Glossary of Volcanic Terms

A scientist from the Hawaii Volcanoes Observatory monitoring gas emissions at Mauna Loa (2005).

USGS photo.

Introduction

Every professional field has its own specialized language, and the study of volcanoes, volcanic processes, volcanic rocks, and volcanic deposits is no different.

The terms in this glossary are adapted when possible from the glossary of geologic terms for Geologic Resources Inventory (GRI) publications. The glossary of geologic terms is largely based on definitions in the Glossary of Geology, 5th edition.

Some definitions in the GRI glossary were adjusted in order to be specific to volcanic resources. Many volcanic terms that are not in the GRI glossary were added to this one. The definitions in this glossary match how the terminology was used throughout the NPS volcanoes subject site.

Volcano Glossary

Search:

Volcano Glossary

Word	Definition

Word	Definition
a'ā	A lava flow with a rough, jagged, or clinkery surface.
absolute age	The geologic age (in years) of a fossil, rock, feature, or event. The term is now in disfavor as it implies a certainty or exactness that may not be possible by present dating methods. See "numeric age."
active volcano	A volcano that is currently erupting or has erupted during the last 10,000 years. Some volcanoes may have dormant periods longer than 10,000 years, but 10,000 years is a convenient cut-off date for activity and is used by convention.
active margin	A tectonically active plate boundary where lithospheric plates are converging, diverging, or sliding past one another.
agglomerate	A consolidated pyroclastic rock made primarily of bombs. Roughly synonymous with spatter.
agglutinate	A welded pyroclastic deposit characterized by vitric material binding the individual clasts that commonly became fused while hot and viscous. Roughly synonymous with spatter
amphibole	A group of silicate (silicon + oxygen) minerals composed of hydrous calcium and magnesium with the general formula (Ca2Mg5)Si8O22(OH)2.
amygdaloidal	Describes rocks with amygdules.
amygdule	A gas cavity or vesicle in an igneous rock that has become filled with secondary minerals.
andesite	A volcanic rock characteristically medium dark in color and containing approximately 57%–63% silica and moderate amounts of iron and magnesium.
anhedral	A crystal lacking well-developed crystal faces.
aphanitic	Describes the texture of fine-grained igneous rock in which different components are not distinguishable by the unaided eye.
Apocalyptic (VEI)	A level 8 eruption on the Volcanic Explosivity Index, a logarithmic scale for explosive volcanic eruptions ranging from 0 for effusive eruptions to 8 for the largest eruptions based on the volume of magma erupted (magnitude) and the eruption column height of explosive eruptions. Apocalypic eruptions have an ejecta volume of >1,000 km3 (200 mi3) and heights of eruption columns at >25 km (16 mi).
ash	Fine-grained material, less than 2 mm (0.08 in) across, ejected from a volcano.
ash fall	Airborne ash that falls from an eruption cloud, and the resulting deposit.
ash flow	A density current, generally a hot mixture of volcanic gases and tephra that travels across the ground surface; produced by the explosive disintegration of viscous lava in a volcanic center, or from a fissure or group of fissures. The solid materials contained in a typical ash flow are generally unsorted and ordinarily include volcanic dust, pumice, scoria, and blocks in addition to ash.
ash-flow tuff	A tuff deposited by an ash flow.
asthenosphere	Earth's relatively weak layer below the rigid lithosphere where isostatic adjustments take place, magmas may be generated, and seismic waves are strongly attenuated; part of the upper mantle.
basalt	A volcanic rock that is characteristically dark in color (gray to black), contains approximately 53% silica or less, and is rich in iron and magnesium.
basaltic andesite	A volcanic rock that is commonly dark gray to black and contains approximately 53% – 57% silica.
blast	A sudden and especially violent explosion produced during a volcanic eruption.
block	A pyroclast ejected in a solid state; it has a diameter greater than 64 mm (2.5 in).
block lava	Lava with a surface consisting of large, angular blocks, usually with an intermediate composition.
bomb	A pyroclast with a diameter greater than 64 mm (2.5 inches) and that has a rounded or ellipsoidal shape that indicates that it was wholly or partially molten during eruption and flight.
breadcrust bomb	A volcanic bomb with a fractured surface that formed because vesicles continued to expand in its core while it was still molten although its crust had been quickly quenched and solidified after eruption. Most breadcrusting forms after landing.
breakout	A new area or region of a lava flow formed where lava from the molten interior of a previously formed lobe breaks out through its sides or upper margin to create a distinct lava flow advance.
breccia (volcanic)	A coarse-grained, generally unsorted volcanic rock consisting of partially welded angular fragments of ejected material.
brecciated	Converted into, characterized by, or resembling a breccia.
caldera	A collapse feature that formed during a large-volume volcanic eruption when the underlying magma chamber was partially emptied and the ground above it subsided into it
caldera complex	The diverse set of rocks and igneous bodies underneath the surface of a caldera including intrusions, dikes, and volcanic deposits.
Cataclismic (VEI)	A level 4 eruption on the Volcanic Explosivity Index, a logarithmic scale for explosive volcanic eruptions ranging from 0 for effusive eruptions to 8 for the largest eruptions based on the volume of magma erupted (magnitude) and the eruption column height of explosive eruptions. Cataclasmic eruptions have an ejecta volume of 1 km3 (0.2 mi3) and heights of eruption columns at 25 km (16 mi).
Catastrophic (VEI)	A level 3 eruption on the Volcanic Explosivity Index, a logarithmic scale for explosive volcanic eruptions ranging from 0 for effusive eruptions to 8 for the largest eruptions based on the volume of magma erupted (magnitude) and the eruption column height of explosive eruptions. Catastrophic eruptions have an ejecta volume of 0.1 km3 (0.02 mi3) and heights of eruption columns at 15 km (9 mi).
cauldron	An inclusive term for all volcanic subsidence structures regardless of shape or size, depth of erosion, or connection with the surface; the term includes caldron subsidences and collapse calderas.
chronology	The arrangement of events in their proper sequence in time.
cinder	A colloquial term for a small nut-size to fist-size, or larger, piece of red or black highly vesicular lava that cooled in air during flight after eruption from a vent. It is roughly synonymous with scoria.
cinder cone	A simple volcano with the shape of a conical hill that consists of accumulations of ash and cinders around a vent.
coignimbrite	Fallout tephra deposited from a pyroclastic flow, composed of near-vent breccias with large lithic clasts, which dropped from pyroclastic flows, and fine-grained ash elutriated (separated based on size) from the top of a pyroclastic flow by the turbulent rise of hot gases.
cold lahar	A lahar that is not associated with a volcanic eruption and is usually initiated by heavy precipitation or snow melt that mobilizes loose volcanic debris.
collapse structure	Any rock structure resulting from the removal of support and consequent collapse by the force of gravity, for example, gravitational sliding on fold limbs, salt solution causing collapse of overlying rocks in salt basins, sinkhole collapse, or collapse into mine workings.
Colossal (VEI)	A level 6 eruption on the Volcanic Explosivity Index, a logarithmic scale for explosive volcanic eruptions ranging from 0 for effusive eruptions to 8 for the largest eruptions based on the volume of magma erupted (magnitude) and the eruption column height of explosive eruptions. Colossal eruptions have an ejecta volume of 100 km3 (20 mi3) and heights of eruption columns at >25 km (16 mi).
columnar joints	Parallel, prismatic columns, polygonal in cross section, in basaltic flows and sometimes in other extrusive and intrusive rocks; form as a result of contraction during cooling.
composite volcano	A volcano made up of both lava flows and pyroclastic deposits that usually experiences multiple eruptions over long periods of time. Sometimes called a "stratovolcano" although the term "composite volcano" is generally preferred.
conduit	The channelway or passage, which may be pipe-shaped, that brings magma from the reservoir or chamber to the vent at the surface where it is erupted.
constructional volcano	A volcano that is a topographic high and was built by eruptions that constructed a volcanic edifice. Cinder cones, composite volcanoes, and shield volcanoes are constructional volcanoes.
continental	Formed on land rather than in the sea. Continental deposits may be of lake, swamp, wind, stream, or volcanic origin.
continental crust	Earth's crust that is rich in silica and aluminum and underlies the continents and the continental shelves; ranges in thickness from about 25 km (15 mi) to more than 70 km (40 mi) under mountain ranges, averaging about 40 km (25 km) thick.
convergent plate boundary	A boundary between two plates that are moving toward each other. Essentially synonymous with “subduction zone” but used in different contexts.
cordillera	An extensive assemblage of more or less parallel chains of mountains (together with their associated valleys, basins, plains, plateaus, rivers, and lakes), especially the main mountain axis of a continent.
cored bomb	A volcanic bomb that has a nucleus of solid material which may be a fragment of previously solidified magma or a xenolith; e.g., foreign rock fragment.
cowdung bomb	A volcanic bomb that was still very fluid so it flattened or splashed upon landing.
crater	A bowl- or funnel-shaped depression that usually lies directly above the vent from which volcanic material is ejected.
crater lake	A volcanic lake found in a crater or caldera, usually formed through the accumulation of rain, snow and ice melt, and groundwater in them.
crust	Earth's outermost layer or shell.
cryptocrystalline	Describes a rock texture in which individual crystals are too small to be recognized or distinguished with an ordinary microscope.
cryptodome	A lava dome intruded just below the Earth's surface
crystalline	The texture of a volcanic rock made of mineral grains.
cumulate	An igneous rock formed by accumulation of crystals that settle out from a magma by the action of gravity.
dacite	A volcanic rock that is characteristically light in color and contains approximately 63% – 68% silica and moderate amounts of sodium and potassium.
debris avalanche	A type of fast-moving landslide that occurs when a sector of a volcanic edi?ce collapses.
debris flow	A moving mass of rock fragments, soil, and mud, with more than half of the particles larger than sand size.
dense rock equivalent (DRE)	A calculation of the volume a volcanic eruption after corrections for for void spaces such as vesicles, commonly presented in cubic kilometers.
devitrification	Conversion of glass to crystalline material.
diabase	An intrusive igneous rock consisting primarily of the minerals labradorite and pyroxene.
diatreme	A breccia-filled volcanic pipe formed by gaseous explosion.
dike	A narrow igneous intrusion that cuts across bedding planes or other geologic structures.
directed blast	A sudden and especially violent explosion that generates a low-density mixture of hot rot debris, ash, and volcanic gases that moves at high speed along the ground surface
divergent plate boundary	A boundary between two plates that are moving apart, characterized by mid-ocean ridges at which sea-floor spreading occurs.
dome	A steep-sided rounded accumulation of highly-viscous silicic lava over a vent. Domes may be part of composite volcanoes, but large ones can make up their own volcanoes. Also termed "lava dome" or "volcanic dome."
dome collapse	The volcanic processes in which a growing lava dome is destructively destabilized by volcanic explositions deither due to instability due to gravity or excess pressure within the dome, producing pyroclastic density currents and sometimes eruption columns.
dormant volcano	A volcano that is not erupting now, but is considered likely to erupt in the future. No precise distinction exists between active and dormant volcanoes. Sometimes dormant volcanoes are described as being potentially active.
driblet	Pyroclastic material ejected from a vent. Synonymous with "spatter."
edifice	The constructional mass of a volcano.
Effusive (VEI)	A level 0 eruption on the Volcanic Explosivity Index, a logarithmic scale for explosive volcanic eruptions ranging from 0 for effusive eruptions to 8 for the largest eruptions based on the volume of magma erupted (magnitude) and the eruption column height of explosive eruptions. Effusive eruptions have an ejecta volume of 0.00001 km3 (0.000002 mi3) and heights of eruption columns at <0.1 km (0.06 mi).
effusive eruption	An eruption dominated by passive emission of lava.
ephemeral lake	A short-lived lake.
equigranular	Said of the texture of a rock having crystals of the same or nearly the same size.
eruption	The expulsion of gases, rock fragments, and/or molten lava from within the Earth through a vent onto the Earth’s surface or into the atmosphere.
eruption cloud	A cloud of volcanic ash, tephra, and gases that forms downwind of an erupting volcano
eruption column	A vertical pillar of superheated volcanic ash, tephra, and gases ejected from a volcanic vent during an explosive eruption. Eruption columns usually spread laterally into eruption clouds higher in the atmosphere
eruptive phase	A period of an eruption that consists of numerous eruptive pulses that generate a pulsating eruptive column or lava flow, and may last from a few hours to days
eruptive pulse	A single explosion that may last a few seconds to minutes.
escarpment	A steep cliff or topographic step resulting from vertical displacement on a fault or as a result of slope movement or erosion. Synonymous with "scarp."
euhedral	A crystal bounded by perfect or well-formed faces; well-formed.
excavational volcano	A volcano with negative relief (e.g., below the general land surface that formed from either violent blasts or from collapse over a magma chamber. Maars, tuff rings, and calderas are excavational volcanoes.
Explosive (VEI)	A level 2 eruption on the Volcanic Explosivity Index, a logarithmic scale for explosive volcanic eruptions ranging from 0 for effusive eruptions to 8 for the largest eruptions based on the volume of magma erupted (magnitude) and the eruption column height of explosive eruptions. Explosive eruptions have an ejecta volume of 0.01 km3 (0.002 mi3) and heights of eruption columns at 5 km (3 mi).
explosive eruption	An eruption dominated by eruption of fragmental (pyroclastic) material.
extension	Deformation of Earth's crust whereby rocks are pulled apart.
extinct volcano	A volcano that is not expected to erupt again. The determination of whether a volcano is extinct may be based on the amount of time since its last eruption. Types of volcanoes such as cinder cones that typically only erupt once are also considered extinct after their eruption ends
extrusive	Describes an igneous rock that has been erupted onto the surface of the Earth. Extrusive rocks include lava flows and pyroclastic material such as volcanic ash.
fall deposits	A a deposit of ash, tephra, and other pryoclastic debris that fell from an eruption cloud to be deposited on the Earth's surface.
fallout	Volcanic ash and/or tephra debris that falls from an eruption cloud to be deposited on the Earth's surface. Also known as ash fall.
fault	A break in rock characterized by displacement of one side relative to the other.
feldspar	A group of abundant silicate (silicon + oxygen) minerals, comprising more than 60% of Earth's crust and occurring in all types of rocks.
fiamme	A flattened structure ignimbrite made of a pumice fragment that has collapsed during welding.
fine-grained	Describes sediment or sedimentary rock and texture in which the individual constituents are too small to distinguish with the unaided eye, specifically sediment or rock whose particles have an average diameter less than 1/16 mm (0.002 in), that is, silt-size particles and smaller. Also, describes a crystalline or glassy rock and texture in which the individual minerals are relatively small, specifically an igneous rock whose particles have an average diameter less than 1 mm (0.04 in).
fire fountain	A spray of molten lava propelled a few tens to hundreds of feet (meters) into the air by rapid expansion of gas bubbles usually occuring during effusive eruptions of basaltic lavas. Also called a "lava fountain."
fissure (volcanic)	An elongated fracture or crack at the surface. Fissures in volcanic areas may be eruptive or non-eruptive.
fissure vent	A volcanic vent that occurs at a fissure.
fissure volcano	A volcano that erupt from an elongated vent(s) (fissures) rather than a central vent
flood basalt	A thick accumulation of lava ?ows erupted within a short period of time and covering a vast geographic area.
flow banding	A type of layering or banding in a lava flow with different layers containing different phenocryst or vesicle content, varying levels of devitrification, or magma composition that forms as a result from friction that occurs during movement of viscous magma. Highly-welded tuffs may also be flow banded if they flow during the welding process.
fracture	The breaking of a mineral other than along planes of cleavage. Also, any break in a rock such as a crack, joint, or fault.
fumarole	A vent or opening at the surface where volcanic gases and vapors are emitted.
geology	The study of Earth, including its origin, history, physical processes, components, and morphology.
geomorphology	The study of the general configuration of surface landforms and their relationships to underlying structures, and of the history of geologic changes as recorded by these surface features.
geothermal	Pertaining to the heat of the interior of the Earth.
glassy	The texture of a volcanic rock that consists of mostly or exclusively of volcanic glass. Synonymous with "vitreous."
glaze	The surface of a lava flow that resembles a fired glass surface or ceramics.
groundmass	The finer grained and/or glassy material between the large crystals of an igneous rock.
Hawaiian	A non-explosive eruption of gusher-like lava fountains ("?re fountains" or "curtains of ?re") that generates red-hot lava rivers of very fluid basaltic lavas. Hawaiian eruptions are typical for shield volcanoes.
historic eruption	A verified eruption that has occurred since 1700 CE (Common Era). The use of 1700 CE as the cutoff between historic and prehistoric eruptions follows its usage as one of the parameters that define historically active volcanoes in Alaska.
holocrystalline	Describes an igneous rock texture composed entirely of crystals.
hornblende	A silicate (silicon + oxygen) mineral of sodium, potassium, calcium, magnesium, iron, and aluminum; commonly black and occurring in distinct crystals or in columnar, fibrous, or granular forms in hand specimens. The most common mineral of the amphibole group.
hornito	A small mound of spatter built on the back of a lava flow, formed by the gradual accumulation of lava clots ejected through an opening in the roof of an underlying lava tube.
hot lahar	A lahar initiated during a volcanic eruption by one or more possible mechanism including melting of snow or ice, liquidification of very wet volcanic landslides, eruptions into crater lakes, or heavy precipitation occuring during an eruption. Also known as a primary lahar.
hot spot	A plume of hot mantle material rising from deep within the Earth over which volcanoes occur.
hot spring	A thermal spring whose temperature is above that of the human body.
hot-spot track	A chain of volcanes formed when a lithospheric plate moves over a mantle plume.
hydrothermal	Of or pertaining to hot water, to the action of hot water, or to the products of this action.
hydrothermal water	Subsurface water whose temperature is high enough to make it geologically or hydrologically significant, whether or not it is hotter than the rock containing it.
hydrovolcanic	A term encompassing all volcanic activity including violently-explosive eruptions driven by steam explosions produced by the interaction of hot magma with surface water or shallow groundwater. Synonym for phreatomagmatic.
hydrothermal alteration	The processes that occur when hot volcanic gases mix with groundwater o form clay minerals, zeolites, and other minerals.
igneous	Describes a rock or mineral that solidified from molten or partly molten material; also, describes processes leading to, related to, or resulting from the formation of such rocks. One of the three main classes or rocks—igneous, metamorphic, and sedimentary.
ignimbrite	A pyroclastic flow deposit.
inflation	Process by which a local area of pāhoehoe lava swells as a result of injection of lava beneath its surface crust.
inflation pit	An unin?ated zone of an inflated lava flow where the solid upper crust and hardened bottom surface were not separated by further injection of liquid lava. Also known as a lava-rise pit.
intermediate	Said of an igneous rock that is transitional between felsic and mafic, generally having a silica content of 54%–65%.
intermediate magma	Describes magma that contains between 62% and 63% silica and is moderately viscous, gas-rich, and sometimes erupts explosively, though it may also produce lava flows.
intrusion	The process of emplacement of magma into preexisting rock. Also, the igneous rock mass formed.
intrusive	Pertaining to intrusion, both the process and the rock body and rock type.
inverted topography	A type of erosional volcanic landscape, predominantly found in arid or semiarid regions, where older volcanic deposits such as lava flows and tuffs stand higher than the general land surface elevation due to differential erosion of more resistent volcanic rocks relative to other rock types so that lava flows and tuffs that once filled valleys are now perched as mesas or highlands.
island arc	An offshore, generally curved belt of volcanoes above a subduction zone.
isotopic age	An age (in years) calculated from the quantitative determination of radioactive elements and their decay products.
isotopic dating	Calculating an age in years for geologic materials by measuring the presence of a short-lived radioactive element (e.g., carbon-14) or by measuring the presence of a long-lived radioactive element plus its decay product (e.g., potassium-40/argon-40). The term applies to all methods of age determination based on nuclear decay of naturally occurring radioactive isotopes.
jameo	A large sink formed by the collapse of the roof of more than one level of a multi-level lava-tube cave.
joint	A break in rock without relative movement of rocks on either side of the fracture surface.
juvenile (volcanology)	Formed directly from magma reaching the surface.
k?puka	An area surrounded by a lava flow.
lahar	A mixture of water and volcanic debris that moves rapidly down the slope of a volcano, characterized by a substantial component (>50%) of fine-grained material that acts as a matrix to give the deposit the strength it needs to carry the bigger clasts.
landslide	A collective term covering a wide variety of slope-movement landforms and processes that involve the downslope transport of soil and rock material en masse under the influence of gravity.
lapilli	Pyroclastic materials ranging between 2 and 64 mm (0.08 and 2.5 in) across with no characteristic shape; may be either solidified or still viscous upon landing. An individual fragment is called a lapillus.
lateral blast	A directed blast with a significant low-angle component.
lateral vent	A vent located on a volcano at a location other than the summit region, typically on the margin or flank of the volcano. Also known as a side vent.
lava	Molten or solidified magma that has been extruded though a vent onto Earth’s surface.
lava blister	A hollow upwelling, usually with a diameter of approximately 1 meter, on the crust of a lava flow formed by expansion of gas or vapor beneath the surface.
lava dome	A steep-sided rounded accumulation of highly-viscous silicic lava over a vent. Domes may be part of composite volcanoes, but large ones can make up their own volcanoes. Also termed "dome" or "volcanic dome."
lava flow	An outpouring of molten rock from a vent onto Earth's surface during an effusive volcanic eruption; also the resulting solidified body of rock.
lava flow field	A body of lava, consisting of one or more lava flows, that is the product of a single eruption.
lava fountain	A spray of molten lava propelled a few tens to hundreds of feet (meters) into the air by rapid expansion of gas bubbles usually occuring during effusive eruptions of basaltic lavas. Also called a "fire fountain."
lava lake	A lake of molten lava, usually basaltic, in a volcanic crater or depression. Also refers to solidified and partly solidified stages.
lava-rise pit	An uninflated zone of an inflated lava flow where the solid upper crust and hardened bottom surface were not separated by further injection of liquid lava. Also known as a inflation pit.
lava tube	A conduit through which lava travels beneath the solidified surface of a lava flow; also, a cavernous segment of the conduit remaining after the flow of lava ceases and the tube drains.
lavacicle	A protrusion of lava that resembles an icicle in form, resulting from the dripping of lava into a lava tube.
laze	Short for "lava haze." A mixture of condensed seawater steam, hydrochloric acid gas, and shards of volcanic glass produced when lava boils seawater to dryness.
levee	A natural banks along the sides of channelized lava flow. A lava levee develop on the chilled margins of a lava flow and may be higher than the surface of the flowing lava flow if flow rate decreases.
lithic	A pyroclast that is a fragment of a preexisting rock of any composition.
lithic tuff	A tuff that is mostly comprised of lithic fragments.
lithification	The conversion of sediment into solid rock.
lithify	To change to stone, or to petrify; especially to consolidate from a loose sediment to solid rock.
lithology	The physical description or classification of a rock or rock unit based on characteristics such as color, mineral composition, and grain size.
lithosphere	Earth's relatively rigid outer shell that consists of the entire crust plus the uppermost mantle. It is broken into about 20 plates, and according to the theory of plate tectonics, movement and interaction of these plates is responsible for most geologic activity.
littoral	Pertaining to the benthic ocean environment or depth zone between high water and low water; also, pertaining to the organisms of that environment. Synonymous with "intertidal."
littoral cone	A mound of hyaloclastic debris constructed by steam explosions at the point where lava enters the sea or other body of water. Littoral cones lack feeding vents connected to subsurface magma supplies.
loess	Windblown silt-sized sediment.
maar	A low-standing pyroclastic cone with a large crater that usually formed from highly-explosive eruptions caused by the interaction of magma with ground or surface waters and a crater that lies below the surrounding ground level.
mafic	Derived from magnesium + ferric (Fe is the chemical symbol for iron) to describe an igneous rock having abundant dark-colored, magnesium- or iron-rich minerals such as biotite, pyroxene, or olivine; also, describes those minerals.
magma	Molten rock beneath Earth's surface capable of intrusion and extrusion.
magma chamber	The reservoir beneath a volcanic vent in the upper part of the crust where magma resides prior to eruption. Also known as a magma reservoir.
magma reservoir	The chamber beneath a volcanic vent in the upper part of the crust where magma resides prior to eruption. Also known as a magma chamber.
magmatic	Of, pertaining to, or related to molten rock material (magma) within the Earth.
magmatic arc	An arcuate line of plutons, volcanic rocks, or active volcanoes formed at a convergent plate boundary.
magmatic gases	Gases or volatiles derived from magma; predominately water vapor, but also including carbon dioxide, sulfur dioxide, carbon dioxide, hydrogen chloride, hydrogen sulfide, and other gases.
magmatism	The development and movement of magma, and its solidification as igneous rock.
mantle	The zone of the Earth below the crust and above the core.
mantle plume	A vertical cylindrical part of Earth's mantle, hotter than its surroundings, within which larger-than-normal amounts of heat are conducted upward to form a hot spot at the Earth's surface.
mass wasting	Dislodgement and downslope transport of a mass of rock and/or unconsolidated material under the direct influence of gravity. In contrast to "erosion," the debris removed is not carried within, on, or under another medium. Synonymous with "slope movement."
massif	A topographic and structural feature, especially in an orogenic belt, commonly formed of rocks more rigid than those of its surroundings; may consist of protruding bodies of basement rocks that consolidated during earlier orogenies, or younger plutonic bodies.
matrix	The fine-grained material between coarse grains in an igneous rock. Roughly synonymous with groundmass.
Mega-colossal (VEI)	A level 7 eruption on the Volcanic Explosivity Index, a logarithmic scale for explosive volcanic eruptions ranging from 0 for effusive eruptions to 8 for the largest eruptions based on the volume of magma erupted (magnitude) and the eruption column height of explosive eruptions. Mega-colossal eruptions have an ejecta volume of 1,000 km3 (200 mi3) and heights of eruption columns at >25 km (16 mi).
metavolcanic	Describes a volcanic rock that shows evidence of metamorphism.
meteoric water	Water of recent atmospheric origin.
mica	A group of abundant silicate (silicon + oxygen) minerals characterized by perfect cleavage, readily splitting into thin sheets. Examples include "biotite" and "muscovite."
micaceous	Consisting of, containing, or pertaining to mica; also, resembling mica, for example, a "micaceous mineral" capable of being easily split into thin sheets.
microcrystalline	Describes a rock texture consisting of crystals visible only with a microscope.
mid-ocean ridge	The continuous, generally submarine and volcanically active mountain range that marks the divergent tectonic margins in Earth's oceans.
mineral	A naturally occurring inorganic crystalline solid with a definite chemical composition or compositional range.
monogenetic	Describes a volcano like a cinder cone, maar, or tuff ring that typically only has one period of volcanic activity.
monogentic volcanic field	A volcanic field consisting mostly or exclusive of monogenetic volcanoes such as cinder cones, maars and tuff rings, and eruptive fissures.
mud pot	A type of hot spring that contains boiling mud, which is usually sulfurous and commonly multicolored that has acid-sulfate water, and are acidic with a low pH.
nuée ardente	A swiftly flowing, turbulent, sometimes incandescent gaseous cloud erupted from a volcano, containing ash and other pyroclastic materials in its lower part. Synonymous with "pyroclastic flow," which is the preferred term.
numeric age	The geologic age of a fossil organism, rock, or geologic feature or event given in units of time, usually years. Commonly used as a synonym of "radiometric age," but may also refer to ages obtained from tree rings, varves, and other dating methods.
obsidian	A black or dark-colored volcanic glass, usually of rhyolite composition, characterized by conchoidal fracture.
oceanic crust	Earth’s crust that underlies the ocean basins and is rich in iron and magnesium; ranges in thickness from about 5 to 10 km (3 to 6 mi).
oceanic trench	A narrow, elongated depression, which may be thousands of kilometers long, of the deep-sea floor associated with a subduction zone, oriented parallel to a volcanic arc and usually to the edge of the adjacent continent; commonly 2 km (1 mi) or more deeper than the surrounding ocean floor.
olivine	A silicate (silicon + oxygen) mineral of magnesium and iron, (Mg,Fe)2SiO4; commonly olive-green and an essential mineral in basalt, gabbro, and peridotite.
orogeny	A mountain-building event.
oxidation	The process of combining with oxygen.
oxide	A chemical compound that contains at least one element combined with oxygen. Also the mineral group. Oxides are also used to report the chemical composition of major elements in volcanic rocks.
p?hoehoe	A lava flow with a smooth, billowy, or ropy surface
palagonite	The product of alteration of volcanic, specifically basaltic, glass with water.
palagonite tuff	A pyroclastic rock consisting of palagonite.
Paroxysmal (VEI)	A level 5 eruption on the Volcanic Explosivity Index, a logarithmic scale for explosive volcanic eruptions ranging from 0 for effusive eruptions to 8 for the largest eruptions based on the volume of magma erupted (magnitude) and the eruption column height of explosive eruptions. Paroxysmal eruptions have an ejecta volume of 10 km3 (2 mi3) and heights of eruption columns at >25 km (16 mi).
passive margin	A continental plate boundary where no plate-scale tectonism is taking place; plates are not converging, diverging, or sliding past one another.
Pele's hair	A thin thread of volcanic glass produced during eruptions of gas-rich fluid lavas in Hawaiian style eruptions. The ?laments form as lava droplets are drawn into threads.
Pele's tears	A small droplet of volcanic glass shaped like glass beads that are frequently attached to filaments of Pele's hair frequently produced in Hawaiian style eruptions.
peperite	A breccialike material in sedimentary rocks, interpreted as a mixture of lava with sediment or as shallow intrusions of magma into wet sediment.
perlite	A volcanic glass having the composition of rhyolite, a perlitic texture, and a generally higher water content than obsidian.
perlitic	Describes the texture of glassy volcanic rocks characterized by numerous curving cracks roughly concentric around closely spaced centers.
phenocryst	A relatively large crystal in a porphyritic igneous rock.
phreatic	Of or relating to groundwater.
phreatic eruption	A volcanic eruption that is driven by steam explosions due to the superheating of groundwater by a magmatic source.
phreatomagmatic	A term encompassing all volcanic activity including violently-explosive eruptions driven by steam explosions produced by the interaction of hot magma with surface water or shallow groundwater. Synonym for hydrovolcanic.
phreatoplinian	A large emission of silicic magmas in the presence of abundant external water that produces pyroclasts that show a greater degree of fragmentation than those produced in Plinian eruptions.
pillow lava	A general term for lavas displaying pillow structures and considered to have formed in a subaqueous environment; such lava is usually basaltic or andesitic in composition. Also called "pillow basalt"
pillow structure	A structure observed in certain extrusive igneous rocks that is characterized by discontinuous bun-shaped masses ranging in size from a few centimeters to a meter or more in greatest dimension. They are considered to be the result of subaqueous extrusion.
pit crater	A small collapse structures, often with vertical sides.
plagioclase	A silicate (silicon + oxygen) mineral of the feldspar group that contains both sodium and calcium ions that freely substitute for one another; characterized by striations (parallel lines) in hand specimens.
plate boundary	A zone of seismic and tectonic activity along the edges of lithospheric plates, resulting from the relative motion among plates.
plate tectonics	A theory of global tectonics in which the lithosphere is divided into about 20 rigid plates that interact with one another at their boundaries, causing seismic and tectonic activity along these boundaries.
Plinian eruption	An extremely explosive eruption that produces ash columns that extend many tens of miles into the stratosphere and that spread out into an umbrella shape and produce widespread deposits of fallout ash.
plug	A vertical, pipelike body of magma that represents the conduit to a former vent. Roughly synonymous with "volcanic neck."
plume	A persistent, pipelike body of hot material moving upward from Earth's mantle into the crust.
pluton	A deep-seated igneous intrusion.
plutonic	Describes an igneous rock or intrusive body formed at great depth beneath Earth's surface.
polygenetic	Describes a volcano like a composite volcano or a shield that typically only has multiple periods of volcanic activity.
porphyritic	Describes an igneous rock of any composition that contains conspicuous phenocrysts (larger crystals) in a fine-grained or glassy groundmass.
porphyry	An igneous rock consisting of abundant coarse-grained crystals in a fine-grained groundmass.
potassium feldspar	A feldspar mineral rich in potassium such as orthoclase, microcline, and sanidine.
prehistoric eruption	An eruption that occurred during the Holocene (e.g., in the last 11,700 years), but prior to 1700 CE.
pressure ridge	An elongated, usually arcuate, ridgs on the crust of a lava flow formed by laterally directed pressure.
pumice	A highly vesicular pyroclast with very low bulk density and thin vesicle walls.
pumiceous	Describes a texture of volcanic rock consisting of tiny gas holes such as in pumice; finer than scoriaceous.
pyroclast	An individual particle or rock or lava fragment ejected during an explosive volcanic eruption; usually classified according to size. From "fire-broken" in the ancient Greek.
pyroclastic	Describes the texture of a rock or deposit made up of pyroclasts. From "fire-broken" in the ancient Greek.
pyroclastic density current	A ground-hugging flow of hot volcanic gases and particles (volcanic ash, pumice, crystals, and small rock fragments) that are propelled by gravity and move extremely rapidly, travelling at speeds more than 200 miles per hour (320 km per hour).
pyroclastic flow	A high-density current of pumice, ash, blocks, and volcanic gas that rapidly moves down the slopes of a volcano.
pyroclastic surge	A low-density current of ash, pumice, crystals, and volcanic gases that is more dilute than a pyroclastic flow.
pyroduct	A lava tube
pyroxene	A group of silicate (silicon + oxygen) minerals composed of magnesium and iron with the general formula (Mg,Fe)SiO3; characterized by short, stout crystals in hand specimens.
quartz	A white or colorless mineral with the composition of silicon dioxide, SiO2.
radiocarbon age	An isotopic age expressed in years and calculated from the quantitative determination of the amount of carbon-14 remaining in an organic material. Synonymous with "carbon-14 age."
radiometric age	An age (in years) calculated from the quantitative determination of radioactive elements and their decay products.
resurgent caldera	A caldera that formed after extremely large (VEI 7 to 8; Mega-colossal to Apocalyptic) Ultra-Plinian eruptions of ash, pumice, and pyroclastic flows and that contains an uplifted area that formed after caldera collapse when the ground surface is pushed up due to magma movement within the magma chamber.
resurgent cauldron	A cauldron in which the cauldron block, following subsidence, has been uplifted, usually in the form of a structural dome.
resurgent dome	An uplifted region within the center of a resurgent caldera due to magma upwelling in the underlying magma reservoir
rhyodacite	A volcanic rock that contains approximately 68%–72% silica and is intermediate in composition between rhyolite and dacite.
rhyolite	A volcanic rock that is characteristically light in color, contains approximately 72% or more of silica, and is rich in potassium and sodium.
ribbon bomb	An elongated ropy-shaped bomb.
ring fracture	The series of fractures where caldera collapse occurs.
rock	An aggregate of one or more minerals (e.g., granite), a body of undifferentiated mineral matter (e.g., obsidian), or a body of solid organic material (e.g., coal).
sanidine	A silicate (silicon + oxygen) mineral of the alkali feldspar group.
scoria	A bomb-size pyroclast that is irregular in form and generally very vesicular. Roughly synonymous with cinder.
scoria cone	A simple volcano with the shape of a conical hill that consists of accumulations of ash and cinders around a vent.
scoriaceous	Describes a texture of volcanic rock consisting of relatively large gas holes such as in vesicular basalt; coarser than pumiceous.
seafloor spreading	A process whereby new oceanic crust is formed by upwelling of magma at the center of mid-ocean ridges and by a moving-away of the new material from the site of upwelling at rates of 1 to 10 cm (2 to 25 in) per year. This movement provides the source of seafloor within the theory of plate tectonics, which also contains a provision for destruction of seafloor by subduction.
Severe (VEI)	A level 1 eruption on the Volcanic Explosivity Index, a logarithmic scale for explosive volcanic eruptions ranging from 0 for effusive eruptions to 8 for the largest eruptions based on the volume of magma erupted (magnitude) and the eruption column height of explosive eruptions. Effusive eruptions have an ejecta volume of 0.001 km3 (0.0002 mi3) and heights of eruption columns at 1 km (0.6 mi).
shield volcano	A very large volcano with very gentle slopes and is made up of basaltic or andesitic lava flows.
silica	Silicon dioxide, SiO2, an essential constituent of many minerals, occurring as crystalline quartz, cryptocrystalline chalcedony, and amorphous opal.
silica tetrahedron	A complex ion formed by four oxygen ions surrounding a silicon ion in a tetrahedral configuration, SiO4; the basic unit of the silicates.
silicate	A mineral group composed of silicon (Si) and oxygen (O) plus an element or elements, for example, quartz, SiO2; olivine, (Mg, Fe)2SiO4; and pyroxene, (Mg,Fe)SiO3; as well as the amphiboles, micas, and feldspars.
siliceous	Describes a rock or other substance containing abundant silica.
silicic	Describes a silica-rich igneous rock or magma.
silicic magma	Describes magma that contains more than 65% silica; generally viscous, gas-rich, and tends to erupt explosively.
sill	An igneous intrusion that parallels the bedding of preexisting sedimentary rock or the foliation of preexisting metamorphic rock.
sinter	The lightweight, porous, opaline variety of silica that is white or nearly white and deposited as an incrustation by precipitation from the waters of geysers and hot springs.
spatter	An accumulation of highly molten bombs near a vent from a lava founded that landed while still molten and welded together to form solid rock, spatter cones, or sometimes feed lava flows.
spatter cone	A low, steep-sided cone of spatter built up on a fissure or vent, usually composed of basaltic material.
spindle bomb	An almond-shaped bomb that results from the tearing apart of a ribbon of lava either during ?ight or upon impact with the ground.
squeeze-up	A small accumulation of viscous lava extruded under pressure from a fracture or opening onto the solidified surface of a lava flow.
stratovolcano	A volcano made up of both lava flows and pyroclastic deposits that usually experiences multiple eruptions over long periods of time. The term "composite volcano" is generally preferred.
Strombolian	A moderately explosive eruption of basaltic magma with moderate gas content. Strombolian eruptions consist of intermittent, discrete explosive bursts which eject pyroclasts as high as hundreds of feet into the air in firework-like incandescent rooster-tails.
Sub-Plinian	An explosive eruption that produces a high eruption columns that are usually less than 12 miles (20km) in height and are unsteady but sustained.
subduction	The process of one lithospheric plate descending beneath another.
subduction zone	A long, narrow belt in which subduction takes place.
subhedral	A crystal partly bounded by crystal faces; intermediate between euhedral and anhedral.
summit caldera	A type of explosive caldera located at or near the summit of a composite volcano.
supervolcano	A volcanic center that has experienced the eruptions ranked at level 8 on the Volcanic Explosivity Index (VEI).
Surtseyan	A phreatomagmatic eruption that under dry conditions would be erupted in the Strombolian-type eruption.
tectonic	Describes a feature or process related to large-scale movement and deformation of Earth's crust.
tephra	The collective term for unconsolidated pyroclastic deposits.
thermal spring	A spring whose water temperature is appreciably higher than the local mean annual atmospheric temperature. A thermal spring may be a "hot spring" or a "warm spring."
trachyte	A silicic volcanic rock with more sodium and potassium than rhyolite.
tree mold	A cylindrical hollow in a lava flow formed by the envelopment of a tree by the flow, solidification of the lava in contact with the tree, and disappearance of the tree by burning and subsequent removal of the charcoal and ash. The inside of the mold preserves the surficial features of the tree.
tuff	A consolidated pyroclastic rock made primarily of ash.
tuff cone	A pyroclastic cone with a crater above the surrounding ground surface formed in a phreatomagmatic eruption tat is taller and steeper than a tuff ring.
tuff ring	A low pyroclastic cone with a crater above the surrounding ground surface formed in a phreatomagmatic eruption.
tuffaceous	Describes non-volcanic, clastic sediments that contain ash-size pyroclasts.
tumulus	A dome or small mound on the crust of a lava flow, caused by pressure due to the difference in the rate of flow between the cooler lava crust and the more fluid underlying lava.
Ultra-Plinian	The largest of all volcanic eruptions and are so voluminous that large calderas form above vacated magma chambers. These eruptions have the highest eruption rates and form highest eruption columns and produce thick pyroclastic flows that cover vast areas and may produce widespread deposits of fallout.
ultramafic	Describes an intrusive igneous rock primarily composed of mafic minerals with a composition less than 45 weight percent SiO2.
vent	The opening or place at Earth's surface through which magma, rock fragments, and/or volcanic gases are emitted.
vesicle	A cavity of variable shape formed by the entrapment of a gas bubble during solidification of molten rock material.
vesicular	Describes the texture of a rock, especially lava, characterized by abundant vesicles formed as a result of the expansion of gases during the fluid stage of the lava.
vesiculation	The processes that occurs when dissolved gases (volatiles) within a magma exsolve to create gas bubbles with the reduction of pressure such as during volcanic eruptions. Vesiculation is a major force propeling volcanic eruptions.
viscosity	The property of a substance to offer internal resistance to flow.
vitreous	Having the luster and appearance of glass.
vitreous luster	Luster that resembles glass. Synonymous to "glassy."
vitric	Describes glassy rock material.
vitrophyre	Any porphyritic igneous rock with a glassy groundmass.
vog	Short for "volcanic smog." A hazy mixture of sulfur dioxide (SO2) gas and aerosols of sulfuric acid and other sulfates created when volcanic gases interact with oxygen and moisture in the atmosphere and sunlight.
volatile	A component of magma or lava that consists of gases such as water or carbon dioxide which are dissolved within the liquid until it is depressurized at or near the surface until it reaches the surface, where they be readily vaporized.
volatile component	Material in magma, such as water or carbon dioxide, whose vapor pressures is sufficiently high to be concentrated as a gas.
volcanic	Pertaining to the activities, structures, or rock types of a volcano. A synonym of extrusive.
volcanic arc	A large-scale (hundreds of kilometers) generally curved belt of volcanoes above a subduction zone.
volcanic center	A region of large, long-lived, volcanic activity that erupts the full range of magma types.
volcanic earthquake	An earthquake caused by movement of magma and gases within and underneath a volcano.
volcanic edifice	A mountain or cone that is the main portion of a volcano. It results from the accumulation of lava, tephra, pyroclastic flows, lahars, and/or other volcanic deposits around a vent or series of vents.
volcanic exhalation	An emission of gas or ash from a vent in a relatively short burst.
Volcanic Explosivity Index	A logarithmic scale for explosive volcanic eruptions ranging from 0 for effusive eruptions to 8 for the largest eruptions based on the volume of magma erupted (magnitude) and the eruption column height of explosive eruptions, which correlates to the eruption intensity. Each interval on the scale represents a ten-fold increase in the size of an eruption, except for events at the bottom of the scale, and also has a descriptive term such as "Catasclysmic" assigned ot it. VEI is generally analogous to magnitude scales for the size of earthquakes.
volcanic field	A cluster of volcanoes or an area covered by volcanic rocks.
volcanic glass	A noncrystalline amorphous solid formed from the rapid cooling or quenching of magma or lava so that crystallation did not take place.
volcanic hazard	A dangerous natural phenomena produced by a volcano.
volcanic neck	The remnant of a volcano's conduit and plumbing system that remain after most of the rest of the volcano has been eroded away.
volcanic plateau	A plateau or area of high elevation produced by volcanic activity.
volcanic process	A volcano-related process that happens on a volcanic landform. Volcanic processes include phenomena that occur before, during, and after volcanic eruptions, and also volcano-related processes that impact them between and after eruptions.
volcanic tremor	A long-lasting release of seismic energy that may be rhythmic or harmonic and is associated with magma in motion.
volcaniclastic	Pertaining to all clastic volcanic materials formed by any process of fragmentation, dispersed by any kind of transporting agent, deposited in any environment, or mixed in any significant portion with nonvolcanic fragments.
volcanism	The processes by which magma and its associated gases rise into Earth's crust and are extruded onto the surface and into the atmosphere.
volcano	A vent where molten rock material emerges from the Earth's interior, and/or the edifice (mountain or cone) built up around that vent or vents during eruption(s).
volcanogenic	Formed by processes directly connected with volcanism.
Vulcanian	A moderately explosive eruption that is intermittent and produces moderate ash columns. Vulcanian eruptions are propelled by exsolution of volatiles in magmas beneath a solidified plug of lava.
warm spring	A thermal spring whose temperature is appreciably above the local mean annual atmospheric temperature, but below that of the human body.
welded tuff	A tuff in which the glass particles within it have been fused together.
welding	The process of fusing hot ash particles and other types of pyroclasts to form a tuff.
xenocryst	A crystal that resembles a phenocryst in igneous rock but is foreign to the body of rock in which it occurs.
xenolith	A rock particle, formed elsewhere, entrained in magma as an inclusion.

Sources

Terms in the volcano glossary were prepared to accompany the Volcanoes, Craters & Lava Flows website. Primary sources, include:

• NPS Geologic Resources Inventory—Glossary of Geologic Terms

• Neuendorf, K. K. E., J. P. Mehl Jr., and J. A. Jackson. 2005. Glossary of geology. Fifth edition. American Geological Institute, Alexandria, Virginia.

• USGS Volcano Hazards Program—Visual Glossary

• USGS Volcano Hazards Program—Glossary

• Haraldur Sigurdsson, Bruce Houghton, Hazel Rymer, John Stix, Steve McNutt. 1999. Encyclopedia of Volcanoes. 1st edition. Elsevier.

• Oregon State University—Volcano World - Definitions

• Other professional sources

Related Links

• NPS—Glossary of Geologic Terms

• The American Geosciences Institute

• 

  Types of Volcanoes
• 

  Volcano Types in Parks
• 

  Anatomy of a Volcano