As the rigid lithosphere pushes down into the hot, high-pressure mantle, it heats up. The increased water content lowers the melting point of the mantle rock in this wedge, causing it to melt into magma. This sort of magma production is called subduction zone volcanism . This process does not produce volcanoes. This kind of boundary can develop later into a subduction zone.
Some plates move against each other rather than push or pull apart.
These transform plate boundaries rarely produce volcanic activity. Click here for a great diagram of plate boundaries. Photo courtesy USGS Most volcanoes pop up at the boundaries between the earth's plates. This interplate volcanic activity is caused by unusually hot mantle material forming in the lower mantle and pushing up into the upper mantle. The mantle material, which forms a plume shape that is from 500 to 1000 km wide, wells up to create a hot spot under a particular point on the earth.