That collision causes a subduction volcano to form. The plate that slips underneath suddenly feels the tremendous pressure of wearing the concrete bodysuit. Some of its rock will literally start to melt, forming more magma and more pressure. The result is a volcano. Meanwhile, in another part of the crust, two plates can pull away from each other, opening a rift in the crust.
Rift volcanoes allow magma to ooze up from the mantle along the rift.
Someday, the two plates may shift their drift and push back together to close the rift. It's also possible for volcanoes to form right in the middle of a crustal plate. Hot-spot volcanoes result from magma melting a hole in the plate and oozing upward. The Hawaiian Islands, for example, are all the result of hot-spot volcano formations. Hawaii is not located along an edge of a crustal plate.