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Andesite A volcanic rock containing 53-63% silica with a moderate viscosity when in a molten state.
Ash Fragments less than 2 millimeters in diameter of lava or rock blasted into the air by volcanic explosions.
Basalt A volcanic rock consisting of less than 53% silica with a low viscosity when in a molten state.
Caldera A large volcanic depression, commonly circular or elliptical when seen from above.
Composite volcano A steep-sided volcano composed of many layers of volcanic rocks, usually of high-viscosity lava and fragmented debris such as lahar and pyroclastic deposits.
Dacite A volcanic rock containing 63-68% silica with a high viscosity when in a molten state.
Dome A steep-sided mound that forms when viscous lava piles up near a volcanic vent. Domes are formed by andesite, dacite, and rhyolite lavas.
Fumarole A vent that releases volcanic gases, including water vapor (steam).
Lahar A flowing mixture of water and rock debris that forms on the slopes of a volcano, sometimes referred to as debris flow or mudflow. The term comes from Indonesia.
Lava Molten rock that erupts from a vent or fissure; see magma.
Magma Molten rock that contains dissolved gas and crystals, formed deep within the Earth. When magma reaches the surface, it is called lava.
Phreatic eruption A type of volcanic explosion that occurs when water comes in contact with hot rocks or ash near a volcanic vent, causing steam explosions.
Pumice A light-colored volcanic rock containing abundant trapped gas bubbles formed by the explosive eruption of magma. Because of its numerous gas bubbles, pumice commonly floats on water.
Pyroclastic flow A hot, fast-moving and high-density mixture of ash, pumice, rock fragments, and gas formed during explosive eruptions.
Pyroclastic surge Same process as pyroclastic flow but of much lower density.
Rhyolite A volcanic rock containing more than 68% silica with a very high viscosity when in a molten state.
Shield volcano A volcano shaped like an inverted warrior's shield with long gentle slopes produced by eruptions of low-viscosity basaltic lava.
Silica The molecule formed of silicon and oxygen (SiO2) that is the basic building block of volcanic rocks and the most important factor controlling the fluidity of magma. The higher a magma's silica content, the greater its viscosity or "stickiness."
Vent The opening at the Earth's surface through which volcanic materials (magma and gas) escape.
Volcano A vent in the surface of the Earth through which magma erupts and also the landform that is constructed by the erupted material.
Volcanic landslide The downslope movement of soil, rock debris, and sometimes glacial ice, with or without water, from the flank of a volcano.

Maintained by John Watson
Updated 06.24.97

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