Geology Reference
In-Depth Information
result from the global plate tectonics process,
which accompanied the origin and evolution
of life on the Earth during the last 2.5-3 Ga
(billion year b.p.).
The stratification shown in Fig.
1.1
was orig-
inated during the last stages of formation of the
solid Earth by a process of gravitational separa-
tion of materials according to their melting points
and densities. In the next pages, we shall build
a continuum mechanics representation of these
layers, in order to establish a rigorous quantita-
tive basis for the description of the geophysical
entities that we call
tectonic plates
.
1.2
Continental Crust
The major part of the continental crust most
probably formed between 4 and 2 Ga during two
main episodes of differentiation at
1.9 Ga ago
and
3.3 Ga (Hawkesworth and Kemp
2006
).
Geochemical modeling suggests that at
4Ga
basaltic magma, having a composition similar to
a mixture of 92 % present day oceanic island
arc basalts (IAB) and 8 % ocean island basalts
(OIB), was extracted from the primitive mantle.
Cooling of this magma ocean led to the formation
of an early relatively thin (30-45 km, Herzberg
and Rudnick
2012
) basaltic crust envelope of
the entire globe. Further differentiation occurred
sometime later, at
3.3 and
1.9 Ga, and con-
sisted into localized processes of re-melting of
the original basaltic layer, with formation of a
13 km thick and lighter
upper continental crust
and an underlying stratum having much greater
thickness and density. The latter became grav-
itationally unstable as a consequence of phase
transitions that produced dense minerals such as
garnet. Therefore, the original
deep continental
crust
resulted from further gravitational differen-
tiation, with foundering of a relevant high-density
part of the residual layer into the mantle. After
this initial phase, concentrated in two pulses dur-
ing the Meso-Archean and the Paleo-Proterozoic,
slow and more regular growth of the continental
crust occurred at the expenses of the surround-
ing oceanic crust, and was essentially driven
by plate tectonic processes, in particular by arc
magmatism.
Fig. 1.1
Geometry and mineral composition of the crust
and the mantle (Mineral abbreviations are explained in
Tabl e
1.1
.
P
is the lithostatic pressure)
An apparent feature of the subdivision shown in
Fig.
1.1
is the trend towards a more complicated
chemical composition and geometry as we
approach the Earth's surface. This greater
complexity is only partially real, and mainly
arises from our increased and often direct
knowledge of the most external layers, whereas
starting from the transition zone we must rely
on a combination of theoretical modelling (both
thermodynamic and geochemical), experimental
petrology laboratory results, and indirect
geophysical (mainly seismological) constraints.
However, it is possible to affirm that most of the
complicate geologic structures that we observe at
crustal scale, as well as the considerable lateral
chemical differentiation, are unique features that
