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
Mount Shasta, California, has erupted at least 10 times in the past 3,400 years and at least 3times in the past 750 years. (Photograph by Lyn Topinka.)
Introduction
Few natural forces are as spectacular andthreatening, or have played such a dominantrole in shaping the face of the Earth,as erupting volcanoes. Volcanism has built someof the world’s greatest mountain ranges, coveredvast regions with lava (molten rock at the Earth’ssurface), and triggered explosive eruptionswhose size and power are nearly impossible forus to imagine today. Fortunately, such calamitouseruptions occur infrequently. Of the 50 orso volcanoes that erupt every year, however, afew severely disrupt human activities. Between1980 and 1990, volcanic activity killed at least26,000 people and forced nearly 450,000 to fleefrom their homes.
Though few people in the United States mayactually experience an erupting volcano, theevidence for earlier volcanism is preserved inmany rocks of North America. Features seen involcanic rocks only hours old are also present inancient volcanic rocks, both at the surface andburied beneath younger deposits. A thick ashdeposit sandwiched between layers of sandstonein Nebraska, the massive granite peaks of theSierra Nevada mountain range, and a variety ofvolcanic layers found in eastern Maine are but afew of the striking clues of past volcanism. Withthis perspective, an erupting volcano is not onlyan exciting and awesome spectacle in its ownright but a window into a natural process thathas happened over and over again throughoutEarth’s history.
The Earth’s crust, on which we live and depend,is in large part the product of millions ofonce-active volcanoes and tremendous volumesof magma (molten rock below ground) that didnot erupt but instead cooled below the surface.Such persistent and widespread volcanism hasresulted in many valuable natural resourcesthroughout the world. For example, volcanic ashblown over thousands of square kilometers ofland increases soil fertility for forests and agricultureby adding nutrients and acting as amulch. Groundwater heated by large, still-hot3magma bodies can be tapped for geothermalenergy. And over many thousands of years,heated groundwater has concentrated valuableminerals, including copper, tin, gold, and silver,into deposits that are mined throughout theworld.
The United States ranks third, behind Indonesiaand Japan, in the number of historicallyactive volcanoes (that is, those for which wehave written accounts of eruptions). In addition,about 10 percent of the more than 1,500 volcanoesthat have erupted in the past 10,000years are located in the United States. Most ofthese volcanoes are found in the Aleutian Islands,the Alaska Peninsula, the Hawaiian Islands, andthe Cascade Range of the Pacific Northwest; theremainder are widely distributed in the westernpart of the Nation. A few U.S. volcanoes haveproduced some of the largest and most dangeroustypes of eruptions in this c