Geoscience Reference
In-Depth Information
Table 10.1
Estimated compositions of the bulk
continental crust and of oceanic crust
Compound
Continental (%)
Oceanic (%)
SiO
2
57.3
49.5
TiO
2
0.9
1.5
Al
2
O
3
15.9
16.0
FeO
9.1
10.5
MgO
5.3
7.7
CaO
7.4
11.3
Na
2
O
3.1
2.8
K
2
O
1.1
0.15
Source
:Taylor and McLennan (1985).
depositional, subduction and endless reworking events. We saw in Chapter 6 how
radiometric isotope methods unravel some of the complexity for us by dating
samples, indicating whether they came from a crustal or mantle source, whether
they were contaminated and where contamination may have occurred. The other
tools used to decipher the history of rocks are, of course, all the methods of
geology and geophysics.
The continental crust, despite its complexity and variation, has a fairly standard
'average' composition (Table 10.1). This composition is more silica-rich than
that of oceanic basalts. In general terms, the composition of the continental
upper crust is similar to that of granodiorite, and the lower crust is probably
granulite. However, this is a gross oversimplification. The crust is far from being
homogeneous and still retains the marks of its origins. Thus, sedimentary material
buried during a thrusting event can be found deep in the crust, and oceanic-type
rocks or even ultramafic rocks have been thrust up to the surface during mountain
building.
Knowledge of the variation of the strength of the crust is based primarily on
laboratory measurements of rock samples. Figure 10.4 shows strength envelopes
1
for continental and oceanic crust under compression. In both cases, the stress at
which failure occurs increases linearly with depth. The sharp reductions that
occur at
35 km depth
for oceanic lithosphere are due to the fact that the rocks deform by solid-state
creep at these depths and temperatures (rocks that are brittle at low tempera-
tures become ductile at higher temperatures). Earthquakes will tend to nucleate
around the brittle-ductile transition. In the continental lithosphere, the upper crust
is strong while the lower crust is weak and will deform viscously or viscoelasti-
cally. However, at the Moho, with the change in composition, there is a region in
which the strength is increased. This maximum depth is dependent upon the strain
∼
15 and 35 km depth for continental lithosphere and
∼
1
See Section 6.6 of Watts (2001) for a full discussion of yield strength.
