Part 1

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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

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.

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.

Volcanoes and the Theory of Plate Tectonics

Only a few of the Earth’s active volcanoes are shown. (Sketch by Ellen Lougae.) PLATES EURASIAN NORTH AMERICAN JUAN DE FUCA PHILIPPINE CARIBBEAN PACIFIC COCOS NAZCA AUSTRALIAN EURASIAN ARABIAN INDIAN AFRICAN SOUTH AMERICAN SCOTIA ANTARCTIC EXPLANATION Plate boundary Active volcanoes

Volcanoes are not randomly distributed over the Earth’s surface. Most are concentrated on the edges of continents, along island chains, or beneath the sea forming long mountain ranges. More than half of the world’s active volcanoes above sea level encircle the Pacific Ocean to form the circum-Pacific “Ring of Fire.” In the past 25 years, scientists have developed a theory—called plate tectonics—that explains the locations of volcanoes and their relationship to other large-scale geologic features.

According to this theory, the Earth’s surface is made up of a patchwork of about a dozen large plates that move relative to one another at speeds from less than one centimeter to about ten centimeters per year (about the speed at which fingernails grow). These rigid plates, whose average thickness is about 80 kilometers, are spreading apart, sliding past each other, or colliding with each other in slow motion on top of the Earth’s hot, pliable interior. Volcanoes tend to form where plates collide or spread apart, but they can also grow in the middle of a plate, as for example the Hawaiian volcanoes.

In the Pacific Northwest, the Juan de Fuca Plate plunges beneath the North American Plate. As the denser plate of oceanic crust is forced deep into the Earth’s interior beneath the continental plate, a process known as subduction, it encounters high temperatures and pressures that partially melt solid rock. Some of this newly formed magma rises toward the Earth’s surface to erupt, forming a chain of volcanoes above the subduction zone.]

PACIFIC PLATE Juan de Fuca Ridge JUAN DE FUCA PLATE NORTH AMERICAN PLATE Mt. Baker Glacier Peak Mt. Rainier MAGMA Magma Conduit

HAWAII NIIHAU KAUAI OAHU MOLOKAI LANAI MAUI KAHOOLAWE PACIFIC PLATE Oceanic Crust Fixed “Hot Spot” Zone of magma formation extends to Kilauea & Mauna Loa Direction of place movement

Recent Eruptions From U.S. Volcanoes

Hawaiian volcanoes

Few places on Earth allow closer or more dramatic views of volcanic activity than Mauna Loa and Kilauea volcanoes on the island of Hawaii. Their frequent but usually non-explosive eruptions make them ideal for scientific study. Kilauea’s eruptions are so intensely monitored that scientists have assembled a detailed picture of the volcano’s magma reservoir “plumbing” system and how it behaves before and during eruptions. Studies of these volcanoes and the surrounding ocean floor continue to improve our understanding of the geologic history of the Hawaiian Island chain and the ability of scientists to determine volcanic hazards that threaten island residents.

EXPLANATION Volcano active during past 2,000 years Potentially active volcano Population centers · 50,000 to 100,000 • 350,000 to 1,000,000 PACIFIC OCEAN NIIHAU KAUAI OAHU •Honolulu MOLOKAI MAUI Haleakala LANAI KAHOOLAWE HAWAII Kohala Mauna Kea ·Hilo Hualalai Mauna Loa Kilauea Lohi

Eruptions of Hawaiian volcanoes are typically non-explosive because of the composition of the magma. Almost all of the magma erupted from Hawaii’s volcanoes forms dark gray to black volcanic rock (called basalt), generally in the form of lava flows and, less commonly, as fragmented lava such as volcanic bombs, cinders, pumice, and ash. Basalt magma is more fluid than the other types of magma (andesite, dacite, and rhyolite). Consequently, expanding volcanic gases can escape from basalt relatively easily and can propel lava high into the air, forming brilliant fountains sometimes called “curtains of fire.”

Lava, whether erupted in high fountains or quietly pouring out, collects to form flows that spread across the ground in thin broad sheets or in narrow streams. The fluid nature of basalt magma allows it to travel great distances from the vent (the place where lava breaks ground) and tends to build volcanoes in the shape of an inverted warrior shield, with slopes less than about 10 degrees. Volcanoes with this kind of profile are called shield volcanoes.

Hawaiian volcanoes erupt at their summit calderas and from their flanks along linear rift zones that extend from the calderas. Calderas are large steep-walled depressions that form when a volcano’s summit region collapses, usually after a large eruption empties or partly empties a reservoir of magma beneath the volcano. Rift zones are areas of weakness within a volcano that extend from the surface to depths of several kilometers. Magma that erupts from the flank of a volcano must first flow underground through one of the volcano’s rift zones, sometimes traveling more than 30 kilometers from the summit magma reservoir before breaking the surface.

Mauna Loa.

Rising more than 9,000 meters from the seafloor, Mauna Loa is one of the world’s largest active volcanoes; from its base below sea level to its summit, Mauna Loa is taller than Mount Everest. It has erupted 15 times since 1900, with eruptions lasting from less than 1 day to as many as 145 days.