Download A volcano is generally a conical shaped hill or mountain built by

Volcanism: How Volcanos are formed
A volcano is generally a conical shaped hill or mountain built by accumulations of
lava flows, tephra, or fragmented rock material ejected by a volcanic explosion.
Also called pyroclastic material. and volcanic ash.
About 95% of active volcanoes occur at the plate subduction zones and at the
mid-oceanic ridges (Figure -1).
Subduction is a process of plate tectonics where one lithospheric plate is pushed
below another into the asthenosphere.
The other 5% occur in areas associated with lithospheric hot spots.
http://www.youtube.com/watch?v=uZp1dNybgfc&feature=fvw
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Location of the Earth's major volcanoes. Most occur along tectonic plate boundaries where plate
subduction creates rising plumes of magma. The volcanoes that do not
occur along plate boundaries are the result of localized asthenosphere hot
spots that melt through the Earth's crust. The Hawaiian Island chain of
volcanoes was create by a hot spot.
Types of Volcanoes
five different types of volcanoes. based on the geomorphic form, magma chemistry, and the
explosiveness of the eruption.
I. Basalt plateau: The least explosive type of volcano is called a basalt plateau. These
volcanoes produce a very fluid basaltic magma with horizontal flows. The form of these
volcanoes is flat to gently sloping and they can occupy an area from 100,000 to 1,000,000
square kilometers, and can be as thick as 1800 meters. Large basalt plateaus are found in the
Columbia River Plateau, western India, northern Australia, Iceland, Brazil, and Antarctica.
II. Shield volcanoes Some basaltic magmas can produce very large slightly sloping volcanoes, 6
to 12°, that have gently flowing magmas called shield volcanoes. Shield volcanoes can be up to
9000 meters tall. The volcanoes of the Hawaiian Islands are typical of this type. Extruded
materials from this type of volcano mainly consist of low viscosity basaltic lava flows.
Figure 3: Low viscosity basaltic lava flow. (Source: U.S. Geological
Survey)
III. A cinder cone is a small volcano, between 100 and 400 meters tall,
made up of exploded rock blasted out of a central vent at a high velocity
(Figure -4). They form when large amounts of gas accumulate within
rising magma. Examples of cinder cones include Little Lake Volcano in
California.
California.: Cinder cone volcano in the Mojave National Preserve,)
http://www.youtube.com/watch?v=DnBggrCdkN0
IV. Composite volcanoes : are made from alternate layers of lava flows and exploded
rock.
Their height ranges from 100 to 3500 meters tall.
The chemistry of the magma of these volcanoes is quite variable ranging from basalt to
granite. They are steep-sided, explosive eruptions.
Magmas that are more granitic tend to be very explosive because of their relatively higher
water content.
Water at high temperatures and pressures is extremely volatile.
Examples of composite volcanoes include Italy's Vesuvius, Japan's Mount Fuji, and
Washington State's Mount Rainier and Mount St. Helens (see Figures -5 and -6).
Figure 10n-5: Mount St. Helens eruption on May 18, 1980. (Source:
U.S. Geological Survey, photograph by Austin Post).
Caldera. The most explosive type of volcano is the caldera. The cataclysmic explosion of
these volcanoes leaves a huge circular depression at the Earth's surface.
This depression is usually less than 40 kilometers in diameter. These volcanoes form when
"wet" granitic magma quickly rises to the surface of the Earth. When it gets to within a few
kilometers of the surface the top of the magma cools to form a dome. Beneath this dome the
gaseous water in the magma creates extreme pressures because of expansion. When the pressure
becomes too great the dome and magma are sent into the Earth's atmosphere in a tremendous
explosion.
A potentially very destructive caldera covering an area of about 2000 square kilometers
exists under Yellowstone National Park in the United States (Figure 7).
Investigations have discovered that over the last 2 million years this volcano has exploded on a
regular interval of about 700,000 years.
The last eruption occurred 630,000 years ago and the next could take place anytime. When the
Yellowstone caldera last erupted, it blasted 1,000 cubic kilometers of volcanic ash and rock into
the atmosphere.
The ash ejected into the atmosphere created climatic havoc on a global scale. The ash would
have blocked sunlight from being received at the ground surface for a few years. A reduction in
the reception of solar radiation would have caused the globle climate to cool significantly. Over
time this ash settled back to the Earth's surface covering more than half of North America.
Figure 10n-7: Map of the location of the Yellowstone caldera. Several
large earthquakes have occurred in the last century in the vicinity of the
caldera indicating that significant volcanic activity is occurring beneath
the ground surface. (Source: U.S. Geological Survey - Yellowstone
Volcano Observatory).
http://www.youtube.com/watch?v=zh9zVXUv-Fs