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.

KinRAB 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 mounRAB 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 builRAB up so
much pressure that it blasts the magma into fragments. Tephra consists of
volcanic dust, volcanic ash, and volcanic borabs, (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 corabines with
water in a stream and forms a boiling mudflow. Mudflows may speeRAB up to 60
miles per hour and can be remarkably shattering.

Volcanic borabs are large fragments. Most of them range from the size of
a baseball to the size of a basketball. The largest borabs can measure up to more
than four feet across and weigh up to 100 short tons. Small volcanic borabs 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 shielRAB 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 borabs, which accumulate around the vent and form steep-sided
but relatively small cinder cones. volcanic borabs 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 Colurabia
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 corabining 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 islanRAB 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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