Summit Calderas

snow covered summit crater
Aniakchak Caldera in Aniakchak National Monument.

NPS photo by Roy Wood.


Summit calderas form on preexisting composite volcanoes at the end of large-volume, climactic eruptions that empty the magma chamber beneath the summit. Caldera-collapse occurs along ring fractures as the summit area founders into the space previously occupied by the shallow magma reservoir.

Summit calderas make some of the most striking volcanic landscapes in the National Park System.

  • Aniakchak Caldera in Alaska, with its series of lava domes, maars, and other vents along with its warm springs and young lava flows within its caldera walls, contains abundant evidence of post-caldera volcanism. The most recent eruption at Aniakchak took place in 1931.

  • The beauty of today’s Crater Lake National Park belies the violence of the lakes origin at the end of the eruption that destroyed Mount Mazama. The preexisting Mazama composite volcano had an elevation of approximately 12,000 ft (3,700 m). The eruption blanketed an area of more than 650,000 square miles (approximately 1.7 square km) across the western United States and southwestern Canada with volcanic ash.

  • The caldera at the summit of Mount Katmai formed during the Valley of Ten Thousand Smokes eruption in 1912. This eruption lasted about 60 hours and transformed a verdant valley rich with wildlife into a desolate expanse covered by pyroclastic flow deposits smoking with superheated fumaroles. The primary vent for this eruption was Novarupta, a separate vent approximately 6 miles (10 km) from Mount Katmai.

National Park Summit Calderas

At least three national parks contain volcanoes with summit calderas:

Aniakchak National Monument, Alaska

Aniakchak caldera is one of the most compelling volcanoes in the National Park System. It is one of the best exposed calderas in the world and also contains a series of domes, cones, maars, lava flows, and other volcanic features.

Aniakchak caldera has a diameter of about 6.6 miles (10.5 km) and averages about 2,000 feet (610 m) deep. It formed 3,660 ± 70 years ago during a Volcanic Explosivity Index (VEI) 6 (Colossal) eruption that was one of the largest eruptions of the Holocene.

The caldera is located at the site of a former composite volcano that was largely destroyed when collapse occurred. This volcano is also the site of a probable previous caldera collapse that occurred about 9,000 years ago.

photo of a distant mountain range with an airplane in the foreground photo of a distant mountain range with an airplane in the foreground

Left image
Distant view of Aniakchak’s caldera profile.
Credit: USGS photo by Game McGimsey.

Right image
Hypothetical reconstruction of the of the pre-caldera composite volcano prior to Aniakchak’s caldera-forming eruption.
Credit: USGS photo by Game McGimsey.

a series of 3 diagrams showing the sequence of eruption and caldera formation

The eruption that formed Aniakchak caldera began with a Plinean phase that propelled ash high into the atmosphere where winds transported it so that it was deposited over most of Alaska and Canada and as far away as Greenland. The ash-flow tuff (ignimbrite) erupted from ring fractures during the caldera collapse phase is as thick as 230-330 feet (70-200 m) in valley bottoms and covers an area just shy of 1,000 square miles (more than 2,500 square km). The pyroclastic flow generated a tsumani when it entered Bristol Bay, which is about 20 mi (32 km) from Aniakchak.

Total volume of the climactic eruption was 17 cubic miles (70 cubic km) Dense Rock Equivalent. (As a comparison, the 1980 eruption of Mount St. Helens was 0.06 cubic miles, 0.25 cubic km DRE.)

Figure (right): Caldera-forming eruption of the Aniakchak composite volcano began with voluminous eruption of pumice and ash in a Plinian to Ultra-Plinian eruption column. Extensive pyroclastic flows were erupted during the climactic phase draining the magma chamber and collapsing the volcanic edifice into the void. Post-caldera eruptions built maars, domes, and composite cones within the caldera.

Modified from USGS illustration.

Crater Lake National Park

Established in 1902, Crater Lake NP was the third national park set aside for its volcanic resources. Crater Lake formed in a Mega-colossal (VEI 7) eruption 7,700 years ago that culminated in caldera collapse. The eruption had an estimated volume of 12 cubic miles (50 cubic km) making it the largest prehistoric eruption in the contiguous United States.

Mount Mazama, the preexisting composite volcano, had been active over about the previous 420,000 years. The upper portions of Mount Mazama were destroyed by the climactic eruption. Today, the caldera walls have much lower elevations than Mazama’s 12,000 ft (3,700 m) peak, and range from 8,156 (2,486 m) ft to 6680 ft (2,036 m) above sea level. The surface of the lake has an average elevation of 6173 ft (1882 m) with the deepest region of the caldera floor having an elevation of approximately 4,370 ft (1,330 m), where it is covered with sediment. Shallower areas of the caldera floor occur where post-caldera eruptions have occurred. The Crater Lake caldera has a diameter of approximately 6 mi (10 km) east-west and 5 mi (8 km) north-south.

3 paintings showing different phases of the eruption of a volcanic peak

The eruption had two main phases that took place over a few days:

  1. Plinian phase: The part of the eruption had a single vent that created an eruption column that rose about 30 mi (50 km) in height. Ash from the eruption was spread over much of the Northwest, including parts of Canada.

  2. Ring-Vent phase: This part of the eruption accompanied caldera collapse and took place from a series of vents along the fractures where collapse occurred. Pyroclastic flows traveled down the slopes of the volcano and as far as 40 mi (70 km) from the vents and partially filled the valleys below the volcano.

Images (right): A series of paintings from the 1930s showing the evolution of Mount Mazama from a tall composite volcano during the Pleistocene to the climactic eruption 7,700 years ago, and finally to the Crater Lake caldera. Paintings by Paul Rockwood who was commissioned through the Works Project Administration. Note that geological research conducted in the years since the paintings were made has indicated that glacial ice wasn’t as extensive at the time of the eruption.

Katmai National Park

The caldera at the summit of Mount Katmai, a composite volcano, has an unusual geologic history. It was caused by an eruption that primarily took place at a vent that was not actually located on Mount Katmai. Instead it formed during the Colossal (VEI 6) eruption of Novarupta, located approximately 6 mi (10 km) away. The Novarupta eruption lasted approximately 60-hours from June 6–8, 1912. It tapped a magma reservoir underneath Mount Katmai, which caused its summit to collapse, leading to the formation of the 1.5-mi (2.5-km) wide caldera. Caldera collapse was accompanied by 14 earthquakes with magnitudes of 6 to 7 and multiple smaller ones. The Novarupta-Katmai eruption was the largest of the 20th century.

photo of a caldera with a crater lake
Katmai caldera in 2020. The lake began forming in the years following the 1912 eruption. In 1951, the lake was 524 ft (160 m) deep. In 2010, the lake was approximately 787 ft (240 m) deep. Today the lake is more than 800 ft (244 m) deep.

USGS Alaska Volcano Observatory photo by Cyrus Read.

Ash from the eruption rose to 19 mi (30 km) into the stratosphere and was carried by the wind around the globe. Falling ash darkened the skies in Kodiak, 100 miles (160 km) away and thick ash deposits formed throughout much of southern Alaska.

The Novarupta-Katmai eruption had three main explosive episodes with the caldera-collapse and eruption of pyroclastic flows that became the Valley of Ten Thousand Smokes occurring in the first one. The two subsequent episodes deposited widespread ash-fall deposits. The eruption concluded with the extrusion of the lava dome that plugged the Novarupta vent. A lava dome also was erupted in the Katmai caldera following caldera collapse.

two maps; one showing location of volcanic features; one showing area of volcanic ashfall
Map of the Katmai area showing the location of the primary vent at Novarupta and the Katmai caldera (left). Map showing the area impacted by ash fall from the Plinian eruption.

USGS images.

Magma Composition

Silicic to intermediate lavas are usually erupted during caldera-forming eruptions. Many of the climactic eruptions that result in caldera formation tap magma reservoirs that are compositionally zoned, with more silicic magmas in the upper part of the chambers and hotter, intermediate magmas present deeper in the reservoir. The thick ignimbrites (ash-flow tuffs) erupted during the climactic eruptions at Aniakchak, Crater Lake, and Novarupta-Katmai are all compositionally-zoned.

Eruption Styles

Composite volcanoes that become the sites of summit calderas experience many eruptions before and after the climactic caldera-forming eruption. Caldera-forming eruptions themselves are typically Plinean to Ultra-Plinean and have a Volcanic Explosivity Index (VEI) value of 6 (Colossal) to 7 (Mega-colossal).


Caldera Walls

Much of the width of summit calderas results from landslides and other mass wasting events that occur along steep unstable walls following subsidence. For example, Crater Lake’s current width is approximately twice as wide as the diameter of the ring vents along which caldera collapse occurred. The ring vents that fed the climactic eruption of Mount Mazama are underneath the waters of Crater Lake.

photo of the rim of a summit caldera with a dusting of snow
Part of Aniakchak’s caldera rim. Pre-caldera volcanic rocks are exposed in Aniakchak’s caldera walls.

NPS photo.

Caldera Lakes

Because calderas are depressions, they may fill with water. Crater Lake in Oregon filled over the course of several hundred years following the climactic eruption 7,700 years ago with precipitation being the water primary source. Aniakchak’s caldera contains a lake in one part of the caldera, but before the caldera walls were breached, water covered most of its floor.

photo of caldera rim and crater lake
The surface of Crater Lake is about 1,000 ft (300 m) below the caldera rim. The Wizard Island cinder cone erupted about 4,800 years ago.

USGS photo by David Ramsey.

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Post-caldera Eruptions

Post-caldera volcanism may include the eruptions of lava flows, or the formation of domes or cinder cones. Explosive eruptions may produce maars or pyroclastic deposits.

black and white photo of a blocky lava flow
A lava flow in Aniakchak caldera in 1931.

Hubbard Collection, Santa Clara University Archives.

photo of crater lake photo of crater lake

Left image
The Wizard Island cone is the only post-caldera eruptive product that extends above the surface of Crater Lake.
Credit: NPS image.

Right image
The bathymetry of Crater Lake reveals evidence of other post-caldera eruptions.
Credit: USGS image.

Thick Pyroclastic Flows (Ignimbrite Deposits)

Extensive pyroclastic flows are typically erupted during caldera collapse. The pyroclastic flows are emitted in a “boiling over” phase of the eruptions and/or collapse of an eruption column.

All three summit calderas in the National Park System produced thick ash-flow tuffs (ignimbrites), with the most iconic being the Valley of Ten Thousand Smokes ignimbrite in Katmai National Park.

photo of a broad valley
The Valley of Ten Thousand Smokes in 2013. NPS photo by M. Fitz. The deposit of the pyroclastic flows changed a U-shaped valley rich with vegetation and wildlife into a nearly barren plain that was covered with thousands of fumaroles that remained active for years after the eruption.

Ash-fall Deposits

Plinian and Ultra-Plinian eruptions before and during caldera collapse produce eruption columns that extend tens of miles into the atmosphere/stratosphere leading to ash deposits that can cover thousands of square miles.

photo of rock cliff topped with layered ash deposits
Deposit of ash and pumice above Llao Rock on the rim of the Crater Lake Caldera from the Plinian phase of the Mazama eruption.

John St. James photo.


Summit volcanoes usually occur on composite volcanoes, like those found above subduction zones such as the ones along the Pacific “Ring of Fire.” Crater Lake is part of the High Cascade Mountains in the Pacific Northwest. Aniakchak and Mount Katmai are part of the Aleutian volcanic arc, formed from the subduction of the Pacific plate underneath the North American plate.

Volcanic Hazards

Calderas present both proximal and distal volcanic hazards. Post-caldera volcanism may consist of effusive or explosive eruptions, and include magmatic and phreatomagmatic eruptions.

Proximal volcanic hazards include ash clouds, ash and tephra fall, pyroclastic flows and surges, lahars, landslides, fumaroles, earthquakes, and tsunamis. Distal hazards include ash clouds, ash and tephra fall, lahars, and tsunamis.

National Park Sites with Summit Calderas

Part of a series of articles titled Volcano Types.

Aniakchak National Monument & Preserve, Crater Lake National Park, Katmai National Park & Preserve

Last updated: April 17, 2023