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Volcano definitions
Source : Volcanoes (A Firefly Guide) isbn 1-55297-683-1
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Ash
Measurement of particle size applied to the finest pyroclastic material, fragments less than 0.08 inches (2mm) in diameter, blasted into the air by volcanic explosions.
Basalt
Volcanic rock poor in silica but rich in magnesium, iron, and calcium. The most common volcanic rock.
Base surge
Turbulent low-density cloud of gas with suspended solid debris that expands radially outward from the base of an eruption column. Comparable to the ring that forms at the base of the cloud formed by a nuclear explosion.
Bomb
Fragment of magma varying in diameter from a few inches to several feet ejected into the air during a volcanic explosion (see tephra)
Caldera
Large depression produced following an eruption by the collapse of the roof of a magma chamber. Usually circular or horseshoe shaped when viewed from above.
Cinder cone
Cone build up by the accumulation of loose bits of magma (scoria) that fall around a vent or crater after being expelled into the air during moderately explosive activity. If still sufficiently hot they meld when they fall to the ground.
Compound volcano
Volcanic structure composed of two or more vents or lava domes or stratovolcanoes, often formed at different times.
Conduit (volcanic)
Passageway, more or less cylindrical in shape through which magma rises from the magma chamber to the surface during volcanic activity.
Crater (volcanic)
Bowl shaped depression or hollow, usually with steep sides at the summit of a volcano or on its flanks which is produced by volcanic activity.
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Dike
Tabular body of intrusive magma that cuts across layers of a volcanic structure.
Eruption column
Vertical cloud of gas and pyroclastic fragments that forms during explosive eruptions and can reach stratospheric heights.
Fall deposit
Layer of fragmentary volcanic material created by fall from an eruption column or during an explosive eruption.
Fissure volcano
Structure composed of the accumulation of volcanic material on the sides of an eruptive fissure, usually associated with a ridge or rift situation.
Fumarole chimney
Vertical structure with a vent through which volcanic gases are discharged.
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Geyser
Intermittent vertical jet of water produced by the heating of underground water by geothermal activity.
Hornito
Small cone or mound produced by the accumulation of spatter ejected by a vent. (see spatter cone)
Hot spot
Area on the planet where material from the mantle rises through a plume to reach a moving plate.
Hummocks
Small hills that rise above deposits from a volcanic avalanche.
Hyaloclastite
Fragmentary material of glassy composition produced by the rapid cooling of magma in contact with water.
Ignimbrite
Particular type of pyroclastic flow deposit containing glassy lenticular structures called flames. Ignimbrites can cover thousands of square miles of area.
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Lahar
Indonesian word for a volcanic mud flow usually formed when an eruption melts part of a glacier on a volcano. These flows involve large quantities of volcanic material and can move at high speeds.
Lapilli
Latin word meaning "small stone". Small fragments of lava (between 0.08 inches (2mm) and 2.5 inches (6.4 cm) across) shot into the air during eruptions when still molten.
Lava
Magma that has reached the surface of the Earth.
Lava Dome
Accumulation of lava in the shape of a steep-sided dome over an eruptive vent.
Lava lake
Lake of molten lava inside a crater tens or hundreds of feet in diameter. Such lakes can remain active for many years.
Lava Plateau
A wide, flat surface formed when lava is expelled from long narrow openings in the crust and spreads rapidly. Layers are formed by multiple eruptive events.
Lithic (volcanic)
Fragments of previously formed rocks (volcanic and non-volcanic) expelled from volcanic vent during explosive activity and encased in pyroclastic deposits.
Maar
Tuff ring that has filled with water. Maars are shallow craters usually with flat bottoms filled with water.
Magma
Molten rock within the earth formed by partial melting of the mantle.
Magma chamber
Reservoir in which magma is accumulated and stored. Located within the structure of a volcano a few miles from the surface.
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Pumice
Frothy volcanic rock heavily vesicled formed by the expansion of gas in erupting lava.
Pyroclast
Literally a "fire rock fragment" simply a piece of material formed by a volcanic explosion or ejected from a volcanic vent.
Pyroclastic flow
Mixture of fragmented volcanic materials and gas traveling at high speeds down the slope of a volcano. It is formed during an eruption or following the collapse of a lava dome.
Scoria
Dark, vesicled volcanic rock produced by moderately explosive activity or lava flows and resulting from blobs of gas-charged lava that have cooled in flight.
Shield volcano
Large volcanic structure with long, gentle slopes built up almost entirely from fluid lava flows.
Somma volcano
Type of volcanic structure composed of two volcanic cones, one of which (the more recent) has formed inside the other.
Spatter
Small fragments of lava ejected from a vent. Spatter is still molten when it falls to the ground thus forming cones or hornitos.
Spatter cone
Small, steep-sided mound or cone built up over a vent.
Stratovolcano
Volcano composed of alternating strata, or layers, of material created from lava and pyroclastic flows.
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Table Mountain Volcano
Volcanic massif with steep sides and horizontal summit created following an eruption of lava under a glacial covering.
Tephra
Generic term used for all pyroclastic materials of all sizes when ejected from a volcano. (Greek word for ashes)
Tuff
Deposited pyroclastic rock that has consolidated.
Tuff cone
Cone with relatively steep sides built up from the accumulation of fine-grained volcanic fragments produced from explosive activity resulting from the interaction of magma and water.
Tuff ring
Volcanic ring whose height is less than its diameter, built of debris around a volcanic vent located near water and produced by explosive activity resulting from the interaction of magma and water.
VOG
A blended word derived from the words volcanic and smog. VOG is formed by sulfur dioxide (and other gases) that mix with oxygen and moisture in direct sunlight during volcanic activity.
Source : Volcanoes (A Firefly Guide) isbn 1-55297-683-1
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