<Dd> Eruptive behavior: explosive or effusive </Dd> <Dd> Distribution: common in hot spots in continental crust (Yellowstone National Park) and in continental rifts </Dd> <P> At any given pressure and for any given composition of rock, a rise in temperature past the solidus will cause melting . Within the solid earth, the temperature of a rock is controlled by the geothermal gradient and the radioactive decay within the rock . The geothermal gradient averages about 25 ° C / km with a wide range from a low of 5--10 ° C / km within oceanic trenches and subduction zones to 30--80 ° C / km under mid-ocean ridges and volcanic arc environments . </P> <P> As magma buoyantly rises it will change from a solid to a liquid, and its temperature will drop by adiabatic cooling . At this point, it may erupt to form lava . Melting can also occur due to a reduction in pressure by a process known as decompression melting . </P>

Which is not a compositional classification of a magma