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PREDICTING ERUPTIONS
A. INTRODUCTION
Predicting a volcanic eruption is different from predicting a
flood or an earthquake in that
volcanoes usually shows signs
of "awakening" before they
erupt, giving geologists time to
monitor the precursory events.
However, forecasting which
dormant ("sleeping")
volcano might "awaken" is similar to
forecasting earthquakes or
floods: the probability (or risk)
depends the recurrence interval of
eruptions (also known as
a volcano's periodicity ).
B. MONITORING
ACTIVE VOLCANOES
Once
a volcano "awakens", geologists monitor warning
signs
(stages)
in an attempt to predict when an eruption
will occur.
1. SEISMIC
ACTIVITY
Earthquakes are often the first sign that a volcano is
awakening and ready to erupt. (Long
Valley, CA).
2. GROUND
DEFORMATION
As
magma approaches the surface, it causes both vertical
and horizontal
deformation of the volcano (measurement).
3.
SUBSURFACE
TEMPERATURES
Rising
magma will also cause temperatures to rise.
4. GAS
EMISSIONS
Changes
in the composition of volcanic gases and their
emission rates can indicate how close an eruption is.
C. MT.
ST. HELENS CASE STUDY
Mount Saint Helens
erupted on May 18, 1980 after 123 years
of dormancy. Although this volcano
was carefully monitored,
the actual eruption caught geologists by
surprise and caused
at least 62 deaths (including one geologist).
1.
MARCH
1980
Seismic activity began in the middle of March.
2. APRIL 1980
A bulge in
the northern
slope became noticeable.
3.
MAY 1980
A large earthquake triggered a landslide that led first to
a
lateral eruption and then a vertical eruption.
Debris
flows filled in the North
Fork Toutle River Valley
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