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51-551: The preserved part of the Stoer Group is made up of three subunits, the Clachtoll, Bay of Stoer and Meall Dearg formations. A basal breccia is present in many areas with large clasts derived from the underlying Lewisian. There is local evidence of weathering of the gneiss beneath the unconformity. Away from the unconformity the breccia becomes crudely stratified within an overall fining upwards sequence passing up into pebbly sandstones,

102-461: A braided river system, trough cross-bedded sandstones and conglomerates . The Bay of Stoer Formation consists of a lower section formed of red trough cross-bedded sandstones with some pebbles, interpreted to be the deposits of a braided river system. The uppermost 100 m of the formation, the Stac Fada and Poll a' Mhuilt members, form a distinctive marker layer within the Stoer Group succession with

153-483: A strike extent of 50 km. The Stac Fada Member is generally about 10 m thick and consists of muddy sandstone facies with abundant clasts of vesicular volcanic glass , locally with accretionary lapilli . The matrix for these volcanic clasts is always non-volcanic suggesting transport from the area where they were erupted. The member also includes large rafts of gneiss and sandstone, up to 15 m in length. The Stac Fada member has been traditionally interpreted to be

204-409: A caldera, possibly an ash-flow caldera. 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. Mars has the tallest volcano in

255-400: 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. 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

306-556: A fine-grained matrix . The word has its origins in the Italian language, in which it means "rubble". A breccia may have a variety of different origins, as indicated by the named types including sedimentary breccia, fault or tectonic breccia, igneous breccia, impact breccia, and hydrothermal breccia. A megabreccia is a breccia composed of very large rock fragments, sometimes kilometers across, which can be formed by landslides , impact events , or caldera collapse. Breccia

357-625: A kilometer in length. Within the volcanic conduits of explosive volcanoes the volcanic breccia environment merges into the intrusive breccia environment. There the upwelling lava tends to solidify during quiescent intervals only to be shattered by ensuing eruptions. This produces an alloclastic volcanic breccia. Clastic rocks are also commonly found in shallow subvolcanic intrusions such as porphyry stocks, granites and kimberlite pipes, where they are transitional with volcanic breccias. Intrusive rocks can become brecciated in appearance by multiple stages of intrusion, especially if fresh magma

408-573: A known impact crater, and/or an association with other products of impact cratering such as shatter cones , impact glass, shocked minerals , and chemical and isotopic evidence of contamination with extraterrestrial material (e.g., iridium and osmium anomalies). An example of an impact breccia is the Neugrund breccia , which was formed in the Neugrund impact . Hydrothermal breccias usually form at shallow crustal levels (<1 km) between 150 and 350 °C, when seismic or volcanic activity causes

459-534: A limestone in the Stoer Group (1199±70 Ma), followed by Ar-Ar dating of the Stac Fada Member ejecta blanket deposit at a slightly lower stratigraphic level (1177±5 Ma). Breccia Breccia ( / ˈ b r ɛ tʃ i ə / BRETCH -ee-ə or / ˈ b r ɛ ʃ i ə / BRESH -ee-ə , Italian: [ˈbrettʃa] ; Italian for 'breach') is a rock composed of large angular broken fragments of minerals or rocks cemented together by

510-467: A magma chamber whose magma is rich in silica . Silica-rich magma has a high viscosity , and therefore does not flow easily like basalt . The magma typically also contains a large amount of dissolved gases, up to 7 wt% for the most silica-rich magmas. 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

561-427: A mudflow. An alternative suggestion has been that the member represents part of the proximal ejecta blanket from an impact crater . This interpretation is supported by the presence of shocked quartz and biotite . The overlying Poll a' Mhuilt member consists of a thin sequence of siltstones and fine sandstones alternating with muddy sandstones, suggesting deposition in a lacustrine environment. The uppermost part of

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612-583: 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 . At some points in geological time , rhyolitic calderas have appeared in distinct clusters. The remnants of such clusters may be found in places such as the Eocene Rum Complex of Scotland,

663-404: A void to open along a fault deep underground. The void draws in hot water, and as pressure in the cavity drops, the water violently boils. In addition, the sudden opening of a cavity causes rock at the sides of the fault to destabilise and implode inwards, and the broken rock gets caught up in a churning mixture of rock, steam and boiling water. Rock fragments collide with each other and the sides of

714-410: Is a large cauldron -like hollow that forms shortly after the emptying of a magma chamber in a volcanic 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

765-414: Is composed of coarse rock fragments held together by cement or a fine-grained matrix. Like conglomerate , breccia contains at least 30 percent of gravel -sized particles (particles over 2mm in size), but it is distinguished from conglomerate because the rock fragments have sharp edges that have not been worn down. These indicate that the gravel was deposited very close to its source area, since otherwise

816-405: 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 . Secondary volcanic vents may form above the ring fracture. As the magma chamber empties, the center of the volcano within

867-489: Is intruded into partly consolidated or solidified magma. This may be seen in many granite intrusions where later aplite veins form a late-stage stockwork through earlier phases of the granite mass. When particularly intense, the rock may appear as a chaotic breccia. Clastic rocks in mafic and ultramafic intrusions have been found and form via several processes: Impact breccias are thought to be diagnostic of an impact event such as an asteroid or comet striking

918-406: Is less common in the mesothermal regime, as the formational event is brief. If boiling occurs, methane and hydrogen sulfide may be lost to the steam phase, and ore may precipitate. Mesothermal deposits are often mined for gold. For thousands of years, the striking visual appearance of breccias has made them a popular sculptural and architectural material. Breccia was used for column bases in

969-529: Is relatively young (1.25 million years old) and unusually well preserved, and it remains one of the best studied examples of a resurgent caldera. The ash flow tuffs of the Valles caldera, such as the Bandelier Tuff , were among the first to be thoroughly characterized. About 74,000 years ago, this Indonesian volcano released about 2,800 cubic kilometres (670 cu mi) dense-rock equivalent of ejecta. This

1020-466: Is uniform in rock type and chemical composition. Caldera collapse leads to the formation of megabreccias, which are sometimes mistaken for outcrops of the caldera floor. These are instead blocks of precaldera rock, often coming from the unstable oversteepened rim of the caldera. They are distinguished from mesobreccias whose clasts are less than a meter in size and which form layers in the caldera floor. Some clasts of caldera megabreccias can be over

1071-673: The Minoan palace of Knossos on Crete in about 1800 BC . Breccia was used on a limited scale by the ancient Egyptians ; one of the best-known examples is the statue of the goddess Tawaret in the British Museum. Breccia was regarded by the Romans as an especially precious stone and was often used in high-profile public buildings. Many types of marble are brecciated, such as Breccia Oniciata. Caldera A caldera ( / k ɔː l ˈ d ɛr ə , k æ l -/ kawl- DERR -ə, kal- )

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1122-500: The San Juan Mountains of Colorado , where the 5,000 cubic kilometres (1,200 cu mi) Fish Canyon Tuff was blasted out in eruptions about 27.8 million years ago. The caldera produced by such eruptions is typically filled in with tuff, rhyolite , and other igneous rocks . The caldera is surrounded by an outflow sheet of ash flow tuff (also called an ash flow sheet ). If magma continues to be injected into

1173-438: The Earth and are normally found at impact craters . Impact breccia, a type of impactite , forms during the process of impact cratering when large meteorites or comets impact with the Earth or other rocky planets or asteroids . Breccia of this type may be present on or beneath the floor of the crater, in the rim, or in the ejecta expelled beyond the crater. Impact breccia may be identified by its occurrence in or around

1224-417: The Earth's volcanic activity (the other 40% is attributed to hotspot volcanism). 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

1275-661: The English term cauldron is also used, 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. 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 , where he first saw the Las Cañadas caldera on Tenerife , with Mount Teide dominating

1326-510: 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

1377-647: The San Juan Mountains of Colorado (formed during the Oligocene , Miocene , and Pliocene epochs) or the Saint Francois Mountain Range of Missouri (erupted during the Proterozoic eon). For their 1968 paper that first introduced the concept of a resurgent caldera to geology, R.L. Smith and R.A. Bailey chose the Valles caldera as their model. Although the Valles caldera is not unusually large, it

1428-720: 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. 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. Io, unusually,

1479-411: 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. 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

1530-577: The caldera atop Fernandina Island collapsed in 1968 when parts of the caldera floor dropped 350 metres (1,150 ft). 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

1581-451: The clasts are so large that the brecciated nature of the rock is not obvious. Megabreccias can be formed by landslides , impact events , or caldera collapse. Breccias are further classified by their mechanism of formation. Sedimentary breccia is breccia formed by sedimentary processes. For example, scree deposited at the base of a cliff may become cemented to form a talus breccia without ever experiencing transport that might round

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1632-664: The collapsed magma chamber, the center of the caldera may be uplifted in the form of a resurgent dome such as is seen at the Valles Caldera , Lake Toba , the San Juan volcanic field, Cerro Galán , Yellowstone , and many other calderas. 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 or Mount Pinatubo in 1991, may result in significant local destruction and

1683-406: The deposits of small alluvial fans. This breccia facies passes vertically and laterally in most places into muddy massive sandstones, true greywackes . The lower part of these sandstones are almost unbedded being replaced upwards by half metre thick beds capped by siltstones often with well-preserved desiccation structures. In parts of the outcrop, the muddy sandstones are succeeded by the deposits of

1734-410: The edges would have been rounded during transport. Most of the rounding of rock fragments takes place within the first few kilometers of transport, though complete rounding of pebbles of very hard rock may take up to 300 kilometers (190 mi) of river transport. A megabreccia is a breccia containing very large rock fragments, from at least a meter in size to greater than 400 meters. In some cases,

1785-403: The emptied or partially emptied magma chamber, leaving a large 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. Compared to the thousands of volcanic eruptions that occur over the course of a century,

1836-464: The formation of a caldera is a rare event, occurring only a few times within a given window of 100 years. Only eight caldera-forming collapses are known to have occurred between 1911 and 2018, with a caldera collapse at Kīlauea , Hawaii in 2018. Volcanoes that have formed a caldera are sometimes described as "caldera volcanoes". The term caldera comes from Spanish caldera , and Latin caldaria , meaning "cooking pot". In some texts

1887-423: The grinding action of two fault blocks as they slide past each other. Subsequent cementation of these broken fragments may occur by means of the introduction of mineral matter in groundwater . Igneous clastic rocks can be divided into two classes: Volcanic pyroclastic rocks are formed by explosive eruption of lava and any rocks which are entrained within the eruptive column. This may include rocks plucked off

1938-463: The human species was reduced to approximately 5,000–10,000 people. 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. There is evidence that human habitation continued in India after the eruption. Some volcanoes, such as the large shield volcanoes Kīlauea and Mauna Loa on

1989-429: 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,

2040-479: The landscape, and then the Caldera de Taburiente on La Palma . 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 in 1815), but also during effusive eruptions on the flanks of a volcano (see Piton de la Fournaise in 2007) or in a connected fissure system (see Bárðarbunga in 2014–2015). If enough magma

2091-561: The magma to produce a mixture of volcanic ash and other tephra with the very hot gases. 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 . Eruptions of this type can spread ash over vast areas, so that ash flow tuffs emplaced by silicic caldera eruptions are

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2142-403: The mesothermal regime, at much greater depths, fluids under lithostatic pressure can be released during seismic activity associated with mountain building. The pressurised fluids ascend towards shallower crustal levels that are under lower hydrostatic pressure. On their journey, high-pressure fluids crack rock by hydrofracturing , forming an angular in situ breccia. Rounding of rock fragments

2193-468: The only volcanic product with volumes rivaling those of flood basalts . For example, when Yellowstone Caldera last erupted some 650,000 years ago, it released about 1,000 km of material (as measured in dense rock equivalent (DRE)), covering a substantial part of North America in up to two metres of debris. Eruptions forming even larger calderas are known, such as the La Garita Caldera in

2244-428: The overpressure of pore fluid within sedimentary basins . Hydrothermal breccias are usually formed by hydrofracturing of rocks by highly pressured hydrothermal fluids. They are typical of the epithermal ore environment and are intimately associated with intrusive-related ore deposits such as skarns , greisens and porphyry -related mineralisation. Epithermal deposits are mined for copper, silver and gold. In

2295-462: 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. 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. One of

2346-523: The rock fragments. Thick sequences of sedimentary ( colluvial ) breccia are generally formed next to fault scarps in grabens . Sedimentary breccia may be formed by submarine debris flows . Turbidites occur as fine-grained peripheral deposits to sedimentary breccia flows. In a karst terrain , a collapse breccia may form due to collapse of rock into a sinkhole or in cave development. Collapse breccias also form by dissolution of underlying evaporite beds. Fault or tectonic breccia results from

2397-456: The sequence consists of trough cross-bedded sandstones thought to have been deposited by braided rivers, similar to the lower part of the Bay of Stoer Formation, possibly with wider channels and a lower paleoslope. A major time break was recognised between the Stoer Group and the overlying Torridon Group from paleomagnetic data . This has been confirmed by radiometric dating, initially Pb-Pb dating of

2448-581: The void, and the angular fragments become more rounded. Volatile gases are lost to the steam phase as boiling continues, in particular carbon dioxide . As a result, the chemistry of the fluids changes and ore minerals rapidly precipitate . Breccia-hosted ore deposits are quite common. The morphology of breccias associated with ore deposits varies from tabular sheeted veins and clastic dikes associated with overpressured sedimentary strata, to large-scale intrusive diatreme breccias ( breccia pipes ), or even some synsedimentary diatremes formed solely by

2499-421: The wall of the magma conduit, or physically picked up by the ensuing pyroclastic surge . Lavas, especially rhyolite and dacite flows, tend to form clastic volcanic rocks by a process known as autobrecciation . This occurs when the thick, nearly solid lava breaks up into blocks and these blocks are then reincorporated into the lava flow again and mixed in with the remaining liquid magma. The resulting breccia

2550-633: The world's best-preserved mineralized calderas is the Sturgeon Lake Caldera in northwestern Ontario , Canada, which formed during the Neoarchean era about 2.7 billion years ago. 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. Explosive caldera eruptions are produced by

2601-464: 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 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

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