Geoscience Reference
In-Depth Information
Chapter 2
Tectonics on a sphere: the geometry
of plate tectonics
2.1 Plate tectonics
The Earth has a cool and therefore mechanically strong outermost shell called
the
lithosphere
(Greek
lithos
, 'rock'). The lithosphere is of the order of 100 km
thick and comprises the crust and uppermost mantle. It is thinnest in the oceanic
regions and thicker in continental regions, where its base is poorly understood.
The
asthenosphere
(Greek
asthenia
,'weak' or 'sick') is that part of the mantle
immediately beneath the lithosphere. The high temperature and pressure which
exist at the depth of the asthenosphere cause its viscosity to be low enough to
allow viscous flow to take place on a geological timescale (millions of years, not
seconds!). If the Earth is viewed in purely mechanical terms, the mechanically
strong lithosphere floats on the mechanically weak asthenosphere. Alternatively,
if the Earth is viewed as a heat engine, the lithosphere is an outer skin, through
which heat is lost by conduction, and the asthenosphere is an interior shell through
which heat is transferred by convection (Section 7.1).
The basic concept of
plate tectonics
is that the lithosphere is divided into a
small number of nearly rigid
plates
(like curved caps on a sphere), which are
moving over the asthenosphere. Most of the deformation which results from the
motion of the plates - such as stretching, folding or shearing - takes place along
the edge, or boundary, of a plate. Deformation away from the boundary is not
significant.
A map of the
seismicity
(earthquake activity) of the Earth (Fig. 2.1) outlines the
plates very clearly because nearly all earthquakes, as well as most of the Earth's
volcanism, occur along the plate boundaries. These
seismic belts
are the zones
in which differential movements between the nearly rigid plates occur. There are
seven main plates, of which the largest is the Pacific plate, and numerous smaller
plates such as Nazca, Cocos and Scotia plates (Fig. 2.2).
The theory of plate tectonics, which describes the interactions of the litho-
spheric plates and the consequences of these interactions, is based on several
important assumptions.
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