How do volcanos from?

There are six main types of volcanoes. Cinder cone, caldera, composite, shield, submarine and lava dome volcano. Basically though, a volcano forms when either a continental plate or oceanic plate subducts under another plate where, due to extreme temperaratures the rock melts. This forms cracks in the plate. Magma will forces its way up through cracks in the earths crust. When it reaches the surface it builds up slowly. when the volcano erupts ash, magma and debris fall around the volcanoes cone. these eruptions will slowly build up the volcano.