Mount Mazama's eruption timeline, an example of caldera formation
Acaldera (/kɔːlˈdɛrə,kæl-/[1]kawl-DERR-ə, kal-) is a largecauldron-like hollow that forms shortly after the emptying of amagma chamber in avolcanic eruption. The ejection of large volumes of magma in a short time can upset the integrity of a magma chamber's structure by in effect removing much of the chamber's filling material. The walls and ceiling of a chamber may now not be able to support its own weight 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 that may have a diameter of dozens of kilometers.[2] Although sometimes described as acrater, the feature is actually a type ofsinkhole, as it is formed throughsubsidence 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 eight caldera-forming collapses are known to have occurred between 1911 and 2018,[3] with a caldera collapse atKīlauea,Hawaii, in 2018.[4] Volcanoes that have formed a caldera are sometimes described as "caldera volcanoes".[5]
The termcaldera comes fromSpanishcaldera, andLatincaldaria, meaning "cooking pot".[6] In some texts the English termcauldron is also used,[7] though in more recent work the termcauldron refers to a caldera that has been deeply eroded to expose the beds under the caldera floor.[6] The termcaldera was introduced into the geological vocabulary by the German geologistLeopold von Buch when he published his memoirs of his 1815 visit to theCanary Islands,[note 1] where he first saw the Las Cañadas caldera onTenerife, with MountTeide dominating the landscape, and then theCaldera de Taburiente onLa Palma.[8][6]
Animation of an analogue experiment showing the origin of a mock volcanic caldera in box filled with flourLandsat image ofLake Toba, on the island ofSumatra,Indonesia (100 km/62 mi long and 30 km/19 mi wide, one of the world's largest calderas). Aresurgent dome formed the island ofSamosir.Topographic map of Cagar Alam Rawa Danau Caldera in Indonesia
A collapse is triggered by the emptying of themagma chamber beneath the volcano, sometimes as the result of a large explosivevolcanic eruption (seeTambora[9] in 1815), but also during effusive eruptions on the flanks of a volcano (seePiton de la Fournaise in 2007)[10] or in a connected fissure system (seeBárðarbunga in 2014–2015). If enoughmagma is ejected, the emptied chamber is unable to support the weight of the volcanic edifice above it. A roughly circularfracture, the "ring fault", develops around the edge of the chamber. Ring fractures serve as feeders for faultintrusions, which are also known asring dikes.[11]: 86–89 Secondary volcanic vents may form above the ring fracture.[12] 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.[6]
Some calderas are known to host richore deposits. Metal-rich fluids can circulate through the caldera, forming hydrothermal ore deposits of metals such as lead, silver, gold, mercury, lithium, and uranium.[13] One of the world's best-preservedmineralized calderas is theSturgeon Lake Caldera innorthwestern Ontario, Canada, which formed during theNeoarcheanera[14] about 2.7 billion years ago.[15] In theSan 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.[16]
Explosive caldera eruptions are produced by a magma chamber whosemagma is rich insilica. Silica-rich magma has a highviscosity, and therefore does not flow easily likebasalt.[11]: 23–26 The magma typically also contains a large amount of dissolved gases, up to 7wt% for the most silica-rich magmas.[17] When the magma approaches the surface of the Earth, the drop inconfining pressure causes the trapped gases to rapidly bubble out of the magma, fragmenting the magma to produce a mixture ofvolcanic ash and othertephra with the very hot gases.[18]
The mixture of ash and volcanic gases initially rises into the atmosphere as aneruption column. However, as the volume of erupted material increases, the eruption column is unable toentrain enough air to remain buoyant, and the eruption column collapses into a tephra fountain that falls back to the surface to formpyroclastic flows.[19] Eruptions of this type can spread ash over vast areas, so that ash flowtuffs emplaced by silicic caldera eruptions are the only volcanic product with volumes rivaling those offlood basalts.[11]: 77 For example, whenYellowstone 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 ofNorth America in up to two metres of debris.[20]
The caldera produced by such eruptions is typically filled in with tuff,rhyolite, and otherigneous rocks.[23] The caldera is surrounded by anoutflow sheet of ash flow tuff (also called anash flow sheet).[24][25]
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 asKrakatoa in 1883[28] orMount Pinatubo in 1991,[29] may result in significant local destruction and a noticeabledrop 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 theLake Toba eruption inIndonesia.
For their 1968 paper[7] that first introduced the concept of a resurgent caldera to geology,[6] 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,[31] and it remains one of the best studied examples of a resurgent caldera.[6] The ash flow tuffs of the Valles caldera, such as theBandelier Tuff, were among the first to be thoroughly characterized.[32]
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 ongoingQuaternary 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[33] proposed that avolcanic winter induced by this eruption reduced the human population to about 2,000–20,000 individuals, resulting in apopulation bottleneck. More recently,Lynn Jorde andHenry Harpending proposed that the human species was reduced to approximately 5,000–10,000 people.[34] 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.[35] There is evidence that human habitation continued inIndia after the eruption.[36]
Sollipulli Caldera, located in central Chile near the border with Argentina, filled with ice. The volcano is in the southern Andes Mountains within Chile's Parque Nacional Villarica.[37]
Some volcanoes, such as the largeshield volcanoesKīlauea andMauna Loa on the island ofHawaii, form calderas in a different fashion. The magma feeding these volcanoes isbasalt, which is silica poor. As a result, the magma is much lessviscous 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 atopFernandina Island collapsed in 1968 when parts of the caldera floor dropped 350 metres (1,150 ft).[38]
Since the early 1960s, it has been known that volcanism has occurred on other planets and moons in theSolar System. Through the use of crewed and uncrewed spacecraft, volcanism has been discovered onVenus,Mars, theMoon, andIo, a satellite ofJupiter. None of these worlds haveplate tectonics, which contributes approximately 60% of the Earth's volcanic activity (the other 40% is attributed tohotspot volcanism).[39] 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.[40]
TheMoon 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.[39] Although caldera-like structures are rare on the Moon, they are not completely absent. TheCompton-Belkovich Volcanic Complex on thefar side of the Moon is thought to be a caldera, possibly anash-flow caldera.[41]
The volcanic activity ofMars is concentrated in two major provinces:Tharsis andElysium. Each province contains a series of giant shield volcanoes that are similar to what we see on Earth and likely are the result of mantlehot spots. The surfaces are dominated by lava flows, and all have one or more collapse calderas.[39] 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.[42]
Because there is noplate tectonics onVenus, heat is mainly lost by conduction through thelithosphere. This causes enormous lava flows, accounting for 80% of Venus' surface area. Many of the mountains are largeshield 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.[39]
Io, unusually, is heated by solid flexing due to thetidal influence ofJupiter and Io'sorbital resonance with neighboring large moonsEuropa andGanymede, which keep its orbit slightlyeccentric. Unlike any of the planets mentioned, Io is continuously volcanically active. For example, the NASAVoyager 1 andVoyager 2 spacecraft detected nine erupting volcanoes while passing Io in 1979. Io has many calderas with diameters tens of kilometers across.[39]
Somma volcano – Volcanic caldera that has been partially filled by a new central cone
Supervolcano – Volcano that has had an eruption with a volcanic explosivity index (VEI) of 8
Volcanic Explosivity Index – Predictive qualitative scale for explosiveness of volcanic eruptionsPages displaying short descriptions of redirect targets
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