Molten rock that has ben extruded from a subterranean reservour(magma chamber) onto the earth's surface. Many factors are involved in determining how forceful is ejected onto the surface, but the silica content seems to be a large determing factor. The higher the silica, the thicker the melt, which causes dissolved gases to become trapped inside crystal lattices within the melt, which means...KABOOM!!
Cascade volcanoes have a relatively high proportion of dissolved gases, which often makes their eruptions explosive