A rock formed from molten magma, deep beneath the Earth’s surface, subsequently coming to the surface as an extrusion, or remaining below ground as an intrusion. The plutonic mode of igneous rock formation is characterized by ‘intrusive emplacement under compressional tectonic stress regimes. The volcanic mode is dominated by the eruption of large volumes of magma, through fault controlled dyke structures in extensional tectonic stress regimes’ (Neuberger and Reuther (1987) Int. J. Earth. Scis 76, 1). The nature of the rock also depends on the rate at which it cooled; intrusions of magma cool slowly, allowing enough time for large crystals to form, while extrusions cool quickly, leaving little time for crystal growth; see R. Le Maitre (1989).