According to the theory of plate tectonics, the Lithosphere (crust and upper mantle) is divided into twelve or so rigid plates, which collide, separate and slide by each other as they move. These movements can cause earthquakes, volcanoes, oceanic trenches, mountain range formation, and many other geological phenomenon.
The plates are moving at a speed that has been estimated at 1 to 10 cm per year. The place where the two plates meet is called a plate boundary. Most of the Earth's seismic activity (volcanoes and earthquakes) occurs at the plate boundaries as they interact. Boundaries have different names depending on how the two plates are moving in relationship to each other:
- Crashing: Convergent Boundaries
At boundaries where plates collide, sometimes the crust buckles and mountains are forced up. The Himalayas are a good example where the Indian plate is pushing against the Eurasian plate and continuing to force up the mountains.
At other convergent boundaries one plate will slide under the other. If an oceanic plate and a continental plate collide, the lighter oceanic plate will go under (or subduct) under the continental plate. Earthquakes and volcanic eruptions can result. The most devasting earthquake occured off the coast of Sumatra, Indonesia on December 26th 2004 when the the India Plate slid under the Burma Plate. The slip did not happen instantaneously but took place in two phases over a period of several minutes. The resulting earthquake triggered the Indian Ocean tsunami which claimed over a quarter of a million lives.
- Pulling apart: Divergent Boundaries
At boundaries where plates diverge, long parallel rifts can form in the Earth's crust. Examples include the East Africa rift in Kenya and Ethiopia, and the Rio Grande rift in New Mexico. Magma (liquid rock) seeps upward to fill the crack between the two plates. In this way, new crust is formed along the boundary. Earthquakes occur along the faults, and volcanoes form where the magma reaches the surface.
In the ocean, magma wells up from the mantle along mid-ocean ridges. As the magma cools, it forms rock which spreads out at either side of the ridge, forming new oceanic crust.
- Sideswiping: Transform Boundaries
Places where plates slide past each other are called transform boundaries. Since the plates on either side of a transform boundary are merely sliding past each other and not tearing or crunching each other, transform boundaries lack the spectacular features found at convergent and divergent boundaries. However, their sliding motion causes lots of earthquakes. The most famous transform boundary in the world is the San Andreas fault in California (See the Earthquakes page). The strongest and most famous earthquake along the San Andreas fault hit San Francisco in 1906.
- Where plates separate (e.g. the Mid Atlantic Ridge) new plates are created and where they collide (e.g. the Himalayas) plates are destroyed.
- Movements along plate boundaries can result in mountains being built, volcanoes, earthquakes and the formation of deep-sea trenches.