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Pyroclastic
Flows |
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Mayon Volcano, Philippines
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Quicktime
video
pyroclastic flow (1.79 M) |
What are pyroclastic flows?
Pyroclastic flows are mixtures of hot rocks and
gasses that move away from the volcano vent at
very high speeds. They can occur when melted rock
or solid rock, or both, erupt from the volcano.
They can also happen if a lava dome collapses on
a steep slope. Most pyroclastic flows have two
layers, a layer of rocks that flows on the ground
and a cloud of ash that erupts into the sky and
is carried by the wind. Ash can be carried a long
way by the wind and then fall out of the sky.
Effects of Pyroclastic flows
A pyroclastic flow will destroy just about everything
in its path. Imagine boulders and hot gasses
traveling 50 miles per hour and greater. (80
kilometers per hour) . Not only are big boulders
a problem, the gas can be between 400 degrees
and 1300 degrees Farenheight. Ouch! (200 – 700
degrees C.) Many things caught in a pyroclastic
flow will burn quickly.
Pyroclastic Flows can be fast or slow, big or
little. But even small ones that only travel a
few miles from the crater can destroy property
and kill or hurt people and animals. Most people
and animals are hurt or killed either because of
burns or by breathing in the toxic gasses.
Usually the pyroclastic flow will follow a valley
and will leave loose rocks and dirt from 3 feet
to 600 feet deep. (1-200 meters) This loose rock
and ash can lead to lahars in two main ways:
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It can dam or block streams which cause a lake
to form behind the dam. If the water keeps rising
over the dam it can break the dam and lots of water
and mud will flow down quickly.
·
During rainstorms the streams and rivers will carry
the ash and rocks downstream. Also, if a hot pyroclastic
flow hits a glacier, the ice will melt quickly
and send a lot of water downstream very fast.
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| Photograph by R.I. Tilling on June
1, 1982 |
Photograph by W.E. Scott on July
1, 1991. |
Photograph by C. Gardner on June
28, 1990 |
Photograph by R.P. Hoblitt on July
12, 1997. |
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