Plate tectonics is the study of how the Earth's plates are driven and shaped by geological forces that keep them in constant motion.

It would appear as though the outermost crust of the Earth is one solid shell. In reality it is cracked into several large pieces or slabs called plates that average 50 miles thick (80 km). These plates float on a partially molten mantle beneath. The molten layer is believed to be in a constant state of convection driven by heat from the Earth's inner core. Convection cells, which are circular in nature, act like conveyer belts that slowly move the plates floating above.

Currently there are 14 main plates and many smaller subplates. The main plates are:

• Pacific plate
• Juan de Fuca plate
• North American plate
• South American plate
• Caribbean plate
• Cocos plate
• Nazca plate
• Scotia plate
• Antarctica plate
• African plate
• Arabian plate
• Eurasian plate
• Indian-Australian plate
• Philippine plate

Continents and seabeds alike sit atop these slabs moving at a rate of about 1—3 inches per year (2.5 — 7.5 cm). As the plates move, pressures build at the plate boundaries. Depending on the plates involved, forces of varying types create geological phenomenon; crust is created, destroyed, or crushed. Earthquakes are often triggered. Continents and plates not only change position but shape over extended periods of time.

Two continental plates colliding or converging will eventually create mountain ranges as the plates compress and push crust upwards.

A thin oceanic plate converging with a thick continental plate will be pushed beneath the continental plate, creating a subduction zone -- an area marked by a deep submarine trench where the oceanic plate is being driven downwards, eventually returning to the molten mantle. As the plate slides into the soft hot interior it pulls the rest of the plate along. This process is referred to as "slab pull" and since 1994 has been theorized to be the main driving force behind plate tectonics.

As crust is destroyed in subduction zones, it is being created in divergent zones. Here plates are pulled away from one another. The best example is at the mid-Atlantic ridge. This is a segment of a much larger mountain chain that encircles the globe under the oceans, winding its way around the continents. The mid-Atlantic ridge lies halfway between the east coast of the United States and Africa, and marks the plate boundaries of the North American and African plates. Volcanic material is constantly welling up from the seafloor at the site of the spreading plates, creating new sea crust as the old crust moves outwards. Prior to 1994, seafloor spreading was thought to be the driving force of plate tectonics.

Plate boundaries might also move laterally past one another, as in the case of the Pacific and North American plates. The former sits under the Pacific Ocean, extending under the edge of California's coast, reaching inland only as far as the San Andreas fault that marks the plate boundary. This plate moves northwesterly at a rate of about 2 inches (5 cm) per year, while the North American plate on the opposite side of the San Andreas fault is moving in a southerly direction. Pressures build until they are sufficiently strong to force the plates to "slip" past one another. The "slip" is experienced as an earthquake and explains why this region of California is so geologically active; though the other types of forces also create earthquakes.

The "Ring of Fire" -- a string of active volcanoes including Mt. St. Helens -— is situated around the Pacific plate where its boundaries meet surrounding plates.

A forerunner of plate tectonics was the theory of continental drift, put forth in 1912 by German meteorologist, Alfred Lothar Wegener. Wegner observed the matching coastlines of Africa and South America and other geological features that indicated continuity. Paleontology records revealed shared coastal fossils. This and other data led Wegener to hypothesize that all continents were once joined in a supercontinent he called Pangea (Greek for "all lands"). Pangea began to slowly break apart 200 million years ago, first into two huge landmasses now called Gondwanaland and Laurasia, and later into the continents we have today. This explained contradictory geological records such as glacial deposits in lands that now lie in arid latitudes, or the coal deposits of tropical plants found in Antarctica. However, Wegener's hypothesis was lacking a viable explanation of how continents could move. For this reason, his theory was rejected. After Wegener's death in 1930, new evidence arose that rekindled interest in continental drift and ultimately led to the theory of plate tectonics.