Plate boundaries
are found at the edge of the lithospheric plates and are of
three types, convergent, divergent and conservative. Wide
zones of deformation are usually characteristic of plate boundaries
because of the interaction between two plates. The three boundaries
are characterized by their distinct motions.
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.
As you can imagine, the formation of the new crust on either
side of the vents would act to push plates apart, as we see
at the Mid-Atlantic Ridge, which helps to move North America
and Europe further and further apart. Mid-ocean ridges are
vast mountain chains in the ocean and are as tall if not taller
than mountain chains on land. The process which actually drives
the motion at these ridges is known as convection. Magma is
pushed upwards through the ridge cracks by convection currents.
As some magma erupts out through the crust, the magma which
does not erupt continues to move under the crust with the
current away from the ridge crest. These continual convection
currents, called convection cells, help to move the plates
away from each other to allow more crust to be created and
the sea floor to grow. This phenomenon is known as sea-floor
spreading.
The
mid-ocean ridges also play a very crucial role in the development
of plate tectonic theory, because of the unique quality that
minerals within the basalt possess. Basalt contains a fair
amount of magnetic minerals, which align to the Earth's magnetic
field upon crystallization. In the past, Earth's magnetic
field has been known to change polarity, causing a reversal
in the magnetic field, which is preserved when the crystals
form. The alignment of these magnetic minerals can be used
to date the crust, since they can be correlated with ages
of known magnetic reversals in Earth's history. This plays
a key role in the development of plate tectonic theory because
it was the first positive proof that the plates were indeed
moving and had been for most of geologic time. By using the
magnetic reversal information preserved in the minerals of
the mid-ocean ridge basalts, scientists were able to prove
that the plates were moving, and that new crust was being
formed and old crust was being destroyed in a continuous process
that had been going on for most of Earth's history. The oldest
crust in the ocean dates back to the early Cretaceous, 100-65
million years ago, which is relatively recent in geologic
time.
If this is the case, where did all the rest of the crust go?
This leads us to the second type of plate boundary, which
is called a convergent boundary or subduction zone. 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.
Subduction
zones are the location of very strong earthquakes, which occur
because the action of the down going slab interacts with the
overriding slab. The "Ring of Fire" around the margins of
the Pacific Ocean is due precisely to the subduction zones
found around the edges of the Pacific plate. Subduction also
is the cause of the volcanic activity in places like Japan:
as the downgoing slab goes deeper beneath the overriding plate,
it becomes hotter and hotter because of its proximity to the
mantle. This causes the slab to melt and form magma, which
moves upward through the crust and eventually forms volcanoes
(island arcs) in oceanic crust or huge intrusive masses (plutons
and batholiths) in continental crust. The Aleutian Islands
are another example of a surface expression of subduction.
Sometimes,
when there is a convergent boundary between two continental
plates, subduction cannot occur. Since continental crust is
more bouyant, or less dense, than oceanic crust, one plate
does not easily override the other. Instead, the plates crumple
as they plow into one another, and a very high mountain range
is created. This is a special type of convergent boundary
called a collisional boundary. The Himalayas in India are
the result of two continental plates (the Indo-Australian
and Eurasian plates) colliding head on.
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.
Email To Friend