Tilling, Robert I. Branch of Igneous and Geothermal Processes, U.S. Geological Survey, Menlo Park, California.
Last reviewed:November 2019
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Molten rock material that is erupted by volcanoes through openings (volcanic vents) in the Earth's surface. Volcanic rock is formed by the accumulation, cooling, and solidification of lava erupted explosively or effusively (nonexplosively). Beneath the Earth's surface, molten rock material is called magma. All magmas and lavas consist mainly of a liquid, along with smaller and variable amounts of solid and gaseous matter. The liquid is molten rock that contains some dissolved gases or gas bubbles; the solids are suspended crystals of minerals or incorporated fragments of preexisting rock. Rapid cooling (quenching) of this liquid upon eruption forms a natural volcanic glass, whereas slower cooling allows more minerals to crystallize from the liquid and preexisting minerals to grow in size. The dissolved gases, a large proportion of which are released rapidly upon eruption, are mostly water vapor, together with lesser amounts of carbon, sulfur, chlorine, and fluorine gases. With very rare exception, the chemical composition of the liquid part of magmas and lavas is dominated by silicon and oxygen, which form polymers or compounds with other common rock-forming elements, such as aluminum, iron, magnesium, calcium, sodium, potassium, and titanium.
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