Plate Boundaries are where two plates meet.
There are three types of plate boundaries:
1) Convergent Boundaries: Convergent means come together. At this boundary the plates are moving toward each other. It is also called a destructive boundary because the older more dense plate melts into the mantle.
2) Divergent Boundaries: Divergent means move apart. At this boundary the plates are moving away from each other. This is also called a constructive boundary because new ocean floor is formed. Ocean Floor Spreading occurs here.
The diagram above shows what occurs at a divergent boundary. Notice the magma explodes from the mantle (volcanic activity). This pushes the plates apart. We end up with a mid-ocean ridge with a rift valley between the ridges. New ocean floor is formed at the bottom of the rift valley. The oldest rock is farthest from the center.
3) Strike-Slip Boundary (Transform Fault): The movement at a strike-slip boundary, also called a transform fault, is sideways. The plates move past each other. These occur at faults or cracks in the crust and are caused by earthquakes.