Home | Session 1 | Plate boundaries 1, 2

Plate boundaries and plate tectonic implications

Presented by Carol Tang
California Academy of Sciences

I. Plate Boundaries
There are three main types of plate boundaries: divergent, convergent and transform faults.

a. 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.

b. 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.

c. 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.

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: 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.

The Hawaiian Island Chain is a classic example (click to zoom in on figure at left—a 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.

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updated January 28, 2002

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