The San Andreas Fault is a fault at the boundary between the Pacific and North American plates.
The surface of the Earth is made up of a series of constantly moving plates. While these movements are extremely slow, over time even a small movement can build up an enormous amount of tension. One way this tension is relieved is by a sudden shifting of the rock that makes up the crust. Sometimes these shifts happen near the surface, and sometimes deeper. These shifts are called earthquakes.
Convergent boundaries
The Nazca plate is subducting (sliding under) the South American plate along the Andes Mountain range.
When two plates are moving toward each other they form a convergent boundary. In a convergent boundary, the lighter plate is pushed beneath the heavier plate. As the lighter plate slides under, it heats up and eventually begins to melt. The melting rock is more buoyant and pushes back toward the surface. As it pushes, it can cause volcanic activity near the plate boundary and deep earthquake activity under the heavier plate. Examples of convergent boundaries can be found on the Oregon and Washington coastlines and the Andes mountain range in South America.
Divergent boundaries
Scientists are studying the effects of divergent boundaries as Iceland is slowly pulled apart by plate tectonics.
When two plates are moving away from each other they form a divergent boundary. When a divergent boundary occurs between oceanic plates, there is usually more volcanic activity and shallow earthquakes. When continental plates are separating, the break is often jagged, and the earthquake activity can be deeper as new continental rock appears and new inland seas are formed. Examples of land-based divergent boundaries can be found in the East African rift valley and down the middle of Iceland.
Transform boundaries
When two plates slide alongside each other, they create a transform boundary. Transform boundaries are usually a side effect of either a convergent or divergent boundary nearby. When the larger motion of plates pulling or pushing causes zig-zag-shaped cracks to form in the crust, these cracks can begin to move horizontally against each other. Most transform boundaries are oceanic and can produce rare, deep earthquakes. A few transform boundaries appear on land; one of the most notable is the San Andreas fault which produces recurring shallow earthquakes and little volcanic activity.
Plate-boundary zones
Multiple microplates form the Mediterranean-Alpine zone.
Some boundaries involve multiple plates and a variety of movements. These areas occur in broad belts and are known as plate-boundary zones. One of the best studied of these zones is the Mediterranean-Alpine zone, which includes several small microplates between the African and Eurasian plates. These areas are notably unpredictable and can express deep earthquakes in one period and more shallow activity in another.
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