Glossary of Volcanic Terms

G. J. Hudak, University of Wisconsin Oshkosh, 2001


AA lava:  A Hawaiian term for lava that has a rough, jagged, spiny, and often clinkery surface.  In thick aa flows, the surface comprises rubble composed of loose, rough lapilli and blocks that generally hides a thick, more massive flow interior (Tilling et al., 1987).  The thickness of the surface crust of aa lavas is controlled by cooling (Kilburn, 2000, p. 291).

Active volcano: A volcano that is currently erupting, one that has erupted during recorded history, or one that has erupted during recorded history and is likely to erupt again (Foxworthy and Hill, 1982). 

Accessory fragment:    A lithic fragment composed of country rock that has been explosively ejected during an eruption (Cas and Wright, 1987, p. 54).  Accessory fragments within pyroclastic deposits may be difficult to distinguish from accidental fragments.  In general terms, referred to as a xenolith.

Accidental fragment:  A clast picked up locally by pyroclastic flows and surges (Cas and Wright, 1987, p. 54).  Accidental fragments may be difficult to distinguish from accessory fragments.  In general terms, referred to as a xenolith.

Accretionary lapilli: Spherical aggregates (commonly with a concentric structure) formed by the accretion of moist ash in eruption clouds (White and Houghton, 2000, p. 495).  Also used for all ash aggregates, including mud lumps (Houghton et al., 2000, p. 513).

Achnelith:  A type of juvenile fragment characterized by smooth, glassy molded surfaces formed from lava spray from extremely fluid mafic eruptions (Walker and Croasdale, 1972).

Agglomerate:  A course, pyroclastic deposit composed of a large proportion of fluidal-shaped volcanic bombs that are formed, in the strictest sense, by a fall deposit in the immediate vicinity of a volcanic vent. It is best applied to describe bomb and scoria deposits that build strombolian cones, and should never be used as a non-generic term for a “volcanic breccia” (Cas and Wright, 1987, p. 359).

Aerosol:  Fine liquid or solid particles suspended in the atmosphere.  Aerosols composed of tiny droplets of sulfuric acid are commonly formed during explosive volcanic eruptions.

Airfall:  Volcanic ash that has fallen through the air from an eruption cloud.  Airfall deposits are characteristically well-sorted and well-layered, and typically exhibit mantle bedding (Foxworthy and Hill, 1982; Cas and Wright, 1987).

Alteration mineral assemblages:  Mineral assemblages found in rocks that result from chemical reactions between the original rock and an agent of alteration (for example, hot volcanic vapors or hydrothermal fluids).

Amygdaloidal: A volcanic texture comprising vesicles (rounded holes resulting when magma cools around gas bubbles) which have been subsequently filled by secondary minerals.

Amygdule: An individual vesicle which has been subsequently filled-in by secondary minerals.

Andesite:   A grey to grey-green colored volcanic rock containing 53% to 63% silica (compositionally between basalt and dacite).  Minerals commonly found in andesite include intermediate composition plagioclase and hornblende.

Andesite magma:   A magma with a chemical composition ranging from 53% to 63% which, upon crystallization, forms an andesite.

Armoured lapilli:  A type of accretionary lapilli composed of a crystal, pumice, or lithic fragment core which is surrounded by a rim of fine to coarse ash (McPhie et al., 1993, p. 29).

Ash: A textural term for volcanic fragments less than 2mm in diameter (Fisher, 1966; Schmid, 1981).  Ash is the typical product of explosive volcanic eruptions.

Ash cloud:  A cloud of ash produced during pyroclastic eruptions (Miller, 1989).  These clouds can result from rapid rising of the hot, buoyant ash-rich eruptive plume, or can be derived by elutriation at the top of a pyroclastic flow (Cas and Wright, 1987).

Ash flow:  A type of pyroclastic flow comprising dominantly ash-sized particles. Hot ash flows may be called “glowing avalanches” or “nuee ardentes”, and if their volume is large enough, may eventually form deposits known as welded tuffs.  These types of flows are extremely dangerous and historically have killed hundreds of thousands of people.

Atmospheric shock wave:  A strong compressional shock wave caused by a combination of volcanic ejecta and sonic waves.

Avalanche:  A large mass of material or mixtures of materials (e.g snow, ice, rock, soil, etc.) that is falling or sliding rapidly due to the force of gravity.  Debris avalanches are avalanches composed of a mixture of earth materials (Foxworthy and Hill, 1982).

Ballistic fragment:  An explosively ejected rock fragment that follows a ballistic (arced) trajectory.

Basalt:  A dark colored (usually dark grey, dark green, or black), low silica content (45% to 53% SiO2) volcanic rock.  Minerals commonly found in basalt include intermediate to calcium-rich plagioclase, pyroxene, and commonly olivine.  Accessory minerals commonly include ilmenite and magnetite.

Basaltic magma:  A low viscosity, low silica (45% to 53% silica) magma that, upon crystallization, forms the volcanic rock basalt.

Base surge:  A turbulent, low-density cloud of rock debris, water, and/or steam that moves over the ground surface at extremely high speeds.  Base surges are commonly the result of directed volcanic explosions.  Base surge deposits are commonly composed of cross-bedded deposits comprising ash and lapilli.

Bimodal:  A term used to describe a material composed of two distinctly compositionally and/or texturally different components.  Commonly used to describe volcanic terrains that have nearly equal proportions of felsic and mafic volcanic rocks.

Blocks:  Fragments of solid rock greater than 64 millimeters in diameter that are ejected during volcanic eruptions.  Blocks are commonly composed of accessory fragments made up of crystallized magma associated with the eruption (e.g. pieces of a lava dome).

Blocky lava:  Lava flows that are characterized by highly fractured surfaces which contain fragments of debris (usually flow fragments) up to several meters in diameter.  The size of the surface fragments in blocky lavas is controlled by the rheology of the lava in the interior of the flow (Kilburn, 2000, p. 291).

Boiling lake:  A lake which has a temperature of nearly 100°C.  Examples include the “Boiling Lake” on Dominica and a lake of mud on Saint Lucia (Bardintzeff and McBirney, 2000, p. 159).

Bombs:  Juvenile fragments of semi-solid or plastic magma ejected during a volcanic eruption.  Based on their shapes after they hit the ground and cool, bombs are given various textural names including breadcrust bombs, cow-dung (cow pie) bombs, spindle bombs (fusiform bombs) and ribbon bombs.

Caldera:  Large, circular to elongate, volcanic collapse depressions that form from the rapid extrusion of magma form a shallow subterranean magma chamber.  In general, the diameter of a caldera is much greater than any of its individual volcanic vents (Williams and McBirney, 1979, p. 207).

Caldera cycle:  A commonly observed evolutionary sequence recognized in many caldera complexes.  From oldest to youngest, the seven stages of the caldera cycle are: 1) regional tumescence and generation of ring fractures; 2) ignimbrite (pyroclastic) eruption(s); 3) caldera collapse; 4) pre-resurgent volcanism and intra-caldera sedimentation; 5) resurgent doming; 6) major ring fracture volcanism; and 7) terminal fumarolic and/or hot spring activity.

Cinders:  A term to describe generally highly vesicular, mafic lava lapilli.

Cinder cone:  A small, generally conical-shaped volcano formed by accumulation of ejected cinders and other volcanic debris that falls back to the earth close (proximal) to the location of the volcanic vent (Gardner et al., 1995).

Clay (minerals):  A group of aluminum-bearing hydrous phyllosilicate minerals (for example, kaolinite).

Clay (textural):  A sedimentary grain size classification for particles less than 1/256 mm in diameter, regardless of mineralogy.

Cognate lithic fragment:  Non-vesiculated juvenile magmatic fragments that have silicified from the erupting magma (Cas and Wright, 1987, p. 54).

Columnar jointing:  A type of fracture pattern resulting from the thermal contraction of hot volcanic rocks after their crystallization which commonly is expressed in elongate, pentagonal or hexagonal columns oriented perpendicular to the cooling surface.  Columnar jointing is common in all compositions of lava flows, although it is generally best developed in mafic (basalt) lava flows and in felsic welded tuffs.

Composite volcano:  A generally steep sided volcano composed of a mixture of lava flows, pyroclastic deposits, and volcaniclastic sedimentary deposits.  Composite volcanoes commonly have increasing slopes toward their summits since they generally have mainly lava flows and sedimentary deposits near their base and pyroclastic (tephra) deposits near their summits.

Conduit:   The underground passage or passages through which magma makes it way to the earth’s surface.

Cooling unit:  A group of hot pyroclastic deposits (ignimbrites) that cools at more or less the same time.  A deposit from a single eruption that shows simple variations in the degree of welding is known as a simple cooling unit.  When many ignimbrites occur over an extremely short period of time, each individual ignimbrite may be deposited, and start to weld over a previous deposit or group of deposits that are cooling and undergoing welding.  The resulting deposits have several zones of partial and dense welding, and since they more or less cool together, are known as compound cooling units (Cas and Wright, 1987, p. 253-255).

Coulée:  A type of rhyolite lava flow that forms when lava issues from one side of a volcanic vent and produces a lava flow which is elongate in plan view (Cas and Wright, 1987, p. 81).

Crater:  A steep sided, usually bowl or funnel shaped depression that commonly occurs at the top of a volcanic cone, and is often a vent for eruptions (Lipman, 2000, p. 643). Volcanic craters may be formed by either explosion or collapse in the vicinityof the volcanic vent.

Curie point:  The temperature at which a body loses (by heating) or preserves (by cooling) its permanent magnetization.  As rocks cool, the electromagnetic field aligns magnetic minerals in the magma, and their orientation is preserved as the rocks cool below the Curie point.

Dacite:  A generally light-colored, relatively silica rich (65% to 68 % SiO2) volcanic rock (extrusive equivalent of a quartz diorite or a tonalite).  Dacitic magmas have a relatively high viscosity, and their associated volcanic eruptions may produce thick, muffin-shaped lava flows (lava domes) or, commonly, may be explosive and produce abundant tephra resulting in ash falls, ash flows, and surges.   Dacites typically contain intermediate plagioclase (andesine or oligoclase) and quartz (>10%) with pyroxene and/or hornblende with minor biotite and/or sanadine (volcanic K-feldspar).

Debris flow:  A type of mass flow comprising a dense, cohesive, flowing mixture of sediment (mud through boulder sized materials, generally >50% by volume), water, and commonly, organic debris.  Debris flows generally move downslope in laminar fashion due to the force of gravity (Vallance, 2000, p. 601; Carey, 2000, p. 627). Debris flows generated at volcanoes are commonly referred to as lahars.

Decompressive melting:  Melting that occurs when rocks undergo a decrease in pressure.  This commonly occurs in the vicinity of hot spots as mantle rocks rise to shallower levels in the earth due to convective rise and upwelling (Sigurdsson, 2000, p. 15). Melting occurs as a result of decreasing pressure, not increasing temperature.

Deposit:  Earth materials that have accumulated by some natural process (Gardner et al., 1995). Deposits may be the result of volcanic (e.g. lavas or pyroclastic), sedimentary (either clastic or chemical), or hydrothermal (precipitation) processes.

Devitrification:   The solid-state transformation of volcanic glass into crystalline materials (AGI, 1976, p. 117).  Devitrification tends to be more prevalent in densely-welded tuffs, but may also occur in less densely-welded or unwelded pyroclastic and/or volcaniclastic deposits.  The main products of devitrification are cristobalite (SiO2) and alkali feldspar (KAlSi3O8) (Cas and Wright, 1987, p. 258).

Diatreme:  A funnel-shaped, pipe-like volcanic conduit, usually filled with volcaniclastic debris, emplaced by the explosive energy of gas-charged magmas.  Diatremes are believed to result from hydrovolcanic fragmentation and subsequent wall rock collapse (Vespermann and Schminke, 2000, p. 683), and may reach depths up to 2500 meters.  Diamond-bearing diatremes are economically important and are referred to as kimberlite pipes. 

Dike:  A discordant, sheetlike body igneous body formed from the injection of magma into a fracture within the brittle crust of the earth (Carrigan, 2000, p. 219: Marsh, 2000, p. 191).  Generally a tabular igneous body which cross-cuts the planar structures in the adjacent rocks.

Directed blast:  A hot, low density mixture of gas, rock debris, and ash that is propelled by a volcanic eruption and generally moves along the ground at high speeds (Miller, 1989).

Dome:  A steep-sided mass of lava that is generally formed immediately above the volcanic vent from which it was extruded.  Domes are generally circular in plan and have a relatively small surface area relative to other types of lava flows.  Domes may be spiny, rounded, or flat on top, and often have rough, blocky surfaces formed by the fragmentation of the dome’s crust during intrusion.  Domes may grow by extrusion of lava onto the outer surface of a previously formed dome (exogenous dome) or may be formed by inflation of a pre-existing dome (endogenous dome).  Domes are most commonly the result of extrusion of viscous lava (primarily of the composition of rhyolite and dacite, but andesite may occur as well).

Dormant volcano:  A volcano that is not currently erupting, but is thought to be likely to erupt in the future.

Downsag caldera:  A type of caldera characterized by inward sloping topography, inward tilted wall rocks, and an apparent absence of large displacement caldera bounding faults (Lipman, 1997).  Downsag calderas are believed to result from small volume eruption from a deep-seated subvolcanic intrusion.

Epithermal mineralization:   A mineral deposit formed from relatively low temperature (generally <350° C) hydrothermal solutions at shallow levels (<2km) in the earth’s crust.  Epithermal mineralization is a common feature on many volcanoes.

Eruption:  The expulsion of volcanic materials (magma, volcanic gases) from a vent or fissure at the earth’s surface.  In a general sense, eruptions are considered to be relatively large explosions which result in the expulsion of volcanic materials at or onto the earth’s surface.

Extinct volcano:  A volcano that is not presently erupting and is unlikely to do so in the future (Foxworthy and Hill, 1982).

Facies:  A part of a rock body that can be differentiated from another part of a related rock body by textural or compositional variations.  The general appearance or composition of one part of a rock body as contrasted with other parts (AGI, 1976, p. 155).

Facies changes:  The textural and compositional changes that occur laterally and/or vertically within related rock bodies.

Fire fountain:  A spray of lava emitted from a vent or a fissure composed of a highly fluid mixture of basaltic magma and gas (Vespermann and Schminke, 2000, p. 683: Spudis, 2000, p. 697).  Deposits from fire fountains produce mantling deposits composed of dense, plastic juvenile fragments and ash known as “agglomerates”.

Flow banding:  A foliation commonly observed in intermediate and felsic lavas, that results from shearing of the lava during laminar flow (Cas and Wright, 1987, p. 78).  In rhyolite flows, flow banding is commonly exhibited by alternating bands comprising volcanic glass and spherulites (small, radiating bodies of devitrified glass).

Fuel-coolant interaction:  The interaction of magma (fuel) with external water (coolant) that may result in thermal explosions (Vespermann and Schminke, 2000, p. 683).

Fumarole:  A vent which releases volcanic gases.  These include steam (H2O), carbon dioxide (CO2), sulfur dioxide (SO2), hydrogen sulfide (H2S), as well as other volatile gases emitted from subterranean magmas.

Fumarolic activity:  Volcanic gas emissions, with or without an accompanying change in the temperature or compositions of the gasses/fluids emitted (USGS Glossary of Volcano and Related Terminology).

Geyser:  A special type of hot spring characterized by intermittent discharged of water and volcanic gases brought about by expansion of a vapor phase (generally steam) in the subsurface.

Graben:  An elongate crustal block that has moved downward relative to bounding fault systems (Foxworthy and Hill, 1982).

Heterolithic:  A clastic (volcaniclstic) deposit containing of a variety of different types of rock fragments.

Hot spot:  An area, generally located in the middle of a lithospheric plate, characterized by anomalous heat flow.  Mantle material rises toward the earth’s surface and undergoes decompressive melting at hot spots which may form volcanoes (as in Hawaii) or cause partial melting of the overlying crust which leads to the formation of volcanoes (e.g. Yellowstone region).

Hot spring:  A thermal spring containing water at a higher temperature than the human body (98°F/37°C)

Hydrothermal:  Pertains to hot water or the action of hot water which has been heated by or in association with magma (Gardner et al., 1995).

Hydrothermal alteration:  Changes in rocks or minerals brought about by metasomatism with hydrothermal fluids (generally hot water).

Hydrothermally altered:  Minerals or rocks that have undergone hydrothermal alteration.

Hydrothermal system:  The system comprising the rocks, fluids, vapors, and conduits associated with hydrothermal activity.  In general, hydrothermal systems have the following components: 1) a shallow magma chamber or cooling intrusion (provides the heat for the system); 2) fluids which can be of magmatic, meteoric, or connate origin, that are heated by the intrusion and flow through the rocks adjacent to (or sometimes within) the heat source; 3) fractures or high permeability zones which allow transfer of fluids from one part of the system to another part of the system. In most cases, this transfer is believed to be the result of buoyancy contrasts between the colder and warmer fluids within the system.

Hydrovolcanic eruptions:  A general term for eruptions caused by the mixing of magma with water (Vespermann and Schminke, 2000, p. 683).  Encompasses hydroclastic, hydromagmatic, and phreatomagmatic eruptions.

Hyaloclastite:  A deposit comprising small, angular glass fragments formed by nonexplosive shattering of lava or magma flowing into water, ice, or water-saturated sediment (Batiza and White, 2000, p. 361: Schmidt and Schmincke, 2000, p. 383). 

Igneous:  Refers to the processes associated with magma, or the rocks formed via the solidification of magma.

Igneous rock:  A variety of rock formed via crystallization from a magma.  The two major classes of igneous rocks are volcanic (crystallized at or near the earth’s surface, for example, basalt) and plutonic (crystallized at depth within the earth, for example, gabbro).

Ignimbrite:  A term used for pyroclastic flow deposits, that is synonymous with “ash tuff” (Lipman, 2000, p. 643).  According to Cas and Wright (1987, p. 98), the term should only be used to describe pumiceous pyroclastic flow deposits.

Island arc:  A curved chain of islands, generally convex towards the open ocean, which is bounded on its convex side by a deep oceanic trench (typically a subduction zone) and generally a deep sea basin (AGI, 1976, p. 234).

Jokulhlaup:  The Icelandic term for “glacial outburst floods” which are commonly caused by subglacial volcanic eruptions.

Juvenile fragment:  Glassy or partially crystallized fragments which represent samples of an erupting magma.  These include fragments such as pumice, scoria, reticulate, achneliths (Pele’s tears, Pele’s hair), and various types of volcanic bombs (Cas and Wright, 1987, p. 47-53).

Lahar:  The Indonesian term for a debris flow or a mudflow originating on a volcano (Harris, 2000, p. 1301).  Lahars are generally composed of volcanic materials, but can contain significant amounts of non-volcanic materials derived from erosion during flow.  Most volcanologists prefer this term to be used for the process and not the sedimentary deposits that it forms, but unfortunately, this distinction has been largely ignored in the geological literature.  Many lahars are composed of sand and coarser materials, and thus, can be distinguished from “mudflows” which predominantly contain silt- or clay-sized grains (Rodolfo, 2000, p. 973). 

Landslide:  A general term for relatively dry, gravity-induced movements of rock, sediment and/or soils (commonly with associated organic debris and/or human-made construction materials (e.g. houses, buildings, roads, etc.)) that are perceptible to the human eye.

Lapilli:  A textural term for fragments in volcanic rocks and volcanic deposits that range from 2mm to 64mm in diameter (Fisher, 1966; Schmid, 1981).

Lateral blast:  A volcanic eruption which is directed horizontally instead of vertically. Lateral blasts may be caused by sudden decompression of a shallow magma chamber residing within the flanks of a volcano (for example, the 1980 eruption of Mt. St. Helens), or along the base or side of a lava dome (for example, the 1902 eruption of Mt. Pelee in Martinique)(Nakada, 2000, p. 945).

Lava:  The term used for magma that has been erupted on to a planet’s surface. 

Lava flow:  An outpouring of lava from a vent or fissure that spreads along the ground surface, as well as the crystallized rock resulting from solidification of the outpouring (Peterson and Tilling, 2000, p. 957). 

Lava lake:  A region typically within the summit of a shield volcano which contains partially crystallized or molten lava which lies immediately above a volcanic conduit which joins the lava lake to the magma chamber.  Strong magma convection within volcanic conduits sustains lava lakes within their respective volcanic vents (Walker, 2000, p. 285).

Lava tube:  A hollow region, commonly found within crystallized pahoehoe lava flows, which was filled with hot, flowing lava during a volcanic eruption.  Lava tubes are formed when the top surface of a channelized lava flow crystallizes, and the magma flowing in the interior of the lava flow drains during and/or immediately following a volcanic eruption.

Lithophysae:  Radial aggregates of fibrous crystals which have formed around an expanding vesicle in a melt which is capable of flowing (Cas and Wright, 1987, p. 84).  Lithophysae are commonly the result of vapor-phase crystallization within a rhyolitic magma.  They should not be confused with spherulites, which are similar-shaped structures formed from devitrification of volcanic glass.

Lithic:  Fragments of previously-formed rocks or dense fragments that occur within volcaniclastic deposits.  Lithic fragments may be accessory fragments, accidental fragments, or juvenile fragments.

Lithospheric plates:  The series of rigid slabs that comprise the earth’s lithosphere (crust and upper mantle.  This term is synonomous with tectonic plates.

Littoral:  An adjective describing physical features or processes associated with shorelines of oceans, seas, or lakes (Peterson and Tilling, 2000, p. 957).

Lobate lava:  A submarine lava comprising elongate, flattish lobes with smooth, outer glassy skins (Batiza and White, 2000, p. 361).

Maar:  A type of monogenetic volcano, generally formed by subterranean phreatic or phreatomagmatic eruptions that occur as magma explosively interacts with ground water or subsurface moisture. Maar craters are cut into the surrounding country rock, vary from 10-500 meters deep, and range from a few hundred meters to 3 km in diameter.  Maar volcanoes are generally surrounded by low, shallowly outward-dipping beds of well-bedded volcanic ejecta that rapidly decrease in thickness away from the vent.  The volcanic deposits are mainly emplaced by base surges and fallout, and commonly contain very little (or in the case of phreatic eruptions, no) juvenile volcanic materials (Vespermann and Schminke, 2000, p. 685: Cas and Wright, 1987, p. 376-377).

Mafic:  A compositional term for igneous rocks which contain 45%-55% SiO2 (by weight).  Mafic rocks are generally dark colored, and are characterized by mineralogy including pyroxene and calcium-rich plagioclase, variable amounts of olivine, and accessory minerals such as ilmenite and magnetite.  Examples of mafic rocks include basalt and gabbro.

Mafic lava:  A lava with a silica content (by weight) ranging from 45-55% (AGI, 1976, p. 447; Peterson and Tilling, 2000, p. 957).

Magma:  A term used to describe subsurface molten rock (Jeanloz, 2000, p. 41).  Magmas are generally considered to be silicate melts (Grove, 2000, p. 133; Wallace and Anderson, 2000, p. 149), but may also be composed of carbonatitic liquids (Spera, 2000, p. 171).  Magmas are composed of up to three components (liquid, crystalline solids, and gas (or supercritical fluid) bubbles; Grove, 2000, p. 133), and may be fully liquid or partially crystalline.  Lavas are magmas that have erupted on to a planet’s surface.

Magma chamber:  A subterranean region composed of magma that may have a conduit or set of conduits leading to a volcanic vent or vents on a planet’s surface.

Magnetic polarity:  The direction of the magnetic poles (either normal or reversed) that is preserved in igneous rocks  after they cool below their Curie temperature (USGS Glossary of Volcano and Related Terminology)

Magnitude:  A numerical measure of the size of an earthquake based on the amount of seismic energy released.  The magnitude of an earthquake is determined by measuring the highest-amplitude waves and correcting for distance and the type of seismometer used (McNutt, 2000, p. 1015).  The seismic magnitude scale is logarithmic, with each increase in one unit on the scale equivalent to a tenfold increase in the wave amplitude.

Mantle:  The part of the earth’s interior lying above the outer core and below the Mohorovicic discontinuity.  The mantle is commonly divided into three parts: the upper mantle (depths down to ~400 km), the transition zone (~400-670 km depth), and the lower mantle (~670-2900 km depth).

Mantle bedding:  Pyroclastic deposits generated by ash fall which maintain a uniform thickness and drape over all but the steepest topography (Cas and Wright, 1987, p. 96).

Mantle plume:  An elliptical, drop-shaped mass of mantle that ascends toward the earth’s crust due to its relatively lower density relative to the adjacent mantle.  The density contrast is commonly the result of higher heat content of the plume, but may also be the result of chemical anomalies within the mantle (Perfit and Davidson, 2000, p. 89: Sigurdsson, 2000, p. 271).  Mantle plumes are associated with intraplate rifting and volcanism.  Mantle plumes are the hypothetical cause of hot spots (Hooper, 2000, p. 345).

Megabreccia:  Coarse, heterolithic breccia deposits formed during caldera collapse, which contain fragments which are generally greater than one meter in diameter (Lipman, 1976).  Megabreccia fragments may be so large that individual fragments may not be readily recognizable on the scale of an outcrop.

Mesa lava:  Generally rhyolitic in composition, a lava flow with an approximately circular plan which forms a biscuit-shaped body (Cas and Wright, 1987, p. 81).

Mesobreccia:  Heterolithic breccia deposits formed during caldera collapse which contain fragments that are generally less than 1 meter in diameter (Lipman, 1976).

Metamorphic rock:  In the strictest sense, rocks that have formed in the solid state in response to pronounced changes in temperature and/or pressure without any change in the bulk chemical composition of the rock.  Metamorphic processes are generally confined to regions within the earth below the zones of weathering, cementation, and diagenesis.

Metamorphism:  In the stricktest sense (isochemical metamorphism), the process by which consolidated rocks undergo textural and mineralogical changes brought about by changes in temperature and/or pressure.  The textural and/or mineralogical changes associated with metamorphism are thermodynamic responses to the physical conditions present in the metamorphic environment.  In general, increasing metamorphism results in dehydration of the rocks, as well as an increase in the grain size of the rocks.

Metasomatism:  A type of metamorphism characterized by the exchange of chemical species between rocks and their associated altering fluids and/or vapors.

Moat sediments:  A general term for sedimentary deposits that occur between the tographic walls and the resurgent central cores of the calderas.  In felsic caldera systems, moat sediments are commonly intruded by, and associated with, lava domes.

Monogenetic volcano:  A volcano that erupts only once (Walker, 2000, p. 283).

Monolithic:  A type of volcaniclastic deposit in which all the clasts present are of the same composition.

Moraine:  A topographic feature or landform composed of an accumulation of sediment that has been carried and subsequently deposited by a glacier.

Mudflow:  A flowing mixture composed of water and mud (clay- and silt-sized sediments).  The term should be used exclusively for mud-dominated mass flows, and should not be used as a substitute for the term “lahar”(Rodolfo, 2000, p. 973-974).  Mudflows are common in both volcanic and non-volcanic environments.

Nested caldera:  A type of caldera which is found within a larger, older caldera structure.

Nueés ardente:  The term used for a “glowing avalanche” resulting from a small-volume block and ash flow produced by the collapse of an actively growing lava dome (LaCroix, 1904).  In recent years, the term has unfortunately been more widely used as a synonym for “ignimbrite”.  Its use should be restricted to the original definition of LaCroix (Cas and Wright, 1987, p. 225).

Orogeny:  A term which describes the process of forming mountains, particularly by folding and thrusting (AGI, 1976, p. 308).

Outwash:  Sediments deposited by glacial meltwater beyond the active glacial ice.  Outwash sediments are commonly characterized by poorly bedded gravels interlayered with well-bedded (and commonly cross-bedded) sands.

Pahoehoe lava:  A Hawaiian term to describe lava flows with smooth, continuous surfaces (Kilburn, 2000, p. 291). Pahoehoe flows may have a variety of surfaces described as smooth, ropy (characterized by rope-like, commonly braided flow folds on the lava flow’s surface) , or shelly (vesicular and cavernous; Cas and Wright, 1987, p. 66-67). Pahoehoe toes and lobes form when largely degassed mafic magma issues from tubes relatively far from the erupting vent.

Peicemeal caldera:  A type of caldera characterized by an internal structure composed of several individual fault-bounded blocks (Lipman, 1997).  Piecemeal calderas may result from non-uniform subsidence of a caldera formed from a single eruption, or may be the result of subsidence following a series of large eruptions (multicyclic; Lipman, 1997; Lipman, 2000, p. 655-656).

Pele’s hair:  A type of achnelith composed of thin, hair-like strands of volcanic glass.  These thin, cylindrical strands of volcanic glass are commonly golden in color, have diameters between 1-500mm in diameter, and may be up to 1 meter in length.  They are formed from stretched magma droplets emitted into the atmosphere during fire fountaining and strombolian eruptions (Vergniolle and Mangan, 2000, p. 447).

Pele’s tears:  A type of achnelith composed of small droplets of shiny black volcanic glass that have been ballistically molded and quenched during flight into spherical, dumbbell, or tadpole shapes.  These droplets generally range from a few millimeters to a few centimeters in size, are generally dense, but locally may be quite vesicular (Vergniolle and Mangan, 2000, p. 447).

Pele:  The mythological Polenesian goddess of volcanoes.  In Hawaii, this temperamental goddess makes her home in Kilauea’s fiery vent, Halemaumau (Sigurdsson and Lopes-Gautier, 2000, p. 1297).

Pelean eruption:  A type of volcanic eruption characterized by a ground hugging glowing avalanche (pyroclastic flow) resulting from a mixture of hot volcanic gases, ash, and incandescent lava fragments. Pelean eruptions may occur when pyroclasts are blown out of a central volcanic vent and then collapse onto the earth’s surface to form a pyroclastic flow (Tilling, 1985).  Pelean eruptions may also occur as a result of the explosive disintegration of a lava dome (as was the case for the lava dome on Mt. Pelee, Martinique in 1902).

Peperite:  A genetic term for a rock formed by in-situ disintigration and mixing of molten magma or lava with wet, poorly consolidated sediment (Batiza and White, 2000, p. 361).  A breccia-like deposit formed from the extrusive or intrusive mixture of lava or magma with wet sediment (Schmidt and Schminke, 2000, p. 383).

Perlite:  Hydrated obsidian, generally light grey in color, that is commonly characterized by rounded, onion-skin-like fractures (perlitic cracks).  Apache’s tears are unhydrated clumps of fresh obsidian that are commonly found within regions containing perlite.

Phreatic eruption:   A steam eruption, commonly associated with water, mud, and other earth materials, that is caused when groundwater, heated by a magma, flashes (and explosively expands) into steam (Harris, 2000, p. 1301).  Phreatic eruptions expel no juvenile (magmatic) material, and are commonly the precursor to magmatic eruptive activity.

Phreatomagmatic eruption:  A type of explosive volcanic eruption that occurs when water (groundwater or surface water) comes in contact with hot magma.  The quenching of the magma by the water causes the magma to violently fragment into juvenile (cognate) particles that are bounded by fracture surfaces and by rounded walls of broken vesicles.  Due to the moisture present, accretionary lapilli are also common in volcanic deposits resulting from phreatomagmatic eruptions (Williams and McBirney, 1979, p. 247-248).

Pillow breccia:  A mixture of coarse, typically glassy fragments and  broken to whole pieces of pillow lava formed from the shattering of pillow lava crusts (Batiza and White, 2000, p. 361).  Pillow breccias commonly form in areas where pillow lavas are not strong enough to maintain their competence along steep submarine slopes or scarps.

Pillow hyaloclastite:  Hyaloclastite deposits immediately surrounding, and intimately associated with, pillow lavas.

Pillow lava:  A type of submarine lava flow consisting of interconnected, elongated lava tubes.  Cross-sections of individual lava tubes resemble pillows with convex upper surfaces and flat or concave lower surfaces (Schmidt and Schminke, 2000, p. 383).  Both radial and concentric cooling fractures may be present along the margins of individual pillows, and these fractures are brought on by thermal contraction during cooling.   Growth of the pillow tubes takes place as the outer, commonly striated outer glassy surface of the pillow tube fractures, and a new tube “buds” from the fracture in a manner similar to the way that toothpaste is squeezed out of a tube.

Pipe-like alteration zone:  A type of narrow, cylindrical or inverted-cone shaped, discordant hydrothermal alteration zone that is typically confined to a narrow region in close proximity to a synvolcanic structure (e. g.  synvolcanic fault).  Pipe–like alteration zones are commonly formed by the highest temperature hydrothermal fluids within a hydrothermal cell (Morton and Franklin, 1987).

Plate (piston)-type caldera:  A type of caldera in which the caldera floor subsides more or less evenly as one coherent block.  Plate-(piston)-type calderas are believed to result from single, large volume pyroclastic eruptions from relatively shallow depth (hypabyssal) magma chambers.

Plinean eruption:  Named for Pliny the Younger (who witnessed the destruction of Pompeii by eruptions from Mt. Vesuvius), a type of violently explosive volcanic eruption that ejects large volumes of tephra high into the atmosphere (Harris, 2000, p. 1301).

Pluton:  A body of rock which has formed beneath the earth from crystallization and consolidation from a magma (AGI, 1976, p. 334).  Plutons may be considered extinct magma chambers (Marsh, 2000, p. 191).  Large plutons (>40 square miles in area) are called “batholiths”.

Ponded flow:  A term used to describe a lava flow that has ponded within a depression or a volcanic vent. A lava lake is a specific type of ponded flow that occurs a volcanic conduit.

Pumice:  Solidified fragments of quenched, highly vesicular (>60%) silicic magma or lava (Cashman et al., 2000, p. 421).  The highly vesicular nature of pumice results from large volumes of gas rapidly expanding within a rapidly cooling magma.  The low density of pumice commonly permits it to float on water for extended periods of time. Hot pumice, however, has been shown experimentally to sink rapidly upon interacting with water (Whitham and Sparks, 1986 ).

Pyroclastic:  Refers to processes resulting from the explosive fragmentation of a magma or lava.  May also be used to describe the deposits formed by explosive volcanic activity and directly deposited by transport processes resulting directly from this activity (Cas and Wright, 1987, p. 8).  Pyroclastic is a Greek term which means “fire-broken” (Harris, 2000, p. 1301).

Pyroclastic fall:  The “rain-out” of pyroclasts through the atmosphere from an eruption jet or eruption plume during an explosive volcanic eruption (Wilson and Houghton, 2000, p. 545; Houghton et al, 2000, p. 555).

Pyroclastic fall deposit:  Volcaniclastic (pyroclastic) deposits formed from the rain-out of clasts through the atmosphere from an eruption jet and/or plume during an explosive eruption (Houghton et al., 2000, p. 555).  Fall deposits typically exhibit mantle bedding, are well sorted, and commonly show well-developed planar stratification (Cas and Wright, 1987, p. 95-96).

Pyroclastic flow:  A dense, hot, dry, high particle concentration mixture of gas and hot rock fragments (ash, pumice, blocks, etc.) that travels along the ground surface, typically at high velocity (generally on the order of hundreds of feet or meters per second; Harris, 2000, p. 1301) away from a volcano.  The high speeds of pyroclastic flows are possible because they flow over a thin layer of hot, commonly expanding and escaping gases.  Most of the material within a pyroclastic flow is contained within concentrated particle dispersion located at the flow’s base (Wilson and Houghton, 2000, p. 545).

Pyroclastic flow deposit:  Pyroclastic (volcaniclastic) deposits that are left by pyroclastic flows (Cas and Wright, 1987, p. 96).  The deposits are usually topographically controlled (infilling valleys and topographic depressions), massive, and poorly sorted.  Depending upon their thickness and heat retention, pyroclastic flow deposits may coalesce into welded tuffs.  Pumice-rich pyroclastic flow deposits are often called “ignimbrites”.

Pyroclastic surge:  A type of turbulent, low density (low particle concentration) pyroclastic cloud or pyroclastic density current.  Being more dilute than pyroclastic flows, surges can sweep over ridges, hills, and other topographic boundaries.  Two kinds of surges are known: wet surges have temperatures <100°C and contain steam that condenses into water droplets that surge along the ground surface with gas and pyroclasts; and dry surges, which have temperatures >100°C, and form by either hydrovolcanic eruptions with low water/magma ratios, or by magmatic eruptions driven solely by expanding magmatic gases (Valentine and Fisher, 2000, p. 571).

Pyroclastic surge deposit:  Pyroclastic deposits that are left by pyroclastic surges.  These deposits mantle topographic features but also generally thicken within topographic depressions.  These deposits are generally well-sorted, and are enriched in crystals and lithic fragments relatively to pyroclastic flow deposits.  Surge deposits commonly exhibit unidirectional sedimentary bedforms, including low angle cross-bedding, dune forms, climbing dune forms, pinch and swell structures, and chute and pool structures (Cas and Wright, 1987, p. 98).

Quenching:  The rapid cooling of magma to form glass (Batiza and White, 2000, p. 361).  Fuel-coolant interactions commonly lead to quenching.  Abrupt quenching may cause a rapid volume decrease which leads to fragmentation of the glass (cooling-contraction granulation).

Reticulite:  An exceptionally porous type of scoria containing porosities ranging from 95-99%98% (Vergniolle and Mangan, 2000, p. 447; McPhie et al., 1993, p. 27).  Commonly referred to as “thread-lace” scoria, reticulite is made up of a honeycomb-like network of thin glass fibers.

Rhyolite:  A volcanic rock containing greater than 68% silica (by weight).  Rhyolites are composed primarily of alkali feldspars (sanadine and orthoclase) and quartz (>10% by volume), with lesser amounts of sodic plagioclase (albite, oligoclase), hornblende, or biotite.  Accessory minerals include zircon, apatite, and tourmaline.  Due to their high silica content (and thus high degree of polymerization), rhyolite lava are very viscous and commonly form lava domes, mesa lavas, or coulees. Rhyolitic magmas with high gas contents typically explode violently to form pyroclastic flows, pyroclastic surges, and pyroclastic falls.

Rhyolite magma:   A magma which contains greater than 68% silica by weight.

Rift:  A linear topographic feature formed by crustal extension.  Rift structures associated with volcanism are commonly composed of a graben with a central high region, which is usually the site of active volcanism (for example, along the mid-ocean ridges).

Ring fracture/Ring fault:  The arcuate bounding faults upon which caldera (cauldron) subsidence takes place. Ring fractures (faults) define the structural limits of calderas. Most observed ring faults are nearly vertical or dip steeply inward (toward the center of the caldera), and this is thought to be a result of doming of the caldera structure following its initial formation (Lipman, 2000, p. 649-650).

Ropy Pahoehoe:  A type of pahoehoe lava characterized by flexible crusts that are bent into tight folds as lava flows.  These tight folds form lava surfaces that appear to be made up of a series of braided ropes (Kilburn, 2000, p. 295).

Satellite vent: A secondary vent on a volcano, commonly located on the volcano’s flank.

Scoria:  Solidified fragments of quenched, highly vesicular (>60%) mafic magma or lava (Cashman et al., 2000, p. 421).  The highly vesicular nature of scoria results from rapid cooling of gas-rich lava.

Scoria cone:  Small volcanic landforms formed from focused (single-vent) subaerial strombolian eruptions of basalt or basaltic-andesite magma.  These features have an inverted cone-shaped profile and are generally circular in plan, although elongate scoria cones can be formed from multiple-vent volcanic eruptions (Cas and Wright, 1987, p. 371-372).

Seismic wave:  A term for elastic earth waves formed by either earthquakes or explosions.  Seismic waves include both surface waves as well as body waves. 

Seismicity:  The phenomenon of earth movement or seismic activity.

Seismograph:  A scientific instrument used to detect and record seismic waves.

Semiconformable alteration zone:  A regional zone of hydrothermal alteration typically characterized by a sheet-like or cloud-like geometry.  Semiconformable alteration zones are generally quite extensive in permeable rock units (e.g. tuffs, medium- to coarse-grained clastic sediments and sedimentary rocks), and are generally patchy in less permeable rock units (e.g. lavas, intrusions).  These zones commonly are found along the periphery of “pipe-like” alteration zones, which are generally confined to regions in close proximity to synvolcanic structures (e.g. synvolcanic faults zones) (Morton and Franklin, 1987).

Shelly pahoehoe:  A type of pahoehoe lava characterized by highly vesicular, extremely fragile crusts that form over hollow lava blisters.  The surfaces of these blisters break easily when stepped upon, giving the impression of walking on eggshells (Kilburn, 2000, p. 295).

Shield volcano:  A broad, low-relief volcano constructed by flows of relatively fluid lava (e.g. basalt: Spudis, 2000, p. 698).  Flank slopes on shield volcanoes are typically < 5° (Zimbelman, 2000, p. 771).

Silica:  The chemical compound silicon dioxide, SiO2.

Silicic:  A term used to describe silica-rich volcanic rock or magma (Miller, 1989).  A chemical classification for a type of rock or magma containing >62% SiO2 (Peterson and Tilling, 2000, p. 958) or 63% SiO2 (Cas and Wright, 1987, p. 16) by weight.

Silicic lava:  A lava with a silica content greater than 62% (by weight).  Synonymous with the term “felsic lava” (Peterson and Tilling, 2000, p. 957).

Sinter:  A type of fragile, commonly white or grey rock formed by precipitation of silica from cooling hydrothermal solutions at or near a hydrothermal vent. Precipitation of siliceous sinter (often with associated sulfide minerals and precious metals) commonly occurs in neutral and acid hydrothermal systems under the influence of biogenic agents such as algae and bacteria (Cas and Wright, 1987, p. 316).

Slabby pahoehoe:  A type of pahoehoe lava with a surface composed of slabs of broken lava crust that are up to meters across and up to several centimeters thick (Kilburn, 2000, p. 295).

Solfatara:  A type of  steam vent or dry fumarole that is characterized by quiet discharge (<20 m/s), and that precipitates a significant amount of sulfur (Hochstein and Browne, 2000, p. 850-851).

Spatter bomb:  A glassy pyroclast greater than 64mm in diameter that takes on a fluidal shape by the force of ejection (Vergniolle and Mangan, 2000, p. 447).

Spherulite:  Typically rounded, radiating arrays of crystal fibers produced by the high temperature devitrification of volcanic glass.  In felsic rocks, the crystal fibers are generally composed of alkali feldspar and a silica polymorph (either quartz or cristobalite), whereas in mafic rocks the fibers commonly consist of plagioclase and/or pyroxene.  Spherulites typically have diameters of 0.1-2.0 cm, but can be much larger (commonly up to 20 cm).  Isolated spherulites are generally spherical, but adjacent spherulites may impinge upon one another to produce long chains that are often aligned with flow foliation (McPhie et al., 1993, p. 24-25).

Spreading center/Spreading ridge:  Places on the ocean floor characterized by active volcanism and where separation of lithospheric plates takes place.

Stratovolcano:  A generally steep sided volcano composed of alternating layers of lava flows, pyroclastic deposits, and commonly, volcaniclastic sedimentary deposits (Walker, 2000, p. 283).  Stratovolcanoes commonly have increasing slopes toward their summits since they generally have mainly lava flows and sedimentary deposits near their base and pyroclastic (tephra) deposits near their summits.  Also called a “composite volcano”.

Stony rhyolite:  Very finely crystalline rhyolite lava (Cas and Wright, 1987, p. 84).

Strombolian eruption:  Volcanic eruptions of basaltic magma, slightly more violent than Hawaiian eruptions, that produce large amounts of scoria and ash around a central vent to form a cone.  Strombolian eruptions are typically pulsating and have periods of several seconds (Wolf and Sumner, 2000, p. 321).  The deposits consist of lava spatter, vesicular bombs, scoria lapilli, and mafic ash (Vespermann and Schminke, 2000, p. 683).  Named after Stromboli, an Italian volcano.

Subduction zone:  A sloping region at collisional plate boundaries where one tectonic plate overrides another tectonic plate.  In most regions, continental crust overrides oceanic crust which is then consumed in the subduction zone (continental – oceanic plate boundary), but in many areas, oceanic crust may be overridden by another plate of oceanic crust (oceanic – oceanic plate boundaries).  Deep oceanic trenches commonly occur as the surface landform associated with subduction zones.  Melting of the subducting slab commonly produces magma which rises to the earth’s surface to produce volcanic arcs.

Surtseyan eruption:  Hydrovolcanic eruptions dominated by jets of wet tephra (scoria and ash) that result in the formation of tuff cones.  The term “surtseyan” is generally used for volcanoes erupting through seawater.  Named after Surtsey, a volcano which emerged from the sea off the coast of Iceland in 1963 (Vespermann and Schminke, 2000, p. 683).

Synvolcanic:  A term used to describe a process or feature that was active or produced during volcanic activity.

Synvolcanic fault:  A fault or geological structure present or produced at the time of volcanic activity.

Tectonic:  A general term used to describe the forces involved in the deformation of the earth’s crust.  Commonly used to also describe the geological structures or features produced by such deformation.

Tectonic plate:  One of the large segments of the earth’s lithosphere (crust and upper mantle, up to 250km thick) that comprise the earth’s outer shell. At the present time, there are 16 major tectonic plates that “float” on top of the asthenosphere, the plastic layer in the earth’s mantle.

Tephra:  A general term used by volcanologists to describe all fragmental volcanic ejecta produced during explosive volcanic eruptions (Dehn and McNutt, 2000, p. 1271).  This includes ash (<2mm diameter fragments), lapilli (2-64 mm diameter fragments and fragements greater than 64 mm in diameter known as bombs (semi-solid or plastic ejecta) or bombs (solid ejecta) (Tilling et al., 1987).

Thread-lace scoria:  See “reticulite”.

Trap-door caldera:  A type of caldera formed when one part of the caldera floor subsides to a greater depth than the other side of the caldera floor.  In general, trap-door calderas have a partial ring fracture (associated with the side of greatest caldera collapse) and a hinge area (associated with the side of least collapse).  Trap-door calderas may represent either calderas that have undergone incomplete collapse, or calderas formed from eruptions from shallow asymmetrical magma chambers (Lipman, 1997: Lipman, 2000, p. 654.

Tremor:  A continuous vibration of the ground around active volcanoes (Vergniolle and Mangan, 2000, p. 447).  Tremors defined on seismographs may have either a regular sine-wave appearance (harmonic tremor) or an irregular, pulsating appearance (spasmodic tremor) (McNutt, 2000, p. 1015).

Tuff:  A lithified volcaniclastic rock composed primarily of ash, with up to minor volumes of lapilli and/or blocks and bombs (Fisher, 1966).  Originally used as a non-genetic rock name, common use today typically implies (incorrectly) that the tephra comprising the rock was deposited while hot.  Similar deposits that have no indication of being hot while deposited are commonly referred to as “tuffaceous” (McPhie et al., 1993, p. 8). 

Tuff cone:  A type of hydroclastic volcano that is generally higher than (generally >50 m high), and has steeper external flanks (commonly >25°) than tuff rings or maars (Vespermann and Schminke, 2000, p. 684).  Craters within tuff cones are generally higher in elevation than the adjacent land surface.  Tuff cones are made up primarily of juvenile clasts deposited from lateral surges, airfall, and associated volcaniclastic remobilization processes.

Tuff ring:  A type of hydroclastic volcano, generally <50m high, defined by craters with low depth/width ratios that sit at or above the elevation of the adjacent land surface.  The rims around tuff rings are composed of juvenile and accidental clasts and are deposited in beds with dips <25° (Vespermann and Schminke, 2000, p. 684).

Tumescence:  The doming or uprising of a volcano commonly due to inflation of a shallow magma chamber.  Regional tumescence commonly occurs prior to a major pyroclastic eruption, but may also occur following an eruption as less volatile magma is emplaced into the shallow crust (Smith and Bailey, 1968).

Tuya:  A flat-topped, steep-sided volcano that erupted into a lake thawed into a glacier by volcanic heat (Smellie, 2000, p. 403).  Commonly referred to as a “table mountain”.

Unconformity:  A surface of erosion that separates younger strata from older rocks (AGU, 1976, p. 448).

Variolite:  A spherulite-like radiating aggregate composed of feathery, needle-like crystals of plagioclase and pyroxene that occur in mafic volcanic rocks (typically basalt).  Variolites may result from devitrification, but are commonly believed to be formed in subaqueous rocks by quench-induced crystallization (Cas and Wright, 1987, p. 420).

VEI index:  The Volcanic Explosivity Index, which is a measure of the size of an eruption based on its magnitude, intensity and destructive power.  The VEI is measured on an eight-point scale, where “8” is the most destructive and powerful eruption (Cioni et al., 2000. p. 477).

Vent:  A surface opening through which volcanogenic materials are erupted (Davidson and DeSilva, 2000, p. 663).  Typically thought of as a hole in a planet from which volcanic products (magma, ash, etc.) are erupted (Spudis, 2000, p. 697).

Vesicle:  A frozen bubble in a volcanic rock.  Vesicles are formed when magma crystallizes around a gas bubble (Spudis, 2000, p. 697).

Vesicular:  A textural term describing volcanic rocks filled with frozen gas bubbles (vesicles).

Vesicular tuff:  Tuffs containing millimeter to centimeter-sized, irregular to round vesicles which are interpreted to form during trapping of air or vapor in wet ash deposits (Vespermann and Schminke, 2000, p. 683).

Vesuvian eruption:  Commonly used as a synonym for a “Plinean” eruption (e.g. Tilling, 1985), , but also used to describe basaltic eruptions which involve long-sustained gas streaming with little ash being released (as in the 1906 eruption of Vesuvius; Cas and Wright, 1987, p. 130).

Viscosity:  A measurement of the ratio of shear stress to the rate of shear strain in a fluid (Williams and McBirney, 1979, p. 20).  In common language, how easily a fluid flows.  Considered the most important physical property of a magma because it largely determines eruptive style as well as volcano morphology. Magma viscosity generally increases as the silica content of the magma increases (due to silica polymerization) and as the temperature of the magma decreases.  Magma viscosity may also be affected by the presence of trace elements (e.g. Ti) or volatiles (e.g. H2O, CO2, SO2, etc.).  In general, common magmas increase in viscosity in the following order: komatiite, basalt, andesite, dacite, rhyodacite, rhyolite.

Volcaniclastic:  A non-genetic term used to describe any fragmental aggregate of volcanic parentage (Cas and Wright, 1987, p. 8).  Rocks formed by the fragmentation of volcanic materials (either magma or volcanic rocks) irrespective of the method of fragmentation.  Pyroclastic rocks and epiclastic rocks are both considered to be “volcaniclastic”.

Volcanic bomb:  Juvenile fragments of semi-solid or plastic magma ejected during a volcanic eruption.  Based on their shapes after they hit the ground and cool, bombs are given various textural names including breadcrust bombs, cow-dung (cow pie) bombs, spindle bombs (fusiform bombs) and ribbon bombs

Volcanic cycle:  A general term used to describe a period of increased volcanic activity.

Volcanic field:  A region comprising a large number of volcanic edifices.  Volcanic fields are usually associated with basaltic volcanism, and commonly comprise a number of small, monogenetic volcanoes (e.g. cinder cones, maars, tuff cones, tuff rings, small shield volcanoes, lava domes).  Fields may form in linear trends associated with tectonic structures (such as faults), on the flanks of larger composite or shield volcanoes, or within calderas (Connor and Conway, 2000, p. 331).

Volcanic landslide: A landslide that occurs along the flank of a volcano.

Volcano:  A mound, hill or mountain constructed by the extrusion of lava and/or pyroclastic material from beneath the ground (Fisher et al., 1997, p. 43). A vent in the earth’s crust from which molten lava, pyroclastic materials, volcanic gases, etc. issue (AGU, 1976, p. 457). 

Vulcanian eruption:  An explosive volcanic eruption generally expelling less than 1km3 of material, but with an eruption column that may reach heights of up to 10-20km (Nakada, 2000, p. 945).  These eruptions last on the order of seconds to minutes (Morrissey and Mastin, 2000, p. 463).

Welding:  The sintering together of hot, glassy fragments, irrespective of shape and size, by compactional lithostatic load (Cas and Wright, 1987, p. 165). 

Welded tuff:  A hard pyroclastic rock compacted by internal heat and pressure from overlying pyroclastic deposits. 








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