• The rigid outer shell of the Earth is called the lithosphere.
• The lithosphere is broken into a series of moving plates.
• The boundary between two plates can be a divergent boundary, a convergent boundary or a transform boundary.
• The lithosphere is composed of the crust, and some of the upper mantle.
• Mature oceanic plate is approximately 80-100 km thick, of which, the upper 5-7 km is oceanic crust.
• Continental plate is somewhat thicker, perhaps 150-200 km thick, and includes continental crust which varies from 5 to 70 km in thickness.
• A single plate may be part oceanic and part continental. In that case, the boundary between ocean crust and continental crust is not a plate boundary.
• The mobile part of the mantle under the lithosphere is called the asthenosphere.

The boundary between two plates can be a divergent boundary, a convergent boundary or a transform boundary. Plate boundaries are most readily identified by seismicity, the location of siginifcant earthquake activity. About 95% of the worlds earthquakes occur on plate boundaries, and all earthquakes occur in the lithosphere.

Divergent Boundaries

• At divergent plate boundaries, two lithospheric plates move apart, and new plate (and crust) are created at the boundary. The new crust is created volcanically. This process always produces ocean crust and thus oceanic plate. Thus divergent boundaries ultimately become mid-ocean ridges.
• As the lithosphere moves away from the ridge, it thickens by a process of underplating in which asthenosphere solidifies onto the underside of the plate as the plate cools.

• The thickening lithosphere subsides as it ages. The ridges are a result of this subsidence. Young lithosphere near the ridge sits up high, while older lithosphere sits lower.
• Ultimately, ocean lithosphere becomes more dense than the asthenosphere beneath it. Thus old oceanic lithosphere will sink into the asthenosphere if it gets the chance.

Convergent Boundaries

• At convergent plate boundaries, two lithospheric plates move together. The denser plate sinks in a process called subduction.
• The subduction process produces magmas through processes we will discuss later.
• Only oceanic plate can subduct. Continental crust is too thick and low density to sink. It acts like a float.
• Subducting plate remains more rigid than the surrounding mantle to depths of about 700 km. This is the maximum depth of earthquakes which only occur in rigid lithosphere. Somewhere beneath this depth, old plates just fade into the mantle.
• At some convergent boundaries, oceanic plate meets continental plate. An example of this is the west coast of southern Mexico, Central America and South America. An oceanic trench forms offshore, and a chain of volcanic mountains forms just inland of the coast. This type of boundary between oceanic crust and continental crust is called an active continental margin since it is a plate boundary.
• At other convergent boundaries, oceanic plate meets other oceanic plate. This results in an oceanic trench and a chain of volcanic islands known as an island arc. Japan, the Philippines and the Aleutians are examples of island arcs.
• At some convergent boundaries, two continents collide. Neither can subduct, so the result is like a big car crash. Both continents are crumpled, large mountain belts are formed, and their insurance rates go up.

Transform Boundaries

• Transform boundaries occur when two plates move parallel to one another in opposite directions, so that there is neither divergence or convergence. This is rare. It occurs for short intervals along mid-ocean ridges. Thus many short transform boundaries are found along, and perpendicular to mid-ocean ridges.
• Transform boundaries on continents are rare. The San Andreas fault in California is a transform boundary. Another transform boundary cuts across the southern island of New Zealand.
• No volcanism occurs at transform boundaries

What Causes Plate Tectonics

• The ultimate cause of plate tectonics is the cooling of the Earth. The core and inner Earth have retained enormous amounts of heat from the initial formation of the Earth. Since the mantle is made of rock, it is a very good insulator.
• Like a hot cup of coffee, the fastest way to cool the Earth is to stir it. Thus the material of the mantle convects enabling the release of heat. The movement of plates is the surface manifestation of that mantle convection.
• Oceanic plates are driven by their own sinking. Thick, cold, dense oceanic lithosphere sinks at subduction zones, pulling the rest of the ocean plate. Since plates have limited tensional strength, this "subduction pull" cannot be the only force acting on an ocean plate. "Ridge push" is the sliding of the young thin part of the plate downhill away from the ridge. A combination of "subduction pull" and "ridge push" move the ocean plates along. These forces are not separate from mantle convection, but are both caused by and help cause that convection. Continental plates are pushed around by oceanic plates.