A caldera is a large-scale volcanic depression formed when a volcano collapses into itself after its magma chamber has been emptied. It is distinguished from a volcanic crater, which is typically smaller and formed by the explosive ejection of material during an eruption. Calderas can be tens of kilometers in diameter and are the result of catastrophic, high-volume eruptions. USGS: Photo Glossary of Volcano Terms - Caldera

Etymology

The term caldera comes from the Spanish word for "cauldron". The term was introduced into the geological vocabulary by the German geologist Christian Leopold von Buch in 1815, after his observations of the Las Cañadas caldera on Tenerife and the Caldera de Taburiente on La Palma in the Canary Islands.

Formation

A caldera-forming eruption begins when a large magma chamber, often rich in silica and gas, builds up pressure beneath a volcano. The eruption empties a significant portion of this chamber, often in a cataclysmic event such as a Plinian eruption. With the magma evacuated, the overlying rock structure loses its support. Geological Society of London: Calderas

The collapse is typically initiated along a series of concentric ring faults that form around the magma chamber's edge. As the roof of the chamber founders, it subsides in one or more blocks into the void below. This collapse can occur in stages or as a single, catastrophic event, often triggering massive pyroclastic flows that travel outwards from the collapsing rim. The resulting depression is the caldera.

Types of Calderas

Calderas are generally classified based on their formation and the type of volcano with which they are associated. Oregon State University: Volcano World - Calderas

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  Crater-Lake Calderas: These are the most widely known type, formed by the collapse of a stratovolcano following a powerful explosive eruption. The resulting depression often fills with water to form a deep lake. Crater Lake in Oregon is a classic example, formed about 7,700 years ago by the collapse of Mount Mazama.

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  Shield Volcano Calderas: These form atop shield volcanoes, such as those in Hawaii and the Galápagos Islands. They are not typically formed by a single explosive eruption but by gradual subsidence as magma is drained from shallow reservoirs to feed flank eruptions. The caldera of Kīlauea on the Big Island of Hawaii is a prominent, active example.

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  Resurgent Calderas: These are the largest and most complex types of calderas, associated with some of the most powerful eruptions on Earth, known as supervolcano eruptions. After the initial collapse, magma begins to re-accumulate beneath the caldera floor, causing the central area to be uplifted into a structural dome or series of domes, a process known as resurgent doming. Examples include the Valles Caldera in New Mexico, the Long Valley Caldera in California, and the Yellowstone Caldera in Wyoming. National Park Service: Calderas

Notable Examples

• –Yellowstone Caldera, United States: An active supervolcano system responsible for several massive eruptions in the last two million years.
• –Mount Tambora, Indonesia: Its 1815 eruption created a 6-kilometer-wide caldera and caused the "Year Without a Summer" in 1816 due to global atmospheric effects. Britannica: Caldera
• –Santorini caldera, Greece: A large, sea-filled caldera in the Aegean Sea formed by the massive Minoan eruption around 1600 BCE.
• –Lake Toba, Indonesia: The site of a massive super-eruption about 74,000 years ago, creating a caldera 100 by 30 kilometers wide that now contains the world's largest volcanic lake.

Extraterrestrial Calderas

Calderas are not unique to Earth. They have been identified on other planets and moons in the Solar System. The shield volcano Olympus Mons on Mars has a complex summit caldera composed of multiple overlapping collapse craters. Calderas have also been observed on Venus and on Jupiter's volcanically active moon, Io.