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Lava flows from andesitic and rhyolitic magmas rarely
cover more than a few square kilometers because of their
high viscosity. More viscous lava comes to the surface
fairly slowly, so lava flows don't travel very far before
cooling and slowing. If large quantities of lava are
extruded quickly, then areas of up to a few hundred
square kilometers can be covered. The most hazardous lava
flows, then, are not from andesitic or rhyolitic magmas,
but from more fluid basaltic eruptions.
|A helicopter lands
near Mt. St. Helens.
Volcanic ash blanketed the
area after the
erupted in 1980.
Tephra deposits cause damage in a variety of ways. Large
fragments of tephra can cause significant damage on
impact, colliding with structures or setting things on
fire. Accumulations of tephra do their damage by burial:
roofs collapse, for example, or crops are killed.
Very fine particles of tephra cause breathing
difficulties and interfere with machinery. Thin layers of
very fine tephra can accumulate thousands of kilometers
from the source. Heavier layers of several centimeters or
more can cover tens of thousands of square kilometers.
Pyroclastic flows cause damage by burial and by
incineration, and because of their speed and gas content
can also cause impact damage and asphyxiation. These
flows are common at stratovolcanoes, since they are
associated with andesitic and rhyolitic magmas.
Pyroclastic flows can travel at speeds of up to 200
meters per second and can extend as far as 200 kilometers
from their source. The heat of the flows can reach
several hundred degrees Celsius. With the capacity to
sweep over barriers as high as a hundred meters or more,
they are a serious danger.
Debris flows can be triggered by a volcanic eruption, by
the normal rumblings and small emissions leading up to an
eruption, by volcanic earthquakes, or simply by gravity
acting on a weakened and overly steep part of the
volcano. Some types of volcanic debris flows are very
similar to rock avalanches. These rarely extend farther
than 100 kilometers from the volcano but can spread
debris over an area of 1,000 square kilometers.
Another common type of debris flow is a lahar, or
volcanic mudflow. This mixture of mud (mainly volcanic
ash from tephra deposits) and water flows quickly down
stream valleys that drain the volcano's slopes. Large
lahars can travel hundreds of kilometers down valleys.
Because of their high density, these mudflows can cause
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