Active: A volcano which is currently erupting or has erupted
in recorded history
Aerosol: A mass of tiny solid or liquid particles suspended
in air or another gas (see volcanic gas).
Aleutian Island Arc: Islands in western Alaska extending in
an arc 1200 miles southwest from the Alaska peninsula.
Ash: Volcanic ash consists of tiny jagged particles of rock
and natural glass blasted into the air by a volcano. Ash may be solid
or molten when first erupted.
Ash Cloud: A cloud formed from tiny ash particles and gases
blasted from the volcano. Wind can carry ash thousands of miles, affecting
far greater areas and many more people than other volcano hazards
Avalanches:Debris avalanches (volcanic landslides) are rapid
downslope movements of rock, snow, and ice. Landslides range in size from
small movements of loose debris on the surface of a volcano to massive
failures of the entire summit or flanks of a volcano. Volcanic avalanches
and landslides can occur even when the volcano is not erupting. Excessive
rainfall and/or earthquakes may start the material moving down hill.
Blast: An explosive eruption producing clouds of hot ash and/or
other volcanic material.
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Blocks: A solid rock fragment greater than 64 mm in diameter
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.
Bombs: Hot lava thrown out in twisted chunks that may change shape
during flight from the volcano or on impact with the earth.
BP: Years (approximate) before present time.
Caldera: A large, basin-shaped depression formed by the inward
collapse of a volcano after or during an eruption (movie).
Cinder cone: A steep sided small cone composed of cinders,
ash and bombs.
Cone-shaped: Shaped like an upside down ice cream cone.
Contact metamorphism: Contact metamorphism occurs when the
heat from an intruding magma changes the mineralogy and texture of the
surrounding pre-existing rock. Heat, rather than pressure, is the primary
cause of the metamorphism. Contact metamorphism is usually restricted
to relatively shallow depths (low pressure) in Earth. This is because
as depth increases, so do pressure and temperature. At depth, there will
not be a large contrast in temperature between the intruding magma and
the surrounding rock.
Crater: A small funnel-shaped depression in the summit of a
volcano at the top of the conduit or pipe through which the magma reaches
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Crust: The thin outermost layer
of the Earth including both the continents and the ocean floors.
Earth's Interior Divisions
Outer Radius, km
Approximate Temperature, C
Crystallization: The process through which crystals separate from
the fluid (magma) state.
Debris avalanches, debris flows: See Avalanche.
Density: A measure of how heavy or light an object is for its
Dormant: An active volcano which is quiet, not presently erupting,
but is expected to erupt in the future. Most of the major Cascade volcanoes
are believed to be dormant rather than extinct.
Earthquake: The shaking of the ground caused by an abrupt shift
of rock along a fault. Within seconds, an earthquake releases stress that
has slowly accumulated within the rock, sometimes over hundreds of years.
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Eject: To throw out, forcefully discharge.
Electrical Discharges: Release of electricity (a form of energy
caused by the motion of electrons).
Eruption: The expelling of material including gases, ash, volcanic
fragments and lava on Earth's surface due to volcanic activity. Eruptions
may be explosive, or quiet lava flows.
Extinct: A volcano that is not expected to erupt again.
Fire fountain: Lava that is shot into the air like a geyser
by the pressure of trapped gases within the magma.
Fissure: A fissure is an elongate fracture or crack at the
surface from which lava erupts.
Geothermal energy: The word "geothermal" literally
means "Earth" plus "heat". To produce electric power
from geothermal resources, underground reservoirs of steam or hot water
are tapped by wells and the steam rotates turbines that generate electricity.
Geysers: Most geysers are hot springs that episodically erupt fountains
of hot water and steam. Such eruptions occur as a consequence of groundwater
being heated to boiling temperature in a confined space (underground).
Glacier: A large mass of ice formed by compressed snow, which moves
slowly under its own weight. Glaciers exist where, over a period of years,
snow remains after summer's end and accumulates year after year.
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Glowing avalanche: Hot ash and larger particles erupted from
the volcano that flow down the sides of the volcano.
Hot spot: A hotspot is a stationary, long-lived (tens of millions
of years) source of basaltic magma coming up through the mantle to the
Hot spring: A spring whose water temperature is above 36.6
°C (98 °F).
Igneous: Rocks solidified from molten magma at or below the
surface of the Earth.
Lahars: Debris flows and/or mudflows produced by loose soil
and rock flowing down the sides of the volcano.
Lateral Blast: A relatively rare explosion of hot, low-density
mixture of rock debris, ash, and gases that moves at high speed out the
side of the vent (laterally) rather than up from the vent (vertically).
(See short movie).
Lava: Molten rock erupted from a volcano. Lava can occur in flows,
domes, fragments and as pillows formed underwater.
Lava Dome: Lava which is forced from the vent much like toothpaste
from a tube, forming a half-ball shape over the vent. A lava dome forms
when the lava is too viscous to flow far from the vent. It continues to
grow upward until it collapses.
Lithosphere: The solid outer shell of the Earth composed of the
crust and the solid outermost layer of the mantle. The lithosphere lies
above the asthenosphere (soft layer of the mantle) and is broken into
crustal plates. (See also Tectonic plates
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Magma: Molten rock below the surface of the Earth that rises
in volcanic vents. Lava is the term for magma after it erupts from a volcano.
Magma Chamber: A space beneath the surface of the earth surrounded
by solid rock and containing magma.
Mana: Maori term signifying a sense of identity, pride and
strength of spirit.
Mantle: The area within the earth that is below the crust
and above the core of the earth.
Mudflows: The downhill movement, often rapid, of soft wet earth
and debris, made fluid by rain or the rapid melting of snow.
Nuées ardentes: A French term applied to a highly heated
mass of gas-charged ash which is expelled with explosive force and moves
with hurricane speed down the mountainside. Nuées ardentes reach
temperatures between 300 to 800 °C, are lighter than glowing avalanches,
and often jump ridges when moving down the flank of a volcano.
Ozone: A form of oxygen that has a pale blue color and a strong
smell. This gas is formed when an electrical discharge passes through
the air. It can be poisonous in large quantities. The ozone layer high
above Earth's surface blocks out some of the harmful rays of the sun.
Plinian Eruption: Plinian eruptions are one of the most explosive
types of eruptions, forming enormous dark columns of tephra and gas high
into the stratosphere (>11 km). They often produce nuées ardentes,
lahars, and caldera collapse. Plinian eruptions are named for Pliny the
Younger (Gaius Plinius Caecilius Secundus), a Roman statesman who carefully
described the disastrous eruption of Vesuvius in 79 A.D. which killed
his father as well as about 2,000 other people.
Plume: A long, feather-shaped cloud of steam or gases.
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Portable volcano observatory: A collection of hardware
and software, which is easiliy moved and installed, to monitor restless
volcanoes. Key components for data gathering, storage, and analysis include
(but are not limited to):
- Personal Computer (PC),
- up to 128 seismometers for measuring earthquake locations and magnitudes,
- electronic distance meters (EDMs), theodolites, reflectors, and Global
Positioning System (GPS) receivers to detect surface deformation,
- correlation spectrometer (COSPEC) to measure sulfur dioxide emission
- low-data-rate, radio-telemetry system to transmit monitoring data
from remote field sites to the portable observatory.
Precipitate: A percipitate is formed when a slightly soluble
substance becomes insoluble and separates from a solution due to heat
or a chemical reaction. The term is used to indicate the act of forming
a solid and for the substance that is precipitated out of a solution.
Pumice: Light rock froth produced by the violent separation of
gas from lava. Because of the many gas bubbles, some of this froth is
so light that it floats on water.
Pyroclastic flows: A high-density mass of gases, hot ash, and larger
material that flows rapidly down the sides of the volcano. Flows tend
to be confined to valleys. Because of the speed at which they travel and
the intense heat, pyroclastic flows and surges are one of the most dangerous
hazards posed by volcanoes.
Pyroclastic surge: A turbulent, low-density cloud of hot rock
debris and gases that moves at extremely high speeds. Because surges are
low density, they tend to spread over large areas and jump ridge crests
Quiescent: A volcano, which is not active, but is still registering
seismic activity. When there is no more seismicity, the volcano is dormant,
but still capable of erupting (see active,
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Reservoir: A place where a large supply of magma collects.
Ring of Fire: A zone around
the perimeter of the Pacific Ocean containing about two-thirds of the
world’s active volcanoes.
Seismograph: An instrument used to measure the shaking caused
by an earthquake.
Shield Volcano: Volcanoes with broad, gentle slopes built by
the eruption of fluid basalt lava.
Spreading Zone: An area of the surface of the earth where the
plates are moving away from one another.
St. Elmo’s Fire: An electrical discharge, like that found in
a neon sign, occuring in nature on pointed objects during electrical storms.
Such phenomenon occasionally occur in the ash cloud of a volcano.
Steam (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.
Stratosphere: The layer of Earth's atmosphere that begins about
11 km (7 mi) above Earth and ends about 50 km (31 mi) above Earth. Clouds
rarely form here and the air is very cold and thin.
Stratovolcano: A large, steep-sided, symmetrical cone built
of alternating layers (strata) of lava, ash, cinders, blocks, and bombs.
Also called composite volcano, these stratovolcanoes form some of Earth's
grandest mountains rising as much as 8,000 feet above their bases (movie).
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Strombolian Eruption: Derived from the volcano Stromboli, these
eruptions, typically intermittent and of short duration, throw out blocks,
bombs and lava flows. Stromboli is one of the Aeolian Islands north of
Sicily and has been almost continuously in eruption for at least the past
Summit: The top of a mountain or volcano.
Tapu: Maori word for something that is sacred.
Tectonic plates: A rigid section of Earth's crust that moves
relative to other such sections on the Earth’s surface (see also crust).
Tephra: The general term used by volcanologists for fragments
of volcanic rock and lava of any size expelled from a volcano.
Tsunami: Term for large, rapidly moving water waves caused by the
displacement of water, usually by earthquakes, landslides and volcanic
eruptions. Tsunamis are also referred to as tidal waves, but they have
no relation to tides.
VEI: Volcanic explosivity index is a scale from
1 to 8 measuring of the size of eruptions. This measurement takes
into account the height of the eruption cloud, amount of material erupted
(ash, tephra, etc.) and distances to which objects of particular size
were thrown. See also Volcano World's How
Big are Eruptions.
Vent: Opening in Earth's crust through which volcano expels
ash, other volcanic products and gases.
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Viscosity: Resistance of a liquid to flow. Thick liquids have
high viscosity, thin liquids have low viscosity.
Volcano: A vent in Earth's crust through which molten or hot
rock, steam, and ash reach the surface, including the cone built by the
Volcanic bombs: See bombs.
Volcanic gas: Dissolved gases contained in the magma are released
into the atmosphere during volcanic eruptions. Gases may also escape continuously
from volcanic vents, fumaroles, and hotsprings. The most common gas released
is steam (H2O), followed by CO2 (carbon dioxide),
SO2 (sulfur dioxide), (HCl) hydrogen chloride and other compounds.
See the USGS for more information on volcanic
Volcanic lightning: Lightning fomed as a result of electrical charges
in the volcano plume due to the negatively charged falling ash particles
and positively charged condensed volcanic gas associated with the plume.
Volcanologists: Scientists who study volcanoes.
Vulcanian eruption: Moderate-sized explosive eruption that ejects
new lava fragments that do not take on a rounded shape during their flight
through the air. The name comes from Vulcano, one of the Aeolian Islands
north of Sicily, believed to be the home of the Roman god of fire, Vulcan.