Glossary of Terms

Term Definition
a'a' A Hawaiian term for lava flows that have a rough, jagged surface.
alkalic Rocks which contain above average amounts of sodium and/or potassium for the group of rocks for which it belongs. For example, the basalts of the capping stage of Hawaiian volcanoes are alkalic. They contain more sodium and/or potassium than the shield-building basalts that make the bulk of the volcano.
andesitic A fine-grained volcanic rock made up of feldspars and ferromagnesian minerals; typically has a silica content of 54 to about 62 percent.
ash Fine fragments (less than 2 millimeters across) of lava or rock formed in an explosive volcanic eruption.
ash flow A turbulent mixture of gas and rock fragments, most of which are ash-sized particles, ejected violently from a crater or fissure. The mass of pyroclastics is normally of very high temperature and moves rapidly down the slopes or even along a level surface.
avalanche A large mass of material or mixtures of material falling or sliding rapidly under the force of gravity. Avalanches often are classified by their content, such as snow, ice, soil, or rock avalanches. A mixture of these materials is a debris avalanche.
basalt Volcanic rock (or lava) that characteristically is dark in color, contains 45% to 54% silica, and generally is rich in iron and magnesium.
block A volcanic block is a solid rock fragment greater than 64 mm in diameter that was ejected from a volcano during an explosive eruption. Blocks commonly consist of solidified pieces of old lava flows that were part of a volcano's cone.
bomb Fragment of molten or semi-molten rock, 2 1/2 inches to many feet in diameter, which is blown out during an eruption. Because of their plastic condition, bombs are often modified in shape during their flight or upon impact.
caldera A large crater formed by collapse or subsidence of the ground surface following a great eruption. During a typical caldera-forming eruption, the magma chamber is partially emptied and large amounts of ash and pyroclastic debris are extruded.
central volcano A volcano constructed by the ejection of debris and lava flows from a central point, forming a more or less symmetrical volcano.
cinder cone A volcanic cone built entirely of loose fragmented material (pyroclastics.)
crater A steep-sided, usually circular depression formed by either explosion or collapse at a volcanic vent.
dacite Volcanic rock (or lava) that characteristically is light in color and contains 62% to 69% silica and moderate a mounts of sodium and potassium.
debris avalanche Rapidly moving, dry flows of disaggregated rock debris, sand, and silt. Volcanic debris avalanches commonly form by some type of structural collapse of the volcano, usually the steep front of the cooled lava dome, or other parts of the upper edifice. A large portion of the volcano may become unstable, break away from the volcanic massif, and become an avalanche. A debris avalanche may be triggered by an eruption or earthquake. Debris avalanches move at velocities ranging from a few tens of meters per second to more than 100 meters per second and behave like complex granular flows or slide flows. Commonly they are quite voluminous (greater than 10 cubic kilometers) and may run out considerable distances (up to 85 kilometers) from their source. The resulting debris-avalanche deposit usually exhibits hummocky surface morphology.
debris flow A mixture of water-saturated rock debris that flows downslope under the force of gravity (also called lahar or mudflow).
dike A tabular igneous intrusion that cuts across the host bedrock.
dome A steep-sided mass of viscous (doughy) lava extruded from a volcanic vent (often circular in plane view) and spiny, rounded, or flat on top. Its surface is often rough and blocky as a result of fragmentation of the cooler, outer crust during growth of the dome.
effusive eruption An eruption dominated by the outpouring of lava onto the ground is often referred to as an effusive eruption (as opposed to the violent fragmentation of magma by explosive eruptions). lava flows generated by effusive eruptions vary in shape, thickness, length, and width depending on the type of lava erupted, discharge, slope of the ground over which the lava travels, and duration of eruption.
ejecta Material that is thrown out by a volcano, including pyroclastic material (tephra) and lava bombs.
eruption cloud Cloud of gas, ash, and other fragments that forms during an explosive volcanic eruption and travels long distances with the prevailing winds.
eruption column The vertical portion of the eruption cloud that rises above a volcanic vent.
fallout A general term for debris that falls to the earth from an eruption cloud.
fault A crack or fracture in the earth's surface. Movement along the fault can cause earthquakes or--in the process of mountain-building--can release underlying magma and permit it to rise to the surface.
fault scarp A steep slope or cliff formed directly by movement along a fault and representing the exposed surface of the fault before modification by erosion and weathering.
felsic An igneous rock having abundant light-colored minerals.
fissure Elongated fractures or cracks on the slopes of a volcano. fissure eruptions typically produce liquid flows, but pyroclastics may also be ejected.
flank eruption An eruption from the side of a volcano (in contrast to a summit eruption.)
fracture The manner of breaking due to intense folding or faulting.
fumarole A small volcanic vent from which gases and vapors are emitted.
GPS A surveying technique that uses signals from a series of artificial satellites to determine position on the Earth's surface.
harmonic tremor A continuous release of seismic energy typically associated with the underground movement of magma. It contrasts distinctly with the sudden release and rapid decrease of seismic energy associated with the more common type of earthquake caused by slippage along a fault.
Holocene A geologic time designation for the last 10,000 years of Earth history.
hornito A small rootless spatter cone that forms on the surface of a basaltic lava flow (usually pahoehoe) is called a hornito. A hornito develops when lava is forced up through an opening in the cooled surface of a flow and then accumulates around the opening. Typically, hornitos are steep sided and form conspicuous pinnacles or stacks. They are "rootless" because they are fed by lava from the underlying flow instead of from a deeper magma conduit.
hummock Rounded or conical mounds within a volcanic landslide or debris avalanche deposit. Hummocks contain a wide range of rock debris, reflecting the variation of deposits that previously formed the flanks of the volcano. Some hummocks contain huge intact blocks tens to hundreds of meters in diameter. Some of the original layering of lava flows and other deposits can be seen in these large hummocks, but most of the large rock fragments are thoroughly shattered. In other hummocks the rock debris is thoroughly mixed as if the material had been in a blender and thoroughly mixed together.
hypocenter The point within the Earth that is the center of an earthquake. The initial point of rupture during an earthquake.
ice piston ice piston, used by AVO in the context of describing a feature at the summit of Mount Redoubt, refers to a crater-like feature made of ice, but with vertical walls. It is formed by a plug of ice dropping down vertically as ice at the base melts and the water flows away.
ignimbrite The rock formed by the widespread deposition and consolidation of ash flows and nuees ardentes. The term was originally applied only to densely welded deposits but now includes non-welded deposits.
infrasound Low-frequency sound waves, below the threshold of human hearing. Erupting volcanoes produce infrasound.
intrusion The process of emplacement of magma in pre-existing rock. Also, the term refers to igneous rock mass so formed within the surrounding rock.
jokulhlaup An Icelandic term that refers without distinction to both water floods and lahars that are generated when a volcano erupts under a glacier.
kipuka An area surrounded by a lava flow.
laccolith A body of igneous rocks with a flat bottom and domed top. It is parallel to the layers above and below it.
lahar An Indonesian term for a debris flow containing angular clasts of volcanic material. For the purposes of this report, a lahar is any type of sedimentwater mixture originating on or from the volcano. Most lahars move rapidly down the slopes of a volcano as channelized flows and deliver large amounts of sediment to the rivers and streams that drain the volcano. The flow velocity of some lahars may be as high as 20 to 40 meters per second and sediment concentrations of greater than 750,000 parts per million are not uncommon. Large volume lahars can travel great distances if they have an appreciable clay content (greater than 3 to 5 percent), remain confined to a stream channel, and do not significantly gain sediment while losing water. Thus, they may affect areas many tens to hundreds of kilometers downstream from a volcano.
lapilli Rock fragments between 2 and 64 mm (0.08-2.5 in) in diameter that were ejected from a volcano during an explosive eruption are called lapilli. lapilli (singular: lapillus) means "little stones" in Italian. lapilli may consist of many different types of tephra, including scoria, pumice, and reticulite.
lava lava is the word for magma (molten rock) when it erupts onto the Earth's surface. Geologists also use the word to describe the solidified deposits of lava flows and fragments hurled into the air by explosive eruptions (for example, lava bombs or blocks). lava is from the Italian word for stream, which is derived from the verb lavare--to wash.
lava delta lava entering the sea often builds a wide fan-shaped area of new land called a lava delta. Such new land is usually built on sloping layers of loose lava fragments and flows. On steep submarine slopes, these layers of debris are unstable and often lead to the sudden collapse of lava deltas into the sea.
lava dome A steep-sided mass of viscous and commonly blocky lava extruded from a vent; typically has a rounded top and roughly circular outline.
lava flow An outpouring of lava onto the land surface from a vent or fissure. Also, a solidified tongue like or sheet-like body formed by outpouring lava.
lava fountain A rhythmic vertical fountainlike eruption of lava.
lava tube A tunnel formed when the surface of a lava flow cools and solidifies while the still-molten interior flows through and drains away.
lithic Of or pertaining to stone.
lithosphere The rigid crust and uppermost mantle of the earth. Thickness is on the order of 60 miles (100 km). Stronger than the underlying asthenosphere.
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). lava entering the ocean heats and boils seawater, often generating steam explosions that hurl tephra onto the shore, including spatter, bombs, blocks, ash,, lapilli, and, rarely, reticulite. As the various tephra accumulates on the shoreline, a well-developed cone may be created.
long-period event Discrete events with very regular, low-frequency (1-5 Hz) waveforms that resonate for many cycles.
maar A volcanic crater that is produced by an explosion in an area of low relief, is generally more or less circular, and often contains a lake, pond, or marsh.
mafic An igneous composed chiefly of one or more dark-colored minerals.
magma Molten rock beneath the Earth's surface.
magma chamber The subterranean cavity containing the gas-rich liquid magma which feeds a volcano.
magnitude A measure of the size of an earthquake, determined by measuring the highest-amplitude waves and correcting for distance and type of instrument. The scale is logarithmic, so each increase of one unit corresponds to amplitude increase of a factor of 10.
mantle The zone of the earth below the crust and above the core.
monogenetic A volcano built by a single eruption.
mud volcano A mud volcano is a small volcano-shaped cone of mud and clay, usually less than 1-2 m tall. These small mud volcanoes are built by a mixture of hot water and fine sediment (mud and clay) that either (1) pours gently from a vent in the ground like a fluid lava flow; or (2) is ejected into the air like a lava fountain by escaping volcanic gas and boiling water. The fine mud and clay typically originates from solid rock--volcanic gases and heat escaping from magma deep below turn groundwater into a hot acidic mixture that chemically changes the rock into mud- and clay-sized fragments.
mudflow A flowage of water-saturated earth material possessing a high degree of fluidity during movement. A less-saturated flowing mass is often called a debris flow. A mudflow originating on the flank of a volcano is properly called a lahar.
nuees ardentes A French term applied to a highly heated mass of gas-charged ash which is expelled with explosive force and moves hurricane speed down the mountainside.
obsidian A black or dark-colored volcanic glass, usually composed of rhyolite.
Pele Tears Small, solidified drops of volcanic glass behind which trail pendants of Pele hair. They may be tear-shaped, spherical, or nearly cylindrical.
peralkaline Igneous rocks in which the molecular proportion of aluminum oxide is less than that of sodium and potassium oxides combined.
phenocryst A conspicuous, usually large, crystal embedded in porphyritic igneous rock.
phreatic eruption An explosive volcanic eruption caused when water and heated volcanic rocks interact to produce a violent expulsion of steam and pulverized rocks. magma is not involved.
phreatomagmatic An explosive volcanic eruption that results from the interaction of surface or subsurface water and magma.
pillow lava Interconnected, sack-like bodies of lava formed underwater.
pit crater Pit craters are circular-shaped craters formed by the sinking or collapse of the ground. Fissures may erupt from the walls or base of a pit crater, but pit craters are not constructional features built by eruptions of lava or tephra. Pit craters may also partially fill with lava to form a lava lake. They are common along rift zones of shield volcanoes; for example, Mauna Loa and Kilauea volcanoes in Hawai`i. No one has observed the formation of a large pit crater, but they are thought to form as a consequence of the removal of support by withdrawal of underlying magma.
Pleistocene A geologic time designation for the period of Earth history from about 1.6 million years ago to about 10,000 years ago.
plinian eruption An explosive eruption in which a steady, turbulent stream of fragmented magma and magmatic gases is released at a high velocity from a vent. Large volumes of tephra and tall eruption columns are characteristic.
plug Solidified lava that fills the conduit of a volcano. It is usually more resistant to erosion than the material making up the surrounding cone, and may remain standing as a solitary pinnacle when the rest of the original structure has eroded away.
plug dome The steep-sided, rounded mound formed when viscous lava wells up into a crater and is too stiff to flow away. It piles up as a dome-shaped mass, often completely filling the vent from which it emerged.
polygenetic Originating in various ways or from various sources.
pumice Light-colored, frothy volcanic rock, usually of dacite or rhyolite composition, formed by the expansion of gas in erupting lava. Commonly seen as lumps or fragments of pea-size and larger, but can also occur abundantly as ash-sized particles.
pyroclastic Pertaining to fragmented (clastic) rock material formed by a volcanic explosion or ejection from a volcanic vent.
pyroclastic flow A dense, hot, chaotic avalanche of rock fragments, gas, and ash that travels rapidly away from an explosive eruption column, down the flanks of the volcano (synonymous with 'ash flow'). pyroclastic flows move at speeds ranging from 10 to several hundred meters per second and are typically at temperatures between 300 and 800 °C (Blong, 1984). pyroclastic flows form either by collapse of the eruption column, or by failure of the front of a cooling lava dome. Once these flows are initiated, they may travel distances of several kilometers or more and easily override topographic obstacles in the flow path. A person could not outrun an advancing pyroclastic flow.
pyroclastic surge A low-density, turbulent flow of fine-grained volcanic rock debris and hot gas. pyroclastic surges differ from pyroclastic flows in that they are less dense and tend to travel as a low, ground-hugging, but highly mobile cloud that an surmount topographic barriers. Surges often affect areas beyond the limits of pyroclastic flows.
Quaternary The period of Earth's history from about 2 million years ago to the present; also, the rocks and deposits of that age.
radiometric age An age estimate in years for rocks and other geologic materials determined by measuring the amount of a radioactive element such as carbon-14, or a radioactive element and its decay product such as potassium-40/argon-40.
reticulite reticulite is basaltic pumice in which nearly all cell walls of gas bubbles have burst, leaving a honeycomb-like structure. Even though it is less dense than pumice, reticulite does not float in water because of the open network of bubbles. The delicate glass threads between the bubbles are so fragile that reticulite was first called "thread-lace scoria" by the great American mineralogist, James Dana. It has also been called limu.
rhyodacite An extrusive rock intermediate in composition between dacite and rhyolite.
rhyolite Volcanic rock (or lava) that characteristically is light in color, contains 69% silica or more, and is rich in potassium and sodium.
Ring of Fire The regions of mountain-building earthquakes and volcanoes which surround the Pacific Ocean.
SAR interferometry Synthetic aperture radar (SAR) interferometry is a powerful remote sensing technique for measuring the distance to the Earth's surface. It requires that the surface is imaged by the radar at least twice from almost the same position in space. The property measured is the phase of the reflected radar signal. The differences in phase between the two images produce interferometric fringes that will represent both the topography of the surface and any changes in position of the surface during the period between the acquisition of the two images (Murray and others, 2000).
scarp A low concave cliff or series of cliffs that marks the detachment zone of a landslide or slope failure.
scoria A cinder-like volcanic rock, usually dark brown, red-brown, or black in color.
seamount A submarine volcano.
seismic swarm A series of earthquakes, occurring in a limited area over a relatively short period of time.
shield volcano A gently sloping volcano in the shape of a flattened dome and built almost exclusively of lava flows.
shoshonite A trachyandesite composed of olivine and augite phenocrysts in a groundmass of labradorite with alkali feldspar rims, olivine, augite, a small amount of leucite, and some dark-colored glass.
silica A chemical combination of silicon and oxygen.
skylight An opening formed by a collapse in the roof of a lava tube.
solfatara A type of fumarole, the gases of which are characteristically sulfurous.
spatter cone A low, steep-sided cone of spatter built up on a fissure or vent. It is usually of basaltic material.
spatter rampart A ridge of congealed pyroclastic material (usually basaltic) built up on a fissure or vent.
stratovolcano (also called a stratocone or composite cone). A steep-sided volcano, usually conical in shape, built of lava flows and fragmental deposits from explosive eruptions.
strike-slip fault A nearly vertical fault with side-slipping displacement.
strombolian An eruption style characterized by pulselike explosive bursts and low-level emission of ash and pyroclastic debris. Usually each burst lasts for only a few seconds, and sustained eruption columns generally do not develop.
subduction zone The zone of convergence of two tectonic plates, one of which usually overrides the other.
surge A ring-shaped cloud of gas and suspended solid debris that moves radially outward at high velocity as a density flow from the base of a vertical eruption column accompanying a volcanic eruption or crater formation.
swarm A group of many earthquakes of similar size occurring closely clustered in space and time with no dominant main shock.
talus A slope formed a the base of a steeper slope, made of fallen and disintegrated materials.
tectonic Refers to earthquakes generated by faulting rather than by volcanic activity.
tephra Any type of rock fragment that is forcibly ejected from the volcano during an eruption.
tephrochronology The collection, preparation, petrographic description, and approximate dating of tephra.
thermal anomaly A thermal anomaly can be defined as an unexpected increase in the radiant temperature of a pixel relative to its neighbors. Its neighbors may be considered as pixels next to it in the same image and same band, or pixels in a different band, or even pixels as they change through time. In other words, the anomaly can occur within the spatial, spectral and temporal domains. The term thermal anomaly does not carry with it any indication of the cause of the anomaly. In this case, the goal is to find thermal anomalies that are the result of volcanic activity. The causes of thermal anomalies include: 1.) Volcanic activity, e.g., warming of the ground, lavas, pyroclastics, gas and ash emissions. 2.) Solar reflections from peaks or meteoric clouds near the volcano, solar heating of rocks. 3.) Thermal contrast increases at lakes when the background drops below lake temperature. 4.) Clouds or vapors from fumaroles may obscure anomalies, or reflect sunlight. 5.) Changes in the atmospheric conditions may favor or hinder (dry vs. wet) detection of warm areas. 6.) Geometric constraints from the satellite's orbit and ground topography, an anomaly may be hidden from view by a crater wall. 7.) Noise in the sensor or in the reception of data. 8.) Natural or anthropogenic signals such as fires.
tilt The angle between the slope of a part of a volcano and some reference. The reference may be the slope of the volcano at some previous time.
trachyandesite An extrusive rock intermediate in composition between trachyte and andesite.
trachybasalt An extrusive rock intermediate in composition between trachyte and basalt.
trachyte A group of fine-grained, generally porphyritic, extrusive igneous rocks having alkali feldspar and minor mafic minerals as the main components, and possibly a small amount of sodic plagioclase.
tremor Low amplitude, continuous earthquake activity often associated with magma movement.
tsunami Widely spaced, fast-moving ocean wave(s) most commonly initiated by sudden displacements of the sea floor during earthquakes or submarine landslides. Volcanic eruptions can also cause tsunamis if unconsolidated volcanic sediment flows rapidly or falls into the water as in a catastrophic slope failure from a steep-sided volcanic cone or edifice, or if explosive eruptions occur at or near sea level. Tsunamis are capable of inundating significant portions of the coastline, especially if the wave energy is focused by narrowing of inlets and bays.
tuff Rock formed of pyroclastic material.
tuff cone A type of volcanic cone formed by the interaction of basaltic magma and water. Smaller and steeper than a tuff ring.
tuff ring A wide, low-rimmed, well-bedded accumulation of hyalo-clastic debris built around a volcanic vent located in a lake, coastal zone, marsh, or area of abundant ground water.
tumulus A doming or small mound on the crest of a lava flow caused by pressure due to the difference in the rate of flow between the cooler crust and the more fluid lava below.
ultramafic Igneous rocks made mostly of the mafic minerals hypersthene, augite, and/or olivine.
unconformity A substantial break or gap in the geologic record where a rock unit is overlain by another that is not next in stratigraphic sucession, such as an interruption in continuity of a depositional sequence of sedimentary rocks or a break between eroded igneous rocks and younger sedimentary strata. It results from a change that caused deposition to cease for a considerable time, and it normally implies uplift and erosion with loss of the previous formed record.
VEI The Volcanic Explosivity Index, or VEI, was proposed in 1982 as a way to describe the relative size or magnitude of explosive volcanic eruptions. It is a 0-to-8 index of increasing explosivity. Each increase in number represents an increase around a factor of ten. The VEI uses several factors to assign a number, including volume of erupted pyroclastic material (for example, ashfall, pyroclastic flows, and other ejecta), height of eruption column, duration in hours, and qualitative descriptive terms.
vent An opening in the Earth's surface through which magma erupts or volcanic gases are emitted.
vesicle A small air pocket or cavity formed in volcanic rock during solidification.
viscosity A measure of resistance to flow in a liquid (water has low viscosity while honey has a higher viscosity.)
volcanic arc A generally curved linear belt of volcanoes above a subduction zone, and the volcanic and plutonic rocks formed there.
volcanic complex A persistent volcanic vent area that has built a complex combination of volcanic landforms.
volcanic cone A mound of loose material that was ejected ballistically.
volcanic neck A massive pillar of rock more resistant to erosion than the lavas and pyroclastic rocks of a volcanic cone.
volcanic tremor Continuous seismic signal with regular or irregular sine wave appearance and low frequencies (0.5-5 Hz). harmonic tremor has a very uniform appearance, whereas spasmodic tremor is pulsating and consists of higher frequencies with a more irregular appearance.
volcaniclastic A volcanic rock or unconsolidated deposit composed of pre-existing fragments, particles or clasts of volcanic origin.
volcano A vent in the surface of the Earth through which magma and associated gases and ash erupt; also, the form or structure (usually conical) that is produced by the ejected material.
Vulcan Roman god of fire and the forge after whom volcanoes are named.
vulcanian A type of eruption consisting of the explosive ejection of incandescent fragments of new viscous lava, usually on the form of blocks.
xenocrysts A crystal that resembles a phenocryst in igneous rock, but is a foreign to the body of rock in which it occurs.
xenoliths A foreign inclusion in an igneous rock.
yBP Years Before Present, also known as "time before present." This time scale is used to specify when events occurred relative to the origin of practical radiocarbon dating in the 1950s.