||What happens when a volcano
think of a volcanic eruption, you probably imagine
red-hot lava flowing down the side of a volcano. Lava is
a serious hazard, but there are others--some of them far
Lava flows are a threat, but they are usually slow enough
that people can get out of the way. Economic loss from
lava flows--including burned buildings and ruined
crops--is more common. Because lava flows normally don't
cover very large areas, the damage is usually limited.
|A lava flow on
Pu'u O'o volcano.
In an explosive eruption, pent-up gases escape violently.
Magma breaks into pieces and bursts from the volcano in a
column of ash and fiery fragments. The cooled fragments
that fall back to Earth are called tephra. In a large
eruption, tephra can cover vast areas with a thick layer
of ash, presenting a much greater hazard than lava flows.
Pyroclastic flows are mixtures of very hot gas and tephra
that cascade down a volcano's sides at high speeds. A
pyroclastic flow covered the city of Herculaneum in A.D.
79, killing many residents. Because pyroclastic flows can
spread destruction over large areas and move at very high
speeds, they are extremely hazardous. Most people are not
aware that this danger exists.
Mud and Debris
Debris flows, fragments of mud and other debris that flow
down the sides of a volcano, are another serious and
little-known hazard. Debris flows often form when part of
the volcano collapses, breaking up and flowing downhill.
If the collapse is a major one, the large flow that
results can travel great distances, often burying
everything in its path. These are particularly dangerous on volcanoes that have glaciers on top, as the eruption instantly melts the ice, causing a massive mud slide, much like what happened during the Mt. St. Helens eruption in 1980.
Katmai video clip (Quicktime, 1:02 minutes, 6,272K)
Lava, ash, and debris flows are the most common and
serious volcanic hazards, but others do exist. Severe
eruptions can disrupt the climate for long periods or
cause atmospheric shock waves. Eruptions can also cause
tsunamis, volcanic earthquakes, or the release of
suffocating gases. These hazards pose serious threats to
both life and property.
View footage from a 1916 expedition to Katmai, an Alaskan volcano.
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