DESCRIPTION:
Calderas and Caldera Formation

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 buildup 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 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.

Processes of Caldera Unrest:
Caldera unrest reflects tectonic, magmatic, and hydrologic processes. For the purposes of this discussion, we define tectonic processes as those that occur in country rock and dominantly involve changes in mechanical energy with little or no movement of mass into or out of the subcaldera environment. Magmatic processes are those that occur within a magma reservoir, and in which thermal energy, magma, and magmatic volatiles can (though need not) move into or out of the subcaldera environment. Hydrologic processes are those involving movement of subcaldera ground water or in which the physical or chemical state of subcaldera ground water is changed. Probably no episode of unrest is purely tectonic, purely magmatic, or purely hydrologic, because tectonic and magmatic changes invariably influence a ground water system and vice versa, and magma (if present) invariably interacts with the local tectonic stress field.

The Aniakchak Caldera, covering some 10 square miles, is one of the great dry calderas in the world. Located in the volcanically active Aleutian Mountains, the Aniakchak last erupted in 1911. The crater includes lava flows, cinder cones, and explosion pits, as well as Surprise Lake, source of the Aniakchak River, which cascades through a 1,500-foot gash in the crater wall.

Link to: Alaska Volcano Observatory Website for MORE Information

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Aerial view Crater Lake Caldera, Oregon.
-- USGS Photo by W.E. Scott

An interesting variation of a composite volcano can be seen at Crater Lake in Oregon. From what geologists can interpret of its past, a high volcano -- called Mount Mazama -- probably similar in appearance to present-day Mount Rainier was once located at this spot. Following a series of tremendous explosions about 6,600 years ago, the volcano lost its top. Enormous volumes of volcanic ash and dust were expelled and swept down the slopes as ash flows and avalanches. These large-volume explosions rapidly drained the lava beneath the mountain and weakened the upper part. The top then collapsed to form a large depression, which later filled with water and is now completely occupied by beautiful Crater Lake. A last gasp of eruptions produced a small cinder cone which rises above the water surface as Wizard Island in, and near the rim, of the lake. Depressions such as Crater Lake, formed by collapse of volcanoes, are known as calderas. They are usually large, steep-walled, basin-shaped depressions formed by the collapse of a large area over, and around, a volcanic vent or vents. Calderas range in form and size from roughly circular depressions 1 to 15 miles in diameter to huge elongated depressions as much as 60 miles long.

Crater Lake Caldera Menu

Hawaiian and other shield volcanoes characteristically have a broad summit, indented with a caldera, a term commonly used for a large depression of volcanic origin. Most calderas form by collapse because of removal of magma from the volcano's reservoir by eruption and/or intrusion. Kilauea's summit caldera is about 2.5 miles long and 2 miles wide. Mokuaweoweo, the summit caldera complex of Mauna Loa is more elongate, measuring about 3 by 1.5 miles. The terms crater or pit crater are applied to similar but smaller collapse features. ...

If the hot-spot theory is correct, the next volcano in the Hawaiian chain should form east or south of the big Island. Abundant evidence indicates that such a new volcano exists at Loihi, a seamount (or submarine peak) located about 20 miles off the south coast of the Big Island. Loihi rises 10,100 feet above the ocean floor to within 3,100 feet of the water surface. Recent detailed mapping shows Loihi to be similar in form to Kilauea and Mauna Loa. Its relatively flat summit apparently contains a caldera about 3 miles across; two distinct ridges radiating from the summit are probably rift zones.

Link to: Hawaiian Volcano Observatory Website for MORE Information

The August 1883 eruption of Krakatau is often cited as a classic example of caldera formation by collapse following eruption of large volumes of pumice (Williams, 1941; Williams and McBirney, 1979; Self and Rampino, 1981, 1982; Francis and Self, 1983). However, other workers have suggested alternate mechanisms for formation of the Krakatau Caldera. Yokoyama (1981, 1982) concluded that the caldera formed by explosive destruction and reaming of the preeruption edifice, and Camus and Vincent (1983) and Francis (1985) favored an origin by large-scale collapse of the northern part of Krakatau Island (similar to the volcanic landslide at Mount St. Helens on 18 May 1980). Regrettable, much of the evidence is sumbmarine and inaccessible, but we are impressed by the similarity of Krakatau and other, better-exposed calderas (for example, Crater Lake) that are thought to have formed by simple collapse following voluminous pumice eruptions. The volume of magma erupted in the plinian eruption (9 cubic kilometers) is adequate to explain the caldera without invoking a landslide origin. ...

Indonesian Volcano Menu

About 760,000 years ago a cataclysmic volcanic eruption in the area blew out 150 cubic miles of magma (molten rock) from a depth of about 4 miles beneath the Earth's surface. Rapidly moving flows of glowing hot ash covered much of east-central California, and airborne ash fell as far east as Nebraska. The Earth's surface sank more than 1 mile into the space once occupied by the erupted magma, forming a large volcanic depression that geologists call a caldera.

Today, Long Valley occupies the eastern half of this 10-mile-wide, 20-mile-long caldera. Magma still underlies the caldera and heats underground water. The heated water feeds local hot springs and natural steam vents and drives three geothermal power plants, producing a combined 40 megawatts of electricity.

The Long Valley Caldera is only one part of a large volcanic system in eastern California that also includes the Mono-Inyo Craters volcanic chain. This chain extends from Mammoth Mountain at the southwest rim of the caldera northward 25 miles to Mono Lake. Eruptions along this chain began 400,000 years ago, and Mammoth Mountain itself was formed by a series of eruptions ending 50,000 years ago. The volcanic system is still active. Scientists have determined that eruptions occurred in both the Inyo Craters and Mono Craters parts of the volcanic chain as recently as 600 years ago and that small eruptions occurred in Mono Lake sometime between the mid-1700's and mid-1800's.

Long Valley caldera is located at the western edge of the Basin and Range Province straddling the eastern frontal fault escarpment of the Sierra Nevada, in which it forms a reentrant or offset commonly referred to as the "Mammoth embayment". The floor of the caldera ranges in elevation from 2,000 meters in its eastern half, where it is dominated by Lake Crowley and sage- and grass-covered Long Valley, to 2,600 meters in its western half, which is hillier and heavily forested. The caldera walls rise steeply to elevations of 3,000 to 3,500 meters on all sides except the east and southeast, where the floor rises only 150 meters before merging with the Volcanic Tableland at 2,300 meters elevation. The Mono-Inyo Craters volcanic chain extends from the western part of Long Valley caldera northward from Mammoth Mountain to Mono Lake, a distance of 50 kilometers.

Link to: Long Valley Observatory Website for MORE Information

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Aerial view of Medicine Lake in the subdued caldera of Medicine Lake Volcano. View is West-Southwest. Mount Shasta is on the skyline.
-- USGS Photo by Julie Donnelly-Nolan, September, 1985

Medicine Lake volcano is a Pleistocene and Holocene shield volcano located in northeastern California about 50 kilometers east of Mount Shasta, near the western margin of the Basin and Range tectonic province. Lava Beds National Monument is located on the northern flank of Medicine Lake volcano and encompasses mostly basaltic and some andesitic lavas. Higher on the volcano, basaltic lava is mostly absent, andesite dominates, and rhyolite and small volumes of dacite are present, the latter mainly near the 7 x 12 kilometer Medicine Lake caldera.

Medicine Lake Caldera Menu

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Yellowstone National Park showing caldera rim and location of 1959 and 1975 quakes
-- Modified from: Dzurisin, et.al., 1995, USGS Open-File Report 95-59

At the heart of Yellowstone's past, present, and future lies volcanism. Catastrophic eruptions occurred here about 2 million years ago, then 1.2 million years ago, and then 600,000 years ago. The latest eruption spewed out nearly 240 cubic miles of debris. What is now the park's central portion then collapsed, forming a 28- by 47- mile caldera (or basin). The magmatic heat powering those eruptions still powers the park's famous geysers, hot springs, fumaroles, and mud pots. The spectacular Grand Canyon of the Yellowstone River provides a glimpse of Earth's interior: its waterfalls highlight the boundaries of lava flows and thermal areas. Rugged mountains flank the park's volcanic plateau, rewarding both eye and spirit.

Yellowstone Caldera Menu

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