Igneous rocks form as molten rock cools and solidifies. Rises to the surface as magma is buoyant. When molten rock breaks through, producing a volcanic eruption, magma reaches the surface and becomes lava. Igneous rocks that form when magma solidifies at the surface are classified as extrusive or volcanic. Common in the western americas and hawaii. Igneous rocks that form below the surface are called intrusive or plutonic. Body of plutonic rock is called a pluton. Common in canada (ie. bc, canadian shield) Magma is believed to be formed from melting in the mantle, more actively at divergent plate boundaries (ie. seafloor spreading) Based on scientific evidence, earth"s crust and mantle are composed primarily of solid, not molten rock. Geothermal gradient is the change in temperature (increase) as depth increases. Volatiles (fluids/gases) or a drop in confining pressure around rocks almost at their melting point cause rocks to melt.