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Volcanoes of the
United States
by Steven R. Brantley
For sale by the US. Government Printing Office
Superintendent of Documents, Mail Stop: SSOP, Washington, DC 20402-9328
ISBN 0-16-045054-3
[Illustration: Mount Shasta, California, has erupted at least 10 times
in the past 3,400 years and at least 3 times in the past 750 years.
(_Photograph by Lyn Topinka._)]
Introduction
Few natural forces are as spectacular and threatening, or have played
such a dominant role in shaping the face of the Earth, as erupting
volcanoes. Volcanism has built some of the world’s greatest mountain
ranges, covered vast regions with _lava_ (molten rock at the Earth’s
surface), and triggered explosive eruptions whose size and power are
nearly impossible for us to imagine today. Fortunately, such calamitous
eruptions occur infrequently. Of the 50 or so volcanoes that erupt every
year, however, a few severely disrupt human activities. Between 1980 and
1990, volcanic activity killed at least 26,000 people and forced nearly
450,000 to flee from their homes.
Though few people in the United States may actually experience an
erupting volcano, the evidence for earlier volcanism is preserved in
many rocks of North America. Features seen in volcanic rocks only hours
old are also present in ancient volcanic rocks, both at the surface and
buried beneath younger deposits. A thick ash deposit sandwiched between
layers of sandstone in Nebraska, the massive granite peaks of the Sierra
Nevada mountain range, and a variety of volcanic layers found in eastern
Maine are but a few of the striking clues of past volcanism. With this
perspective, an erupting volcano is not only an exciting and awesome
spectacle in its own right but a window into a natural process that has
happened over and over again throughout Earth’s history.
The Earth’s crust, on which we live and depend, is in large part the
product of millions of once-active volcanoes and tremendous volumes of
_magma_ (molten rock below ground) that did not erupt but instead cooled
below the surface. Such persistent and widespread volcanism has resulted
in many valuable natural resources throughout the world. For example,
volcanic ash blown over thousands of square kilometers of land increases
soil fertility for forests and agriculture by adding nutrients and
acting as a mulch. Groundwater heated by large, still-hot magma bodies
can be tapped for geothermal energy. And over many thousands of years,
heated groundwater has concentrated valuable minerals, including copper,
tin, gold, and silver, into deposits that are mined throughout the
world.
The United States ranks third, behind Indonesia and Japan, in the number
of historically active volcanoes (that is, those for which we have
written accounts of eruptions). In addition, about 10 percent of the
more than 1,500 volcanoes that have erupted in the past 10,000 years are
located in the United States. Most of these volcanoes are found in the
Aleutian Islands, the Alaska Peninsula, the Hawaiian Islands, and the
Cascade Range of the Pacific Northwest; the remainder are widely
distributed in the western part of the Nation. A few U.S. volcanoes have
produced some of the largest and most dangerous types of eruptions in
this century, while several others have threatened to erupt.
[Illustration: Map of United States showing areas where active volcanoes
are located and shown later in more detail.]
ALASKA See page 24
UNITED STATES See page 16
HAWAIIAN ISLANDS See page 10
Scientists at the U.S. Geological Survey (USGS) engage in a variety of
research activities in order to reduce the loss of life and property
that can result from volcanic eruptions and to minimize the social and
economic turmoil that can result when volcanoes threaten to erupt. These
activities include studies of the physical processes before, during, and
after a volcanic eruption, assessments of volcano hazards, and public
outreach to translate scientific information about volcanoes into terms
that are meaningful to the public and public officials.
Monitoring volcanoes for signs of activity, another vital component, is
carried out by USGS earth scientists at three volcano observatories,
which were established to study active volcanoes in Hawaii (1912), the
Cascades (1980), and Alaska (1988). These researchers record
earthquakes, survey the surfaces of volcanoes, map volcanic rock
deposits, and analyze the chemistry of volcanic gas and fresh lava to
detect warning signs of impending activity and determine the most likely
type of activity that will affect areas around a volcano. During the
past 10 years, several warnings of eruptions were issued by the USGS and
monitoring of recently active volcanoes in the United States was
expanded. Predicting the time and size of volcanic eruptions, however,
remains a difficult challenge for scientists.
[Illustration: Scientist collects lava sample from lava flow entering
the sea on Kilauea Volcano. (_Photograph by J.D. Griggs._)]
[Illustration: Scientists conducting field studies on active volcanoes
in the United States.]
[Illustration: Field studies.]
[Illustration: Field studies.]
[Illustration: Field studies.]