17.3 Plate Boundaries
The evidence of seafloor spreading suggested that the
continental and oceanic crusts move as enormous slabs that
are called tectonic plates. Tectonic plates are huge pieces of
crust and rigid upper mantle that fit together at their edges
to cover Earth’s surface. There are about 12 major plates
and several smaller ones. These plates move very slowly,
only a few cm a year which is similar to the rate at which
fingernails grow. Tectonic plates move in different
directions and at different rates relative to one another and
they interact with one another at their boundaries. There
are three types of boundaries.
Divergent Boundaries – this is a region where two tectonic plates are moving apart. Most divergent
boundaries are found on the sea floor at rift valleys. It is at this center rift where seafloor spreading
occurs. Because new ocean crust is being created here by magma, there is a lot of volcanic activity,
earthquakes and intense heat associated with divergent boundaries. Most divergent boundaries occur on
the ocean floor, however some do form on continents. There is one forming right now in East Africa. This
rifting might eventually form an ocean basin. New crust is created at divergent boundaries.
Convergent boundaries - is where two tectonic plates are moving toward each other and collide. When
the two plates collide, the denser plate will descends beneath the less dense plate in a process called
subduction. There are three types of convergent boundaries classified according to the type of crust
involved. Crust is destroyed at convergent boundaries.
Oceanic – oceanic this is when one oceanic plate,
which is denser than the other, subducts beneath
another oceanic plate. The subduction process
creates an ocean trench. The subducted plate
descends into the mantle where it will be
recycled. During this subduction process,
water is also being subducted and this will
lower the melting temps of the plates causing it
to melt at shallower depths. The magma,
which is less dense, will rise back to the surface
where it often erupts and forms an arc of
volcanic islands that parallel the trench.
Oceanic – continental Subduction zones are
also found where an oceanic plate converges
with a continental plate. The denser plate will
subduct. This type of convergence will also
produce a trench and volcanic arc. Instead of
producing and island arc of volcanoes, it
produces a chain of volcanoes along the edge of
the continental plate. Therefore, you will get a
mountain range with many volcanoes.
Continental – continental this is when two
continental plates collide. Oceanic crusts often carry
continental plates along with it as the move. Over
time when the oceanic crust is completely
subducted, the continental plate that it was
pulling behind it will collide with the
continental plate at the subduction zone. The
reason the continental plate will not subduct at
the subduction zone is because both
continental plates have the same density. As a
result of this collision, the continental plates
will become crumpled, folded, and uplifted to
form vast mountain ranges.
Transform boundaries – This is a region where two plates slide horizontally past one another. Transform
boundaries are characterized by long faults, hundreds of miles long, and shallow earthquakes. Crust is
deformed or transformed along a transform boundary.