a'a'A Hawaiian term for
lava flows that have a rough, jagged surface.
alkalicRocks 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.
andesiticA fine-grained volcanic rock made up of feldspars
and ferromagnesian minerals; typically has a
silica content of 54 to about 62 percent.
ashFine fragments (less than 2 millimeters across) of
lava or rock formed in an explosive volcanic eruption.
ash flowA 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.
avalancheA 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.
basaltVolcanic rock (or
lava) that characteristically is dark in color, contains 45% to 54%
silica, and generally is rich in iron and magnesium.
blockA 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.
bombFragment 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.
calderaA 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 volcanoA
volcano constructed by the ejection of debris and
lava flows from a central point, forming a more or less symmetrical volcano.
cinder coneA
volcanic cone built entirely of loose fragmented material (pyroclastics.)
cohesive laharA type of volcanic
debris flow that
contains more than 3-5 percent clay in the deposit matrix. Such lahars are thought to form from volcanic debris avalanches. Cohesive lahars are very mobile and may flow long distances (several tens of kilometers or more).
craterA steep-sided, usually circular depression formed by either explosion or collapse at a volcanic
vent.
daciteVolcanic rock (or
lava) that characteristically is light in color and contains 62% to 69%
silica and moderate a mounts of sodium and potassium.
debris avalancheRapidly 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 flowA mixture of water-saturated rock debris that flows downslope under the force of gravity (also called
lahar or
mudflow).
dikeA tabular igneous
intrusion that cuts across the
host bedrock.
domeA 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
eruption cloudCloud of gas,
ash, and other fragments that forms during an explosive volcanic eruption and travels long distances with the prevailing winds.
eruption columnThe vertical portion of the eruption
cloud that rises above a volcanic
vent.
falloutA general term for debris that falls to the earth from an
eruption cloud.
faultA 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 scarpA 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.
felsicAn igneous rock having abundant light-colored minerals.
fissureElongated fractures or cracks on the slopes of a
volcano. Fissure eruptions typically produce liquid flows, but pyroclastics may also be ejected.
flank eruptionAn eruption from the side of a
volcano (in contrast to a summit eruption.)
fractureThe manner of breaking due to intense folding or faulting.
fumaroleA small volcanic
vent from which gases and
vapors are emitted.
GPSA surveying technique that uses signals from a series of artificial satellites to determine position on the Earth's surface.
harmonic tremorA 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.
HoloceneA 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.
hummockRounded 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.
hypocenterThe point within the Earth that is the center
of an earthquake. The initial point of rupture
during an earthquake.
ice pistonIce 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.
ignimbriteThe 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.
infrasoundLow-frequency sound waves, below the threshold of human hearing. Erupting volcanoes produce
infrasound.
intrusionThe process of emplacement of
magma in pre-existing rock. Also, the term refers to igneous rock mass so formed within the surrounding rock.
jokulhlaupAn Icelandic term that refers without distinction to both water floods and lahars that are generated when a
volcano erupts under a glacier.
kipuka
laccolithA body of igneous rocks with a flat bottom and domed top. It is parallel to the layers above and below it.
laharAn 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.
lapilliRock 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.
lavaLava 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 deltaLava 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 domeA steep-sided mass of viscous and commonly blocky
lava extruded from a
vent; typically has a rounded top and roughly circular outline.
lava flowAn 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 fountainA rhythmic vertical fountainlike eruption of
lava.
lava tubeA tunnel formed when the surface of a
lava flow cools and solidifies while the still-molten interior flows through and drains away.
lithicOf or pertaining to stone.
lithosphereThe 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 eventDiscrete events with very regular, low-frequency (1-5 Hz) waveforms that resonate for many cycles.
maarA 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.
maficAn igneous composed chiefly of one or more dark-colored minerals.
magmaMolten rock beneath the Earth's surface.
magma chamberThe subterranean cavity containing the gas-rich liquid
magma which feeds a
volcano.
magnitudeA 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.
mantleThe zone of the earth below the crust and above the core.
monogeneticA
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.
mudflowA 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.
noncohesive laharA type of volcanic
debris flow that contains less than 3-5 percent clay in the deposit matrix. Such lahars form when meltwater produced by the interaction of
pyroclastic flows and snow or ice picks up locally available sediment on the flanks of a
volcano, or in stream channels developed on the volcano. Noncohesive lahars usually evolve
downstream into watery sediment-laden flows
called hyperconcentrated flows, or floods.
nuees ardentesA 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.
obsidianA black or dark-colored volcanic glass, usually composed of
rhyolite.
Pele HairA natural spun glass formed by blowing-out during quiet fountaining of fluid
lava, cascading lava falls, or turbulent flows, sometimes in association with pele tears. A single strand, with a diameter of less than half a millimeter, may be as long as two meters.
Pele TearsSmall, solidified drops of volcanic glass behind which trail pendants of Pele hair. They may be tear-shaped, spherical, or nearly cylindrical.
peralkalineIgneous rocks in which the molecular proportion of aluminum oxide is less than that of sodium and potassium oxides combined.
phenocrystA conspicuous, usually large, crystal embedded in porphyritic igneous rock.
phreatic eruptionAn 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.
phreatomagmaticAn explosive volcanic eruption that results from the interaction of surface or subsurface water and
magma.
pillow lavaInterconnected, sack-like bodies of
lava formed underwater.
pit craterPit 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.
PleistoceneA geologic time designation for the period of Earth history from about 1.6 million years ago to about 10,000 years ago.
plinian eruptionAn 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.
plugSolidified
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 domeThe 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.
polygeneticOriginating in various ways or from various sources.
pumiceLight-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.
pyroclasticPertaining to fragmented (clastic) rock material formed by a volcanic explosion or ejection from a volcanic
vent.
pyroclastic flowA 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 surgeA 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.
QuaternaryThe period of Earth's history from about 2 million years ago to the present; also, the rocks and deposits of that age.
radiometric ageAn 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.
reticuliteReticulite 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.
rhyodaciteAn extrusive rock intermediate in composition between
dacite and
rhyolite.
rhyoliteVolcanic rock (or
lava) that characteristically is light in color, contains 69%
silica or more, and is rich in potassium and sodium.
Ring of FireThe regions of mountain-building earthquakes and volcanoes which surround the Pacific Ocean.
SAR interferometrySynthetic 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).
scarpA low concave cliff or series of cliffs that marks the detachment zone of a landslide or slope failure.
scoriaA cinder-like volcanic rock, usually dark brown, red-brown, or black in color.
seamount
seismic swarmA series of earthquakes, occurring in a limited area over a relatively short period of time.
shield volcanoA gently sloping
volcano in the shape of a flattened
dome and built almost exclusively of
lava flows.
shoshoniteA
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.
silicaA chemical combination of silicon and oxygen.
skylightAn opening formed by a collapse in the roof of a
lava tube.
solfataraA type of
fumarole, the gases of which are characteristically sulfurous.
spatter coneA low, steep-sided cone of spatter built up on a
fissure or
vent. It is usually of basaltic material.
spatter rampart
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 faultA nearly vertical
fault with side-slipping displacement.
strombolianAn 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 zoneThe zone of convergence of two
tectonic plates, one of which usually overrides the other.
surgeA 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.
swarmA group of many earthquakes of similar size occurring closely clustered in space and time with no dominant main shock.
talusA slope formed a the base of a steeper slope, made of fallen and disintegrated materials.
tectonicRefers to earthquakes generated by faulting rather than by volcanic activity.
tephraAny type of rock fragment that is forcibly ejected from the
volcano during an eruption.
tephrochronologyThe collection, preparation, petrographic description, and approximate dating of
tephra.
thermal anomalyA
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.
tiltThe 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.
trachyandesiteAn extrusive rock intermediate in composition between
trachyte and andesite.
trachybasaltAn extrusive rock intermediate in composition between
trachyte and
basalt.
trachyteA 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.
tremorLow amplitude, continuous earthquake activity often associated with
magma movement.
tsunamiWidely 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
tuff cone
tuff ringA 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.
tumulusA 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.
ultramaficIgneous rocks made mostly of the
mafic minerals hypersthene, augite, and/or olivine.
unconformityA 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.
VEIThe 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.
ventAn opening in the Earth's surface through which
magma erupts or volcanic gases are emitted.
vesicleA small air pocket or cavity formed in volcanic rock during solidification.
viscosityA measure of resistance to flow in a liquid (water has low
viscosity while honey has a higher viscosity.)
volcanic arcA generally curved linear belt of volcanoes above a
subduction zone, and the volcanic and plutonic rocks formed there.
volcanic complexA persistent volcanic
vent area that has built a complex combination of volcanic landforms.
volcanic coneA mound of loose material that was ejected ballistically.
volcanic neck
volcanic tremorContinuous 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.
volcaniclasticA volcanic rock or unconsolidated deposit composed of pre-existing fragments, particles or clasts of volcanic origin.
volcanoA
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.
VulcanRoman god of fire and the forge after whom volcanoes are named.
vulcanianA type of eruption consisting of the explosive ejection of incandescent fragments of new viscous
lava, usually on the form of blocks.
xenocrystsA crystal that resembles a
phenocryst in igneous rock, but is a foreign to the body of rock in which it occurs.
xenolithsA foreign inclusion in an igneous rock.