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Molten or partially molten rock from which igneous rocks form, usually consisting of silicate liquid. Magma migrates either at depth or to the Earth's surface, where it is ejected as lava. The interactions of several physical properties, including chemical composition, viscosity, content of dissolved gases, and temperature, determine the characteristics of magma. Numerous events that can occur during crystallization influence the resulting rock: separation of early crystals from liquid prevents reaction between them; magma can cool too rapidly for reaction to occur; and loss of volatiles may remove some components from the magma.
This entry comes from Encyclopædia Britannica Concise. For the full entry on magma, visit Britannica.com.