A caldera is a depression created after a volcano releases the majority of the contents of its magma chamber in an explosive eruption. Without any structural support below, the land around the erupting volcanic vent or vents collapses inwardly, creating the bowl-shaped caldera.

Depending on their intensity and duration, volcanic eruptions can create calderas as much as 100 kilometers (62 miles) wide. A caldera-causing eruption is the most devastating type of volcanic eruption. It permanently alters the environment of the surrounding area.

A caldera is not the same thing as a crater. Craters are formed by the outward explosion of rocks and other materials from a volcano. Calderas are formed by the inward collapse of a volcano. Craters are usually more circular than calderas. (Calderas may have parts of their sides missing because land collapses unevenly.) Craters are also usually much smaller than calderas, only extending to a maximum of one kilometer (less than a mile) in diameter.

Types of Calderas

Crater-Lake Calderas

Crater-lake calderas result from the collapse of a stratovolcano after a Plinian eruption, the most explosive type of volcanic eruption. Plinian eruptions release massive amounts of lava, volcanic ash, and rocks.

Crater Lake, in the U.S. state of Oregon, is in a crater-lake caldera, not a crater. The Crater Lake caldera is about 10 kilometers (six miles) wide. Crater Lake is the deepest lake in the United States. It resulted from an eruption that occurred more than 7,000 years ago at the ancient Mount Mazama. The eruption completely emptied Mount Mazama’s magma chamber. The magma chamber’s roof then collapsed and filled with water from rain and snow, creating the lake.

Subsequent eruptions of Mount Mazama built up Wizard Island, a volcanic cone that forms an island on the western end of the lake. Deception Island, located off the coast of Antarctica, is another crater-lake caldera.

Unlike Mount Mazama, the Deception volcano is still active. The Deception volcano experienced a violent eruption roughly 10,000 years ago that caused its summit to collapse and flood with seawater, forming a caldera about seven kilometers (4.4 miles) wide.

The caldera gives the island its unique horseshoe shape, which opens to the sea through a narrow channel. Deception Island’s unique geologic structure makes it one of the only places in the world where vessels can sail directly into an active volcano.

Shield Volcano Calderas 

Shield volcano calderas do not result from singular explosive eruptions. They instead subside in gradual stages, due to the episodic release of lava. This less-explosive release of lava, known as lava fountaining, is characteristic of shield volcanoes.

As a shield volcano periodically releases lava, it produces nested or terraced depressions rather than a large bowl-shaped caldera. As a result, shield volcano calderas are usually less than five kilometers (3.1 miles) in diameter.

Composed of dormant and active shield volcanoes, the U.S. islands of Hawai'i have a number of impressive shield volcano calderas. Moku'aweoweo, the caldera at the summit of Mauna Loa on the “Big Island” of Hawai'i, is actually comprised of three distinct depressions that overlap. In total, the caldera complex has an area of roughly 15 square kilometers (six square miles) and reaches a depth of 180 meters (600 feet).

The Galápagos Islands are also a series of shield volcanoes with some summit calderas. Fernandina Island, the most volcanically active island in the chain, has a deep elliptical caldera that measures 4-by-6.5 kilometers (2.5-by-4 miles).

In 1968, a massive volcanic eruption produced one of the largest caldera collapses in recent history. Like most shield volcano calderas, Fernandina caldera collapsed incrementally and asymmetrically, sinking in as much as 350 meters (1,150 feet) in some parts.

Resurgent Calderas

Resurgent calderas are the largest volcanic structures on Earth, ranging from 15 to 100 kilometers (nine to 62 miles) in diameter. They are not associated with one particular volcano, but instead result from the widespread collapse of vast magma chambers.

This caldera collapse is produced by incredibly destructive eruptions known as pyroclastic sheet flows, the likes of which have not occurred in historic times. The Toba Caldera on the Indonesian island of Sumatra is the newest resurgent caldera, created roughly 74,000 years ago by the largest volcanic eruption in the last 25 million years.

This massive eruption ejected 2,800 cubic kilometers (1,740 cubic miles) of debris. The eruption left a caldera 100 kilometers (62 miles) long, 29 kilometers (18 miles) wide, and 508 meters (1,666 feet) deep, making it the largest volcanic structure on Earth. The caldera is now home to Lake Toba and Samosir Island.

Samosir was formed by the uplift of the caldera floor due to magma pressure below. This uplift is common to all resurgent calderas as new magma fills in the empty magma chamber over thousands of years.

The Yellowstone Caldera, located in Yellowstone National Park in the U.S. state of Wyoming, is the result of the Yellowstone Supervolcano. This enormous volcano complex last erupted about 640,000 years ago. The Yellowstone Caldera is more than 72 kilometers (45 miles) long.

Recent scientific analyses have revealed that the caldera’s magma chamber is 2.5 times larger than previously thought, measuring an incredible 90 kilometers by 30 kilometers (55 miles by 20 miles) and 10 kilometers (six miles) deep. These new measurements put the magma chamber at roughly the same size it was during its last eruption.