Volcano Terms and Definition

Savage Earth

Waves of Destruction
Out of the Inferno
Restless Planet
Hell's Crust
From the legendary fury of Mt. Vesuvius to the devastating convulsions of Kobe, Japan, Savage Earth tells the stories of these great natural disasters, the scientists who struggle to understand and predict them, and the people whose lives are forever changed by their merciless force

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Digtal Microscope

Aniakchak Caldera formed during an enormous explosive eruption that expelled more than 50 km 3 of magma about 3,450 years ago. The caldera is 10 km in diameter and 500-1,000 m deep. Subsequent eruptions formed domes, cinder cones, and explosion pits on the caldera floor.

A caldera is a large, usually circular depression at the summit of a volcano formed when magma is withdrawn or erupted from a shallow underground magma reservoir. The removal of large volumes of magma may result in loss of structural support for the overlying rock, thereby leading to collapse of the ground and formation of a large depression. Calderas are different from craters, which are smaller, circular depressions created primarily by explosive excavation of rock during eruptions. .

Mauna Loa Volcano, Hawaii

Snow-covered Moku`aweoweo Caldera atop Mauna Loa shield volcano (Mauna Kea in background). The caldera is 3 x 5 km across, 183 m deep, and is estimated to have collapsed between 600-750 years ago. Several pit craters along the upper southwest rift zone of Mauna Loa (lower right) also formed by collapse of the ground. For more information about the world's largest volcano, see Mauna Loa Volcano from the USGS Hawaiian Volcano Observatory.


Kaguyak volcano, Alaska

This lake-filled caldera formed atop a former stratovolcano (note remnant of upper part of older cone at right center). The caldera formed about 1,100 years ago and is 2.5 km in diameter. The prominent peninsula and small island consists of lava domes erupted after the caldera formed.

Ugashik volcano, Alaska

Aerial view, looking southwest, of Ugashik caldera. The caldera is 5 km in diameter and is partially filled by at least five lava domes erupted after the caldera formed.

Crater Lake, Oregon

Crater Lake partially fills an 8-10 km-wide caldera that formed when the top of the Cascade volcano known as Mount Mazama collapsed during an enormous explosive eruption about 7,700 years ago. The eruption expelled about 50 km 3 of magma. This cataclysmic event was followed by a series of post-caldera eruptions until about 5,000 years ago. These smaller events erupted lava flows, a lava dome, and tephra from vents on the caldera floor. Such activity built Wizard Island (above) on the western caldera floor. A 1997 report describes the various types of volcano and earthquake hazards in the Crater Lake area, estimates the likelihood of future events, suggests ways to reduce risk, and includes a map of hazard zones.

Volcano Glossary



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U.S. Department of the Interior