Aa: Hawaiian word used to describe a lava flow whose surface is broken into rough angular
fragments.
Active volcano: A volcano that is erupting. Also, a volcano that is not presently erupting but that has
erupted within historical time and is considered likely to do so in the future.
Ash: Fine particles of pulverized rock blown from an explosion vent. Ash may be either solid or molten when first erupted.
Ashfall: Volcanic ash that has fallen through the air from an eruption cloud. A deposit so
formed is usually well sorted and layered.
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.
Avalanche: A large mass of material or mixtures of material falling or sliding rapidly under the force
of gravity.
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: The Spanish word for cauldron, a basin-shaped volcanic depression; by definition, at least a
mile in diameter. Such large depressions are typically formed by the subsidence of volcanoes. Crater
Lake occupies the best-known caldera in the Cascades.
Central Vent: A central vent is an opening at the Earth's surface of a volcanic conduit of cylindrical
or pipelike form.
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.)
Composite Volcano: A steep volcanic cone built by both lava flows and pyroclastic eruptions.
Compound Volcano:A volcano that consists of a complex of two or more vents, or a volcano that has
an associated volcanic dome, either in its crater or on its flanks. Examples are Vesuvius and Mont
Pelee.
Conduit: A passage followed by magma in a volcano.
Crater: A steep-sided, usually circular depression formed by either explosion or collapse at a volcanic
vent.
Debris avalanche: A rapid and unusually sudden sliding or flowage of unsorted masses of rock and
other material. As applied to the major avalanche involved in the eruption of Mount St. Helens, a rapid
mass movement that included fragmented cold and hot volcanic rock, water, snow, glacier ice, trees,
and some hot pyroclastic material.
Dome: A steep-sided mass of lava extruded from a volcanic vent. Its surface is often rough and blocky as a result of fragmentation of the cooler, outer crust during growth of the dome.
Dormant volcano: The term is used to describe a volcano which is presently
inactive but which may erupt again.
Eruption: The process by which solid, liquid, and gaseous materials are ejected into the earth's
atmosphere and onto the earth's surface by volcanic activity. Eruptions range from the quiet overflow
of liquid rock to the tremendously violent expulsion of pyroclastics.
Eruption cloud: The column of gases, ash, and larger rock fragments rising from a crater or other
vent. If it is of sufficient volume and velocity, this gaseous column may reach many miles into the
stratosphere, where high winds will carry it long distances.
Eruptive vent: The opening through which volcanic material is emitted.
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.
Fissures: Elongated fractures or cracks on the slopes of a volcano. Fissure eruptions typically
produce liquid flows, but pyroclastics may also be ejected.
Fumarole: A vent or opening through which issue steam, hydrogen sulfide, or other gases. The craters
of many dormant volcanoes contain active fumaroles.
Harmonic tremor: A release of seismic energy associated with the underground
movement of magma.
Hot Spot: A volcanic center, 60 to 120 miles across and persistent for at least a few
tens of million of years, that is thought to be the surface expression of a persistent rising plume of hot
mantle material. Hot spots are not linked to arcs, and may not be associated with ocean ridges.
Hyaloclastite: A deposit formed by the flowing or intrusion of lava or magma into water, ice, or
water-saturated sediment, and its consequent granulation or shattering into small angular fragments.
Intensity: A measure of the effects of an earthquake at a particular place. Intensity depends not only
on the magnitude of the earthquake but also on the distance from the epicenter and the local geology.
Intrusion: The process of emplacement of magma in pre-existing rock. Also refers to igneous rock
mass so formed within the surrounding rock.
Joint: A surface of fracture in a rock.
Lahar: A flow of water-saturated volcanic debris down the slope of a volcano in response to
gravity. A type of mudflow.
Lava: Magma which has reached the surface through a volcanic eruption. The term is applied to streams of liquid rock that flow from a crater or fissure.
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 lake: A lake of molten lava, usually basaltic, in a volcanic crater or depression. The term
refers to solidified and partially solidified stages as well as to the molten, active lava lake.
Magma: Molten rock beneath the surface of the earth.
Magma chamber: The subterranean cavity containing the gas-rich liquid magma which feeds a
volcano.
Mantle: The zone of the earth bel ow the crust and above the core.
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.
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.
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.
Pyroclastic: Pertaining to fragmented (clastic) rock material formed by a volcanic explosion or
ejection from a volcanic vent.
Pyroclastic flow: Flowage of a mixture of hot gases and unsorted pyroclastic
material that can move at a maximum speed of 100 miles an hour.
Repose:The interval of time between volcanic eruptions.
Rift zone: A zone of volcanic features associated with underlying dikes. The location of the rift is
marked by cracks, faults, and vents.
Ring of Fire: The regions of mountain-building earthquakes and volcanoes which surround the Pacific
Ocean.
Seamount:A submarine volcano.
Stratovolcano: A volcano composed of both lava flows and pyroclastic material.
Strombolian eruption: A type of volcanic eruption characterized by jetting of clots or fountains of
fluid basaltic lava from a central crater.
Vent: The opening at the earth's surface through which volcanic materials issue forth. Usage of vent.
Vesicle: A small air pocket or cavity formed in volcanic rock during solidification.
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.
Volcanic arc: A generally curved linear belt of volcanoes above a subduction zone, and the volcanic
and plutonic rocks formed there.
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.
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