Plate Tectonics

The theory of plate tectonics originated in the 1960s. It describes how many moving, crustal plates cover Earth’s surface. Seafloor spreading showed that Earth’s crust moves sideways. Scientists wanted to understand the motion of all Earth’s plates.

Plates are made of a rigid layer of uppermost mantle and a layer of either oceanic or continental crust above. Some plates are made only of oceanic crust. Others are made of part oceanic and part continental crust. There are 7 major plates and many minor ones.  The seven major plates are:

• African plate

• Antarctic plate

• Eurasian plate

• Indo-Australian plate

• North American plate

• Pacific plate

• South American plate

Some of the minor plates are:

• Arabian plate

• Caribbean plate

• Philippine plate

• Scotia plate

• Nazca plate

Plate Motions

There are three main kinds of plate motions. At their boundaries, plates can move apart (divergent boundaries), move together (convergent boundaries), or slide past each other (transform boundaries).

A) Divergent boundaries are places where plates are pulling apart. You learned that magma is pushed up through faults in a rift valley at a mid-ocean ridge system. The magma spreads, cools and hardens to form new oceanic crust at this divergent boundary.

B) Convergent boundaries are areas where plates collide. Several things can happen at these boundaries depending on the types of plates that collide.

C) Transform boundaries are areas where two plates slide by each other. No new lithosphere is created. No old lithosphere is being destroyed, or recycled. The main result of transform boundaries is horizontal plate movement.