There are three basic types of plate boundaries:
Divergent boundaries form where plates move apart and the crust expands. When two pieces of crust move away from each other, molten mantle rock rises into the opening. This process is called sea floor spreading.
Convergent boundaries occur where two plates are pushing against each other. crust is lost or destroyed in these areas. There are two basic types of convergent boundaries.
-
Collision
When two continental plates collide, they may crumple against each other, forming a thick mass. The best example of a collision boundary is between the Indian and Asian plates: the Himalaya.
Subduction
When two plates converge, on of them, the subducting plate, may bend and move under the other. It is also pulled down by gravity. This process usually involves one oceanic and one continental plate, but sometimes happens between two oceanic plates.
As a subducting plate slides beneath the overlying plate, it melts under graeat pressure. This molten rock eventually erupts through the overlying plate in the form of a chain of volcanoes, several hudnred miles inland and parallel to the plate boundary.