Aerial view of the Mono-Inyo Craters Volcanic Chain, California. Several eruptions occurred along both chains as recently as about 550 to 600 years ago. (Photograph by C. Dan Miller.)
Mono Craters Wilson Butte Northern part of Inyo Volcanic Chain Obsidian Dome Glass Creek Dome
Restless calderas
The largest and most explosive volcanic eruptions eject tens to hundreds of cubic kilometers of magma onto the Earth’s surface. When such a large volume of magma is removed from beneath a volcano, the ground subsides or collapses into the emptied space, to form a huge depression called a caldera. Some calderas are more than 25 kilometers in diameter and several kilometers deep.
Calderas are among the most spectacular and active volcanic features on Earth. Earthquakes, ground cracks, uplift or subsidence of the ground, and thermal activity such as hot springs, geysers, and boiling mud pots are common at many calderas. Such activity is caused by complex interactions among magma stored beneath a caldera, ground water, and the regional build-up of stress in the large plates of the Earth’s crust. Significant changes in the level of activity at some calderas are common; these new activity levels can be intermittent, lasting for months to years, or persistent over decades to centuries. Although most caldera unrest does not lead to an eruption, the possibility of violent explosive eruptions warrants detailed scientific study and monitoring of some active calderas.
Recently, scientists have recognized volcanic unrest at two calderas in the United States, Long Valley Caldera in eastern California and Yellowstone Caldera in Yellowstone National Park, Wyoming. Whether unrest at these calderas simply punctuates long periods of quiet or is the early warning sign of future eruptions is an important but still unanswered question.
Yellowstone River plummets through the famous Grand Canyon of Yellowstone. Carved by the river, the “yellow” rocks of the canyon are rhyolite lava flows that have been altered by hot water. The lava flows were erupted after the most recent caldera-forming eruption about 600,000 years ago.
Long Valley Caldera, California.
Long Valley Caldera lies on the eastern front of the Sierra Nevada, about 300 kilometers east of San Francisco. A huge explosive eruption about 700,000 years ago formed the caldera and produced pyroclastic flows that traveled 65 kilometers from the vent and covered an area of about 1,500 square kilometers. Ash from the caldera-forming eruption fell as far east as Nebraska. Within the past 40,000 years, eruptions have been restricted to a linear zone of vents, including the Mono-Inyo Craters Volcanic Chain, that extends about 50 kilometers north from the northwest part of the caldera.