Deep in the Earth, temperatures and pressures increase gradually but cannot melt rocks completely. The fusion liquids, which are very hot - from 700° to 1 300 ° celsius - are called “magmas”.

Some magmas pour out at the surface of the Earth. They cool rapidly, forming rocks known as “volcanic rocks” (eg. basalt).

Other magmas remain deep underground. Their slow cooling leads to the development of big crystals. They form “plutonic rocks” (eg. granite). Later foldings and long erosion process generate granite outcrops.