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COMPOSITE VOLCANO
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Composite Volcanoes
Sometimes called Stratovolcanoes, Composite volcanoes form when runny
lava escapes through a fissure and flows a long way. Composite volcanoes
are tall cone-shaped mountains that are typically steeply-sided, symmetrical
cones of large dimensions. The essential feature of a composite volcano is a
conduit system through which magma from a reservoir deep in the earth's
crust rises to the surface. The volcano is built up by the accumulation of
material erupted through the conduit and increases in size as lava, cinders,
ash etc. are added to its slopes.
Composite volcanoes erupt in different ways at different times. These
volcanoes are built in layers by 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), tends to
form composite cones. During some eruptions, cinders, bombs and blocks
form a mountain or add height to one that earlier volcanic eruptions had
built. During other eruptions, lava flows cement these rocks together. Most
composite volcanoes have a crater at the summit which contains a central
vent or a clustered group of vents. Lava either flow through breaks in the
crater wall or from fissures on the flanks of the cone. Lava, solidified
within the fissures, form dikes that act as ribs which help to strengthen the
cone. They may rise as much as 8,000 feet above their bases. Depending on
the type of volcanic material it is composed of, some can grow to such
heights that their slopes become unstable and are susceptible to collapse
from the pull of gravity.
When volcanic activity ceases, erosion begins to destroy the cone. After
thousands of years, the cone is stripped away and the hardened magma
filling the conduit (the volcanic plug) and fissures (the dikes) become
exposed, and it too is slowly reduced by erosion. Finally, all that is left is
the plug or "volcanic neck" and dike complex projecting above the land
surface.
Some composite volcanoes occur in chains and are separated by several
tens of kilometers. There are many composite volcano chains on earth,
notably around the Pacific rim, known as the "Rim of Fire".
Other examples of composite volcanoes and their locations are:
Mount St. Helens - Washington State
Mount Fuji - Japan
Mount Rainier - Washington State
Mount Cotopaxi - Ecuador
Mount Vesuvius - Italy
Mount Shasta - California
Mayon Volcano - Luzon Island, Philippines
Mount Hood - Oregon