Divergent is where two plates are moving away from each
other. New sea floor is made here. The mid-ocean ridges Richard talked
about in the earlier lecture are examples
of divergent margins. (See figure, right, of the mid-Atlantic ridge;
click to zoom in.) But you cannot keep making new crust or else the
Earth is going to get bigger. Somewhere you must destroy crust.
Convergent margins are where two plates come together and it's
where crust and sea floor can be destroyed through subduction. Subduction
zones and trenches are other terms for convergent margins. (See figure,
left, of subduction of plates to form trenches; click to zoom in.)
Subduction is when one plate goes under another and as the pressure
and heat builds up, that bottom plate will get melted down. Convergent
margins are where you can get mountains built up due to the pressure
of two plates pushing against each other.
Transform faults are the third type of plate boundary, in which
plates are only moving side by side and thus no new material is created
or destroyed. These faults are found at offsetting mid-ocean ridges.
There are whole series of parallel transform faults along these ridges.
Sometimes, like in the San Andreas Fault (see figure, left; click
to zoom in), you can have a transform fault that is on land (but still
between two ridges offshore). The San Andreas is not straight and
so it builds up pressure and releases it in big earthquakes.
One can map where the earthquakes will be (on transform faults where
the plates are moving in separate directions on either side), along
ridges, and along subduction zones. (See figure, right; click to zoom
in.) Thus theoretically, there are earthquakes on all plate boundaries.
Shallow earthquakes occur at ridges and transform faults, while deep
ones can only be found associated with subduction trenches.
Volcanoes can occur along convergent margins because as the lower
plate gets melted, that molten material can be pushed up to the surface
and form volcanoes. Another way to get volcanoes is through hot spots,
which are also related to plate tectonics (see figure, left, showing
the locations of hot spots; click to zoom in). These hot spots are
places from where molten material can make it up to the surface of
the Earth. It is believed that the hot spots stay in one place for
long time intervals and as the plate above them moves, they form a
string of volcanic islands.
Hawaiian Island Chain is a classic example (click to zoom in on figure
at lefta model of the Pacific Plate moving over the fixed Hawaiian
"hot spot"). Looking at the age of the volcanoes, one should
be able to recreate the direction that the plate is moving (remember
the hot spot stays in one place). So where should the newest island
be forming? It is called Loihi and it has appeared on the sea floor
already. In millions of years, it could reach the surface of the sea
and become a new island.