Geology Reference
In-Depth Information
The crust
10 km, into which almost the whole crustal inventory
of the radioactive incompatible elements K, U and Th
has been concentrated.
It is notable that the other terrestrial planets have
dominantly basaltic crusts. Why have none of them
developed andesitic or 'granitic' crust resembling the
continents on Earth? This unique feature of the Earth
probably stems from another: the existence on the sur-
face of liquid water (see reference to the
habitable zone
below). The igneous minerals of the basaltic ocean
crust react with seawater to form hydrous secondary
minerals. Subduction of altered oceanic crust trans-
ports this bound water deep into subduction zones
where the hydrous minerals dehydrate (cf. Figure 2.3),
thereby releasing water into the overlying mantle
wedge. In the presence of water vapour, mantle perid-
otite undergoes partial melting at lower temperatures
and - as experiments show - produces melts that are
more SiO
2
-rich (andesitic) than when melted 'dry'.
This explains the dominance of andesite in many
island arcs, and provides the precursor for the form-
ation of granitic upper continental crust.
The Earth's crust falls into two broad divisions
(Figure 11.7). The basaltic crust of the ocean basins,
being a product of partial melting of mantle perid-
otite, has a higher Mg content (although much lower
than the mantle itself) and lower Si (less than 50%)
than the continents. It has a relatively short lifetime:
its passage from ocean ridge to subduction zone,
where it is delivered back into the mantle, typically
takes less than 200 million years (the age of the oldest
known oceanic lithosphere). During this time oce-
anic crust becomes partially hydrated by chemical
interaction with ocean water and it acquires a blanket
of sediment. This modified crustal package is returned
to the Earth's interior through subduction, and parts
of it reappear at the surface as constituents of sub-
duction-related island-arc volcanics and Cordilleran
plutonic rocks.
Unlike the ephemeral ocean floor, the
sialic
conti-
nental crust has been accumulating throughout
known geological time, although probably not at a
uniform rate. None of the Earth's earliest crust has
survived, having been reworked by the intense plan-
etesimal bombardment that continued until about
3.8 Ga ago. The oldest recognizable remnants of early
crust are about 4.0 Ga old (although greater ages have
been obtained from detrital zircon grains in younger
sedimentary rock) and the present continental crust
has been accumulating since that time. It is being
extended today by the lateral accretion of island arcs
on to continental margins, and by deep-seated ign-
eous intrusions into continental roots. Different
mechanisms may have operated in the past, for exam-
ple during the huge increase in the volume of the
continental crust that seems to have occurred between
3.0 and 2.0 billion years ago, at the close of the
Archaean era. Average continental crust is equivalent
to andesite in composition, having a SiO
2
content of
around 57%, significantly higher than oceanic crust
(49.5%, Figure 11.7).
Repeated melting and metamorphism within the
continental crust have led to its internal different-
iation into a lower, more refractory continental crust
depleted in incompatible elements; and an upper
crustal layer, roughly 10 km thick, which is enriched in
Na, K and Si (Figure 11.7). Most of the heat flow we
measure in continental areas originates in this top
The early atmosphere
Accreting planets capture a
primary atmosphere
of neb-
ular gases (mainly hydrogen), but this would have
been lost from the Earth as soon as the nebula thinned
and cleared (Zahnle
et al
., 2007). Of more interest is the
secondary atmosphere
consisting of volatiles outgassed
from within the Earth itself by the high temperatures
associated with accretion. Its inferred composition is
highly sensitive to the composition of the parental
material from which the Earth is presumed to have
accreted: an Earth resembling CI carbonaceous chon-
drites would have outgassed a hot dense atmosphere
dominated by H
2
O, CO
2
, N
2
and H
2
(which of these
predominates depends on the temperature), whereas
an Earth formed by accretion of other types of chon-
drite would have generated a more reduced atmos-
phere dominated by CH
4
, H
2
, N
2
and CO (Schaefer and
Fegley, 2010). Regardless of which starting material is
adopted, it is clear that the Earth's early atmosphere
contained no free oxygen.
This early atmosphere must have been lost during
the Moon-forming giant impact, which melted most
of the Earth's mantle and even vaporized part of it,
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