Part 4

Sierra Nevada Mono craters Inyo craters Mammoth Lakes Resurgent dome Long Valley Caldera

Three moderate earthquakes south of the caldera and one beneath the caldera on May 25-26, 1980, marked the beginning of unrest that continues into the 1990’s. Swarms of earthquakes beneath the caldera, changes in several hot springs, and the formation of new springs have occurred since 1980. Precise surveys have also shown that the central part of the caldera has risen by more than 50 centimeters since 1975. This unrest is probably related to the stretching (east-west extension) of the Earth’s crust that is known to be occurring in the region around the caldera, and it probably also involves the rise of magma beneath the caldera. Scientists do not know if this unrest will lead to volcanic activity, but the geologically recent eruptions along the Mono-Inyo Craters Volcanic Chain suggest that future eruptions are possible.

Yellowstone Caldera, Yellowstone National Park, Wyoming.

Yellowstone Caldera is one of the largest and most active calderas in the world. The spectacular geysers, boiling hot springs, and mud pots that have made Yellowstone famous—and even the strikingly beautiful Grand Canyon of Yellowstone through which the Yellowstone River plunges—owe their existence to the tremendous volcanic forces that have affected the region during the past 2 million years. Cataclysmic eruptions 2.0, 1.3, and 0.6 million years ago ejected huge volumes of rhyolite magma; each eruption formed a caldera and extensive layers of thick pyroclastic-flow deposits. The youngest caldera is an elliptical depression, nearly 80 kilometers long and 50 kilometers wide, that occupies much of Yellowstone National Park. The caldera is buried by several extensive rhyolite lava flows erupted between 75,000 and 150,000 years ago.

IDAHO Hebgen Lake West Yellowstone MONTANA Silver Gate Gardiner WYOMING Yellowstone National Park Mammoth M 7.5 Aug. 18 1959 Norris Geyser Basin Mud Pots Old Faithful West Thumb Geyser Basin Yellowstone Lake CALDERA RIM COLORADO Resurgent dome

The Earth’s crust beneath Yellowstone National Park is still restless. Precise surveys have detected an area in the center of the caldera that rose by as much as 86 centimeters between 1923 and 1984 and then subsided slightly between 1985 and 1989. Scientists do not know the cause of these ups and downs but hypothesize that they are related to the addition or withdrawal of magma beneath the caldera, or to the changing pressure of the hot ground water system above Yellowstone’s large magma reservoir. Also, Yellowstone National Park and the area immediately west of the Park are historically among the most seismically active areas in the Rocky Mountains. Small-magnitude earthquakes are common beneath the entire caldera, but most are located along the Hebgen Lake fault zone that extends into the northwest part of the caldera. A magnitude 7.5 earthquake occurred along this zone in 1959.

Active Volcanoes: Windows Into the Past

Molten rock has erupted onto the surface of the Earth throughout its 4.5-billion-year history. Although many of these ancient rocks were removed by erosion, volcanic deposits can be found beneath younger rocks in many parts of the United States. To a geologist, such long-lasting volcanic rocks look like those formed by today’s active volcanoes. Many ancient volcanic rocks, however, change somewhat with time, as they become firmly consolidated, buried by younger deposits, and sometimes folded and faulted by the continuous shifting of the Earth’s crust. Even minerals of volcanic rocks may change, if after burial they encounter high pressures and temperatures.

Most active volcanoes are built on older volcanic deposits erupted from ancient volcanoes, and visitors to the present-day volcanoes walk or drive across these products of past volcanism. For example, anyone driving across the Cascade Range, sunbathing at Waikiki, or fishing on the Alaska Peninsula is there because old volcanic rocks form the landscape.

One step further back in time from today’s active volcanoes are people who picnic in the White Mountains of New Hampshire, enjoy the autumn colors in the Blue Ridge of Shenandoah National Park, and hike in the rugged Big Bend National Park of Texas. Many of the rocks in these areas were formed by eruptions or by intrusion of magma into the Earth’s crust many millions of years ago. Because volcanic activity has been so important in shaping the Earth, watching active volcanoes today provides a window through which we can glimpse and reconstruct the early volcanic history of our planet.

As we increase our knowledge about volcanic processes, by studying volcanoes erupting today as well as those that have lain dormant for hundreds to thousands of years, we increase our ability to predict when and how volcanoes will erupt. Accurate predictions, presented in terms that are meaningful to public officials, will minimize the number of lives lost and the social and economic upheaval that an eruption can cause.

Glossary

Andesite A volcanic rock containing 53-63% silica with a moderate viscosity when in a molten state. Ash Fragments less than 2 millimeters in diameter of lava or rock blasted into the air by volcanic explosions. Basalt A volcanic rock consisting of less than 53% silica with a low viscosity when in a molten state. Caldera A large volcanic depression, commonly circular or elliptical when seen from above. Composite A steep-sided volcano composed of many layers of volcano volcanic rocks, usually of high-viscosity lava and fragmented debris such as lahar and pyroclastic deposits. Dacite A volcanic rock containing 63-68% silica with a high viscosity when in a molten state. Dome A steep-sided mound that forms when viscous lava piles up near a volcanic vent. Domes are formed by andesite, dacite, and rhyolite lavas. Fumarole A vent that releases volcanic gases, including water vapor (steam). Lahar A flowing mixture of water and rock debris that forms on the slopes of a volcano, sometimes referred to as debris flow or mudflow. The term comes from Indonesia. Lava Molten rock that erupts from a vent or fissure; see magma. Magma Molten rock that contains dissolved gas and crystals, formed deep within the Earth. When magma reaches the surface, it is called lava. Phreatic A type of volcanic explosion that occurs when eruption water comes in contact with hot rocks or ash near a volcanic vent, causing steam explosions. Pumice A light-colored volcanic rock containing abundant trapped gas bubbles formed by the explosive eruption of magma. Because of its numerous gas bubbles, pumice commonly floats on water. Pyroclastic flow A hot, fast-moving and high-density mixture of ash, pumice, rock fragments, and gas formed during explosive eruptions. Pyroclastic Same process as pyroclastic flow but of much surge lower density. Rhyolite A volcanic rock containing more than 68% silica with a very high viscosity when in a molten state. Shield volcano A volcano shaped like an inverted warrior’s shield with long gentle slopes produced by eruptions of low-viscosity basaltic lava. Silica The molecule formed of silicon and oxygen (SiO₂) that is the basic building block of volcanic rocks and the most important factor controlling the fluidity of magma. The higher a magma’s silica content, the greater its viscosity or “stickiness.” Vent The opening at the Earth’s surface through which volcanic materials (magma and gas) escape. Volcano A vent in the surface of the Earth through which magma erupts and also the landform that is constructed by the erupted material. Volcanic The downslope movement of soil, rock debris, and landslide sometimes glacial ice, with or without water, from the flank of a volcano.

The metric units used in this publication can be converted to English units by using the approximate conversions given below:

Length

1 kilometer 0.6 of a mile 1 meter 39.37 inches 1 centimeter 0.4 inch 1 millimeter 0.04 inch

Area

1 sq. kilometer 0.4 sq. mile 1 sq. meter 1.2 sq. yards 1 sq. centimeter 0.155 sq. inch

Temperature

To convert °Celsius to °Fahrenheit, multiply °C by 1.8 and add 32. To convert °Fahrenheit to °Celsius, subtract 32 from °F and divide the result by 1.8.

Further Reading

Decker, Robert, and Decker, Barbara, 1989, Volcanoes: San Francisco, Freeman, 285 p. (An information-packed introduction to the study of volcanoes written in an easy-to-read style.)

Editors, 1982, Volcano: in the series Planet Earth, Alexandria, Virginia, Time-Life Books, 176 p. (A well illustrated and readable general survey of volcanoes and their activity.)

McClelland, Lindsay, Simkin, Tom, Summers, Marjorie, Nielsen, Elizabeth, and Stein, T.C., editors, 1989, Global Volcanism 1975-1985: Englewood Cliffs, New Jersey, Prentice-Hall, 656 p. (A full account of volcanism on Earth based on eyewitness accounts from geologists and other scientists, reporters, travelers, and other keen observers.)

Simkin, Tom, and Seibert, Lee, 1994, Volcanoes of the World (Second edition): Stroudsburg, Pa., Hutchinson Ross, 233 p. (A comprehensive regional directory of worldwide volcanic activity during the past 10,000 years; activity presented in table format.)

Tilling, R. I., 1982, Volcanoes: Reston, Virginia, U.S. Geological Survey general-interest publication, 46 p. (A general introduction for the nonspecialist to the study of volcanoes, with focus on the nature, types, workings, products, and hazards of volcanoes.)

Tilling, R. I., 1984, Eruptions of Mount St. Helens: Past, present, and future: Reston, Virginia, U.S. Geological Survey general-interest publication, 46 p. (A nontechnical summary, illustrated by many color photographs and diagrams, of the abundant scientific data available for the volcano, with emphasis on the catastrophic eruption on May 18, 1980, which caused the worst volcanic disaster in U.S. history.)

Tilling, R. I., 1987, Eruptions of Hawaiian volcanoes: Past, present, and future: Reston, Virginia, U.S. Geological Survey general-interest publication, 54 p. (A nontechnical summary, illustrated by color photographs and drawings, of the eruption history, style, and products of two of Hawaii’s active volcanoes, Kilauea and Mauna Loa.)