Glossary of Volcanic Features
- atolls
coral reefs in the open ocean, often ring-shaped; need 18-30oC warm water to grow; found at latitudes less than 30o; originally grew on outer rim of volcanic islands; volcanoes subsided and eroded while corals continued to grow; volcano eventually lost structure above water
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- black smokers
hot mineral-rich water escapes through tall chimneys
near mid-ocean ridges, creating a unique "extreme environment" in which creatures
life near hot (> 100oC) water
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- calderas
a large crater-like depression that forms as a result of a large volcanic eruption.
Calderas are more than 1km across (e.g. Santorini, Crater Lake, Krakatau),
usually smaller in diameter than the
original base of the volcano.
A possible scenario is a humongous explosion but extensive effusive eruptions may also
cause the formation of a caldera when the magma chamber empties out extensive eruptions and
the roof of the magma chamber collapses under the weight of the overlying volcano.
The course book associates calderas with high viscosity,
high volatiles, very large volume eruptions. This is not always the case!
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- cataclysmic
from the Greek word for flood
violent volcanic eruptions that change the Earth's surface. Usually such eruptions destroy the main volcanic structure and leave a large caldera behind. If occurring at sea, such an eruption triggers catastrophic tsunamis.
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- cinder or spatter or scoria cones
eruptions of gas-rich frothy lava produce sprays
of marble-size tephra that collect on steep cones. The slopes approach
the "angle of repose", the steepest slope a pile can sustain without
collapsing (about 40o) (e.g. Haleakala, Hawaii; Sunset Crater, AZ)
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- effusive
one of the two eruption styles of volcanoes.
effusive volcanism produces mainly lava flows; low-viscosity basaltic lavas;
lava fountains form when lots of dissolved gases are present.
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- explosive
one of the two eruption styles of volcanoes.
typically pyroclastic eruptions; high-viscosity
very silicic lava; gas in the magma chamber expands but cannot
escape eventually leading to explosion; phreatic eruptions when
significant amounts of water/steam is involved
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- fissures
very low-viscosity lava escapes through long cracks without
forming a volcano (forms plateaus instead); tens of kilometers long (e.g. Iceland)
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- fumaroles
volcanic gas exhalations through holes in the ground; usually visible (when lots of
H2O involved) and smelly (when sulphur involved)
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- geysers
circulating ground water that gets heated by hot rock
underground and then erupts at the surface (e.g. Yellowstone, Iceland); some geysers
can be quite regular (e.g. Old Faithful at 75 min) but usually they are not
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- hot springs
pleasant warm spas that may occur even in cold climate; often carry minerals;
water heated below ground by either heat coming through a fault or by deep-seated rock
(e.g. magma chamber)
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- lava dome
high-viscosity lava pushes up volcanic vent, often clogging
the crater; can get dangerous when volcanic gases cannot escape; results in
violent eruptions with pyroclastic flows
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- mofettes
like fumaroles, except that the only volcanic gas involved is CO2; also, gas may not be hot
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- pyroclast
from the Greek words for fire and fragment
any material, from fine dust and ash to large blocks of rock, ejected from the vent of an erupting volcano
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- resurgent calderas
much larger than the original volcano; created during humongous
eruptions; eventually caldera floor starts to rise due to the removal of fill within the
caldera as well as ascending hot material beneath (e.g. Long Valley Caldera, CA; Yellowstone, WY; Valles, NM)
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- tephra (ejectamenta)
from the Greek word for ashes
fragmental material thrown out by a volcano, including ash, lapilli, cinders,
bombs, and pumice.
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- VEI
This acronym stands for Volcanic Explosivity Index and gives a measure for the severity of a volcanic eruption. The scale goes from 0 to 8, with 0 being harmless eruptions and 8 being humongous, catastrophic eruptions. See Lecture 9 for details.
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- volcano
The word volcano comes from ''Vulcan'', the ancient Roman god of fire. He was thought to be responsible for the spewing of molten rock at the surface of some islands in Southern Italy (the Aeolian Islands). One of these islands still today is called Volcano.
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