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GLOSSARY OF SELECTED TERMS

ash: fine fragments (less than 2 millimeters across) of lava or rock formed in an explosive volcanic eruption

basalt: general term for dark-colored mafic igneous rock, usually extrusive, containing less than 52 weight percent silica

bomb: boulder-size chunk of partly solidified lava explosively ejected from a volcano

caldera: a large, roughly circular depression usually caused by volcanic collapse or explosion

cinder cone: small, steep-sided conical hill built mainly of cinder, spatter, and volcanic bombs

fallout: a general term for debris which falls to the earth from an eruption cloud

fault: a fracture or zone of fractures along which there has been displacement of the sides relative to one another

fissure: a roughly linear or sinuous crack or opening on a volcano; a type of vent which commonly produces lava fountains and flows

fumarole: a small opening or vent from which hot gases are emitted

Holocene: geologic epoch extending from the last 10,000 years to present

incandescent: glowing red or orange due to high temperature

intracaldera: refers to something within the caldera

lava: when molten rock reaches the earth's surface, it is called lava

magma: molten rock below the surface of the earth

phreatic activity: an explosive eruption caused by the sudden heating of ground water as it comes in contact with hot volcanic rock or magma

phreatic ash: fine fragments of volcanic rock expelled during phreatic activity; this ash is usually derived from existing rock and not from new magma

regional earthquake: earthquake generated by fracture or slippage along a fault; not caused by volcanic activity

satellite cone: a subsidiary volcanic vent located on the flank of a larger volcano

seismic swarm: a flurry of closely spaced earthquakes or other ground shaking activity; often precedes an eruption

shield volcano: a broad, gently sloping volcano usually composed of fluid, lava flows of basalt composition (e.g. Kilauea)

stratovolcano: (also called a stratocone or composite cone) a steep-sided volcano, usually conical in shape, built of lava flows and fragmental deposits from explosive eruptions

strombolian: type of volcanic eruption characterized by intermittent bursts of fluid lava, usually basalt, from a vent or crater

subplinian: style of explosive eruptions characterized by vertical eruption columns and widespread dispersal of tephra

tephra: a general term covering all fragmental material expelled from a volcano (ash, bombs, cinders, etc.)

vent: an opening in the earth's surface through which magma erupts or volcanic gasses are emitted

volcano-tectonic earthquakes: earthquakes generated within a volcano from brittle rock failure resulting from strain induced by volcanic processes

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