A caldera is a large, basin-shaped volcanic depression, typically more than one kilometer in diameter, formed when a volcano's magma chamber empties during a massive eruption and the overlying rock collapses into the void. The word comes from the Spanish "caldera," meaning "cauldron" — an apt description for these enormous volcanic basins.
The Scale of a Caldera Eruption
Calderas represent some of the most powerful volcanic events in Earth's history. The eruptions that create them can be thousands of times more powerful than typical volcanic eruptions, ejecting enough material to affect global climate. Yellowstone's caldera, for example, was formed by an eruption 1,000 times more powerful than the 1980 Mount St. Helens eruption.
How Calderas Form
- Magma Chamber: Large body of molten rock accumulates underground
- Eruption: Massive explosive eruption rapidly empties the chamber
- Collapse: Roof of the emptied chamber collapses under its own weight
- Result: A broad, shallow depression much larger than a typical crater
Caldera formation begins with the accumulation of a large magma chamber close to the surface. When pressure triggers an eruption, the chamber can empty rapidly, ejecting cubic kilometers of ash, pumice, and lava. With the support removed, the ground above the emptied chamber collapses like a trapdoor, forming the caldera.
The contrast with other volcano forms is instructive. A stratovolcano such as Fuji builds upward over thousands of eruptions and a shield volcano such as Mauna Loa spreads outward, while a caldera is what is left when one of them loses its foundations; the main forms are compared in our guide to types of volcanoes. Unlike a volcanic crater, which is formed by an outward explosion, a caldera is formed by an inward collapse. This is why calderas are typically much larger than craters — some exceed 80 kilometers in diameter.
Types of Calderas
- Explosive/Collapse Caldera: Formed by catastrophic eruption and collapse, e.g. Yellowstone
- Shield Volcano Caldera: Formed by gradual drainage of lava, e.g. Kilauea
- Erosion Caldera: Enlarged by erosion after initial formation
- Resurgent Caldera: Floor rises again as new magma accumulates beneath
The most dramatic calderas form from explosive eruptions. These supervolcanic events eject hundreds to thousands of cubic kilometers of material. The Toba eruption in Indonesia 74,000 years ago created a caldera 100 km long and may have caused a global volcanic winter lasting years.
Shield volcano calderas form more quietly through the gradual drainage of lava lakes and slow subsidence. Kilauea's summit caldera in Hawaii has formed and refilled multiple times, most recently collapsing dramatically during the 2018 eruption.
Famous Calderas
- Yellowstone Caldera (USA): 72 x 55 km, last erupted 640,000 years ago
- Crater Lake (USA): Caldera filled with water to 594 m depth
- Santorini (Greece): Caldera from the Minoan eruption, ~1600 BCE
- Ngorongoro Crater (Tanzania): World's largest unbroken caldera, 19 km wide
- Lake Toba (Indonesia): 100 km long, from the largest eruption in 2 million years
Crater Lake in Oregon occupies a caldera formed when Mount Mazama erupted and collapsed roughly 7,700 years ago. The lake has no inlet or outlet — it is filled entirely by rain and snowmelt. At 594 meters deep, it is the deepest lake in the United States and one of the clearest in the world.
The Ngorongoro Crater in Tanzania is the world's largest unbroken and unflooded caldera, measuring 19 kilometers across. Its steep walls enclose a grassland ecosystem that supports roughly 25,000 large animals including lions, elephants, black rhinos, and flamingos.
A Caldera Is Not a Crater
The two are routinely confused and form in opposite ways. A volcanic crater is excavated outward by an explosion at the vent, and is typically a few hundred metres across. A caldera is what remains when the roof of an emptied magma chamber collapses inward, and can be tens of kilometres wide.
The distinction matters because it tells you what happened. A crater means material was thrown out from a point; a caldera means the ground itself subsided over a void, usually after an eruption large enough to drain the chamber beneath.
The word comes from the Spanish for cauldron, borrowed from the Canary Islands, where the Caldera de Taburiente on La Palma gave the landform its name in nineteenth-century geology.
The Ones That Could Erupt Again
A handful of calderas are described as supervolcanoes, meaning they have produced eruptions of magnitude 8 on the Volcanic Explosivity Index, ejecting more than 1,000 cubic kilometres of material.
Yellowstone is the best studied. Its caldera, covered in detail in our Yellowstone supervolcano guide, measures roughly 55 by 72 kilometres, and three major eruptions have occurred at roughly 2.1 million, 1.3 million and 640,000 years ago. The ground above the chamber rises and falls measurably year to year, and the geysers and hot springs are surface expressions of the same heat.
Campi Flegrei, west of Naples, is more immediately concerning because roughly half a million people live inside it. Its ground level has risen and fallen by metres within decades, a process called bradyseism, and the town of Pozzuoli has been partly evacuated more than once.
Toba and the Eruption That Left a Mark on Everyone
Lake Toba in Sumatra fills a caldera about 100 kilometres long, formed in an eruption roughly 74,000 years ago that is among the largest known in the last two million years.
The Toba catastrophe hypothesis argues that the resulting volcanic winter reduced the human population to a few thousand breeding individuals, creating a genetic bottleneck still detectable today. The evidence is contested, and archaeological sites in India and Africa show continuity through the period, but the eruption's scale is not in doubt.
Samosir, the island in the middle of the lake, is a resurgent dome: the caldera floor pushed back up by magma refilling the chamber beneath, which is a normal part of the cycle rather than a sign of imminent activity.
Calderas Full of Water
Because a caldera is a closed depression, it frequently fills, and some of the world's most striking lakes sit in them. Crater Lake in Oregon, despite its name, occupies the caldera of Mount Mazama, which collapsed about 7,700 years ago; at 594 metres it is the deepest lake in the United States and among the clearest in the world. Wizard Island, a cinder cone built by smaller eruptions after the collapse, rises from the lake's western side.
Lake Taupo in New Zealand fills a caldera whose eruption around 232 AD was described in Chinese and Roman records as unusual skies. Lake Toba, Lake Atitlan in Guatemala and Kelimutu's coloured lakes in Indonesia are all caldera lakes.
Not all of them are freshwater. Santorini's caldera is open to the Aegean, flooded when the sea broke through after the Minoan eruption, which is why ships can sail into the middle of a volcano and anchor beside its walls.
Living Inside One
Calderas are often fertile, sheltered and geothermally useful, which is why people settle in them despite the obvious risk. Volcanic soils weather quickly into productive ground, and the caldera walls provide shelter from wind.
The Ngorongoro Crater in Tanzania, in fact a caldera, holds one of the densest concentrations of large mammals anywhere inside its 260 square kilometres, and Maasai pastoralists graze cattle alongside them under a conservation arrangement.
Iceland and New Zealand both draw substantial electricity and district heating from geothermal fields associated with caldera systems, which turns the same underlying heat that makes the setting dangerous into the reason the settlement is viable.
How Geologists Watch Them
Monitoring a caldera means watching for three things at once: ground deformation, seismicity and gas. Satellite radar interferometry can detect surface movement of a centimetre or less, which is how the inflation at Campi Flegrei and the breathing of Yellowstone are tracked.
Swarms of small earthquakes at shallow depth often indicate magma or fluid moving, and changes in the ratio of carbon dioxide to sulphur dioxide in vent gases can signal fresh magma rising. None of these predicts an eruption on its own, and false alarms are common: Campi Flegrei's 1980s uplift prompted evacuations and no eruption followed.
The difficulty is that the recurrence intervals are far longer than the observational record. Yellowstone's three great eruptions are spaced hundreds of thousands of years apart, so a century of instruments samples almost nothing of the cycle.
The Ones That Are Not on Land
Most of the planet's calderas are submarine and unmapped. Kolumbo, seven kilometres north-east of Santorini, erupted in 1650 and killed around seventy people on the island through gas and tsunami, and was only surveyed in detail in the 2000s.
The 2022 eruption of Hunga Tonga-Hunga Ha'apai, a submarine caldera in the South Pacific, produced the most powerful explosion recorded by modern instruments, sent a plume into the mesosphere and generated a pressure wave that circled the planet several times.
That event is the clearest recent demonstration of why undersea calderas matter: it was barely visible as an island beforehand, and its eruption severed Tonga's single fibre-optic cable and cut the country off for weeks.