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Deep within the Earth it is so hot that some rocks slowly melt
and become a thick flowing substance called magma.
Because it is lighter than the solid rock around it, magma rises and collects in
magma chambers. Eventually some of the magma pushes through
vents and fissures
in the Earth's surface. A volcanic eruption occurs! Magma that has erupted
is called lava.
Some volcanic eruptions are explosive and other are not.
How explosive an
eruption is depends on how runny or sticky the magma is. If magma is thin and
runny, gases can escape easily from it. When this type of magma erupts, it
flows out of the volcano. Lava flows rarely kill people because they move
slowly enough for people to get out of their way. Lava flows, however, can
cause considerable destruction to buildings in their path.
If magma is thick and sticky, gases cannot escape easily. Pressure builds up
until the gases escape violently and explode. In this type of eruption, the
magma blasts into the air and breaks apart into pieces called tephra.
Tephra can range in size from tiny particles of ash to house-size boulders.
Explosive volcanic eruptions can be dangerous and deadly.
They can blast out
clouds of hot tephra from the side or top of a volcano. These fiery clouds race
down mountainsides destroying almost everything in their path. Ash erupted into
the sky falls back to Earth like powdery snow, but snow that won't melt. If
thick enough, blankets of ash can suffocate plants, animals, and humans. When
hot volcanic materials mix with water from streams or melted snow and ice,
mudflows form.
Mudflows have buried entire communities located near erupting volcanoes.
Because there may be hundreds or thousands of years
between volcanic eruptions,
people may not be aware of a volcano's dangers. When Mount St. Helens in the
State of Washington erupted in 1980, it had not erupted for 123 years. Most
people thought Mount St. Helens was a beautiful, peaceful mountain and not a
dangerous volcano.
-- Excerpts from:
Volcanoes! -- U. S. Department of the Interior,
U. S. Geological Survey, Teaching Packet
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