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 Volcanos
are vents
or fissures in the earth's crust through which gases, molten
rock, or lava , and solid fragments are discharged. They are
one of Earth's most dramatic and violent agents of change. Not
only can their powerful explosive eruptions drastically alter
land and water for many miles around a volcano, but tiny liquid
droplets of sulfuric acid that erupts into the stratosphere
can change our planet's climate temporarily. Eruptions often
force people living near volcanos to abandon their land and
homes, sometimes forever. Those living farther away are likely
to avoid complete destruction, but their cities and towns, crops,
industrial plants, transportation systems, and electrical grids
can still be damaged by tephra (particles emitted into the air),
lahars (mud flows), and flooding (created by snow and ice that
was melted by hot lava and rock). And
although most volcano hazards are triggered directly by an eruption,
some occur when a volcano is quiet.
 A
volcano is most commonly a conical hill or mountain built around a
vent that connects with reservoirs of molten rock below the surface
of the Earth. The term volcano also refers to the opening or vent
through which the molten rock and associated gases are expelled. Driven
by buoyancy and gas pressure the molten rock, which is lighter than
the surrounding solid rock, forces its way upward and may ultimately
break through zones of weaknesses in the Earth's crust. If this happens,
an eruption begins, and the molten rock may pour from the vent as
nonexplosive lava flows, or it may shoot violently into the air as
dense clouds of lava fragments. Larger fragments fall back around
the vent, and accumulations of fallback fragments may move downslope
as ash flows under the force of gravity. Some of the finer ejected
materials may be carried by the wind only to fall to the ground many
miles away. The finest ash particles may be injected miles into the
atmosphere and carried many times around the world by stratospheric
winds before settling out.
Volcanos
can be classified as active, dormant, or extinct. These terms are
to be taken in the relative sense. Many volcanos, although they have
not erupted for many decades, and are not currently erupting, still
may not be considered dormant. Scientists would rather use the term
"Historically Active or Dormant" or "Hyocene Volcano
(last 10,000 years)". These terms put a volcano's activity into
perspective with regards to other historical events, rather comparing
eruption intervals using human memories. Volcanos would have to register
no activity for a very long time to be considered dormant. At the
same time, some volcanoes are extremely active. At least 20 volcanoes
will probably be erupting as you read these words (Italy's Stromboli,
for example, has been erupting for more than a thousand years), and
roughly 60 erupted each year through the 1990s. For up-to-date volcano
activity, see the "Weekly Volcano Report" link below.
Many people live in the vicinity of
active volcanoes. They continually have to be on guard for their lives.
Will there be an eruption today? Will they have enough time to find
safety? These questions bring the topic of volcano prediction to the
table. Scientists can study a volcano's past eruptions to help predict
future activity. Areas affected by lava flows, debris flows, tephra,
or pyroclastic flows can be mapped, making disaster planning more
effective. In addition to this type of long-range forecasting, scientists
are becoming more and more skilled at spotting the warning signs of
an eruption.
Before an eruption, magma moves into
the area beneath the volcano and collects in a magma chamber, or reservoir.
As it comes closer to the surface, the magma releases gases. These
events can offer valuable clues about the likelihood of an eruption.
For example, the movement of magma produces small earthquakes and
vibrations (seismicity). Magma gathering in a chamber causes slight
swelling of the volcano's slopes. Gases released near the volcano
can be measured for changes in quantity and makeup.Volcanologists
are becoming very skilled at predicting the likelihood of an eruption.
Still, a number of barriers remain. It's very difficult to pinpoint
exactly when an eruption will happen. Often, moving magma doesn't
result in an eruption, but instead cools below the surface. Monitoring
potential eruptions is expensive, but if lives can be saved, it is
well worth the expense.
 With many volcanoes
erupting only every few hundred or thousand years, it's not possible
to monitor every site. Volcanic eruptions don't occur without warning,
however. With this in mind, steps should be taken to prepare for a
possible eruption, should you live near a potentially active volcano.
All information obtained from the United
States Geological Survey, Learner.org, and the Smithsonian Institute.
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