Volcano Hazards Program Glossary

Terms/Definitions

Term: 
Agglutinate
Definition: 
A welded pyroclastic deposit; term is commonly used for deposits of bombs fused while hot and viscous. Agglutinate typically occurs in spatter cones.
Term: 
Andesite
Definition: 
Volcanic rock (or lava) characteristically medium dark in color and containing 54 to 62 percent silica and moderate amounts of iron and magnesium.
Term: 
Ash
Definition: 
Fine fragments (less than 2-4 mm in diameter) of volcanic rock formed by a volcanic explosion or ejection from a volcanic vent.
Term: 
Basalt
Definition: 
Volcanic rock (or lava) that characteristically is dark in color (gray to black), contains 45 to 53 percent silica, and is rich in iron and magnesium. Basaltic lavas are more fluid than andesites or dacites, which contain more silica.
Term: 
Basaltic andesite
Definition: 
Volcanic rock, commonly dark gray to black, with about 53-57 percent silica.
Term: 
Base surge
Definition: 
A ring-shaped cloud of gas and suspended solid debris that moves radially outward at high velocity from the base of a vertical eruption column. Can accompany phreatomagmatic eruptions.
Term: 
Bishop Tuff
Definition: 
Deposit of rhyolite ash and pumice erupted during formation of Long Valley caldera.
Term: 
Caldera
Definition: 
A large basin-shaped volcanic depression with a diameter many times larger than included volcanic vents; may range from 2 to 50 km (1 to 30 mi) across.
Term: 
Cinder cone
Definition: 
A conical hill, often steep, formed by accumulation of solidified fragments of lava that fall around the vent of a single basaltic or andesitic eruption.
Term: 
Composite volcano
Definition: 
Steep, conical volcanoes built by the eruption of viscous lava flows, tephra, and pyroclastic flows. They are usually constructed over tens to hundreds of thousands of years and may erupt a variety of magma types (basalt to rhyolite). They typically consist of many separate vents. Synonym is stratovolcano.
Term: 
Cumulo-volcano
Definition: 
A dome-shaped volcano constructed of multiple lava domes and flows.
Term: 
Dacite
Definition: 
Volcanic rock (or lava) that characteristically is light in color and contains 62 to 69 percent silica and moderate amounts of sodium and potassium.
Term: 
Debris avalanche
Definition: 
Moving masses of rock, soil and snow that occur when the flank of a mountain or volcano collapses and slides downslope.
Term: 
Dike
Definition: 
A tabular igneous intrusion, typically much longer than it is wide.
Term: 
Dome
Definition: 
A steep-sided mass of viscous and often blocky lava extruded from a vent
Term: 
Edifice
Definition: 
The main portion of a volcano built by eruption of lava, tephra, pyroclastic flows, lahars, and related volcanic deposits. Some shield volcanoes have lava flows that extend beyond the edifice.
Term: 
Effusive eruption
Definition: 
An eruption that produces mainly lava flows and domes (as opposed to an explosive eruption).
Term: 
Ejecta
Definition: 
Material explosively ejected from a volcano.
Term: 
Eruption column
Definition: 
The ascending, vertical part of the mass of erupting debris and volcanic gas that rises directly above a volcanic vent. Higher in the atmosphere, columns usually spread laterally into plumes or umbrella clouds.
Term: 
Explosive eruption
Definition: 
An energetic eruption that produces mainly ash, pumice, and fragmental ballistic debris (as opposed to an effusive eruption).
Term: 
Extensional tectonic environment
Definition: 
A region of the earth's crust or lithosphere that is spreading apart due to tectonic forces acting on the surrounding areas. Back-arc basins are regions of extension behind subduction zones and are often locations where volcanoes form.
Term: 
Facies
Definition: 
A body of rock with characteristics that are the same. Typically the facies is a unit of rock that was formed under conditions reflecting a specific emplacement process e.g., a single lava flow, an ash-flow tuff, a lahar.
Term: 
Fallout
Definition: 
A general term for all the ash and debris that falls to earth (also known as ashfall) from an eruption cloud.
Term: 
Fumarole
Definition: 
A vent from which steam and volcanic gasses issue.
Term: 
Holocene
Definition: 
The youngest geologic time period, considered to include the past approximately 12,000 years. It is almost equivalent to postglacial time.
Term: 
Hydrothermal explosion
Definition: 
Explosion that can occur when hot water within a volcano's hydrothermal (hot water) system flashes to steam, breaking rocks and throwing them into the air.
Term: 
Igneous
Definition: 
Refers to rocks formed by solidification from magma.
Term: 
ka
Definition: 
Thousand years ago.
Term: 
Lahar
Definition: 
Lahar is an Indonesian word for a rapidly flowing mixture of rock debris and water that originates on the slopes of a volcano. Lahars are also referred to as volcanic mudflows or debris flows.
Term: 
Lava
Definition: 
General term for magma (molten rock) that has been erupted onto the surface of the Earth and maintains its integrity as a fluid or viscous mass, rather than exploding into fragments.
Term: 
Mafic
Definition: 
Describes magma that contains lower amounts of silica and is generally less viscous and less gas-rich than silicic magma. Tends to erupt effusively, as lava flows. Includes andesites (57-63 percent SiO2), basaltic andesites (53-57 percent SiO2), and basalts (47-53 percent SiO2).
Term: 
Magma
Definition: 
Molten rock beneath the surface of the Earth.
Term: 
Moat
Definition: 
The annular lowland between the resurgent dome and the walls of a large caldera.
Term: 
Paleomagnetic studies
Definition: 
Investigations of the orientation and(or) intensity of the Earth's magnetic field in the past, as recorded in geologic materials. The magnetic poles wander about the Earth's axis of rotation, and the paleomagnetic pole position at the time of cooling of a volcanic rock is frozen by magnetic minerals. An empirical calibration of this "secular variation" over time allows eruption ages to be constrained and isolated outcrops to be correlated with one another.
Term: 
Petrologic
Definition: 
Petrology is the study of rocks, including their occurrence, composition, and origin.
Term: 
Phreatic eruption
Definition: 
An eruption that primarily involves steam explosions, usually ground water flashed into steam by the heat of subsurface magma.
Term: 
Phreatomagmatic eruption
Definition: 
An eruption that involves both magma and water, which typically interact explosively, leading to concurrent ejection of steam and pyroclastic fragments.
Term: 
Postglacial
Definition: 
Refers to the time since the end of the last major ice age - this varies by location.
Term: 
Pumice
Definition: 
Highly vesicular volcanic ejecta, typically silicic in composition. It is essentially magma that has been frothed up by escaping gases and then cooled and solidified during eruption. Rhyolitic pumice is typically of low enough density that it floats on water. Near a vent, hot pumice can accumulate and form a pumice cone.
Term: 
Pyroclast
Definition: 
General term applied to volcanic products or processes that involve explosive ejection and fragmentation of erupting material. Literally means fire-broken.
Term: 
Pyroclastic flow
Definition: 
A hot (typically >800°C), chaotic mixture of rock fragments, gas, and ash that travels rapidly (tens of meters per second) away from a volcanic vent or collapsing flow front.
Term: 
Resurgent dome
Definition: 
The central highland in many large calderas formed by gradual upwarping of the caldera floor after caldera collapse as a result of renewed magma intrusion.
Term: 
Rhyodacite
Definition: 
Volcanic rock (or lava) that is intermediate in composition between rhyolite and dacite.
Term: 
Rhyolite
Definition: 
Volcanic rock (or lava) that characteristically is light in color, contains 69 or more percent of silica, and is rich in potassium and sodium. Low-silica rhyolite contains 69 to 74 percent silica. High-silica rhyolite contains 75 to 80 percent silica. Rhyolitic lavas are viscous and tend to form thick blocky lava flows or steep-sided piles of lava called lava domes. Rhyolite magmas tend to erupt explosively, commonly also producing abundant ash and pumice.
Term: 
Scoria
Definition: 
Vesicular volcanic ejecta, essentially magma that has been frothed up by escaping gases. It is a textural variant of pumice, with scoria typically being less vesicular, denser, and usually andesitic or basaltic.
Term: 
Silica
Definition: 
Silicon dioxide, the most abundant rock-forming compound on Earth and the predominant molecular constituent of volcanic rocks and magmas. It tends to polymerize into molecular chains, increasing the viscosity of the magma. Basaltic magma, having lower SiO2, is fairly fluid, but with increasing contents of SiO2, andesite, dacite, and rhyolite magmas become progressively more viscous. Because it is more difficult for dissolved gas to escape from more viscous magma, higher silica magmas generally erupt more explosively.
Term: 
Silicic
Definition: 
Describes magma that contains more than ~63 percent silica and is generally viscous, gas-rich, and tends to erupt explosively. Includes rhyolite and dacite.
Term: 
Shield volcano
Definition: 
A broad shield-shaped volcano that is built up by successive, mostly effusive, eruptions of low-silica lava.
Term: 
Spatter cone
Definition: 
A steep-sided cone constructed of agglutinate at a mafic vent. Most spatter cones are small (typically 10 m or less in height) and commonly form in linear groups along a fissure.
Term: 
Stratovolcano
Definition: 
Steep, conical volcanoes built by the eruption of viscous lava flows, tephra, and pyroclastic flows. They are usually constructed over tens to hundreds of thousands of years and may erupt a variety of magma types (basalt to rhyolite). They typically consist of many separate vents. Synonym is composite volcano.
Term: 
Tectonic
Definition: 
Relating to structural deformation of the Earth's crust
Term: 
Tephra
Definition: 
Any type and size of rock fragment that is forcibly ejected from the volcano and travels an airborne path during an eruption (including ash, bombs, and scoria).
Term: 
Vent
Definition: 
Any opening at the Earth's surface through which magma erupts or volcanic gases are emitted.
Term: 
Ash-flow tuff
Definition: 
The deposit of a hot, chaotic mixture of pumice, ash, and gas (pyroclastic flow) that travels rapidly (as fast as tens of meters per second) away from a volcanic vent during an explosive eruption.
Term: 
Monogenetic
Definition: 
Resulting from one process or formation or derived from one source, or originating or developing at one place and time. e.g. a volcano built up by a single eruption.
Term: 
Lithic
Definition: 
In volcanology: rocks that are not from the primary eruptive magma, but are instead pulled from the walls of the magma storage zone or conduit during an eruption. Previously formed rocks not from the primary magma source.
Term: 
Stratigraphy (Stratigraphic)
Definition: 
The science of rock strata, or layers. It is concerned with all characters and attributes of rocks as sequentially timed layers and their interpretation in terms of mode of origin and geologic history. The arrangement of strata signifies chronologic order of sequence.
Term: 
Unconsolidated
Definition: 
A unit of rocks that are loosely arranged or not layered, or whose particles are not cemented together, occurring either at the surface or at depth.
Term: 
Alluvium
Definition: 
A general term for deposits made up of a mixture of loose rocks that were produced by mechanical means (detritus). Deposits of alluvium are typically formed as a result of flooding or transportation in a stream or river. e.g. an alluvial fan.
Term: 
Tuff
Definition: 
A general term for all consolidated (hardened and/or compacted) pyroclastic (explosive, volcanic origination) rocks. Synonymous with tuffaceous.
Term: 
Seismic tomography
Definition: 
Seismic tomography is similar to a CAT scan. When earthquakes occur, they sent out seismic waves, and by recording the patterns of the wave arrivals from many earthquakes on many seismometers, seismologists can calculate the wave speeds through the crust. Fast and slow wave speeds can then be interpreted to geologically understand the tectonic and/or volcanic structure of the earth.
Term: 
Obsidian
Definition: 
Obsidian is dense volcanic glass, usually rhyolite in composition and typically black in color. Obsidian forms in lava flows where the lava cools so fast that crystals do not have time to grow.
Term: 
Fissure
Definition: 
In geology, a fissure is a fracture or crack in rock along which there is a distinct separation; fissures are often filled with mineral-bearing materials. On volcanoes, a fissure is an elongate fracture or crack at the surface from which lava erupts.
Term: 
Batholith
Definition: 
A large mass of rock formed by magmatic processes that has more than 100 km<sup2>2</sup> (40 mi<sup>2</sup>) of surface exposure and no known floor.
Term: 
Lava tube
Definition: 
Lava tubes are natural conduits through which lava travels beneath the surface of a lava flow. Tubes form by the crusting over of lava channels and pahoehoe flows.
Term: 
Pillow lava
Definition: 
Mounds of elongate lava <i>pillows</i> formed by repeated oozing and quenching of hot basalt.
Term: 
Littoral cone
Definition: 
A cone of lava fragments built on the surface of a lava flow pouring into a body of water, usually the sea, is called a littoral cone (littoral" refers to a shoreline)."
Term: 
Tumulus
Definition: 
A tumulus is created when the upward pressure of slow-moving molten lava within a flow swells or pushes the overlying crust upward. The surfaces of pahoehoe flows on flat or gentle slopes often exhibit elliptical, domed structures called tumuli.
Term: 
Block and ash flow
Definition: 
Flow of ash and angular rock fragments larger than 26 cm or 10 in.
Term: 
Deformation
Definition: 
Changes to the surface of a volcano that occur due to magma movement underneath the surface. Most volcano deformation can only be detected and measured with precise surveying techniques such as with a Global Positioning System (GPS), tiltmeter, Interferometric Synthetic Aperture Radar (InSAR), or an Electronic Distance Meter (EDM).
Term: 
Maar
Definition: 
A maar is a low-relief, broad volcanic crater formed by shallow explosive eruptions. The explosions are usually caused by the heating and boiling of groundwater when magma invades the groundwater table. Maars often fill with water to form a lake.
Term: 
Sinter
Definition: 
Also known as <i>siliceous</i> 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.
Term: 
Ma
Definition: 
Million years ago.
Term: 
Directed blast
Definition: 
A hot, low-density mixture of rock debris, ash, and gases that moves at high speed along the ground surface. Directed blasts are generated by explosions.
Term: 
Pleistocene
Definition: 
The first epoch of the Quaternary period (the most recent geologic time period) that lasted from 2,588,000 to 11,700 years ago, spanning the world's recent period of repeated glaciations. It is the period of time before the Holocene.
Term: 
Quaternary
Definition: 
The most recent geologic time period and spans 2,588,000 years ago until the present.
Term: 
Back-arc basin
Definition: 
The area behind a volcanic arc and within the overriding plate in a subduction zone environment.
Term: 
PAHOEHOE
Definition: 
Basaltic lava that has a smooth, hummocky, or ropy surface. A pahoehoe flow typically advances as a series of small lobes and toes that continually break out from a cooled crust.
Term: 
Seismicity
Definition: 
The phenomenon of earthquakes caused by the brittle fracturing of rocks in the earth's crust. Synonymous with seismic activity.
Term: 
Isopach
Definition: 
A line on a map that connects geologic units of equal rock thickness. In volcanology, this typically relates to the thickness of an ash or tephra deposit from an explosive eruption.
Term: 
Intrusive
Definition: 
Molten rock that is forced into pre-existing rocks, or an igneous rock that has crystalized (turned to solid) below the earth's surface.
Term: 
Ophiolite
Definition: 
An assemblage of crustal rocks that represents a section of the Earth's oceanic crust and upper mantle that has been uplifted and exposed above sea level.
Term: 
Magma chamber
Definition: 
The location beneath the vent of a volcano where molten rock (magma) is stored prior to eruption. Also known as a magma storage zone or magma reservoir.
Term: 
Lava fountain
Definition: 
A jet of lava sprayed into the air by the rapid formation and expansion of gas bubbles in the molten rock is called a lava fountain. Lava fountains typically range from about 10 to 100 m in height, but occasionally reach more than 500 m.
Term: 
Subaerial
Definition: 
Formed in the open air, or on the Earth's surface, and not underwater or underground.
Term: 
Subduction
Definition: 
Subduction is the process of the oceanic lithosphere colliding with and descending beneath the continental lithosphere.
Term: 
Mantle
Definition: 
The mantle is the part of the earth's interior between the metallic outer core (below the mantle) and the crust (above the mantle).
Term: 
Crust
Definition: 
The crust is the outermost major layer of the earth, ranging from about 10 to 65 km in thickness worldwide. The uppermost 15-35 km of crust is brittle enough to produce earthquakes.
Term: 
Volcaniclastic
Definition: 
A body of rock that is composed of fragments of volcanically derived rocks or minerals that were then transported some distance from their place of origin.
Term: 
Sedimentary rocks
Definition: 
Sedimentary rocks are formed from pre-existing rocks or pieces of once-living organisms. Clastic sedimentary rocks are made up of pieces (clasts) of pre-existing rocks. Pieces of rock are loosened by weathering, then transported to some basin or depression where sediment is trapped. If the sediment is buried deeply, it becomes compacted and cemented, forming sedimentary rock.
Term: 
Tertiary
Definition: 
Geologic period from 65 million to 2.6 million years ago. This time period began with the extinction of the non-avian dinosaurs and signifies the end of the Cretaceous Period and the start of the Cenozoic era.
Term: 
K-Ar
Definition: 
Symbol for potassium-argon; typically used when referring to the potassium-argon age dating method, which measures the radioactive decay of potassium into argon and provides an absolute age for rock samples older than a few thousand years.
Term: 
Breadcrust Bomb
Definition: 
A breadcrust bomb is a volcanic bomb with a cracked and checkered surface, sometimes resembling the surface of a loaf of bread. The cracks develop when the outer surface of a partially molten lava fragment cools to form a brittle surface and then subsequently cracks as the hot interior expands due to the continued growth of gas bubbles.
Term: 
Lava flow
Definition: 
Streams of molten rock that erupt relatively non-explosively from a volcano, then move downslope until they stop, cool, and solidify.
Term: 
AA
Definition: 
A'a (pronounced ah-ah") is a Hawaiian term for lava flows that have a rough rubbly surface composed of broken lava blocks called clinkers."
Term: 
Benchmark
Definition: 
A location on the surface of the earth at a specific latitude and longitude that is used as a point of reference. A marker is often installed at a benchmark location, so that the same exact point can be revisited over and over again to compare and assess changes through time.
Term: 
Bomb
Definition: 
Volcanic bombs are lava fragments that were ejected while viscous (partially molten), and blocks are solid rock fragments; both are larger than 64 mm in diameter and are ejected during an explosive eruption.
Term: 
CE
Definition: 
Abbreviation for Common Era" or "Current Era". CE refers to the year since the start of the current era in the Julian or Gregorian calendar
Term: 
Cirque
Definition: 
A bowl-shaped, amphitheater-like depression eroded into the head or the side of a glacier valley. Typically, a cirque has a lip at its lower end. The term is French and is derived from the Latin word circus.
Term: 
Coulee
Definition: 
An elongate lava dome that typically forms as a high-silica lava erupts on the steep flank of a volcano. The dome axis perpendicular to the volcano's slope is shorter than the axis parallel to the slope, creating an elliptical shape. The leading edge of the coulee is often blocky and steep.
Term: 
Cryptodome
Definition: 
A body of magma that rises from depth and intrudes into the edifice of a volcano, but does not erupt on the surface. Cryptodome formation can results in a bulge or welt on the surface of a volcano.
Term: 
Debris dam
Definition: 
Volcanic material deposited in a valley to form unstable natural dams that block pre-existing drainages; lakes may form behind the impoundment.
Term: 
Diamict
Definition: 
A very poorly sorted sediment typically associated with glacial deposits.
Term: 
DOAS
Definition: 
DOAS instruments measure sulfur dioxide emission rates by detecting the amount of ultraviolet sunlight absorbed by SO<sub>2</sub> molecules within a volcanic plume.
Term: 
Drainage
Definition: 
The land area drained by a stream, river or other body of surface water.
Term: 
Emission
Definition: 
The release of volcanic gases from the earth, which escape into the atmosphere.
Term: 
FTIR
Definition: 
Fourier Transform Infrared Spectrometers measure the amount of heat absorbed by volcanic gases.
Term: 
Glacier
Definition: 
A mass of ice that shows evidence of flow over a period of years as indicated by the presence of flow lines, crevasses, and other geologic evidence.
Term: 
Hydrology
Definition: 
The science encompassing the behavior of water as it occurs in the atmosphere, on the surface of the ground, and underground. 
Term: 
Ignimbrite
Definition: 
The rock formed by widespread deposition and consolidation of pyroclastic density currents (pyroclastic flows). Also known as welded tuff or ash flow tuff.
Term: 
Lateral Blast
Definition: 
A lateral (sideways) explosion with a significant low-angle component that is directed towards an area that can cover as much as 180 degrees. Because they carry rock debris at high speeds, lateral blasts can devastate areas tens to hundreds of square kilometers within a few minutes, and they can destroy manmade structures and kill all living things by abrasion, impact, burial, and heat.
Term: 
Lava lake
Definition: 
Large volume of molten lava, usually basaltic, in a vent, crater, or broad depression, which forms a lake.
Term: 
Plinian
Definition: 
Plinian eruptions are large explosive events that form enormous dark columns of tephra and gas high into the stratosphere (>11 km). Such eruptions are named for Pliny the Younger, who carefully described the disastrous eruption of Vesuvius in 79 A.D. This eruption generated a huge column of tephra into the sky, pyroclastic flows and surges, and extensive ash fall. Many thousands of people evacuated areas around the volcano, but about 2,000 were killed, including Pliny the Older.
Term: 
Pyroclastic surge
Definition: 
Ground-hugging clouds of ash, rock, and volcanic gas that move at hurricane velocities and have temperatures of several hundred degrees Celsius.
Term: 
Seismic swarm
Definition: 
A series of minor earthquakes occurring in the same area and time, none of which may be identified as the main shock or with the same fault.
Term: 
Submarine volcano
Definition: 
A volcano located below sea level on the ocean floor, commonly composed of basalt.
Term: 
Thermal
Definition: 
Of or relating to heat. At volcanoes, thermal features are observed to determine whether temperatures are changing (e.g. fumaroles, vents, lava surfaces, etc...). These changes help scientists to understand volcanic processes.
Term: 
Tsunami
Definition: 
A gravitational sea wave produced by a large-scale, short-duration disturbance of the ocean floor, such as a submarine earthquake, slump, subsidence, or volcanic eruption.
Term: 
VEI
Definition: 
The Volcanic Explosivity Index (VEI) is a relative measure of the explosiveness of volcanic eruptions. It was devised by Chris Newhall of the United States Geological Survey and Stephen Self at the University of Hawaii in 1982.
Term: 
Lava lake
Definition: 
Lava lakes are large volumes of molten lava, usually basaltic, contained in a vent, crater, or broad depression. Scientists use the term to describe both lava lakes that are molten and those that are partly or completely solidified. Lava lakes can form (1) from one or more vents in a crater that erupts enough lava to partially fill the crater; (2) when lava pours into a crater or broad depression and partially fills the crater; and (3) atop a new vent that erupts lava continuously for a period of several weeks or more and slowly builds a crater higher and higher above the surrounding ground.
Term: 
Spatter cone
Definition: 
A steep-sided cone constructed of agglutinate at a mafic vent. Most spatter cones are small (typically 10 m or less in height) and commonly form in linear groups along a fissure.
Term: 
pali
Definition: 
Steep slope, hill, cliff, or precipice.