Geology Reference
In-Depth Information
1
Composition of the Crust
and the Mantle
Abstract
The external layers of the solid Earth, from the crust to the lower
mantle, are the main actors involved in the plate tectonics drama. In this
chapter, I describe their chemical composition and introduce the principal
geodynamic processes occurring within and between these layers.
For example, seismology studies and research on
high-temperature (
HT
) and high-pressure (
HP
)
minerals indicate that the upper mantle mostly
consists of two phases: Mg
2
SiO
4
(the fosterite
end-member of olivine) and Mg
2
Si
2
O
6
(orthopy-
roxene). The chemical composition of mineral
assemblages is usually described by the so-
called
components
, which represent the minimum
number of chemical formulae that are needed to
describe the set of phases composing the rock.
For example, both olivine and orthopyroxene
can be represented by a mixture of MgO and
SiO
2
, because Mg
2
SiO
4
D
2MgO
C
SiO
2
and
Mg
2
Si
2
O
6
D
2MgO
C
2SiO
2
. We emphasize that
this decomposition is arbitrary and represents
only a useful way to describe the
chemical
composition of a rock through its constitutive
elements, independently from the real crystalline
structure of the mineral phases included in the
solid state mixture.
A basic mineralogical characterization and
stratigraphy of the external layers of the solid
Earth is illustrated in Fig.
1.1
, while composition,
density, and other physical parameters of the
main crustal and mantle minerals are listed in
Tab le
1.1
Crust and Mantle Minerals
Plate tectonics focuses on the dynamics and
kinematics of the most external layers of the solid
Earth: the relatively thin
oceanic crust
(7-10 km
thickness), the
continental crust
(5-70 km
thickness), the
mantle lithosphere
, whose lower
boundary can be found at depths ranging between
80 and 250 km, the underlying
asthenosphere
(up to 410 km depth), the
transition zone
(410-670 km depth), and the very huge
lower
mantle
, which extends to a depth of
2,900 km
(Fig.
1.1
). All these rock layers are formed
by solid-state mixtures of minerals that are
chemically and structurally stable only within
determined intervals of pressure and temperature.
Rocks can eventually contain liquid phases in the
existing pores between grains or in cracks.
In general, chemical composition, crystalline
structure, and the physical state of Earth
materials change from point to point, so that
rocks are strongly heterogeneous systems.
However, any rock system can be ideally
resolved into a finite number of
phases
that
are
physically
and
chemically
homogeneous.
1.1
. The meaning of these mechanical
