The present distribution of the continents has
taken place in the last 65 million years. The
drift of continents still continues. The ridges
down the middle of ocean floors have been
emitting lava actively.
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These midoceanic
ridges, are cracks on the floor of ocean
where molten rocks push up to form new
crust. The crust spreads away from the ridge
and the ocean basin widens. This phenomena
is known as Sea Floor Spreading. The Atlantic
Ocean is getting wider by several centimetres
a year, the Pacific Ocean is getting smaller, and
the Red Sea is part of a crack in the crust that
will widen to produce a new ocean millions of
years in future. The widening South Atlantic
Ocean has separated Africa and South America.
PLATE THEORY
According to the global plate tectonic theory, the
lithosphere is broken into a number of
moderately rigid plates.
The plates
move continuously and have relative direction
of motion. Based on the relative motion of plates,
three kinds of plate boundaries or marginal
zones are recognised �
(i) zones or margins of
divergence or spreading;
(ii) zones or margins of
convergence; and
(iii) fracture zones or
transform faults.
Zones of divergence are boundaries along
which plates separate and in this process of
separation molten material upwells. This is
commonly observed along linear ocean ridges
where new lithosphere is created in the form
of new ocean floors. Active volcanism and
shallow focus earthquakes mark such
boundaries.
Zones of convergence are boundaries along
which the edge of one plate overrides the other.
The overridden plate slips down into the
mantle and is absorbed. This process is known
as subduction. Besides volcanism and shallow
to deep focus earthquakes, these boundaries
also produce deep trenches/ basins and
folded mountain chains.
There is neither creation nor destruction
along the transform fault. The
lithospheric plates slide past each other.
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AKHAND PRATAP SINGH
