Volcano Glossary 

Aa: Hawaiian word used to describe a lava flow whose surface is broken into rough angular fragments.

 Accretionary lava ball: A rounded mass, ranging in diameter from a few centimeters to several meters, [carried] on the surface of a lava flow such as aa, or on cinder-cone slopes, [and formed] by the molding of viscous lava around a core of already solidified lava.

 Active volcano: An erupting volcano, or volcano likely to erupt.

 Andesite: Volcanic rock (or lava) characteristically medium dark in color and containing 54 to 62 percent silica and moderate amounts of iron and magnesium.

 Ash: Fine particles of pulverized rock blown from an explosion vent. Measuring less than 1/10 inch in diameter, ash may be either solid or molten when first erupted. By far the most common variety is vitric ash, glassy particles formed by gas bubbles bursting through liquid magma.

 Ashfall (Airfall): Volcanic ash that has fallen through the air from an eruption cloud. A deposit so formed is usually well sorted and layered.

 Ash flow: A turbulent mixture of gas and rock fragments ejected violently from a crater or fissure. Also called a pyroclastic flow.

 Basalt: Volcanic rock (or lava) that characteristically is dark in color, contains 45 to 54 percent silica, and generally is rich in iron and magnesium.

 Block: Angular chunk of solid rock ejected during an eruption.

 Bench: The unstable, newly-formed front of a lava delta.

 Bomb: Fragment of molten or semi-molten rock, 2 1/2 inches to many feet in diameter, which is blown out during an eruption. Because of their plastic condition, bombs are often modified in shape during their flight or upon impact.

 Caldera: The Spanish word for cauldron, a basin-shaped volcanic depression; by definition, at least a mile in diameter. Such large depressions are typically formed by the subsidence of volcanoes. Crater Lake occupies the best-known caldera in the Cascades.

 Capping stage: Refers to a stage in the evolution of a typical Hawaiian volcano during which alkalic basalt and related rocks build a steeply sloping cap on the main shield of the volcano. Eruptions are less frequent but more explosive, also the caldera may be covered.

Central Vent: A central vent is an opening at the Earth's surface of a volcanic conduit of cylindrical or pipelike form.

 Central Volcano: A volcano constructed by the ejection of debris and lava flows from a central point, forming a more or less symmetrical volcano.

 Cinder cone: A volcanic cone built entirely of loose fragmented material (pyroclastics.) a conical hill formed by the accumalation of volcanic debris around a vent

 Compound Volcano:A volcano that consists of a complex of two or more vents, or a volcano that has an associated volcanic dome, either in its crater or on its flanks. Examples are Vesuvius and Mont Pelee.

Conduit: A passage followed by magma in a volcano.

Crater: A steep-sided, usually circular depression formed by either explosion or collapse at a volcanic vent.

 Curtain of fire: A row of coalescing lava fountains along a fissure; a typical feature of a Hawaiian-type eruption.

 Dacite: Volcanic rock (or lava) that characteristically is light in color and contains 62 to 69 percent silica and moderate a mounts of sodium and potassium.

 Debris avalanche: A rapid and unusually sudden sliding or flowage of unsorted masses of rock and other material. As applied to the major avalanche involved in the eruption of Mount St. Helens, a rapid mass movement that included fragmented cold and hot volcanic rock, water, snow, glacier ice, trees, and some hot pyroclastic material. Most of the May 18 deposits in the upper valley of the North Fork Toutle River and in the vicinity of Spirit Lake are from the debris avalanche.

 Debris Flow: A mixture of water-saturated rock debris that flows downslope under the force of gravity (also called lahar or mudflow).

 Devonian: A period of time in the Paleozoic Era that covered the time span between 400 and 345 million years.

 Dormant volcano: Literally, "sleeping". The term is used to describe a volcano which is presently inactive but which may erupt again. Most of the major Cascade volcanoes are believed to be dormant rather than extinct.

 Ejecta: Material that is thrown out by a volcano, including pyroclastic material (tephra) and, from some volcanoes, lava bombs.

 Episode: An episode is a volcanic event that is distinguished by its duration or style.

 Eruption: The process by which solid, liquid, and gaseous materials are ejected into the earth's atmosphere and onto the earth's surface by volcanic activity. Eruptions range from the quiet overflow of liquid rock to the tremendously violent expulsion of pyroclastics.

 Eruption cloud: The column of gases, ash, and larger rock fragments rising from a crater or other vent. If it is of sufficient volume and velocity, this gaseous column may reach many miles into the stratosphere, where high winds will carry it long distances.

 Eruptive vent: The opening through which volcanic material is emitted.

Extinct volcano: A volcano that is not presently erupting and is not likely to do so for a very long time in the future.

 Fissures: Elongated fractures or cracks on the slopes of a volcano. Fissure eruptions typically produce liquid flows, but pyroclastics may also be ejected.

 Flank eruption: An eruption from the side of a volcano (in contrast to a summit eruption.)

 Fumarole: A vent or opening through which issue steam, hydrogen sulfide, or other gases. The craters of many dormant volcanoes contain active fumaroles.

 Guyot: A type of seamount that has a platform top. Named for a nineteenth-century Swiss-American geologist.

Horizontal blast: An explosive eruption in which the resultant cloud of hot ash and other material moves laterally rather than upward.

Hot Spot: A volcanic center, 60 to 120 miles (100 to 200 km) across and persistent for at least a few tens of million of years, that is thought to be the surface expression of a persistent rising plume of hot mantle material. Hot spots are not linked to arcs, and may not be associated with ocean ridges.

 Hyaloclastite: A deposit formed by the flowing or intrusion of lava or magma into water, ice, or water-saturated sediment, and its consequent granulation or shattering into small angular fragments.

 Intrusion: The process of emplacement of magma in pre-existing rock. Also refers to igneous rock mass so formed within the surrounding rock.

 Kipuka: An area surrounded by a lava flow.

Lahar: A torrential flow of water-saturated volcanic debris down the slope of a volcano in response to gravity. A type of mudflow.

Lapilli: Literally, "little stones"; round to angular rock fragments measuring 1/10 inch to 2 1/2 inches in diameter, which may be ejected in either a solid or molten state.

 Lava: Magma which has reached the surface through a volcanic eruption. The term is most commonly applied to streams of liquid rock that flow from a crater or fissure. It also refers to cooled and solidified rock.

Lava Dome: A steep-sided mass of viscous (doughy) lava extruded from a volcanic vent, often circular in plane view and spiny , rounded, or flat on top. Its surface is often rough and blocky as a result of fragmentation of the cooler, outer crust during growth of the dome.

 Lava Flow: An outpouring of lava onto the land surface from a vent or fissure. Also, a solidified tongue like or sheet like body formed by outpouring lava.

 Lava lake (pond): A lake of molten lava, usually basaltic, in a volcanic crater or depression. The term refers to solidified and partially solidified stages as well as to the molten, active lava lake.

 Lava Shields: A shield volcano made of basaltic lava.

 Lava tube: A tunnel formed when the surface of a lava flow cools and solidifies, while the still-molten interior flows through and drains away.

 Limu o Pele (Pele seaweed): Delicate, translucent sheets of spatter filled with tiny glass bubbles.

 Maars: A volcanic crater that is produced by an explosion in an area of low relief, is generally more or less circular, and often contains a lake, pond, or marsh.

Magma: Molten rock beneath the surface of the earth.

 Magma chamber: The subterranean cavity containing the gas-rich liquid magma which feeds a volcano.

 Monogenetic: A volcano built by a single eruption.

Mudflow: A flowage of water-saturated earth material possessing a high degree of fluidity during movement. A less-saturated flowing mass is often called a debris flow. A mudflow originating on the flank of a volcano is properly called a lahar.

Nuee ardente: A French term applied to a highly heated mass of gas-charge ash which is expelled with explosive force, and hurricane speed, down the mountainside.

 Obsidian: A black or dark-colored volcanic glass, usually composed of rhyolite.

Pahoehoe: A Hawaiian term for lava with a smooth, billowy, or ropy surface.

 Pali:Hawaiian word for steep hills or cliffs.

 Pele hair: A natural spun glass formed by blowing-out during quiet fountaining of fluid lava, cascading lava falls, or turbulent flows, sometimes in association with pele tears. A single strand, with a diameter of less than half a millimeter, may be as long as two meters.

 Pele tears: Small, solidified drops of volcanic glass behind which trail pendants of Pele hair. They may be tear-shaped, spherical, or nearly cylindrical.

 Phreatic eruption (explosion): An explosive volcanic eruption caused when water and heated volcanic rocks interact to produce a violent expulsion of steam and pulverized rocks. Magma is not involved.

 Phreatomagmatic: An explosive volcanic eruption that results from the interaction of surface or subsurface water and magma.

 Pillow lava: Interconnected, sack-like bodies of lava formed underwater.

 Pit crater: A crater formed by sinking in of the surface; not primarily a vent for lava.

Plinian eruption:An explosive eruption in which a steady, turbulent stream of fragmented magma and magmatic gases is released at a high velocity from a vent. Large volumes of tephra and tall eruption columns are characteristic.

 Plug: Solidified lava that fills the conduit of a volcano. It is usually more resistant to erosion than the material making up the surrounding cone, and may remain standing as a solitary pinnacle when the rest of the original structure has eroded away.

 Plug dome: The steep-sided, rounded mound formed when viscous lava wells up into a crater and is too stiff to flow away. It piles up as a dome-shaped mass, often completely filling the vent from which it emerged.

Pumice: Light-colored, frothy volcanic rock, usually of dacite or rhyolite composition, formed by the expansion of gas in erupting lava. Commonly seen as lumps or fragments of pea-size and larger, but can also occur abundantly as ash-sized particles.

Pyroclastic: Pertaining to fragmented (clastic) rock material formed by a volcanic explosion or ejection from a volcanic vent.

 Pyroclastic flow: Lateral flowage of a turbulent mixture of hot gases and unsorted pyroclastic material (volcanic fragments, crystals, ash, pumice, and glass shards) that can move at high speed (50 to 100 miles an hour.) The term also can refer to the deposit so formed.

Repose:The interval of time between volcanic eruptions.

Renewed volcanism stage: Refers to a stage in the evolution of a typical Hawaiian volcano during which, after a long period of quiescence, lava and tephra erupt intermittently. Erosion and reef building continue.

 Rhyodacite: An extrusive rock intermediate in composition between dacite and rhyolite.

 Rhyolite: Volcanic rock (or lava) that characteristically is light in color, contains 69 percent silica or more, and is rich in potassium and sodium.

 Rift system: The oceanic ridges formed where tectonic plates are separating and a new crust is being created; also, their on-land counterparts like the East African Rift.

 Rift zone: A zone of volcanic features associated with underlying dikes. The location of the rift is marked by cracks, faults, and vents.

 Ring of Fire: The regions of mountain-building earthquakes and volcanoes which surround the Pacific Ocean.

 Scoria: A bomb-size (> 64 mm) pyroclast that is irregular in form and generally very vesicular. It is usually heavier, darker, and more crystalline than pumice.

Seafloor spreading: The mechanism by which new seafloor crust is created at oceanic ridges and slowly spreads away as plates are separating.

 Seamount:A submarine volcano.

Shield volcano: A gently sloping ( usually ten degrees and less) volcano in the shape of a flattened dome, built almost exclusively of lava flows.

 Silica: A chemical combination of silicon and oxygen .

 Skylight: An opening, formed by collapse, in the roof of a lava tube.

 Solfatara: A type of fumarole, the gases of which are characteristically sulfurous.

Spatter cone: A low, steep-sided cone of spatter built up on a fissure or vent; it is usually of basaltic material.

 Spatter rampart: A ridge of congealed pyroclastic material, usually basaltic, built up on a fissure or vent.

 Specific gravity: The density of a mineral divided by the density of water.

 Spines: Horn-like projections formed upon a lava dome.

 Stratovolcano: A volcano with sides of at least 20 degrees composed of both lava flows and pyroclastic material.

 Strombolian eruption: A type of volcanic eruption characterized by jetting of clots or fountains of fluid basaltic lava from a central crater.

 Surge: A ring-shaped cloud of gas and suspended solid debris that moves radially outward at high velocity as a density flow from the base of a vertical eruption column accompanying a volcanic eruption or crater formation.

Tephra: Materials of all types and sizes that are erupted from a crater or volcanic vent and deposited from the air.

 Tilt: The angle between the slope of a part of a volcano and some reference. The reference may be the slope of the volcano at some previous time.

Trachyandesite: An extrusive rock intermediate in composition between trachyte and andesite.

 Trachybasalt: An extrusive rock intermediate in composition between trachyte and basalt.

 Trachyte: A group of fine-grained, generally porphyritic, extrusive igneous rocks having alkali feldspar and minor mafic minerals as the main components, and possibly a small amount of sodic plagioclase.

 Tsunami: A great sea wave produced by a submarine earthquake, volcanic eruption, or large landslide.

 Tuff: Rock formed of pyroclastic material.

 Tuff cone: A type of volcanic cone formed by the interaction of basaltic magma and water. Smaller and steeper than a tuff ring.

Tuff ring: A wide, low-rimmed, well-bedded accumulation of hyalo-clastic debris built around a volcanic vent located in a lake, coastal zone, marsh, or area of abundant ground water.

Vent: The opening at the earth's surface through which volcanic materials issue forth.

 Vesicle: A small air pocket or cavity formed in volcanic rock during solidification.

Viscosity: A measure of resistance to flow in a liquid (water has low viscosity while honey has a higher viscosity.)

 Volcano: A vent in the surface of the Earth through which magma and associated gases and ash erupt; also, the form or structure, usually conical, that is produced by the ejected material.

 Volcanic arc: A generally curved linear belt of volcanoes above a subduction zone, and the volcanic and plutonic rocks formed there.

 Volcanic complex: A persistent volcanic vent area that has built a complex combination of volcanic landforms.

 Volcanic neck: A massive pillar of rock more resistant to erosion than the lavas and pyroclastic rocks of a volcanic cone.

Vulcan: Roman god of fire and the forge, after whom volcanoes are named.

 Vulcanian: A type of eruption consisting of the explosive ejection of incandescent fragments of new viscous lava, usually on the form of blocks.

Glossary compiled from:

Audubon Society, The Once and Future Mountain (July, 1980)

Bullard, Fred M., Volcanoes of the Earth (London: University of Texas Press, 1976)

Decker and Decker, Volcanoes (W.H. Freeman and company, 1980)

Foxworthy and Hill, Volcanic Eruptions of Mount St. Helens: The First 100 Days (U.S. Geological Survey)

Korosec, The 1980 Eruption of Mount St. Helens (Washington State Department of Natural        Resources) MacDonald, Volcanoes (Prentice-Hall)

Tilling, Eruptions of Mount St. Helens: Past, Present and Future (U.S. Geological Survey)

 Takahashi, T.J., and Griggs, J.D., Hawaiian Volcanic Features: A Photoglossary (U.S.             Geological Survey Professional Paper 1350, v. 2, 1987).

Links   Famous Volcanoes    Info    Projects   Pictures   Main    Volcanic Eruptions