The movement is explained by a theory called plate tectonics. Let us see
what that means. The earth's crust and upper mantle, which make up the
lithosphere, consist of 20 rigid plates holding up the landmasses, that
are in slow continuous motion.
The tectonic system operates from Earth's internal heat. The
layer below the lithosphere, called the asthenosphere, is more plastic
than both the overlying lithosphere and the underlying lower mantle because
of an optimum balance of temperature and pressure. Above the plastic asthenosphere,
relatively cool and rigid lithospheric plates split and move apart as
single mechanical units. Molten rock from the asthenosphere wells up to
fill the void and creates new lithosphere.
Convection circulation occurs in the asthenosphere where the rising material
fills the gap between the spreading plates and moves outwards and away
to areas where the plates descend into the lower mantle at the sites of
the oceanic trenches. Some plates contain blocks of thick, low-density
continental crust, which cannot sink into the mantle. As a result, where
a plate carrying continental crust collides with another plate, the continental
margins are deformed and thrown up into mountain ranges. The plate margins
are the most active areas on earth. They are the sites of the most intense
volcanism, seismic activity, and crustal deformation. Most earthquake
mountains are volcanoes, which occur where these plates slide past one
another or collide.
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