flag slovak   flag slovak
Department of Mineralogy and Petrology
Faculty of Natural Sciences
Comenius University in Bratislava

Magmatic rock

Magmatic rocks (from Greek ignis = fire) is a rock, originating by cooling and solidification of magma or lava. The magma is generated by partial melting of Earth´s mantle or crust. The melting is caused due to a combination of several factors, such as temperature increase, pressure decrease, or influx of a volatile phase (H22), which depresses the melting point of a solid rock. A magmatic rock originating by crystallization from magma emplaced at depth or near surface is either plutonic (named after Pluto – the Roman god of the underworld) or intrusive rock. The magma poured on surface is called lava and rocks originating by solidification of the lava are volcanic rocks (named after Vulcano - the Roman god of fire), effusive or extrusive.

Magma is a mobile mixture of solid, liquid and volatile phases. Their amount at stable conditions depends on three intensive variables – pressure, temperature and composition (concentrations of individual chemical compounds). Each rock consists of a homogeneous liquid at a sufficiently high temperature above liquidus, but this case is rather rare in nature. Natural melts are mostly composed of mixture of silicates composed predominantly of O, Si, Al, Ca, Na and H ions. Very rare is a carbonatite melt with dominant C, O, Ca and Na ions. Mafic and ultramafic silicate magmas frequently contain immiscible droplets of sulfidic melts. In many cases, the magma is composed of two chemically distinct phases, e.g. two immiscible silicate phases, one Fe-rich and the second Si-rich, silicate and carbonatite, or silicate and sulfidic melts). The melt present in the magma is the reason of its mobility. Schematic examples of different types of magmas divided according to their phase composition are shown in Fig. 1.

Fig. 1. Schematic drawings of possible magma types according to phase composition. A) One-phase system composed of a single melt with or without dissolved volatile components. B) Two-phase system composed of melt and gas bubbles. C) Another type of two-phase system, from which olivine crystallizes as a solid phase. D) Two-phase system consisting of two immiscible melts of different compositions and densities. E) Four-phase system composed of melt, gas bubbles and two solid phases – olivine and plagioclase. Each solid phase is considered as a separate phase. More than five phases create a polycomponent system.