Convection currents in the mantle cause the plates to move together.
If one plate is made from oceanic crust and the other from continental crust, the denser oceanic crust sinks under the lighter continental crust, in subduction.
The oceanic plate being forced down causes earthquakes.
The oceanic plate is destroyed by friction and heat from the mantle as it melts to form magma.
The hot liquid rock can rise and result in violent volcanic eruptions.
Another type of destructive plate margin is where two continental plates meet at a collision boundary. They collide instead of sinking beneath the other.
Constructive Plate Margins
Convection currents drag plates apart.
Cracks and fractures form between the plates where there is no solid crust.
Magma forces its way into the cracks and makes its way to the surface to form volcanoes.
Volcanic islands and mid-ocean ridges are created by lava.
Conservative Plate Margins
Plates are sliding past each other, moving in similar (not the same) directions.
As one plate is moving faster than the other and in a slightly different direction, they tend to get stuck.
Eventually, the build-up of pressure causes them to be released.
This sudden release of pressure causes an earthquake.
In a conservative plate boundary, crust is neither being destroyed nor made.