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.
[Illustration: 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.
This volcanic chain consists of many vents that have erupted in the past
several thousand years. Eruptions from vents as recently as 550 years
ago produced lava flows, pyroclastic flows, and ash, all of rhyolitic
composition. Geologic mapping shows that some eruptions were preceded by
ground cracking, suggesting that the ground was pulled apart or
stretched as magma neared the surface.
[Illustration: Sketch of Long Valley Caldera and the Mono-Inyo Craters
Volcanic Chain in central California, viewed from the southeast.
(_Sketch by Tau Rho Alpha._)]
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.
[Illustration: South Inyo Crater, Long Valley Caldera, California.
Explosive eruptions formed the crater about 500 years ago. (_Photograph
by Steven R. Brantley._)]
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.
[Illustration: Map of most recent Yellowstone Caldera and its main
thermal features. After the caldera formed, many vents erupted thick
rhyolite lava flows, and the central part of the caldera was pushed
upward to form resurgent domes. The star marks the magnitude 7.5 Hebger
Lake earthquake.]
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.
[Illustration: Castle Geyser erupting a column of hot water, Yellowstone
National Park. (_Photograph by Steven R. Brantley._)]