Geoscience Reference
In-Depth Information
Chapter 8
Let's take it from the top: the crust
and upper mantle
The crust
ZOE: Come and I'll peel off.
BLOOM:
(feeling his occiput dubiously
with the unparalleled embarrassment
of a harassed pedlar gauging the
symmetry of her peeled pears)
Somebody would be dreadfully
jealous if she knew.
The major divisions of the Earth's interior -- crust,
mantle and core -- have been known from seis-
mology for about 80 years. These are based on the
reflection and refraction of P- and S-waves. The
boundary between the crust and mantle is called
the Mohorovicic discontinuity (M-discontinuity
or Moho for short) after the Croatian seismol-
ogist who discovered it in 1909. It separates
rocks having P-wave velocities of 6--7 km/s from
those having velocities of about 8 km/s. The term
'crust' has been used in several ways. It initially
referred to the brittle outer shell of the Earth
that extended down to the asthenosphere ('weak
layer'); this is now called the lithosphere ('rocky
layer'). Later it was used to refer to the rocks
occurring at or near the surface and acquired a
petrological connotation. Crustal rocks have dis-
tinctive physical properties that allow the crust
to be mapped by a variety of geophysical tech-
niques. Strictly speaking, the crust and the Moho
are seismological concepts but petrologists speak
of the 'petrological Moho,' which may actually
occur in the mantle! The Moho may represent a
transition from mafic to ultramafic rocks or to
a high-pressure assemblage composed predomi-
nately of garnet and clinopyroxene. It may thus
be a chemical change or a phase change or both,
and differs from place to place. The lower conti-
nental crust can become denser than the mantle,
and may founder and sink into the mantle.
The present surface crust represents 0.4% of
the Earth's mass and 0.6% of the silicate Earth. It
James Joyce, Ulysses
The broad-scale structure of the Earth's interior
is well known from seismology, and knowledge
of the fine structure is improving continuously.
Seismology not only provides the structure, it
also provides information about the composi-
tion, mineralogy, dynamics and physical state. A
1D seismological model of the Earth is shown
in Figure 8.1. Earth is conventionally divided
into crust, mantle and core, but each of these
has subdivisions that are almost as fundamen-
tal (Tables 8.1 and 8.2). Bullen subdivided the
Earth's interior into shells, from A (the crust)
through G (the inner core). The lower mantle,
starting at 1000 km depth, is the largest subdi-
vision, and therefore it dominates any attempt
to perform major-element mass balance calcula-
tions. The crust is the smallest solid subdivision,
but it has an importance far in excess of its rel-
ative size because we live on it and extract our
resources from it, and, as we shall see, it con-
tains a large fraction of the terrestrial inventory
of many elements.
