The residing magma starts to cool , with the higher melting point components such as olivine crystallizing out of the solution , particularly near to the cooler walls of the chamber , and forming a denser conglomerate of minerals which sinks ( cumulative rock ) . Upon cooling , new mineral phases saturate and the rock type changes ( e.g. fractional crystallization ) , typically forming ( 1 ) gabbro , diorite , tonalite and granite or ( 2 ) gabbro , diorite , syenite and granite . If magma resides in a chamber for a long period , then it can become stratified with lower density components rising to the top and denser materials sinking . Rocks accumulate in layers , forming a layered intrusion . Any subsequent eruption may produce distinctly layered deposits ; for example , the deposits from the 79 AD eruption of Mount Vesuvius include a thick layer of white pumice from the upper portion of the magma chamber overlaid with a similar layer of grey pumice produced from material erupted later from lower in the chamber
79 AD eruption of Mount Vesuvius
Eruption of Mount Vesuvius in 79
Magma chamber