Geology Reference
In-Depth Information
Oceanic
Continental
Crust
Lithosphere
Crust
Lithosphere
Figure 2.3. Distinctions between crust and lithosphere, and between their conti-
nental and oceanic forms.
reaching nearly double this value under the Himalayas. It is smaller in some regions
where the crust has been horizontally extended. The average thickness seems to
be determined by a long-term balance between horizontal shortening and erosion.
This was first perceived by Hess [6], who argued that continental crust tends to be
eroded down to sea level and pushed up by the plate tectonic forces that he was
among the first to conceive.
The thickness of the oceanic crust was not determined until the 1950s and
1960s, using seismic surface waves and later ocean-going seismic profiling [7].
To most geologists' surprise, it turned out to be much thinner than continental
crust, averaging about 7 km. It is thicker in some places where there is an 'oceanic
plateau', and it is thinner in only a few places. Otherwise it is remarkably uniform
in thickness.
Both kinds of crust were detected and defined seismically. Seismic waves travel
more slowly in the crust than in the underlying mantle rocks. This contrast in wave
velocity causes reflections and refractions, and these allow seismologists to infer
the presence of interfaces or 'discontinuities' below the Earth's surface.
The difference in seismic velocity between crust and mantle implies a difference
in composition. This inference is not a simple one, as there was debate for some
time about whether the continental moho might be due to a phase change, in
which the minerals comprising the rocks are squeezed into denser crystal structures
due to the increase of pressure with increasing depth. Eventually detailed studies
resolved the debate in favour of a change in composition. There was also a debate
about the oceanic moho. Hess [6], for example, proposed that the oceanic crust was
made of serpentinite, which is a hydrated form of the predominant upper-mantle
mineral olivine. There are places where hydrated mantle is known, but drilling has
established that most of the oceanic crust has a basaltic composition, different from
the underlying mantle, whose dominant rock type is peridotite.
The concept of the lithosphere was established by early in the twentieth century.
During the nineteenth century, geologists established evidence for continuing uplift
and subsidence during geological history, as distinct from everything being frozen
