Page 46 - Global Tectonics

THE INTERIOR OF THE EARTH 33

logical studies have revealed that the lower mantle has been recognized that travels to and from the inner core
thermal and/or compositional heterogeneity, probably as a P wave, but traverses the inner core as an S wave.

as a result of the penetration of subducted oceanic The amplitude of a phase reflected off the inner core
lithosphere through the 660 km discontinuity (Section also suggests that it must have a finite rigidity and thus

2.8.3). be a solid.
The lowest 200–300 km of the mantle, Layer D″ Shock wave experiments have shown that the major
(Section 12.8.4), is often characterized by a decrease in constituents of both the inner and outer core must com-
seismic velocity, which is probably related to an increased prise elements of an atomic number greater than 23,
temperature gradient above the mantle-core boundary. such as iron, nickel, vanadium, or cobalt. Of these ele-
This lower layer shows large lateral changes in seismic ments, only iron is present in suffi cient abundance in
velocity, indicating it is very heterogeneous. Ultra-low the solar system to form the major part of the core.
velocity zones, which show a 10% or greater reduction Again, by considering solar system abundances, it
in both P and S wave velocities relative to the surround- appears that the core should contain about 4% nickel.
ing mantle, have been interpreted to reflect the pres- This iron–nickel mixture provides a composition for the

ence of partially molten material (Williams & Garnero, outer core that is 8–15% too dense and it must therefore
1996). These zones are laterally very heterogeneous and contain a small quantity of some lighter element or
quite thin (5–40 km vertical thickness). Laboratory elements. The inner core, however, has a seismic veloc-
experiments suggest that the liquid iron of the core ity and density consistent with a composition of pure
reacts with mantle silicates in Layer D″, with the pro- iron.
duction of metallic alloys and nonmetallic silicates from There are several candidates for the light elements
perovskite. Layer D″ thus is important because it present in the outer core, which include silicon, sulfur,
governs core–mantle interactions and also may be the oxygen, and potassium (Brett, 1976). Silicon requires an
source of deep mantle plumes (Sections 12.8.4, 12.10). over-complex model for the formation of the Earth and

sulfur conflicts with the idea that the interior of the
Earth is highly depleted in volatile elements. Oxygen
2.9 THE CORE appears to be the most likely light element as FeO is
probably sufficiently soluble in iron. The presence of

potassium is speculative, but is interesting in that it
would provide a heat source in the core that would be
The core, a spheroid with a mean radius of 3480 km, active over the whole of the Earth’s history. It would
occurs at a depth of 2891 km and occupies the center of also help to explain an apparent potassium defi ciency in
the Earth. The core–mantle boundary (Gutenberg dis- the Earth compared to meteorites.
continuity) generates strong seismic refl ections and
thus probably represents a compositional interface.
The outer core, at a depth of 2891–5150 km, does not

transmit S waves and so must be fluid. This is confi rmed 2.10 RHEOLOGY
by the generation of the geomagnetic field in this region

by dynamic processes and by the long period variations OF THE CRUST

observed in the geomagnetic field (Section 3.6.4). The
convective motions responsible for the geomagnetic
field involve velocities of ∼10 m a , fi ve orders of mag- AND MANTLE
4
−1

nitude greater than convection in the mantle. A fl uid
state is also indicated by the response of the Earth to
the gravitational attraction of the Sun and Moon. 2.10.1 Introduction
The boundary between the outer core and inner
core at 5150 km depth is sharp, and not represented by Rheology is the study of deformation and the fl ow of

any form of transition zone. The inner core is believed materials under the influence of an applied stress
to be solid for several reasons. Certain oscillations of the (Ranalli, 1995). Where temperature, pressure, and the
Earth, produced by very large earthquakes, can only be magnitudes of the applied stresses are relatively
explained by a solid inner core. A seismic phase has low, rocks tend to break along discrete surfaces to form