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Volcanism is the phenomenon connected
with volcanoes and volcanic activity. It
includes all phenomena resulting from and
causing magma within
the crust or mantle of a planet to rise
through the crust and form volcanic
rocks on the surface.
TYPES OF VOLCANOES
COMPOSITE VOLCANOES
 Composite volcanoes are constructed from multiple eruptions, sometimes
recurring over hundreds of thousands of years, sometimes over a few
hundred. Andesite magma, the most common but not the only magma type
forming composite cones, produces lava more brittle than basaltic lava because
of its higher viscosity. Although andesitic composite cones are constructed
dominantly of fragmental debris, some of the magma intrudes the cones
as dike or sills. In this way, multiple intrusive events build a structural
framework that knits together the voluminous accumulation of volcanic
rubble, which can stand higher than cones composed solely of fragmental
material. Composite cones can grow to such heights that their slopes become
unstable and susceptible to collapse from the pull of gravity. Famous examples
of composite cones are Mayon Volcano Philippines, Mount Fuji in Japan, and
Mount Rainier, Washington, U.S.A. Some composite volcanoes attain two to
three thousandmeters in height above their bases. Most composite volcanoes
occur in chains and are separated by several tens of kilometers. There are
numerous composite volcano chains on earth, notably around the Pacific rim,
known as the "Rim of Fire".
Shield Volcanoes
 Shield volcanoes are large volcanoes that are built almost entirely of
fluid lava flows. It has broad sloping sides and is usually surrounded by
gently sloping hills in a circular or fan shaped pattern, that looks like a
warrior's shield.
 The volcano is produced by the action of the gas (steam or water vapor)
with heat from the earth's core. This action melts rock turning it into
magma. The pressure from the heat of the gas pushes the magma
upwards till it explodes. Molten magma shoots upward from deep
below the ocean floor and breaks through the drifting plates to form
shield volcanoes. Lava flows gently and continuously out of the central
volcanic vent or group of vents. This lava is very runny, and can't be
piled up into steep mounds. It gradually accumulates and cools around
the volcano. The eruptions are characterized by low explosivity lavafountaining that forms cinder cones and spatter cones at the vent. The
volcanoes are built up slowly by the accretion of thousands of highly
fluid lava flows called basalt lava. The lava spread widely over great
distances, then cools as thin gently dipping sheets. Lavas also erupt
from vents along fractures (rift zones) that form on the flanks of the
cone. Some of the largest volcanoes in the world are Shield volcanoes.
Cinder Cone Volcanoes
Cinder cone volcanoes are the most common kind of
volcanoes. They are steep sided cones of basaltic
fragments and are smaller and simpler than
composite volcanoes. Streaming gases carry liquid
lava blobs into the atmosphere that fall back to
earth around a single vent to form the cone. The
volcano forms when ash, cinders and bombs pile
up around the vent to form a circular or oval cone.
Spatter Cones
A spatter cone is formed of molten lava ejected from a vent
somewhat like taffy. Expanding gases in the lava fountains tear
the liquid rock into irregular gobs that fall back to earth, forming
a heap around the vent. The still partly liquid rock splashes down
and over the sides of the developing mound is called spatter.
Because spatter is not fully solid when it lands, the individual
deposits are very irregular in shape and weld together as they
cool, and in this way particularly differ from cinder and ash.
Spatter cones are typical of volcanoes with highly fluid magma,
such as those found in the Hawaiian Islands. The spatter that
builds up the cone can either be agglutinated or welded, the
former meaning that the individual spatters pose one above each
other with a lesser degree of welding occurring, while welded
spatter is almost fluid when it lands and therefore welds easily.
Complex Volcanoes
 Complex volcanoes are mixed landforms. In most
cases, they occur because of changes either in eruptive
habit or in location of the principal vent area. A
stratovolcano may form a large explosion crater that
later
becomes filled by a lava dome, or several new cones
and craters may develop on a caldera's rim. One
stratovolcano cone may overlap another and have
multiple summits.
Spatter Cones
Cinder Cones
Complex Volcano
Kinds of volcanic eruption
Strombolian Eruption
Strombolian eruptions are often associated with small lava lakes, which can build
up in the conduits of volcanoes. They are one of the least violent of the
explosive eruptions, although they can still be very dangerous if bombs or lava
flows reach inhabited areas. Strombolian eruptions are named for the volcano
that makes up the Italian island of Stromboli, which has several erupting
summit vents. These eruptions are particularly spectacular at night, when the
lava glows brightly.
Vulcanian Eruption
 A Vulcanian eruption is a short, violent, relatively small explosion of
viscous magma (usually andesite, dacite, or rhyolite). This type of
eruption results from the fragmentation and explosion of a plug of lava
in a volcanic conduit, or from the rupture of a lava dome (viscous lava
that piles up over a vent). Vulcanian eruptions create powerful
explosions in which material can travel faster than 350 meters per
second (800 mph) and rise several kilometers into the air. They
produce tephra, ash clouds, and pyroclastic density currents (clouds of
hot ash, gas and rock that flow almost like fluids).
Plinian Eruption
 The largest and most violent of all the types of volcanic
eruptions are Plinian eruptions. They are caused by the
fragmentation of gassy magma, and are usually associated
with very viscous magmas (dacite and rhyolite). They
release enormous amounts of energy and create eruption
columns of gas and ash that can rise up to 50 km (35 miles)
high at speeds of hundreds of meters per second. Ash from
an eruption column can drift or be blown hundreds or
thousands of miles away from the volcano. The eruption
columns are usually shaped like a mushroom (similar to a
nuclear explosion) or an Italian pine tree; Pliny the
Younger, a Roman historian, made the comparison while
viewing the 79 AD eruption of Mount Vesuvius, and
Plinian eruptions are named for him.
Lava Domes
Lava domes form when very viscous, rubbly lava (usually andesite,
dacite or rhyolite) is squeezed out of a vent without exploding. The
lava piles up into a dome, which may grow by inflating from the
inside or by squeezing out lobes of lava (something like toothpaste
coming out of a tube). These lava lobes can be short and blobby,
long and thin, or even form spikes that rise tens of meters into the
air before they fall over. Lava domes may be rounded, pancakeshaped, or irregular piles of rock, depending on the type of lava they
form from.
Surtseyan Eruption
 Surtseyan eruptions are a kind of hydromagmatic eruption, where
magma or lava interacts explosively with water. In most cases,
Surtseyan eruptions occur when an undersea volcano has finally grown
large enough to break the water's surface; because water expands when
it turns to steam, water that comes into contact with hot lava explodes
and creates plumes of ash, steam and scoria. Lavas created by a
Surtseyan eruption tend to be basalt, since most oceanic volcanoes are
basaltic.
Products of Volcanic Eruptions
Sources:
http://geology.com/volcanoes/types-of-volcaniceruptions/
http://www.britannica.com/EBchecked/topic/130036/co
mplex-volcano
http://www.k12.hi.us/~kapunaha/student_projects/volc
_blowout/cinder_cone_volcano.htm