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'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.


A welded pyroclastic deposit; term is commonly used for deposits of bombs fused while hot and viscous. Agglutinate typically occurs in spatter cones.


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.


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


Fine fragments (less than 2-4 mm in diameter) of volcanic rock formed by a volcanic explosion or ejection from a volcanic vent.

Ash-flow tuff

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.

Back-arc basin

The area behind a volcanic arc and within the overriding plate in a subduction zone environment.


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.

Basaltic andesite

Volcanic rock, commonly dark gray to black, with about 53-57 percent silica.

Base surge

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.


A large mass of rock formed by magmatic processes that has more than 100 km2 (40 mi2) of surface exposure and no known floor.


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.

Bishop Tuff

Deposit of rhyolite ash and pumice erupted during formation of Long Valley caldera.

Block and ash flow

Flow of ash and angular rock fragments larger than 26 cm or 10 in.


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.

Breadcrust Bomb

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.


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.

Cinder cone

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.


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.

Composite volcano

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.


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.


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.


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.


A dome-shaped volcano constructed of multiple lava domes and flows.


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

Debris avalanche

Moving masses of rock, soil and snow that occur when the flank of a mountain or volcano collapses and slides downslope.

Debris dam

Volcanic material deposited in a valley to form unstable natural dams that block pre-existing drainages; lakes may form behind the impoundment.


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).


A very poorly sorted sediment typically associated with glacial deposits.


A tabular igneous intrusion, typically much longer than it is wide.

Directed blast

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.


DOAS instruments measure sulfur dioxide emission rates by detecting the amount of ultraviolet sunlight absorbed by SO2 molecules within a volcanic plume.


A steep-sided mass of viscous and often blocky lava extruded from a vent


The land area drained by a stream, river or other body of surface water.


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.

Effusive eruption

An eruption that produces mainly lava flows and domes (as opposed to an explosive eruption).


Material explosively ejected from a volcano.


The release of volcanic gases from the earth, which escape into the atmosphere.

Eruption column

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.

Explosive eruption

An energetic eruption that produces mainly ash, pumice, and fragmental ballistic debris (as opposed to an effusive eruption).

Extensional tectonic environment

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.


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.


A general term for all the ash and debris that falls to earth (also known as ashfall) from an eruption cloud.


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.


Fourier Transform Infrared Spectrometers measure the amount of heat absorbed by volcanic gases.


A vent from which steam and volcanic gasses issue.


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.


The youngest geologic time period, considered to include the past approximately 12,000 years. It is almost equivalent to postglacial time.


The science encompassing the behavior of water as it occurs in the atmosphere, on the surface of the ground, and underground. 

Hydrothermal explosion

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.


Refers to rocks formed by solidification from magma.


Molten rock that is forced into pre-existing rocks, or an igneous rock that has crystalized (turned to solid) below the earth's surface.


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.


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.


Thousand years ago.


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.

Lateral Blast

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.


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.

Lava flow

Streams of molten rock that erupt relatively non-explosively from a volcano, then move downslope until they stop, cool, and solidify.

Lava fountain

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.

Lava lake

Large volume of molten lava, usually basaltic, in a vent, crater, or broad depression, which forms a lake.

Lava tube

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.


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.

Littoral cone

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).


Million years ago.


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.


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).


Molten rock beneath the surface of the Earth.

Magma chamber

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.


The mantle is the part of the earth's interior between the metallic outer core (below the mantle) and the crust (above the mantle).


The annular lowland between the resurgent dome and the walls of a large caldera.


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.


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.


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.


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.

Paleomagnetic studies

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 in” 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.


Petrology is the study of rocks, including their occurrence, composition, and origin.

Phreatic eruption

An eruption that primarily involves steam explosions, usually ground water flashed into steam by the heat of subsurface magma.

Phreatomagmatic eruption

An eruption that involves both magma and water, which typically interact explosively, leading to concurrent ejection of steam and pyroclastic fragments.

Pillow lava

Mounds of elongate lava pillows formed by repeated oozing and quenching of hot basalt. First, a flexible glassy crust forms around the newly extruded lava, forming an expanded pillow. Next, pressure builds until the crust breaks and new basalt extrudes like toothpaste, forming another pillow. This sequence continues until a thick sequence may be deposited. When geologists find pillow basalts in ancient rock sequences, they may conclude that the area was once under water.


The first epoch of the Quaternary period (the most recent geologic time period) that lasted from 2,588,000 to 11,700 years ago. It is the period of time before the Holocene.


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.


Refers to the time since the end of the last major ice age - this varies by location.


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.


General term applied to volcanic products or processes that involve explosive ejection and fragmentation of erupting material. Literally means "fire-broken."

Pyroclastic flow

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.


The most recent geologic time period and spans 2,588,000 years ago until the present.

Resurgent dome

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.


Volcanic rock (or lava) that is intermediate in composition between rhyolite and dacite.


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.


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.

Sedimentary rocks

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.

Seismic swarm

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.

Seismic tomography

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.


The phenomenon of earthquakes caused by the brittle fracturing of rocks in the earth's crust. Synonymous with seismic activity.

Shield volcano

A broad shield-shaped volcano that is built up by successive, mostly effusive, eruptions of low-silica lava.


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.


Describes magma that contains more than ~63 percent silica and is generally viscous, gas-rich, and tends to erupt explosively. Includes rhyolite and dacite.


Also known as siliceous sinter. The lighweight, 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 cone

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.

Stratigraphy (Stratigraphic)

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.


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.


Formed in the open air, or on the Earth's surface, and not underwater or underground.


Subduction is the process of the oceanic lithosphere colliding with and descending beneath the continental lithosphere.

Submarine volcano

A volcano located below sea level on the ocean floor, commonly composed of basalt.


Relating to structural deformation of the Earth's crust


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).


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.


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.


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.


A general term for all consolidated (hardened and/or compacted) pyroclastic (explosive, volcanic origination) rocks. Synonymous with tuffaceous.


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.


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.


Any opening at the Earth's surface through which magma erupts or volcanic gases are emitted.


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.