Environmental Geology lecture outline
- Environmental aspects of volcanism, case histories, and prediction
of eruptions.
So what's
the concern? What specifically should
be included in a hazard map for a volcanic region?
Some instructive case histories:
Figure of Mt. St. Helens looking
to the east and taken in 1996. The red arrow shows the northward
direction of the lateral blast, while the Tuttle River valley
is show with the lahar deposits in the foreground. This is the
edge of the blast zone, and dead trees litter the slopes in the
lower left corner.
Mt. St. Helens: the classic. May 18th, 1980
- part of Cascade arc associated with subduction
off NW U.S. coast.
- large eruption initiated by mass wasting
of oversteepened slope due to shallow magma inflation.
- lateral blast to the N that extended 15 km.
- lahars that traveled 10s of km down drainage,
speeds up to 55 km/hr.
- 54 fatalities, $3 billion loss (mostly in
logging potential).
- many evacuated prior to more explosive eruption.
Krakatoa: the big one. August 22nd, 1881, dormant
for 200 years.
- part of Java-Sumatra arc.
- 18 cubic km of ejecta, energy = 5000 megaton
hydrogen bombs.
- 4 by 4.5 mile caldera
- 100-120' tsunamis generated, 36,000 fatalities
- phreato-magmatic eruption.
- what would the effect be if occurred today.
Mount Pelee - Martinique, West Indies, 1902
- eastern Caribbean subduction zone.
- moderate Vulcanian eruptions in 1792, 1851.
- early lahar killed 150.
- May 8th, large explosion and nuee ardente
kills 30,000 inhabitants.
- politics may have prevented evacuation.
Nevado del Ruiz, Nov. 13 1985
- Columbia, S. America, subduction related,
complex Carribean tectonics.
- lahars in excess of 30 km down drainage mobility
caused >23,000 fatalities, $200 million loss.
- hazard map predicted course of events. Stories
differ as to why no action was taken.
Pinatubo, Phillipines - the video.
Dante's Peak was a movie!
Prediction of volcanic eruptions:
Possible precursors can monitor:
- earthquake swarms, harmonic tremors, due
to magma movement.
- tiltmeters or geodetic surveys responding
to magma inflation at a shallow level.
- change in gas content (specifically S/Cl
ratio and amount of S).
- thermal anomalies in caldera lakes.
- magnetic anomalies.
- Pinatubo as an example of a success.
- Regular success with flows on Hawaii.
- Mammoth Lake as not a success.
Can an
eruption be controlled or prevented?
In class laboratory. What do you need to include in a grant application
to monitor a volcanic construct?
Volcanos
on the web. A lot of good case histories here.
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