Volcanoes of the United States
Volcanoes of the
by Steven R. Brantley
For sale by the US. Government Printing Office
Superintendent of Documents, Mail Stop: SSOP, Washington, DC 20402-9328
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 century, whileseveral others have threatened to erupt.
Scientists at the U.S. Geological Survey(USGS) engage in a variety of research activitiesin order to reduce the loss of life and propertythat can result from volcanic eruptions and tominimize the social and economic turmoil thatcan result when volcanoes threaten to erupt.These activities include studies of the physicalprocesses before, during, and after a volcaniceruption, assessments of volcano hazards, andpublic outreach to translate scientific informationabout volcanoes into terms that are meaningfulto the public and public officials.
Monitoring volcanoes for signs of activity,another vital component, is carried out by USGSearth scientists at three volcano observatories,which were established to study active volcanoesin Hawaii (1912), the Cascades (1980), andAlaska (1988). These researchers record earthquakes,survey the surfaces of volcanoes, mapvolcanic rock deposits, and analyze the chemistryof volcanic gas and fresh lava to detect warningsigns of impending activity and determine themost likely type of activity that will affect areasaround a volcano. During the past 10 years,several warnings of eruptions were issued by theUSGS and monitoring of recently active volcanoesin the United States was expanded. Predictingthe time and size of volcanic eruptions, however,remains a difficult challenge for scientists.
Volcanoes and the Theory of Plate Tectonics
Major tectonic plates of the Earth.
Volcanoes are not randomly distributed overthe Earth’s surface. Most are concentratedon the edges of continents, along islandchains, or beneath the sea forming long mountainranges. More than half of the world’s active7volcanoes above sea level encircle the PacificOcean to form the circum-Pacific “Ring of Fire.”In the past 25 years, scientists have developed atheory—called plate tectonics—that explains thelocations of volcanoes and their relationship toother large-scale geologic features.
According to this theory, the Earth’s surface ismade up of a patchwork of about a dozen large8plates that move relative to one another atspeeds from less than one centimeter to aboutten centimeters per year (about the speed atwhich fingernails grow). These rigid plates,whose average thickness is about 80 kilometers,are spreading apart, sliding past each other, orcolliding with each other in slow motion on top ofthe Earth’s hot, pliable interior. Volcanoes tendto form where plates collide or spread apart, butthey can also grow in the middle of a plate, asfor example the Hawaiian volcanoes.
Recent Eruptions From U.S. Volcanoes
Few places on Earth allow closer or moredramatic views of volcanic activity thanMauna Loa and Kilauea volcanoes on theisland of Hawaii. Their frequent but usually non-explosiveeruptions make them ideal for scientificstudy. Kilauea’s eruptions are so intensely monitoredthat scientists have assembled a detailedpicture of the volcano’s magma reservoir “plumbing”system and how it behaves before and duringeruptions. Studies of these volcanoes andthe surrounding ocean floor continue to improveour understanding of the geologic history of theHawaiian Island chain and the ability of scientiststo determine volcanic hazards that threatenisland residents.
Eruptions of Hawaiian volcanoes are typicallynon-explosive because of the composition of themagma. Almost all of the magma erupted fromHawaii’s volcanoes forms dark gray to blackvolcanic rock (called basalt), generally in theform of lava flows and, less commonly, asfragmented lava such as volcanic bombs,cinders, pumice, and ash. Basalt magma is morefluid than the other types of magma (andesite,dacite, and rhyolite). Consequently, expandingvolcanic gases can escape from basalt relativelyeasily and can propel lava high into the air, formingbrilliant fountains sometimes called “curtainsof fire.”
Lava, whether erupted in high fountains orquietly pouring out, collects to form flows thatspread across the ground in thin broad sheetsor in narrow streams. The fluid nature of basaltmagma allows it to travel great distances fromthe vent (the place where lava breaks ground)and tends to build volcanoes in the shape of aninverted warrior shield, with slopes less thanabout 10 degrees. Volcanoes with this kind ofprofile are called shield volcanoes.
Hawaiian volcanoes erupt at their summitcalderas and from their flanks along linear riftzones that extend from the calderas. Calderasare large steep-walled depressions that formwhen a volcano’s summit region collapses,usually after a large eruption empties or partlyempties a reservoir of magma beneath the volcano.Rift zones are areas of weakness within avolcano that extend from the surface to depthsof several kilometers. Magma that erupts fromthe flank of a volcano must first flow undergroundthrough one of the volcano’s rift zones,sometimes traveling more than 30 kilometersfrom the summit magma reservoir before breakingthe surface.
Rising more than 9,000 meters fromthe seafloor, Mauna Loa is one of the world’slargest active volcanoes; from its base below sealevel to its summit, Mauna Loa is taller thanMount Everest. It has erupted 15 times since1900, with eruptions lasting from less than 1 dayto as many as 145 days.
The most recent eruption began before dawnon March 25, 1984. Brilliant lava fountains lit thenight-time sky as fissures opened across thefloor of the caldera. Within hours, the summit activitystopped and lava began erupting from aseries of vents along the northeast rift zone.When the eruption stopped 3 weeks later, lavaflows were only 6.5 kilometers from buildings inthe city of Hilo. Mauna Loa erupts less frequentlythan Kilauea, but it produces a much greatervolume of lava over a shorter period of time.
Kilauea’s longest rift-zoneeruption in historical time began on January 3,1983. A row of lava fountains broke out from itseast rift zone about 17 kilometers from the summitcaldera; within a few months, the activitysettled down to a single vent. Powerful fountainingepisodes hurled molten rock 450 meters intothe air and built a cone of lava fragments thatquickly became the tallest landmark on the riftzone.
The eruption changed style abruptly in July1986 when lava broke out through a new vent.Instead of regular episodes of high lava fountaining,lava spilled continuously onto Kilauea’s surface.The steady outpouring of lava formed alake of molten rock that became perched atop asmall shield volcano. By June 1991, the shieldwas about 60 meters tall and 1,600 meters indiameter, and lava from the eruption hadcovered 75 square kilometers of forest andgrassland, added 120 hectares of new land tothe island, and destroyed 179 homes.
Although most of Kilauea’s historical rift eruptionswere much briefer, prolonged eruptive activityin the east rift zone from 1969 to 1974formed a similar shield, Mauna Ulu (Hawaiian for“Growing Mountain”), and an extensive lavafield on the volcano’s south flank. The geologicrecord shows that such large-volume eruptionsfrom the rift zones and the summit area,