A caldera (/kɔːlˈdɛrə,kæl-/[1]kawl-DERR-ə, kal-) is a large
cauldron-like hollow that forms shortly after the emptying of a
magma chamber in a
volcano eruption. An eruption that ejects large volumes of magma over a short period of time can cause significant detriment to the structural integrity of such a chamber, greatly diminishing its capacity to support its own roof, and any substrate or rock resting above. The ground surface then collapses into the emptied or partially emptied magma chamber, leaving a large depression at the surface (from one to dozens of kilometers in diameter).[2] 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. Compared to the thousands of volcanic eruptions that occur over the course of a century, the formation of a caldera is a rare event, occurring only a few times within a given window of 100 years.[3] Only seven caldera-forming collapses are known to have occurred between 1911 and 2016.[3] More recently, a caldera collapse occurred at
Kīlauea, Hawaii in 2018.[4]
Etymology
The term caldera comes from
Spanishcaldera, and
Latincaldaria, meaning "cooking pot".[5] In some texts the English term cauldron is also used,[6] though in more recent work the term cauldron refers to a caldera that has been deeply eroded to expose the beds under the caldera floor.[5] The term caldera was introduced into the geological vocabulary by the German geologist
Leopold von Buch when he published his memoirs of his 1815 visit to the
Canary Islands,[note 1] where he first saw the Las Cañadas caldera on
Tenerife, with Mount
Teide dominating the landscape, and then the
Caldera de Taburiente on
La Palma.[7][5]
Caldera formation
A collapse is triggered by the emptying of the
magma chamber beneath the volcano, sometimes as the result of a large explosive
volcanic eruption (see
Tambora[8] in 1815), but also during effusive eruptions on the flanks of a volcano (see
Piton de la Fournaise in 2007)[9] or in a connected fissure system (see
Bárðarbunga in 2014–2015). If enough
magma is ejected, the emptied chamber is unable to support the weight of the volcanic edifice above it. A roughly circular
fracture, the "ring fault", develops around the edge of the chamber. Ring fractures serve as feeders for fault
intrusions which are also known as
ring dikes.[10]: 86–89 Secondary volcanic vents may form above the ring fracture.[11] As the magma chamber empties, the center of the volcano within the ring fracture begins to collapse. The collapse may occur as the result of a single cataclysmic eruption, or it may occur in stages as the result of a series of eruptions. The total area that collapses may be hundreds of square kilometers.[5]
Mineralization in calderas
Some calderas are known to host rich
ore deposits. Metal-rich fluids can circulate through the caldera, forming hydrothermal ore deposits of metals such as lead, silver, gold, mercury, lithium, and uranium.[12] One of the world's best-preserved
mineralized calderas is the
Sturgeon Lake Caldera in
northwestern Ontario, Canada, which formed during the
Neoarcheanera[13] about 2.7 billion years ago.[14] In the
San Juan volcanic field, ore veins were emplaced in fractures associated with several calderas, with the greatest mineralization taking place near the youngest and most silicic intrusions associated with each caldera.[15]
Explosive caldera eruptions are produced by a magma chamber whose
magma is rich in
silica. Silica-rich magma has a high
viscosity, and therefore does not flow easily like
basalt.[10]: 23–26 The magma typically also contains a large amount of dissolved gases, up to 7
wt% for the most silica-rich magmas.[16] When the magma approaches the surface of the Earth, the drop in
confining pressure causes the trapped gases to rapidly bubble out of the magma, fragmenting the magma to produce a mixture of
volcanic ash and other
tephra with the very hot gases.[17]
The mixture of ash and volcanic gases initially rises into the atmosphere as an
eruption column. However, as the volume of erupted material increases, the eruption column is unable to
entrain enough air to remain buoyant, and the eruption column collapses into a tephra fountain that falls back to the surface to form
pyroclastic flows.[18] Eruptions of this type can spread ash over vast areas, so that ash flow
tuffs emplaced by silicic caldera eruptions are the only volcanic product with volumes rivaling those of
flood basalts.[10]: 77 For example, when
Yellowstone Caldera last erupted some 650,000 years ago, it released about 1,000 km3 of material (as measured in dense rock equivalent (DRE)), covering a substantial part of
North America in up to two metres of debris.[19]
The caldera produced by such eruptions is typically filled in with tuff,
rhyolite, and other
igneous rocks.[22] The caldera is surrounded by an outflow sheet of ash flow tuff (also called an ash flow sheet).[23][24]
Because a silicic caldera may erupt hundreds or even thousands of cubic kilometers of material in a single event, it can cause catastrophic environmental effects. Even small caldera-forming eruptions, such as
Krakatoa in 1883[27] or
Mount Pinatubo in 1991,[28] may result in significant local destruction and a noticeable
drop in temperature around the world. Large calderas may have even greater effects. The ecological effects of the eruption of a large caldera can be seen in the record of the
Lake Toba eruption in
Indonesia.
For their 1968 paper[6] that first introduced the concept of a resurgent caldera to geology,[5] R.L. Smith and R.A. Bailey chose the Valles caldera as their model. Although the Valles caldera is not unusually large, it is relatively young (1.25 million years old) and unusually well preserved,[30] and it remains one of the best studied examples of a resurgent caldera.[5] The ash flow tuffs of the Valles caldera, such as the
Bandelier Tuff, were among the first to be thoroughly characterized.[31]
About 74,000 years ago, this Indonesian volcano released about 2,800 cubic kilometres (670 cu mi)
dense-rock equivalent of ejecta. This was the largest known eruption during the ongoing
Quaternary period (the last 2.6 million years) and the largest known explosive eruption during the last 25 million years. In the late 1990s,
anthropologist Stanley Ambrose[32] proposed that a
volcanic winter induced by this eruption reduced the human population to about 2,000–20,000 individuals, resulting in a
population bottleneck. More recently,
Lynn Jorde and
Henry Harpending proposed that the human species was reduced to approximately 5,000–10,000 people.[33] There is no direct evidence, however, that either theory is correct, and there is no evidence for any other animal decline or extinction, even in environmentally sensitive species.[34] There is evidence that human habitation continued in
India after the eruption.[35]
Non-explosive calderas
Some volcanoes, such as the large
shield volcanoesKīlauea and
Mauna Loa on the island of
Hawaii, form calderas in a different fashion. The magma feeding these volcanoes is
basalt, which is silica poor. As a result, the magma is much less
viscous than the magma of a rhyolitic volcano, and the magma chamber is drained by large lava flows rather than by explosive events. The resulting calderas are also known as subsidence calderas and can form more gradually than explosive calderas. For instance, the caldera atop
Fernandina Island collapsed in 1968 when parts of the caldera floor dropped 350 metres (1,150 ft).[37]
Extraterrestrial calderas
Since the early 1960s, it has been known that volcanism has occurred on other planets and moons in the
Solar System. Through the use of crewed and uncrewed spacecraft, volcanism has been discovered on
Venus,
Mars, the
Moon, and
Io, a satellite of
Jupiter. None of these worlds have
plate tectonics, which contributes approximately 60% of the Earth's volcanic activity (the other 40% is attributed to
hotspot volcanism).[38] Caldera structure is similar on all of these planetary bodies, though the size varies considerably. The average caldera diameter on Venus is 68 km (42 mi). The average caldera diameter on Io is close to 40 km (25 mi), and the mode is 6 km (3.7 mi);
Tvashtar Paterae is likely the largest caldera with a diameter of 290 km (180 mi). The average caldera diameter on Mars is 48 km (30 mi), smaller than Venus. Calderas on Earth are the smallest of all planetary bodies and vary from 1.6–80 km (1–50 mi) as a maximum.[39]
The
Moon has an outer shell of low-density crystalline rock that is a few hundred kilometers thick, which formed due to a rapid creation. The craters of the Moon have been well preserved through time and were once thought to have been the result of extreme volcanic activity, but are currently believed to have been formed by meteorites, nearly all of which took place in the first few hundred million years after the Moon formed. Around 500 million years afterward, the Moon's mantle was able to be extensively melted due to the decay of radioactive elements. Massive basaltic eruptions took place generally at the base of large impact craters. Also, eruptions may have taken place due to a magma reservoir at the base of the crust. This forms a dome, possibly the same morphology of a shield volcano where calderas universally are known to form.[38] Although caldera-like structures are rare on the Moon, they are not completely absent. The
Compton-Belkovich Volcanic Complex on the
far side of the Moon is thought to be a caldera, possibly an
ash-flow caldera.[40]
The volcanic activity of
Mars is concentrated in two major provinces:
Tharsis and
Elysium. Each province contains a series of giant shield volcanoes that are similar to what we see on Earth and likely are the result of mantle
hot spots. The surfaces are dominated by lava flows, and all have one or more collapse calderas.[38] Mars has the tallest volcano in the Solar System,
Olympus Mons, which is more than three times the height of Mount Everest, with a diameter of 520 km (323 miles). The summit of the mountain has six nested calderas.[41]
Because there is no
plate tectonics on
Venus, heat is mainly lost by conduction through the
lithosphere. This causes enormous lava flows, accounting for 80% of Venus' surface area. Many of the mountains are large
shield volcanoes that range in size from 150–400 km (95–250 mi) in diameter and 2–4 km (1.2–2.5 mi) high. More than 80 of these large shield volcanoes have summit calderas averaging 60 km (37 mi) across.[38]
Io, unusually, is heated by solid flexing due to the
tidal influence of
Jupiter and Io's
orbital resonance with neighboring large moons
Europa and
Ganymede, which keep its orbit slightly
eccentric. Unlike any of the planets mentioned, Io is continuously volcanically active. For example, the NASA Voyager 1 and Voyager 2 spacecraft detected nine erupting volcanoes while passing Io in 1979. Io has many calderas with diameters tens of kilometers across.[38]
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