<Tr> <Td> Rhyolite magma </Td> <Td> 2180--2250 </Td> </Tr> <P> It is usually very difficult to change the bulk composition of a large mass of rock, so composition is the basic control on whether a rock will melt at any given temperature and pressure . The composition of a rock may also be considered to include volatile phases such as water and carbon dioxide . </P> <P> The presence of volatile phases in a rock under pressure can stabilize a melt fraction . The presence of even 0.8% water may reduce the temperature of melting by as much as 100 ° C. Conversely, the loss of water and volatiles from a magma may cause it to essentially freeze or solidify . </P> <P> Also a major portion of almost all magma is silica, which is a compound of silicon and oxygen . Magma also contains gases, which expand as the magma rises . Magma that is high in silica resists flowing, so expanding gases are trapped in it . Pressure builds up until the gases blast out in a violent, dangerous explosion . Magma that is relatively poor in silica flows easily, so gas bubbles move up through it and escape fairly gently . </P>

Which does not affect the formation of magma