Geoscience Reference
In-Depth Information
the crust and mantle at several tenths of a degree per year. A complete revolution
would therefore take many centuries. This differential rotation of the inner core
may affect many aspects of the workings of the planet, including the magnetic
field. Conservation of the total angular momentum of the Earth means that any
slowing of the rotation of the mantle must be balanced by an increase in the rota-
tion of the atmosphere, oceans and core and vice versa. The atmosphere changes
on a short timescale, whereas the core will respond over decades. Changes in the
observed length of a day are well explained by atmospheric variation and there is
no reason to suppose that this rotation measured over the last thirty years is not
a long-term feature of the inner core.
8.1.5 The composition of the Earth
The continental crust varies greatly in the variety of its igneous, sedimentary
and metamorphic rocks. However, on average, the continental crust is silica-rich
and, very loosely speaking, of granitoid composition (see Section 10.1.3). In
contrast, the oceanic crust is basaltic and richer in mafic (Mg, Fe-rich) minerals
(see Section 9.1).
Our knowledge of the composition of the deep interior of the Earth is largely
constrained by seismic and density models; we have little direct evidence regard-
ing the compositions of the mantle and core. Chemistry is important, but unfortu-
nately we lack
in situ
measurements, since no one has yet drilled that deeply. Our
chemical knowledge of the deep interior has to be inferred from the chemistry
of the volcanic and intrusive rocks derived from liquids that originated in the
mantle, from structurally emplaced fragments of mantle, from nodules brought
up during volcanism and from geochemical models of the various seismic and
density models discussed in the previous sections.
This lack of direct evidence might suggest that the mantle composition is pretty
much unknown, but geochemistry is a sophisticated and powerful branch of Earth
science and has developed many techniques in which we use the compositions of
rocks available to us on the surface to model the composition at depth. In addition,
experiments at high temperatures and pressures have enabled the behaviour of
minerals thought to exist in the deep mantle to be studied in some detail. There are
many compositional models of the upper mantle, some of which are more popular
than others. These models include imaginary rocks such as pyrolite, which is a
mixture of basalt and residual mantle material. The main constituent of the mantle
is magnesian silicate, mostly in the form of olivine. The core is iron-rich, and the
core-mantle boundary is a very major compositional boundary.
Olivine in the mantle lies between two end members: forsterite (Fo), which is
Mg
2
SiO
4
, and fayalite (Fa), which is Fe
2
SiO
4
. Normal mantle olivine is very
forsteritic, probably in the range Fo
91
-Fo
94
,where 91 and 94 represent per-
centages of Mg in (Mg, Fe)
2
SiO
4
. Other trace components of mantle olivine
include nickel (Ni) and chromium (Cr). The other major mantle minerals include
