What is a Caldera?
Caldera Volcanoes (Also called Bathtubs Volcanoes)
Calderas are some of the most spectacular features on Earth. They are large volcanic craters that form by two different methods: 1) an explosive volcanic eruption; or, 2) collapse of surface rock into an empty magma chamber.
The accompanying image is a satellite view of one of the most famous calderas - Crater Lake in Oregon. Crater Lake was formed about 7700 years ago when an enormous volcanic eruption of Mount Mazama emptied a large magma chamber below the mountain. The fractured rock above the magma chamber collapsed to produce a massive crater over six miles across. Centuries of rain and snow filled the caldera, creating Crater Lake. With a depth of 1949 feet (594 meters), Crater Lake is the deepest lake in the United States and the ninth-deepest lake in the world.
A caldera is a large cauldron-like depression that forms following the evacuation of a magma chamber/reservoir. When large volumes of magma are erupted over a short time, structural support for the crust above the magma chamber is lost. The ground surface then collapses downward into the partially emptied magma chamber, leaving a massive depression at the surface (from one to dozens of kilometers in diameter). Although sometimes described as a crater, the feature is actually a type of sinkhole, as it is formed through subsidence and collapse rather than an explosion or impact. Only seven known caldera-forming collapses have occurred since the start of the 20th century, most recently at Bárðarbunga volcano in Iceland.
A large, bowl-shaped volcanic depression, a calderaforms when the top of a volcanic cone collapses into the space left after magma is ejected during a violentvolcanic eruption. Its diameter is many times that of the original vent. The term is Spanish for "caldron."
It was Charles Darwin who said "Nothing, not even the wind that blows, is so unstable as the crust of this earth." and that brings us to Bathtub Volcanoes.
These volcanoes are the deadliest, stickiest, nasttiest, gassiest of the entire lot.
Their eruptions can be earth shaking, cataclysmic- they can have the power of many nuclear explosions. They may bring famine, pestilence, tsunamis, plaques, great pyroclastic flows, darkness for many days or months, global climatic change, and, possibly, very pretty sunsets.
A caldera is a volcanic feature formed by the collapse of a volcano into itself, making it a large, special form of volcanic crater.
A caldera collapse is usually triggered by the emptying of the magma chamber beneath the volcano, as the result of a large volcanic eruption.
If enough magma is erupted, the emptied chamber will not be able to support the weight of the volcanic edifice (the mountain) above.
Fractures will form around the edge of the chamber, usually in a roughly circular shape.
These ring fractures may in fact serve as volcanic vents.As the magma chamber empties, the center of the volcano within the Santa Ana volcano in El Salvador - a shield caldera ring fractures begins to collapse.
The collapse may occur as the result of a single massive eruption, or it may occur in stages as the result of a series of eruptions.
The total amount of collapse may be hundreds or thousands of meters.
Caldera volcanoes are ones where the diameter of the circular to oval crater exceeds 1 mile. These form when so much lava is erupted (blown out) so rapidly it partially empties the underlying magma chamber. When this happens the summit of the volcanic structure collapses into the emptied magma chamber. Typically the erupted material occurs as airfall or pyroclastic flows.
There have been no recent eruptions from caldera volcanoes- Valley of Ten Thousand Smokes was the last and before it there was Krakatoa (1886) and Tambora (1815)- which caused the year without a summer. Three most famous examples in North America are Yellowstone,Crater Lake, and Long Valley.
Overall at least 138 caldera volcanos, whose crater exceeds 5 miles in diameter, are known. Unlike Crater Lake , many of these have calderas so large and irregular they remained undetected until high-quality aerial or satellite images became available. One of these, LaGarita, located in the San Jauan mountains of southern Colorado is some 20 miles wide and 60 miles long, Yellowstone, with a caldera of 24 by 40 miles is another example. Both these calderas were formed due to the large scale evacuation of a near surface magma chamber (think of the amount of material that has to be blown out to create a hole many miles in diameter and 1,000's of feet deep),and the collapse of the overlying rocks into it.
The eruptions from these caldera volcanoes are largely explosive leading to extensive air fall and pyroclastic flow deposits.
Where they occur:
Hotspots- Yellowstone
Island Arcs- Krakatoa, Tambora, Toba, Taupau
Continental Mountains- Crater Lake,
Precambrian Example- Sturgeon Lake- Ontario- a large caldera volcano some 2.7 billion years old
Characteristics of Caldera Volcano
Calderas are some of the most spectacular features on Earth. They are large volcanic craters that form by two different methods:
1) an explosive volcanic eruption; or,
2) collapse of surface rock into an empty magma chamber.
The accompanying image is a satellite view of one of the most famous calderas - Crater Lake in Oregon. Crater Lake was formed about 7700 years ago when an enormous volcanic eruption of Mount Mazama emptied a large magma chamber below the mountain. The fractured rock above the magma chamber collapsed to produce a massive crater over six miles across. Centuries of rain and snow filled the caldera, creating Crater Lake. With a depth of 1949 feet (594 meters), Crater Lake is the deepest lake in the United States and the ninth-deepest lake in the world.
Two kinds of Caldera Volcanoes:
Stratovolcanoes that turn into calderas due to cataclysmic eruptions- Crater lake, Tambora, Krakatoa.
Negative volcanoes- no sign of a volcanic ediface- usually just a large lake or a depression with lots of hot springs- Taupo, Long Valley, Yellowstone, Toba, Sturgeon Lake
These are the most famous Calderas Volcano
Aniakchak Caldera, Alaska
Aniakchak Caldera in Alaska: Aniakchak Caldera, located in the Aleutian Range of Alaska, formed during an enormous explosive eruption that expelled more than 50 km3 of magma about 3,450 years ago. The caldera is 10 kilometers in diameter and 500-1,000 meters deep. Subsequent eruptions formed domes, cinder cones, and explosion pits on the caldera floor. Enlarge image.
Aniakchak Caldera, Alaska
Snow-Covered Calderas - Mauna Loa, Hawaii
Mauna Loa Volcano: Snow-covered Moku'aweoweo Caldera atop Mauna Loa shield volcano (Mauna Kea in background). The caldera is 3 x 5 km across, 183 m deep, and is estimated to have collapsed between 600-750 years ago. Several pit craters along the upper southwest rift zone of Mauna Loa (lower right) also formed by collapse of the ground. Image and caption by USGS. Enlarge image.
Snow-Covered Calderas - Mauna Loa, Hawaii
References:
https://www.merriam-webster.com/dictionary/caldera
http://www.d.umn.edu/~rmorton/ronshome/Volcanoes/calderas.html
https://www.sciencedaily.com/terms/caldera.htm
http://geology.com/articles/caldera/
https://en.wikipedia.org/wiki/Caldera