Plate
tectonics is a theory of
geology that has been developed to explain the observed evidence for large
scale motions of the Earth''s lithosphere. The theory encompassed and superseded
the older theory of continental drift from the first half of the 20th century
and the concept of sea floor spreading developed during the 1960s.
The outermost part of the Earth''s interior is made up of two layers: above
is the lithosphere, comprising the crust and the rigid uppermost part of the mantle.
Below the
lithosphere lies the asthenosphere. Although solid, the
asthenosphere has relatively low viscosity and shears strength and can flow like a liquid on
geological time scales. The deeper mantle below the asthenosphere is more rigid
again. This is, however, not due to cooler temperatures but due to high
pressure.
The lithosphere is broken up into what are called tectonic
plates —
in the case of Earth, there are seven major and many minor plates (see list
below). The lithospheric plates ride on the asthenosphere. These plates move in
relation to one another at one of three types of
Plate boundaries: convergent
or collision boundaries, divergent or spreading boundaries, and transform boundaries.
Earthquakes, volcanic activity, mountain-building, and oceanic trench formation
occur along plate boundaries. The lateral movement of the plates is typically
at speeds of 0.65 to 8.50 centimeters per year.
More abstracts about the Plate tectonics