Geoscience Reference
In-Depth Information
Continental crust
underlies land areas and also many of the areas covered by
shallow seas. Geophysical work shows that this crust is typically about 35 km thick, but
may be 80-90 km thick below some high plateaus and mountain ranges. The highest
mountains in Britain are barely noticeable on a scale diagram comparing crustal thick-
nesses (Fig. 31). Continental crust is made of rather less dense materials than the
oceanic crust or the mantle, and this lightness is the reason why land surfaces and shal-
low sea floors are elevated compared to the deep oceans. Much of the continental crust
is very old (up to 3-4 billion years), having formed early in the Earth's life when light-
er material separated from denser materials within the Earth and rose to the surface.
Oceanic crust
forms the floors of the deep oceans, typically 4 or 5 km below
sea level. It is generally 5-10 km thick and is distinctly denser than continental crust.
Oceanic crust only forms land where volcanic material has been supplied to it in great
quantity (as in the case of Iceland), or where other important local forces in the crust
have caused it to rise (as is the case in parts of Cyprus). Oceanic crust is generally
relatively young (only 0-200 million years old), because its higher density and lower
elevation ensures that it is generally
subducted
and destroyed at plate boundaries that
are
convergent
(see below).
Figure 29 shows the major pattern of tectonic plates on the Earth today. The Mer-
cator projection of this map distorts shapes, particularly in polar regions, but we can
see that there are seven very large plates, identified by the main landmasses located on
their surfaces. The Pacific plate lacks continental crust entirely, whereas the other six
main plates each contain a large continent (Eurasia, North America, Australia, South
America, Africa and Antarctica) as well as oceanic crust. There are a number of oth-
er middle-sized plates (e.g. Arabia and India) and large numbers of micro-plates, not
shown on the world map.
Figures 29 and 32 also identify the different types of plate boundary, which are
distinguished according to the relative motion between the two plates.
Convergent
plate boundaries involve movement of the plates from each side towards the suture (or
central zone) of the boundary. Because the plates are moving towards each other, they
become squashed together in the boundary zone. Sometimes one plate is pushed be-
low the other in a process called
subduction
, which often results in a deep ocean trench
and a zone of mountains and/or volcanoes, as well as earthquake activity (Fig. 32). The
earthquake that happened on the morning of 26 December 2004 under the sea off west-
ern Sumatra was the strongest anywhere in the world for some 40 years. It seized world
attention particularly because of the horrifying loss of life caused by the tsunami waves
that it generated. This earthquake was the result of a sudden lithosphere movement of
several metres on a fault in the convergent subduction zone where the Australian plate
has been repeatedly moving below the Eurasian plate.