Geology Reference
In-Depth Information
Satellite remote sensing
Ground deformation studies
Gravity and magnetic
field studies
GPS measurements
Direct gas
sampling
Temperature
measurements
Study of past
volcanic deposits
Lava sample
collection
Remote sensing
of gases
Seismic measurements
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Figure 5.19
Volcanic Monitoring Some important techniques used to monitor volcanoes.
Geologists successfully gave timely warnings of
impending eruptions of Mount St. Helens in Washington
and Mount Pinatubo in the Philippines, but in both cases,
the climactic eruptions were preceded by eruptive activity of
lesser intensity. In some cases, however, the warning signs are
much more subtle and diffi cult to interpret. Numerous small
earthquakes and other warning signs indicated to USGS
geologists that magma was moving beneath the surface of
the Long Valley caldera in eastern California, so in 1987, they
issued a low-level warning, and then nothing happened.
Volcanic activity in the Long Valley caldera occurred as
recently as 250 years ago, and there is every reason to think
that it will occur again. Unfortunately, the local populace
was largely unaware of the geologic history of the region, the
USGS did a poor job in communicating its concerns, and
premature news releases caused more concern than was jus-
tifi ed. In any case, local residents were outraged because the
warnings caused a decrease in tourism (Mammoth Moun-
tain on the margins of the caldera is the second largest ski
area in the country) and property values plummeted. Moni-
toring continues in the Long Valley caldera, and the signs of
renewed volcanism, including earthquake swarms, trees be-
ing killed by carbon dioxide gas apparently emanating from
magma, and hot spring activity, cannot be ignored. In April
2006, three members of a ski patrol were killed by carbon di-
oxide gas that accumulated in a low area.
Chapter Summary
Volcanism encompasses those processes by which magma
rises to the surface as lava fl ows and pyroclastic materials
and gases are released into the atmosphere.
Gases make up only a few percent by weight of magma.
Most is water vapor, but sulfur gases may have far-
reaching climatic effects.
Aa lava fl ows have surfaces of jagged, angular blocks, whereas
the surfaces of pahoehoe fl ows are smoothly wrinkled.
Several other features of lava fl ows are spatter cones, pres-
sure ridges, lava tubes, and columnar joints. Lava erupted
under water typically forms bulbous masses known as
pillow lava.
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