Summit Calderas: How Aniakchak, Crater Lake and Katmai Collapsed into Themselves

A summit caldera forms on an existing composite volcano at the end of a huge, climactic eruption that empties the magma chamber beneath its summit. With nothing left to hold it up, the top of the volcano sinks along ring fractures into the space the magma occupied. The result is some of the most striking volcanic scenery in the National Park System. At least three national parks have one.

Aniakchak Caldera, Aniakchak National Monument. National Park Service.

Aniakchak, Alaska

Aniakchak is one of the best-exposed calderas in the world: about 6.6 miles (10.5 km) across and 2,000 feet (610 m) deep on average, with lava domes, cones, maars, lava flows, warm springs and young lava inside its walls. It last erupted in 1931.

It formed 3,660 ± 70 years ago in a VEI 6 ("colossal") eruption, one of the largest of the Holocene, which destroyed the composite volcano that stood there; a probable earlier collapse happened about 9,000 years ago.

Aniakchak’s caldera in profile. U.S. Geological Survey.

A reconstruction of the volcano before the caldera-forming eruption. U.S. Geological Survey.

The eruption began with a Plinian phase, a towering column of pumice and ash that winds carried over most of Alaska and Canada and as far as Greenland. Then, as the summit collapsed, pyroclastic flows poured from the ring fractures, leaving ignimbrite (ash-flow tuff) up to 230–330 feet thick in valley bottoms over nearly 1,000 square miles; where they reached Bristol Bay, about 20 miles away, they set off a tsunami. The total volume was 17 cubic miles (dense-rock equivalent), against 0.06 cubic miles for Mount St. Helens in 1980.

How Aniakchak’s caldera formed: eruption column, collapse with pyroclastic flows, then new domes, maars and cones inside. Modified from a U.S. Geological Survey illustration.

Crater Lake, Oregon

Crater Lake National Park, established in 1902, was the third national park set aside for its volcanic features. The lake fills a caldera formed 7,700 years ago in a VEI 7 ("mega-colossal") eruption of about 12 cubic miles, the largest prehistoric eruption in the contiguous United States. It destroyed the top of Mount Mazama, a volcano active for about 420,000 years and roughly 12,000 ft (3,700 m) high, and spread ash over more than 650,000 square miles of the western United States and southwestern Canada.

The eruption had two phases over a few days: a Plinian column from a single vent, about 30 miles (50 km) high, spreading ash across the Northwest; then a ring-vent phase as the summit collapsed, with pyroclastic flows racing up to 40 miles from the vents and partly filling the valleys.

Mount Mazama to Crater Lake, painted in the 1930s by Paul Rockwood for a Works Progress Administration commission. Later research found less glacial ice at the time than shown. National Park Service.

Today the rim stands between 8,156 ft and 6,680 ft, the lake surface at about 6,173 ft, and the deepest, sediment-covered floor at about 4,370 ft. The caldera is about 6 miles across east–west and 5 north–south.

Katmai, Alaska

Mount Katmai's caldera has an odd history: the eruption that made it happened mostly at Novarupta, a vent about 6 miles (10 km) away. Over about 60 hours, 6–8 June 1912, the VEI 6 eruption, the largest of the 20th century, drained a magma reservoir beneath Katmai, whose summit collapsed into a caldera 1.5 miles wide, with 14 earthquakes of magnitude 6 to 7. Ash rose 19 miles into the stratosphere and circled the globe; it darkened the sky in Kodiak, 100 miles away. Pyroclastic flows turned a green valley full of wildlife into the Valley of Ten Thousand Smokes, a barren plain of fumaroles that steamed for years. Lava domes later plugged Novarupta and rose in the caldera.

Katmai’s caldera in 2020; its lake, begun after 1912, was 524 ft deep in 1951, about 787 ft in 2010 and is now more than 800 ft. U.S. Geological Survey Alaska Volcano Observatory.

Novarupta and the Katmai caldera (left); the area covered by ash fall (right). U.S. Geological Survey.

What calderas have in common

• Magma. Caldera-forming eruptions usually bring up silicic to intermediate magma from chambers that are zoned: more silicic magma on top, hotter intermediate magma below. The great ignimbrites of all three parks are zoned.
• Eruption style. Before and after the climax, the volcano erupts many times; the climax itself is typically Plinian to ultra-Plinian, VEI 6 to 7.
• Walls. Much of a caldera's width comes from landslides off its steep, unstable walls after collapse: Crater Lake is about twice as wide as the ring vents beneath its water.
• Lakes. Calderas can fill with water. Crater Lake filled over several hundred years, mostly from rain and snow; Aniakchak's floor was largely flooded until its wall was breached.
• Later eruptions. Lava flows, domes and cinder cones can follow, or explosive maars. Wizard Island, a cinder cone about 4,800 years old, is the only post-caldera vent rising above Crater Lake; the lake floor holds others.
• Ignimbrite and ash. All three parks have thick ash-flow tuffs, the Valley of Ten Thousand Smokes the most famous; Plinian columns spread ash over thousands of square miles.

Older volcanic rock exposed in Aniakchak’s caldera wall. National Park Service.

Crater Lake lies about 1,000 ft below the rim; Wizard Island erupted about 4,800 years ago. U.S. Geological Survey.

Wizard Island. National Park Service.

The lake floor shows other post-caldera eruptions. U.S. Geological Survey.

The Valley of Ten Thousand Smokes in 2013. National Park Service.

Where and why

Summit calderas form on composite volcanoes above subduction zones, such as those of the Pacific "Ring of Fire." Crater Lake is in the High Cascades; Aniakchak and Katmai are in the Aleutian arc, where the Pacific plate dives beneath the North American plate.

Calderas bring hazards both near and far. Close by: ash clouds and fall, pyroclastic flows and surges, lahars, landslides, fumaroles, earthquakes and tsunamis. Far away: ash clouds and fall, lahars and tsunamis. See the National Park Service's geohazard guidance, and for more: The Novarupta-Katmai Eruption of 1912 (USGS) and the geology of Mount Mazama and Crater Lake (USGS).

Sources

• National Park Service, "Summit Calderas"; rewritten in hubnx's own words. Photographs, diagrams and maps: National Park Service and U.S. Geological Survey. A photograph credited to a private photographer is not reproduced here.