Extrusive volcanism

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  • Created on: 31-03-13 16:32
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Extrusive volcanism ­ a summary
Landform / structure Description Process /notes
A volcano is an opening, or rupture, in a
planet's surface or crust, which allows hot
magma, ash and gases to escape from below
the surface. The word volcano is derived from
the name of Vulcano island off Sicily which in
turn, was named after Vulcan, the Roman god
of fire. Volcanoes are generally found where
tectonic plates are diverging or converging. A
mid-oceanic ridge, for example the
Mid-Atlantic Ridge, has examples of volcanoes
caused by divergent tectonic plates pulling
apart; the Pacific Ring of Fire has examples of
volcanoes caused by convergent tectonic
plates coming together. By contrast,
Volcano volcanoes are usually not created where two
tectonic plates slide past one another.
Volcanoes can also form where there is
stretching and thinning of the Earth's crust
(called "non-hotspot intraplate volcanism"),
such as in the African Rift Valley, the Wells
Gray-Clearwater volcanic field and the Rio
Grande Rift in North America and the European
Rhine Graben with its Eifel volcanoes.
Volcanoes can be caused by mantle plumes.
These so-called hotspots, for example at
Hawaii, can occur far from plate boundaries.

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A pyroclastic flow (also known scientifically
as a pyroclastic density current is a common
and devastating result of certain explosive
Pyroclastic flow volcanic eruptions. The flows are fast-moving
currents of hot gas and rock (collectively
known as tephra), which travel away from the
volcano at speeds generally as great as
700 km/h (450 mi/h). The gas can reach
temperatures of about 1,000 °C. The flows
normally hug the ground and travel downhill,
or spread laterally under gravity.…read more

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Volcanic cones are among the simplest
volcanic formations in the world. They are built
by ejecta from a volcanic vent, piling up
around the vent in the shape of a cone with a
Volcanic cones central crater. Volcanic cones are of different
types, depending upon the nature and size of
the fragments ejected during the eruption.
Types typically differentiated are spatter
cones, ash cones, tuff cones, and cinder ones.…read more

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Spatter cones are typical of
volcanoes with highly fluid magma, such as
those found in the Hawaiian Islands.
An ash cone is composed of particles of silt to
sand size. Explosive eruptions from a vent
where the magma is interacting with
groundwater or the sea (as in an eruption off
the coast) produce steam and are called
phreatic. The interaction between the magma,
Ash cones expanding steam, and volcanic gases results in
the ejection of mostly small particles called
ash.…read more

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Cinder cones rarely rise more than 300 to 750
m or so above their surroundings, and, being
unconsolidated, tend to erode rapidly unless
further eruptions occur. Cinder cones are
numerous in western North America as well as
throughout other volcanic terrains of the
world. Parícutin, the Mexican cinder cone
which was born in a cornfield on February 20,
1943, and Sunset Crater in Arizona in the US
are classic examples of cinder cones.…read more

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Flood basalts
have erupted at random intervals throughout
geological history and are clear evidence that
Flood basalts the Earth undergoes periods of enhanced
activity rather than being in a uniform steady
state. One explanation for flood basalts is that
they are caused by the combination of
continental rifting and its associated
decompression melting, in conjunction with a
mantle plume also undergoing decompression
melting, producing vast quantities of a
tholeiitic basaltic magma.…read more

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A stratovolcano, sometimes called a
composite volcano is a tall, conical volcano
with many layers (strata) of hardened lava,
tephra, and volcanic ash. Stratovolcanoes are
Composite (strato) or characterized by a steep profile and periodic,
andesitic explosive eruptions. The lava that flows from
volcano stratovolcanoes tends to be viscous; it cools
and hardens before spreading far. The magma
forming this lava is often felsic, having
high-to-intermediate levels of silica (as in
rhyolite dacite, or andesite), with lesser
amounts of less-viscous mafic magma.…read more

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Cañadas Caldera Teide, Tenerife and the
Caldera de Taburiente, La Palma, both in the
Canary Islands. When he published his memoirs
he introduced the term "caldera" into the
geological vocabulary. A collapse is triggered
by the emptying of the magma chamber
beneath the volcano, usually as the result of a
large volcanic eruption. If enough magma is
ejected, the emptied chamber is unable to
support the weight of the volcanic edifice
above it.…read more


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