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Composition
Types of deposits
Types of volcanoes
Distribution
Prediction
Impact of eruptions
Supervolcanoes
Volcano: A mound of material that is extruded to the
Earth’s surface from a vent that is connected to a magma
chamber via a feeder conduit.
Volcanoes are classified according to their form.
The form of a volcanoes depends on the type of material
that it is made up of.
The nature of the extruded material (and the volcano
itself) depends on the properties of the magma.
Magma: Molten rock within the Earth.
Magma is called lava when it reaches the surface.
The composition of magma determines the type of rock
that forms when it cools and its behavior during an
eruption.
Main controls on behavior:
chemical composition (largely silica dioxide - SiO2 content)
and
gas content (largely water vapor and CO2).
SiO2 content controls the viscosity of a magma.
Viscosity: a measure of how easily a fluid flows. Water has
a low viscosity, molasses has a much higher viscosity.
Viscosity, in turn, controls the amount of gas that can be
trapped in the magma.
The greater the viscosity the more gas in the magma.
There are three basic types of magma:
Basaltic Magma
Andesitic Magma
Rhyolitic Magma
The names are based on the rock type that forms when the
magma crystallizes.
Magma
Type
Chemical
Composition
Basaltic
45-55% SiO2;
High in Fe,
Mg, Ca; Low in
K, Na.
Temperature Viscosity
(degrees C)
1000 - 1200
Low
Gas
Content
Low
Andesitic 55-65% SiO2;
Intermediate
Fe, Mg, Ca,
Na, K
800-1000
Intermediate Intermediate
Rhyolitic 65-75% SiO2;
Low in Fe, Mg,
Ca; High in K,
Na
650-800
High
High
Overall, the behaviour of the magma determines the type
of volcano that develops:
Low SiO2 magmas, with little gas and low viscosity, flows
readily through their vents and across the land surface
when the lava escapes the vents.
High SiO2 magmas, gaseous and with high viscosity, tend
to plug their vents until the force of escaping magma blows
the vent clear; such magmas cause explosive volcanoes.
Types of volcanic deposits
(photos from USGS)
Volcanoes also vary in terms of the types of deposits that
they produce.
Lava: Hot (up to 1200 degrees C), fluid, molten rock
that flows along the land surface.
Lava can flow like viscous water, including forming lava
falls.
Pahoehoe: Lava with a ropelike surface texture due to
partial cooling as the lava flowed. Relatively hot, low
viscosity lava.
Pahoehoe
A thick deposit of pahoehoe lava
Aa: Blocky, rough lava flow. Due to high viscosity lava
that flowed pushing chunks of solid and semi-solid
blocks.
Lava tube: A tube
formed by cooling and
solidifying of the lava
walls while fluid lava
continued to flow inside.
Pillows: A form of closed lava tube (with a bulbous
end) that forms when a lava flows into water (e.g., a
lake or ocean) and cools very rapidly.
http://oceanexplorer.noaa.gov/explorations/04fire/background/volcanism/media/pillow_lava_video.html
Pyroclastic material: Debris formed by a volcanic
explosion. Results when magma is very viscous.
Tephra: The general term for all pyroclastic material
that is ejected from a volcano. Different terms apply
according to the size of the tephra. (syn. Ejecta)
Ash: tephra that is finer than 2 mm in diameter.
Lapilli: from 2 mm to 64 mm in diameter.
Blocks: hard fragments greater
than 64 mm in diameter.
Bombs: soft, partially melted fragments greater than
64 mm in diameter.
Tuff: A deposit made up of ash.
Welded tuff: A deposit of pyroclastic material that was
laid down while still very hot and particles become
fused together.
Ash fall: Fallout of very fine ash from the air.
Volcanic ash fall during
mid-day with the
eruption of Mount
Pinatubo in the
Philippines.
Ash flow: Pyroclastic debris that flows downslope.
Lahar: A water saturated slurry of ash and other
volcanic debris that flows downslope.
Nuée Ardente (glowing cloud): A hot, gaseous cloud of
ash that flows down slope.
Flow speeds can reach 160
km/hr and temperatures can
exceed 600 degrees C.
http://volcano.und.nodak.edu/vwdocs/volc_images/img_mt_pelee.html