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 (6 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 7 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 5 kilometers (3.1 miles) in diameter.  
 
Composed of dormant and active shield volcanoes, the islands of Hawaii have a number of impressive shield volcano calderas. Moku'aweoweo, the caldera at the summit of Mauna Loa on the “Big Island” of Hawaii, is actually comprised of three distinct depressions that overlap. In total, the caldera complex has an area of roughly 15 square kilometers (6 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 (9 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 (6 mi) deep. These new measurements put the magma chamber at roughly the same size it was during its last eruption.
Types of Calderas
Pay no attention to the name—Crater Lake, Oregon, is not a crater. It's a caldera.

Yellowstone is Still Hot
Yellowstone National Park sits over a hotspot, a place in the Earth's crust where magma sits close to the surface. The Yellowstone Supervolcano, which sits over the hotspot, has had three caldera-forming eruptions: 2.1 million years ago, 1.3 million years ago and 640,000 years ago.

Magma continues to build beneath Yellowstone. It's a question of when, not if, it erupts again. (Chances are good that we have 90,000 years or so before that happens, but geologists monitor the area just in case!)

Calderas in Space
Earth isn't the only planet that has calderas. Other planets have them as well, including Venus and Mars. The moon also has calderas.

active volcano
Noun

volcano that has had a recorded eruption since the last glacial period, about 10,000 years ago.

alter
Verb

to change.

ancient
Adjective

very old.

asymmetric
Adjective

not identical on both sides.

Noun

large depression resulting from the collapse of the center of a volcano.

channel
Noun

deepest part of a shallow body of water, often a passageway for ships.

characteristic
Noun

physical, cultural, or psychological feature of an organism, place, or object.

Noun

edge of land along the sea or other large body of water.

complex
Noun

group of structures or parts that are somehow related.

comprise
Verb

to contain or be made up of.

Noun

bowl-shaped depression formed by a volcanic eruption or impact of a meteorite.

debris
Noun

remains of something broken or destroyed; waste, or garbage.

depression
Noun

indentation or dip in the landscape.

destructive
Adjective

harmful.

diameter
Noun

width of a circle.

distinct
Adjective

unique or identifiable.

dormant
Adjective

state of minimal growth or activity.

duration
Noun

length of time.

eject
Verb

to get rid of or throw out.

elliptical
Adjective

having the shape of an oval.

enormous
Adjective

very large.

environment
Noun

conditions that surround and influence an organism or community.

episodic
Adjective

occurring sporadically or irregularly.

explosion
Noun

violent outburst; rejection, usually of gases or fuel

Noun

overflow of a body of water onto land.

geologic
Adjective

having to do with the physical formations of the Earth.

gradual
Adjective

rising or falling by a small amount.

incremental
Adjective

increasing in regular additions.

intensity
Noun

measure of magnitude.

Noun

body of land surrounded by water.

Noun

body of water surrounded by land.

lava
Noun

molten rock, or magma, that erupts from volcanoes or fissures in the Earth's surface.

lava fountain
Noun

phenomenon where lava is forcefully but not violently ejected from a volcano through a fissure or vent.

locate
Verb

to find or identify a place.

magma chamber
Noun

underground reservoir that holds molten rock.

massive
Adjective

very large or heavy.

periodic
Adjective

occasional.

permanent
Adjective

constant or lasting forever.

Plinian eruption
Noun

powerful, violent volcanic explosion characterized by pyroclastic flows and ejection of material high into the atmosphere.

Noun

current of volcanic ash, lava, and gas that flows from a volcano.

Noun

liquid precipitation.

resurgent caldera
Noun

depression caused by the collapse of an underground magma chamber not affiliated with a particular volcano; the largest volcanic structures on Earth.

rock
Noun

natural substance composed of solid mineral matter.

shield volcano
Noun

large, gently sloping volcano made from fluid lava.

snow
Noun

precipitation made of ice crystals.

stratovolcano
Noun

steep volcano made of hardened lava, rock, and ash. Also known as a composite volcano.

subsequent
Adjective

coming after.

subside
Verb

to return to a lower level.

summit
Noun

highest point of a mountain.

terrace
Noun

flat surface created on a steep hillside.

uplift
Noun

elevation of the Earth's surface due to tectonic or other natural activity.

vast
Adjective

huge and spread out.

vessel
Noun

craft for traveling on water, usually larger than a rowboat or skiff.

violent
Noun

strong, destructive force.

Noun

fragments of lava less than 2 millimeters across.

Noun

hill created by a volcanic eruption.

volcanic eruption
Noun

activity that includes a discharge of gas, ash, or lava from a volcano.

volcanic vent
Noun

opening in the Earth's crust where lava and gases escape to the Earth's surface or atmosphere.