Most volcanoes occur near the edges of plates. When plates push together, one plate slides beneath the other. This is a subduction zone. When the plunging plate gets deep enough inside the mantle, some of the rock on the overlying plate melts and forms magma that can move upward and erupt at the Earth's surface. At rift zones, plates are moving apart and magma comes to the surface and erupts.
Volcanoes grow because of repeated eruptions.
There are three main kinds, or shapes, of volcanoes based on the type of materials they erupt. Stratovolcanoes build from eruptions of lava and tephra that pile up in layers, or strata, much like layers of cake and frosting. These volcanoes form symmetrical cones with steep sides. Shield volcanoes form from eruptions of flowing lava. The lava spreads out and builds up volcanoes with broad, gently sloping sides.