
core
The velocity range of the middle crustal layer gener-
−1
ally is taken to be 6.4–6.7 km s . The typical velocity 4000
range of the lower crust, where a middle crust is
−1
present, is 6.8–7.7 km s (Mooney et al., 1998). Exam-
Inner
ples of the velocity structure of continental crust in a core
tectonically active rift, a rifted margin, and a young 6000 V s Vp
orogenic belt are shown in Figs 7.5, 7.32a, and 10.7, 0 2 4 6 8 10 12 14
respectively. Velocity, V (km s ¯1)
The oceanic crust has principally been studied by
Figure 2.16 Seismic wave velocities as a function of
explosion seismology. The Moho is always present and
depth in the Earth showing the major discontinuities. AK
the thickness of much of the oceanic crust is remark-

135 Earth model specified by Kennett et al., 1995 (after
ably constant at about 7 km irrespective of the depth of
Helffrich & Wood, 2001, with permission from Nature
water above it. The internal layering of oceanic crust
412, 501–7. Copyright © 2001 Macmillan Publishers
and its constancy over very wide areas will be discussed
Ltd.).
later (Section 2.4.4).
In studying the deeper layering of the Earth, seismic

believed to be in a fluid state. The geomagnetic fi eld
waves with much longer travel paths are employed. The
(Section 3.6.4) is believed to originate by the circulation
velocity structure has been built up by recording the
of a good electrical conductor in this region. At a depth
travel times of body waves over the full range of pos-
of 5150 km the P velocity increases abruptly and S
sible epicentral angles. By assuming that the Earth is
waves are once again transmitted. This inner core is
radially symmetrical, it is possible to invert the travel
thus believed to be solid as a result of the enormous
time data to provide a model of the velocity structure.

confining pressure. There appears to be no transition
A modern determination of the velocity–depth curve
zone between inner and outer core, as was originally
(Kennett et al., 1995) for both P and S waves is shown
believed.
in Fig. 2.16.
Velocities increase abruptly at the Moho in both con-
tinental and oceanic environments. A low velocity zone
(LVZ) is present between about 100 and 300 km depth,
although the depth to the upper boundary is very vari- 2.3 COMPOSITION
able (Section 2.12). The LVZ appears to be universally
present for S waves, but may be absent in certain regions OF THE EARTH
for P waves, especially beneath ancient shield areas.
Between 410 and 660 km velocity increases rapidly in a
stepwise fashion within the mantle transition zone that All bodies in the solar system are believed to have been
separates the upper mantle from the lower mantle. formed by the condensation and accretion of the prim-
Each velocity increment probably corresponds to a itive interstellar material that made up the solar nebula.
mineral phase change to a denser form at depth (Section The composition of the Sun is the same as the average
2.8.5). Both P and S velocities increase progressively in composition of this material. Gravitational energy was
the lower mantle. released during accretion, and together with the radio-
The Gutenberg discontinuity marks the core–mantle active decay of short-lived radioactive nuclides eventu-
boundary at a depth of 2891 km, at which the velocity ally led to heating of the proto-Earth so that it
of P waves decreases abruptly. S waves are not transmit- differentiated into a radially symmetric body made up
ted through the outer core, which is consequently of a series of shells whose density increased towards its