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A caldera is a large volcanic depression formed mainly when the ground collapses after magma is withdrawn from beneath a volcano. Some calderas develop during catastrophic explosive eruptions, while others form through repeated lava eruptions, gradual subsidence or collapse inside shield volcanoes.
This guide explains how calderas form, how they differ from volcanic craters, the main types of calderas, what happens after collapse, why caldera unrest occurs and whether every caldera is a supervolcano.
What Is a Caldera?
A caldera is a broad volcanic depression created mainly by collapse of the ground above a magma reservoir. Calderas are usually much larger than ordinary volcanic craters and may span several kilometers or, in exceptional cases, tens of kilometers.
They form when magma moves out of an underground storage region faster than the overlying rock can remain supported. The roof above the reservoir then fractures and sinks, producing a large collapse basin.
A caldera may contain:
- One or more volcanic vents
- Lava domes
- Cinder cones
- Hot springs and fumaroles
- Crater lakes
- Resurgent uplift
- Earthquake swarms
- Areas of ground deformation
- Later lava flows and explosive deposits
Some calderas are obvious circular depressions. Others are partly buried, eroded, flooded, covered by later eruptions or difficult to recognize without geological mapping and satellite data.
Caldera vs. Crater: What Is the Difference?
A crater and a caldera are both volcanic depressions, but they differ mainly in size and formation process.
Volcanic crater
A crater is generally a relatively small depression surrounding a volcanic vent. It may form through explosions, excavation of material, collapse of a conduit or repeated activity around a central opening.
Crater dimensions usually range from tens to hundreds of meters, although some are larger.
Volcanic caldera
A caldera is usually much larger and forms primarily through collapse above a magma reservoir after substantial magma withdrawal.
Calderas commonly measure several kilometers across and may contain multiple craters, vents, domes and younger volcanoes within the larger depression.
Caldera vs. Crater at a Glance
- Crater: smaller depression centered on a vent
- Caldera: large collapse basin above a magma reservoir
- Crater formation: explosions, excavation or local collapse
- Caldera formation: large-scale subsidence after magma withdrawal
A crater can form inside a caldera, and a caldera can contain many separate craters.
How Do Calderas Form?
Caldera formation begins within a volcanic plumbing system. Magma accumulates in one or more underground reservoirs, sometimes over thousands or hundreds of thousands of years.
Collapse becomes possible when a significant volume of magma leaves the reservoir through eruption or underground migration.
The main sequence commonly includes:
- Magma accumulates beneath a volcanic system.
- Pressure, gas content or magma supply changes.
- An eruption begins through vents or ring fractures.
- Magma and volcanic material leave the reservoir.
- The unsupported roof begins to fracture.
- Large blocks of crust sink into the partially emptied reservoir.
- A broad depression forms at the surface.
- Later eruptions and hydrothermal activity modify the new caldera.
This process may occur rapidly during a major eruption or develop through several collapse episodes.
Not every caldera forms through one enormous explosion. Basaltic calderas can form through repeated magma withdrawal and summit collapse during prolonged lava eruptions.
How Does Caldera Collapse Happen?
The roof above a magma reservoir does not usually fall as one perfectly intact block. Collapse may occur through faults, fractures and segments that move at different times and speeds.
As magma leaves the reservoir, stress concentrates in the overlying rock. Ring-shaped fractures may develop around the collapsing area, creating pathways for magma, ash and gases.
Collapse styles include:
Piston-like collapse
A relatively coherent block sinks downward into the reservoir, somewhat like a piston. Natural calderas are rarely perfectly symmetrical, but this model helps explain some large collapse structures.
Piecemeal collapse
The caldera floor breaks into several blocks that subside unevenly. Different sections may tilt, rotate or sink at different rates.
Trapdoor collapse
One side of the caldera floor subsides more than the other, producing asymmetrical deformation.
Progressive collapse
Subsidence occurs in several stages as magma withdrawal continues or separate sections of the reservoir are emptied.
During major explosive eruptions, collapse can intensify eruption rates by forcing magma upward along fractures. The eruption and collapse may therefore reinforce one another.
Main Types of Calderas
Calderas are classified by their tectonic setting, magma composition, eruption history and collapse mechanism. The categories overlap, and some caldera systems display characteristics of more than one type.
The main groups include:
- Explosive collapse calderas
- Basaltic shield-volcano calderas
- Resurgent calderas
- Nested calderas
- Volcano-tectonic depressions
Explosive Collapse Calderas
Explosive calderas form during large eruptions that release major volumes of ash, pumice, gases and pyroclastic flows.
Many are associated with silica-rich magma such as dacite or rhyolite. These magmas can be viscous and gas-rich, allowing pressure to build within the volcanic system.
As eruption columns rise and pyroclastic flows spread outward, magma is rapidly removed from beneath the volcano. The roof then collapses, producing the caldera.
Typical products
Explosive caldera eruptions may generate:
- Widespread ashfall
- Pumice deposits
- Pyroclastic density currents
- Ignimbrites
- Volcanic gases
- Caldera-collapse breccias
- Ring-fracture eruptions
Ignimbrites
Ignimbrites are deposits left by hot, ground-hugging mixtures of ash, pumice and gases. Large caldera eruptions can produce sheets of ignimbrite covering thousands of square kilometers.
Some deposits weld together while still hot, forming dense volcanic rock known as welded tuff.
Basaltic and Shield-Volcano Calderas
Basaltic calderas commonly form at the summits of shield volcanoes. They are usually associated with fluid basaltic magma rather than highly explosive, silica-rich eruptions.
Collapse may occur when magma drains from a summit reservoir into:
- A flank eruption
- A long fissure system
- A dike intrusion
- A deeper or lateral magma pathway
As magma leaves the summit storage region, the caldera floor can subside in repeated steps.
These collapses may occur during weeks or months of intense lava eruption. Earthquakes, fault movement and dramatic changes in crater shape can accompany the process.
Hawaiian examples
Kīlauea and Mauna Loa contain summit calderas formed through repeated episodes of magma withdrawal, collapse, lava filling and renewed subsidence.
The 2018 Kīlauea eruption demonstrated how magma draining toward lower flank vents could trigger major summit collapse even though the largest lava eruption occurred many kilometers away.
Explore these systems in Hawaiian Volcanoes & Hotspot Volcanism.
Resurgent Calderas
A resurgent caldera is a large caldera in which part of the floor later rises again because of renewed magma intrusion, heating, pressurization or movement within the underlying system.
The uplifted area is called a resurgent dome. It is not necessarily a lava dome and does not automatically mean an eruption is imminent.
Resurgence may occur over thousands of years and can create:
- Broad uplifted blocks
- Fault zones
- Earthquake activity
- Hot springs and fumaroles
- New vents
- Lava domes
- Local subsidence around the uplifted center
Examples of resurgent caldera systems include Long Valley in California, Valles in New Mexico and parts of Yellowstone.
Ground uplift in a caldera can attract attention, but deformation alone does not prove that a large eruption is approaching. Scientists evaluate it alongside earthquakes, gas emissions, heat flow and other monitoring data.
Caldera Lakes
Calderas often become natural basins where water accumulates. Rainfall, snowmelt, rivers and groundwater can gradually fill the depression and create a caldera lake.
Famous examples include:
- Crater Lake in Oregon
- Lake Toba in Indonesia
- Lake Taupō in New Zealand
- Lake Taal in the Philippines
- Lake Aira in Japan
- Lake Ilopango in El Salvador
Caldera lakes can conceal active vents, hydrothermal systems and younger volcanic structures beneath the water.
Hazards associated with caldera lakes
Potential hazards include:
- Phreatic explosions
- Gas release
- Rapid heating
- Water-level changes
- Landslides
- Lake tsunamis
- Flooding after rim failure
- Explosive interaction between magma and water
Not every caldera lake is highly dangerous, but water can significantly influence eruption style and local hazards.
What Happens After a Caldera Forms?
Caldera collapse does not necessarily mark the end of volcanic activity. Many calderas remain active for thousands or hundreds of thousands of years.
Later activity may include:
- Lava flows
- Lava domes
- Small explosive eruptions
- Cinder-cone formation
- Hydrothermal explosions
- Earthquake swarms
- Ground uplift and subsidence
- Gas emissions
- Geothermal activity
Post-caldera lava
New magma may rise through ring faults, central fractures or zones of weakness inside the collapse basin. This can build domes, cones or extensive lava flows.
Hydrothermal systems
Heat remaining beneath the caldera can drive circulation of groundwater. This may produce hot springs, geysers, fumaroles, mud pots and mineral deposits.
Nested calderas
A younger caldera can form partly or entirely inside an older one. Repeated collapse episodes may create a complex pattern of overlapping depressions.
The active volcanic landscape inside a caldera may therefore be much younger than the original collapse event.
Caldera Unrest and Monitoring
Large calderas can experience long periods of unrest without producing a major eruption. Unrest may reflect movement of magma, gases or hydrothermal fluids within a complex underground system.
Scientists monitor calderas using:
- Seismometers
- GPS stations
- Satellite radar
- Gas measurements
- Thermal cameras
- Groundwater chemistry
- Gravity measurements
- Geological mapping
Earthquake swarms
Swarms may occur when magma or pressurized fluids move through fractures. Many swarms end without an eruption.
Ground deformation
Uplift can occur when magma or fluids enter an underground reservoir. Subsidence may reflect cooling, pressure loss or movement of fluids away from an area.
Caldera floors may rise and fall repeatedly over time.
Gas changes
Changes in carbon dioxide, sulfur dioxide and other gases can provide clues about magma depth and hydrothermal activity.
Hydrothermal unrest
Heated groundwater can cause explosions, changes in hot springs and localized deformation without new magma reaching the surface.
No single sign proves that an eruption is imminent. Scientists combine multiple datasets to identify whether unrest is caused by magma, hydrothermal fluids, tectonic stress or normal background activity.
Learn how these signals are interpreted in Volcano Monitoring & Forecasting.
Caldera Hazards
Caldera systems can produce a wide range of hazards. The most likely hazard is not always another giant caldera-forming eruption.
Potential hazards include:
Pyroclastic flows
Large explosive eruptions can generate fast-moving currents of hot ash, gas and rock that devastate areas around the caldera.
Ashfall
Ash may spread across large regions, damaging buildings, machinery, agriculture, water supplies and aviation.
Lava flows and domes
Post-caldera eruptions can build lava domes or produce lava flows inside and beyond the collapse basin.
Hydrothermal explosions
Pressurized groundwater can flash into steam and violently excavate craters. These explosions may occur without a magmatic eruption.
Volcanic gases
Carbon dioxide, sulfur dioxide and hydrogen sulfide can accumulate near vents, depressions and geothermal areas.
Earthquakes and ground deformation
Movement of faults, magma and fluids can generate earthquake swarms and damage infrastructure.
Landslides and lake waves
Steep caldera walls can fail into lakes and generate local waves or flooding.
Large explosive eruptions
Some caldera systems are capable of major explosive eruptions, but these events are generally much less frequent than smaller eruptions, earthquakes or hydrothermal disturbances.
Explore individual processes in Volcanic Hazards Explained.
Are All Calderas Supervolcanoes?
No. A caldera is a geological structure, while a supervolcano is an informal term associated with a volcanic system capable of producing an exceptionally large eruption.
Calderas form across a broad range of sizes and eruption styles. Many result from moderate explosive eruptions or repeated basaltic collapse rather than a super-eruption.
A volcanic system is generally discussed in connection with super-eruptions only when geological evidence shows that it produced an eruption near the highest end of the Volcanic Explosivity Index.
Caldera vs. Supervolcano
- Caldera: a large volcanic collapse depression
- Supervolcano: an informal label for a system associated with an extremely large eruption
- Every supervolcano has a caldera: usually associated with one
- Every caldera is a supervolcano: no
Even large calderas are far more likely to produce smaller eruptions or prolonged unrest than another maximum-scale eruption.
Continue with Supervolcanoes Explained for a detailed look at super-eruptions, VEI 8 events and realistic volcanic risk.
Famous Calderas Around the World
Yellowstone Caldera, United States
Yellowstone contains overlapping calderas formed by several large prehistoric eruptions. Today, its most visible activity comes from earthquakes, ground deformation and an enormous hydrothermal system of geysers, hot springs and fumaroles.
Explore the complete Yellowstone Supervolcano & Hydrothermal System.
Toba Caldera, Indonesia
Toba formed during one of the largest known Quaternary eruptions. Lake Toba now fills much of the depression, while Samosir Island represents uplift and volcanic activity within the caldera.
Taupō Caldera, New Zealand
Taupō is a large rhyolitic caldera system responsible for several powerful eruptions. Much of the caldera lies beneath Lake Taupō.
See New Zealand Volcanoes Explained.
Campi Flegrei, Italy
Campi Flegrei is a densely populated caldera west of Naples. It is known for bradyseism—cycles of uplift and subsidence—along with earthquakes, fumaroles and hydrothermal activity.
See Italian Volcanoes Explained.
Long Valley Caldera, United States
Long Valley formed during a large eruption in eastern California. Later lava domes, earthquakes, deformation and hydrothermal activity show that the system continued evolving after collapse.
Aira Caldera, Japan
Aira Caldera lies beneath Kagoshima Bay and contains Sakurajima, one of Japan’s most active volcanoes.
See Japanese Volcanoes Explained.
Santorini Caldera, Greece
Santorini consists of volcanic islands surrounding a flooded caldera. Its eruptive history includes the major Bronze Age eruption often called the Minoan eruption.
Crater Lake Caldera, United States
Crater Lake formed after the collapse of Mount Mazama. The deep lake now occupies the caldera, with Wizard Island representing later volcanic activity.
Kīlauea Caldera, Hawaii
Kīlauea’s summit caldera has repeatedly collapsed, filled and changed during magma withdrawal and renewed eruption.
Comparison of Major Caldera Types
| Caldera type | Typical magma | Main formation process | Common later activity |
|---|---|---|---|
| Explosive collapse caldera | Often dacitic or rhyolitic | Large explosive eruption followed by roof collapse | Domes, ash eruptions, hydrothermal activity and resurgence |
| Basaltic shield caldera | Mostly basaltic | Magma drainage during lava eruption or dike intrusion | Lava lakes, fissures, repeated collapse and refilling |
| Resurgent caldera | Variable | Initial collapse followed by renewed uplift | Doming, earthquakes, faults, vents and geothermal activity |
| Nested caldera | Variable | Repeated collapse events within an older caldera | Younger craters, domes and overlapping volcanic centers |
| Lake-filled caldera | Variable | Collapse basin later filled by water | Hydrothermal activity, island volcanoes and lake hazards |
Frequently Asked Questions About Calderas
What is a volcanic caldera?
A volcanic caldera is a large depression formed mainly when the ground collapses after a substantial volume of magma leaves an underground reservoir.
How is a caldera different from a crater?
A crater is usually a smaller depression surrounding a volcanic vent. A caldera is much larger and forms primarily through collapse above a magma reservoir.
Does a caldera form only after a super-eruption?
No. Calderas form after eruptions of many different sizes. Basaltic shield volcanoes can also develop calderas through repeated magma withdrawal and summit collapse.
Is every caldera a supervolcano?
No. A caldera is a collapse structure, while supervolcano is an informal term associated with volcanic systems capable of exceptionally large eruptions.
Can a caldera erupt again?
Yes. New vents, lava domes, cones and fissures can form within or around a caldera long after the original collapse event.
Can people live inside a caldera?
Yes. Many calderas contain towns, farms, roads and cities. Living there does not necessarily mean an eruption is imminent, but active systems may require monitoring and hazard planning.
Why do some calderas contain lakes?
Calderas create enclosed basins where rain, snowmelt, rivers and groundwater can accumulate. The resulting lake may partially or completely fill the depression.
What is a resurgent caldera?
A resurgent caldera is one in which part of the floor rises after collapse because of renewed magma intrusion, heating or pressure changes beneath the surface.
Does ground uplift mean a caldera will erupt?
No. Uplift can result from magma, gas or hydrothermal fluids and may occur without an eruption. Scientists interpret deformation together with earthquakes, gases, heat flow and other monitoring data.
What is the largest caldera on Earth?
The answer depends on how caldera boundaries and ancient volcanic structures are defined. Several enormous caldera systems and volcanic depressions exist, including Toba, Yellowstone and large ancient calderas preserved in volcanic provinces.
Can caldera collapse cause earthquakes?
Yes. Fracturing and movement along ring faults can generate many earthquakes during collapse. Earthquake swarms can also occur during later unrest.
What hazards can occur at a caldera?
Possible hazards include ashfall, pyroclastic flows, lava eruptions, hydrothermal explosions, volcanic gases, earthquakes, landslides, ground deformation and lake-related flooding or waves.
