I fenomeni vulcanici - Sei

132 I FENOMENI ENDOGENI
CONTENT AND LANGUAGE INTEGRATED LEARNING CLIL
Short-Term Prediction
In contrast to regional predictions, short-term predictions attempt
to forecast the specific time and place of an impending
eruption. They are based on instruments that monitor an active
volcano to detect signals that the volcano is about to erupt.
The signals include changes in the shape of the mountain and
surrounding land, earthquake swarms indicating movement
of magma beneath the mountain, increased emissions of ash
or gas, increasing temperatures of nearby hot springs, and any
other signs that magma is approaching the surface.
In 1978, two U.S. Geological Survey (USGS) geologists, Dwight
Crandall and Don Mullineaux, noted that Mount St. Helens
had erupted more frequently and violently during the past
4,500 years than any other volcano in the contiguous 48 states.
They predicted that the volcano would erupt again before the
end of the century.
In March 1980, about two months before the great May eruption,
puffs of steam and volcanic ash rose from the crater of
Mount St. Helens, and swarms of earthquakes occurred beneath
the mountain. This activity convinced other USGS geologists
that Crandall and Mullineaux’s prediction was correct.
In response, they installed networks of seismographs,
tiltmeters, and surveying instruments on and around the
mountain.
2 Aerial view of pyroclastic flow deposits from the Mount Pinatubo eruption 1991.
Swarm: fig. a group of bees
Ash: the grey substance left when something is burnt completely
Nearby: that is a short distance away
Steam: the gas or vapour that rises from boiling water
In the spring of 1980, the geologists warned government agencies
and the public that Mount St. Helens showed signs of an
impending eruption. The U.S. Forest Service and local law enforcement
officers quickly evacuated the area surrounding the
mountain, averting a much larger tragedy.
Using similar kinds of information, geologists predicted the 1991
Mount Pinatubo eruption in the Philippines, saving many lives 2 .
Although the June 25, 1997, eruption of the Soufrière Hills volcan
on the Caribbean island of Montserrat killed 19 people and destroyed
many homes and farms, predictions of the eruption by
the Montserrat Volcano Observatory saved many additional lives.
The island was long known to harbor an active volcano, and the
observatory was established to monitor the volcano and to predict
eruptions.
The observatory recorded continued swelling of the dome,
swarms of earthquakes, and many large and small eruptions, including
pyroclastic flows, through the rest of 1996 and the first
half of 1997. On June 25, 1997, major pyroclastic flows reached
to within 50 meters of the airport. Surges and flows devastated
several communities, killing 19 people.
Although some people returned to homes and farms during
lulls in the long eruptive sequence, there is little doubt that the
warnings issued by the observatory and evacuations ordered by
the government saved many lives.
True or false
from Thompson-Turk, Earth Science and Environment, 4E. © 2007.
1. Regional predictions attempt to forecast the
specific time and place of an impending eruption. T F
2. The potential violence of a volcanic eruption
is related to the geological environment
of the volcano. T F
3. Earthquake swarms, emissions of ahes and gases
and puffs of steam are important signal
in short-term prediction of volcanic eruptions. T F
4. Short-time predictions always save human lives. T F
5. If an active volcano lies on continental crust,
the eruption may be gentle because granitic
magma may form. T F
6. If an active volcano lies on oceanic crust,
the eruption may be gentle because basaltic
magma may form. T F
to Harbor: fig. to host
Dome: the rounded summit of a hill or mountain; (geology) a rounded
uplifted landform or underground structure
Lull: a temporary period of quiet or inactivity
© SEI – 2012