Geology Reference
In-Depth Information
in place since early in Earth's history. This required the interior of the Earth to be
deformable, though not necessarily liquid. Seismology established that the mantle
is in fact not liquid, so the mantle was inferred to be a deformable solid. This
history will be discussed in more detail in Chapter 4. Despite the evidence for
continuing deformations, it was also evident that structures less than about 100 km
in horizontal extent seemed to be supported without continuing deformation. These
observations can be reconciled if the outer 100-200 km of the Earth is strong and
usually not deforming, even on geological timescales.
This outer, strong layer became known as the
lithosphere
. In 1914 Barrell [8]
proposed the term
asthenosphere
for the deformable region below the lithosphere.
Thus the lithosphere is defined in terms of its
strength
. Since it is thicker than the
crust, it must comprise the crust and the top part of the mantle.
The greater strength of the lithosphere was inferred to be due to lower temper-
atures near the Earth's surface, and this is confirmed in the modern picture, as we
will see as we go along. The lower temperatures also cause seismic velocities to
be higher, and modern techniques and instruments have allowed the lithosphere to
be resolved seismically, although it is more subtle and harder to distinguish than
the crust. In this way we have learnt that the oceanic lithosphere is up to around
100 km thick, though it is thinner near mid-ocean ridges. On the other hand, the
continental lithosphere is rather variable, and over 200 km thick under older parts
of the continental crust.
To summarise, the
lithosphere
is defined by its strength, which is sufficient to
prevent it from deforming significantly on geological timescales. Its strength is
inferred to be due to its lower temperature. It is up to about 100 km thick in oceanic
regions, and from about 100 km to over 200 km thick in continental regions. The
crust
is defined by its lower seismic wave velocities. The lower velocities are
inferred to be due to it having a different composition than the mantle underneath.
In oceanic regions it is about 7 km thick and has a basaltic composition. Its density
is about 2900 kg/m
3
, in contrast to the upper mantle density of 3300 kg/m
3
.In
continental regions it averages 35-40 km thick. Its composition is quite variable,
roughly from basaltic to granitic, and averaging to an intermediate rock type like
andesite. Its density is also rather variable, and averages around 2700 kg/m
3
. These
properties are summarised in Table 2.1.
2.3 Topography
The Earth's topography has some striking features. To appreciate it fully, we need
to see it without the oceans, as it is shown in Figure 2.4. In this view it is very
clear that there are two predominant elevations of the Earth's surface, that of the
continents and that of the ocean basins. This bimodal distribution of elevation is
