VOLCANISM
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
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 vents 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 lava-fountaining 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.
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, pancake-shaped, 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-volcanic-eruptions/
http://www.britannica.com/EBchecked/topic/130036/complex-volcano
http://www.k12.hi.us/~kapunaha/student_projects/volc_blowout/cinder_cone_volcano.htm
Rocha, Marites B.
Haliba, Aini L.
(A7)