magma, molten or partially molten rock from which igneous rocks form. It usually consists of silicate liquid, although carbonate and sulfide melts occur as well. Magma migrates either at depth or to Earth’s surface and is ejected as lava. Suspended crystals and fragments of unmelted rock may be transported in the magma; dissolved volatiles may separate as bubbles and some liquid may crystallize during movement. Several interrelated physical properties determine the characteristics of magma, including chemical composition, viscosity, dissolved gases, and temperature.
There are three major types of magma. Basaltic (or mafic) magma predominates in nonexplosive volcanic eruptions. It is a high-temperature magma (1,200 °C [about 2,200 °F]) characterized by flowing lava, and it is made up of about 45–55 percent silica (SiO2) by weight. In contrast, the temperature of rhyolitic (or felsic) magma is much lower (750–850 °C [about 1,400–1,560 °F]), but its silica content is higher, ranging from about 65 to 75 percent by weight. Rhyolitic magmas are characteristic of the most explosive eruptions, which also produce ash falls and pyroclastic flows. Andesitic magma is intermediate in temperature (800–1,000 °C [about 1,470–1,830 °F]) and silica content. (See also felsic and mafic rocks.)
As magma cools, crystals form in a systematic manner, which is most simply expressed in the form of Bowen’s reaction series; early high-temperature crystals will tend to react with the liquid to form other minerals at lower temperatures. Two series are recognized: (1) a discontinuous reaction series, which from high to low temperatures is composed of olivine, orthopyroxene, clinopyroxene, amphibole, and biotite; and (2) a continuous reaction series, represented by high-temperature calcium-rich plagioclase to low-temperature sodium-rich plagioclase. Numerous variations can occur during crystallization to influence the resulting rock. Such variations include separation of early crystals from liquid, preventing a reaction; cooling of magma too rapidly for reactions to occur; and loss of volatiles, which may remove some components from the magma. Transport and emplacement of magma is strongly affected by its viscosity and by the fracture characteristics of rocks through which it moves. Viscosity is reduced by water and a lower silica content.