Plate boundaries

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Constructive or divergent plate boundary
The first sort of plate boundary is called a divergent boundary, or spreading center. At these boundaries, two
plates move away from one another. As the two move apart, mid-ocean ridges are created as magma from the
mantle upwells through a crack in the oceanic crust and
cools. This, in turn, causes the growth of oceanic crust on
either side of the vents. As the plates continue to move,
and more crust is formed, the ocean basin expands and a
ridge system is created. Divergent boundaries are
responsible in part for driving the motion of the plates.
Convergent boundary
These are plate margins where one plate is overriding
another, thereby forcing the other into the mantle
beneath it. These boundaries are in the form of
trench and island arc systems. All the old oceanic
crust is going into these systems as new crust is
formed at the spreading centers. Convergent
boundaries also explain why crust older than the
Cretaceous cannot be found in any ocean basin-- it
has already been destroyed by the process of
subduction.
Conservative
The third type of plate boundary is called a conservative or transform
boundary. It is called conservative because plate material is neither created nor
destroyed at these boundaries, but rather plates slide past each other. The
classic example of a transform plate boundary is the San Andreas fault in
California. The North American and Pacific Plates are moving past each other at
this boundary, which is the location of many earthquakes. These earthquakes
are caused by the accumulation and release of strain as the two plates slide
past each other. Another example of a transform boundary is seen at the
mid-ocean ridges, where the spreading centers are offset by transform faults
anywhere from a few meters to several kilometers in length.

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