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					Volcano



      This paper will define and discuss the volcano to include: types of
volcanoes,

formation of a volcano, and elements of a volcano; such as, lava, rock
fragments, and gas.

This paper also tells a little bit about volcanic activity in different
parts of the world.



What is a volcano?

      A volcano is a vent in the earth from which molten rock and gas
erupt. The molten

rock that erupts from the volcano forms a hill or mountain around the
vent. The lava may

flow out as a viscous liquid or it may explode from the vent as solid or
liquid particles.


Kinds of Volcanic Materials

      Three basic materials that may erupt from a volcano are; 1. lava,
2. rock

fragments, and 3. gas.



Lava

      Lava is the name for magma that has been released onto the Earth's
surface. When

lava comes to the Earth's surface, it is red hot and may have
temperatures of more than

2012 degrees Fahrenheit. Fluid lava flows swiftly down a volcano's
slopes.

Sticky lava flows more slowly. As the lava cools, it may harden into many
different

formations. Highly fluid lava hardens into smooth, folded sheets of rock
called pahoehoe.
Stickier lava cools into rough, jagged sheets of rock called aa.
Pahoehoe and aa cover

large areas of Hawaii, where the terms originated. The stickiest lava
forms flows of

boulders and rubble called block flows. It may also form mounds of lava
called domes.

      Other lava formations are spatter cones and lava tubes. Spatter
cones are steep hills

that can get up to 100 feet high. They build up from the spatter of
geyser-like eruptions of

thick lava. Lava tubes are tunnels formed from fluid lava. As the lava
flows, its exterior

covering cools and hardens. But the lava below continues to flow. After
the flowing lava

drains away, it leaves a tunnel.


Rock Fragments

      Rock fragment are usually called tephra and are formed from sticky
magma. This

magma is so sticky that its gas can not easily escape when the magma
approaches the

surface or central vent. Finally, the trapped gas builds up so much
pressure that it blasts the

magma into fragments. Tephra consists of volcanic dust, volcanic ash, and
volcanic bombs,

(from smallest to largest size particle).

      Volcanic dust consists of particles less than one one-hundredth
inch in diameter.

Volcanic dust can be carried for great distances. In 1883, the eruption
of Krakatau in

Indonesia shot dust 17 miles into the air. The dust was carried around
the Earth several

times and produced brilliant red sunsets in many parts of the world. Some
scientists
assume large quantities of volcanic dust can affect the climate by
reducing the amount of

sunlight that reaches the Earth.

      Volcanic ash is made up of fragments less than one fifth inch in
diameter. Nearly all

volcanic ash falls to the surface and becomes welded together as rock
called volcanic tuff.

Sometimes, volcanic ash combines with water in a stream and forms a
boiling mudflow.

Mudflows may speeds up to 60 miles per hour and can be remarkably
shattering.

      Volcanic bombs are large fragments. Most of them range from the
size of a

baseball to the size of a basketball. The largest bombs can measure up to
more than four

feet across and weigh up to 100 short tons. Small volcanic bombs are
generally called

cinders.


Gas

      Gas pours out of volcanoes in large quantities during almost all
eruptions. The gas

is made up particularly of steam, but may also include carbon dioxide,
nitrogen, sulfur

dioxide, and other gases. Most of the steam comes from a volcano's magma,
but some

steam may also be produced when rising magma heats water in the ground.
Volcanic gas

carries a large sum of volcanic dust. This alliance of gas and dust looks
like black smoke


Types of Volcanoes

      The magmas that are the most liquefied erupt quietly and flow from
the vent to form

sloping shield volcanoes, a name that is conceived because they look like
the shields of
ancient German warriors. The lava that flows from shield volcanoes is
usually only one to

ten meters thick, but the lava may extend for great distances away from
the vent. The

volcanoes of Hawaii and Iceland are typical shield volcanoes.

      Magma with high gas contents and high viscosities are usually more
explosive than

the lava that flows from shield volcanoes. This gas-rich lava in many
occurrences is blown

very high into the air during an eruption. The magma falls as volcanic
bombs, which

accumulate around the vent and form steep-sided but relatively small
cinder cones.

volcanic bombs range in size from fine-grained ash to house-size blocks.
Cinder cones

most commonly consist of volcanic fragments any where from ash to small-
pebble size

which is less than three centimeters in diameter.

      Most of the tallest volcanoes are composite volcanoes, which are
also called

stratovolcanoes. These form a cycle of quiet eruptions of fluid lava
followed by explosive

eruptions of viscous lava. The fluid lava creates an erosion resistant
shell over the

explosive debris, which forms, strong, steep-sided volcanic cones.

      In the past, giant eruptions of extremely fluent basaltic lava from
extensive systems

of fissures in the Earth have occurred. These series of eruptions formed
large plateaus of

basaltic lava. In India, the Deccan basalts cover 260,000 square
kilometers, and in Oregon

and Washington the Columbia Plateau basalts cover approximately 130,000
square

kilometers. No eruptions of this extent have ever been observed during
historical times.
Even more voluminous accumulations of basaltic lava, nevertheless, are
currently being

formed at the mid-ocean ridges.



How a volcano is formed

The Beginning

      A volcano begins as lava inside the Earth. This lava is created
from extreme

temperatures in the Earth's interior. Most magma forms 50 to 100 miles
beneath the Earth's

surface. Some magma develops at depths of 15 to 30 miles below the
Earth's surface.

      The magma, which is now filled with gas from combining with the
other rock inside

the Earth, progressively rises toward the Earth's surface because it is
less dense than the

solid rock around it. As the magma rises, it melts gaps in the
surrounding rock and forms a

large room as close as two miles to the surface. The magma room that is
formed is the

reservoir from which volcanic materials erupt.


The Eruption

      The gas-filled lava in the reservoir is now under great pressure
from the weight of

the solid rock around it. the pressure causes the gas to blast or melt a
channel in a fractured

or weakened part of the rock. The magma now moves through the channel to
the surface.

When the magma gets near the surface, the gas in the magma is released.
The gas and

magma blast out an opening called the central vent. Most of the lava and
other volcanic
materials then erupt through this vent. The materials gradually pile up
around the vent, and

form a volcanic mountain, or a volcano. After the eruption stops, a
bowllike crater usually

forms at the top of the volcano. The vent lies at the bottom of the
crater.

      Once a volcano has formed, not all the lava from later eruptions
reaches

the surface through the central vent. As the magma rises, some of it may
break through the

channel wall and branch out into smaller channels in the rock. The magma
in these channels

may escape through a vent made in the side of the volcano, or it may rest
below the surface.

Volcanoes are very wondrous and amazing. They are one of the most
destructive and one of

the most beautiful things on this Earth. They contain gas, lava, and
tunnels that go many

miles into the Earth. They can form new islands or gigantic mountains.
The materials that

volcanoes erupt can help scientists understand about the inner Earth.
 Bibliography


Bullard, Fred M. Volcanoes of the Earth. Austin:   University of Texas
Press,     1962.

Decker, Robert and Barbara. Volcanoes. San Francisco:   W.H. Freeman and
      company, 1981.

Decker, Robert and Barbara. Volcanoes. New York:   W.H. Freeman and
      company, 1981.

Macdonald, Gordon A. Volcanoes. New Jersey:   Prentice-Hall, inc., 1972.

"Volcano", The World Book Encyclopedia, 1993, Volume 20, pages 438-440.

				
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