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76-578: The Edziza Formation ( / ə d ˈ z aɪ z ə / əd-zy-zə ) is a stratigraphic unit of Pleistocene age in northwestern British Columbia , Canada. First described in 1984, the Edziza Formation was mapped as one of several geological formations of the Mount Edziza volcanic complex . It overlies at least four other geological formations of this volcanic complex that differ in age and composition. The main volcanic rock comprising

152-778: A correlation with age of the forming lava, with in the case of the Hawaiian chain, steepness increasing with age, as later lavas tend to be more alkali so are more viscous, with thicker flows, that travel less distance from the summit vents. In height they are typically about one twentieth their width. Although the general form of a "typical" shield volcano varies little worldwide, there are regional differences in their size and morphological characteristics. Typical shield volcanoes found in California and Oregon measure 3 to 4 mi (5 to 6 km) in diameter and 1,500 to 2,000 ft (500 to 600 m) in height, while shield volcanoes in

228-426: A crudely radial pattern towards the summit crater. They individually range in thickness from a few centimetres to nearly 2 kilometres (1.2 miles) and occur both within the stratovolcano and in the underlying basement rocks. Cupolas occur throughout Tenchen Valley as 3-to-9-metre-wide (9.8-to-29.5-foot) projections through thick pyroclastic deposits and resemble inverted sand castings up to 12 metres (39 feet) high where

304-545: A far greater distance than those of other eruptive types before solidifying, forming extremely wide but relatively thin magmatic sheets often less than 1 m (3 ft) thick. Low volumes of such lavas layered over long periods of time are what slowly constructs the characteristically low, broad profile of a mature shield volcano. Also unlike other eruptive types, Hawaiian eruptions often occur at decentralized fissure vents , beginning with large "curtains of fire" that quickly die down and concentrate at specific locations on

380-484: A formation in another region and a formation may reduce in rank for member or bed as it "pinches out". A bed is a lithologically distinct layer within a member or formation and is the smallest recognisable stratigraphic unit. These are not normally named, but may be in the case of a marker horizon . A member is a named lithologically distinct part of a formation. Not all formations are subdivided in this way and even where they are recognized, they may only form part of

456-457: A further 13 km (8 mi) below the waterline and into the crust, approximately 80,000 km (19,000 cu mi) of rock. Kīlauea , another Hawaiian shield volcano, is one of the most active volcanoes on Earth, with its most recent eruption occurring in 2021. The Galápagos Islands are an isolated set of volcanoes, consisting of shield volcanoes and lava plateaus, about 1,100 km (680 mi) west of Ecuador. They are driven by

532-422: A prolonged basis. At least three trachyte domes of the Edziza Formation occur on Mount Edziza, all of which were created by the rapid effusion of viscous trachytic lava. The nearly circular Nanook Dome forms the southeastern buttress of the summit crater and is the largest of the three lava domes with a diameter of about three-quarters of a kilometre. It may have been the source for the two upper lava lakes inside

608-583: A prominent feature on these volcanoes and account for their seemingly random volcanic structure. They are fueled by the movement of the Pacific Plate over the Hawaii hotspot and form a long chain of volcanoes, atolls , and seamounts 2,600 km (1,616 mi) long with a total volume of over 750,000 km (179,935 cu mi). The chain includes Mauna Loa, a shield volcano which stands 4,170 m (13,680 ft) above sea level and reaches

684-560: A shear cliff across the full width of the Tenchen cirque headwall. The uppermost unit is largely buried under overhanging ice, but it appears to consist of agglutinated volcanic blocks. Exposed in the underlying basement rocks on the deeply eroded eastern flank of Mount Edziza are subvolcanic trachyte cupolas , sills , dikes and irregular intrusive masses linked to the magma plumbing system. Dikes are present as swarms and are usually vertical or subvertical, having been distributed in

760-440: A single point. The main shield then forms, burying the smaller ones formed by the early eruptions with its lava. Icelandic shields are mostly small (~15 km (4 cu mi)), symmetrical (although this can be affected by surface topography), and characterized by eruptions from summit calderas. They are composed of either tholeiitic olivine or picritic basalt . The tholeiitic shields tend to be wider and shallower than

836-461: Is Nyamuragira . Eruptions at the shield volcano are generally centered within the large summit caldera or on the numerous fissures and cinder cones on the volcano's flanks. Lava flows from the most recent century extend down the flanks more than 30 km (19 mi) from the summit, reaching as far as Lake Kivu . Erta Ale in Ethiopia is another active shield volcano and one of the few places in

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912-565: Is a set of two or more associated groups and/or formations that share certain lithological characteristics. A supergroup may be made up of different groups in different geographical areas. A sequence of fossil -bearing sedimentary rocks can be subdivided on the basis of the occurrence of particular fossil taxa . A unit defined in this way is known as a biostratigraphic unit, generally shortened to biozone . The five commonly used types of biozone are assemblage, range, abundance, interval and lineage zones. Shield volcano A shield volcano

988-563: Is a set of two or more formations that share certain lithological characteristics. A group may be made up of different formations in different geographical areas and individual formations may appear in more than one group. Groups are occasionally divided into subgroups, but subgroups are not mentioned in the North American Stratigraphic Code, and are permitted under International Commission on Stratigraphy guidelines only in exceptional circumstances. A supergroup

1064-667: Is a shield volcano on the southwestern part of Isabela Island and is one of the most active in the Galapagos, with the last eruption between May and June 2008. The Geophysics Institute at the National Polytechnic School in Quito houses an international team of seismologists and volcanologists whose responsibility is to monitor Ecuador's numerous active volcanoes in the Andean Volcanic Belt and

1140-468: Is a type of volcano named for its low profile, resembling a shield lying on the ground. It is formed by the eruption of highly fluid (low viscosity ) lava , which travels farther and forms thinner flows than the more viscous lava erupted from a stratovolcano . Repeated eruptions result in the steady accumulation of broad sheets of lava, building up the shield volcano's distinctive form. Shield volcanoes are found wherever fluid, low- silica lava reaches

1216-523: Is alkali feldspar which comprises small tabular phenocrysts. Permeating the completely altered material are chalcedony veinlets less than 1 millimetre (0.039 inches) thick which are cut by more extensively distributed veins 1 to 5 centimetres (0.39 to 1.97 inches) wide. Tiny cubes of pyrite occupy many of the larger veins while the surrounding wall rock contains disseminated pyrite and marcasite . The conduit breccia and surrounding rocks were likely altered by intense fumarolic activity that occurred on

1292-486: Is also the youngest geological formation of the Mount Edziza volcanic complex involving more than 10 cubic kilometres (2.4 cubic miles) of volcanic material. Basalt flows of the 4.4 million-year-old Nido Formation are overlain by the Edziza Formation on the eastern flank of Mount Edziza between Tenchen Creek and The Pyramid where they overlie a ridge of Mesozoic basement rocks. The Edziza Formation overlies

1368-453: Is an expansion of Kakiddi Creek. Stratigraphically , the Edziza Formation is the fifth youngest unit of the Mount Edziza volcanic complex, having been deposited over the Nido, Pyramid , Ice Peak and Pillow Ridge formations. It has a volume of 18 cubic kilometres (4.3 cubic miles), making it the fifth most voluminous geological formation of the Mount Edziza volcanic complex. The Edziza Formation

1444-531: Is common in Hawaiʻi; most Hawaiian eruptions begin with a so-called "wall of fire" along a major fissure line before centralizing to a small number of points. This accounts for their asymmetrical shape, whereas Icelandic volcanoes follow a pattern of central eruptions dominated by summit calderas , causing the lava to be more evenly distributed or symmetrical. Most of what is currently known about shield volcanic eruptive character has been gleaned from studies done on

1520-562: Is embedded with sparse phenocrysts of anorthoclase and sodic ferrohedenbergite. Edziza trachyte is mineralogically similar to trachyte of the Kakiddi Formation which was erupted 0.3 million years ago during the final stages of the fourth magmatic cycle . However, the Kakiddi trachyte flows travelled along gently sloping valleys for at least 10 kilometres (6.2 miles), suggesting that they were extruded more fluidly than those of

1596-399: Is relatively smooth and flows with a ropey texture, and ʻaʻā flows which are denser, more viscous (and thus slower moving) and blockier. These lava flows can be anywhere between 2 and 20 m (10 and 70 ft) thick. ʻAʻā lava flows move through pressure— the partially solidified front of the flow steepens because of the mass of flowing lava behind it until it breaks off, after which

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1672-592: Is sparsely overlain by the Edziza Formation, but much larger portions of the Edziza Formation overlap with the western and northern edges of Pillow and Tsekone ridges, respectively. Hawaiite and patches of air-fall tephra assigned to the Holocene Big Raven Formation overlie the northern end of the Edziza Formation where they are part of the Desolation Lava Field . The Edziza Formation consists mainly of trachyte that

1748-534: Is taken from the German term Schildvulkan , coined by the Austrian geologist Eduard Suess in 1888 and which had been calqued into English by 1910. Shield volcanoes are distinguished from the three other major volcanic types— stratovolcanoes , lava domes , and cinder cones —by their structural form, a consequence of their particular magmatic composition. Of these four forms, shield volcanoes erupt

1824-479: The Galápagos hotspot , and are between approximately 4.2 million and 700,000 years of age. The largest island, Isabela , consists of six coalesced shield volcanoes, each delineated by a large summit caldera. Española , the oldest island, and Fernandina , the youngest, are also shield volcanoes, as are most of the other islands in the chain. The Galápagos Islands are perched on a large lava plateau known as

1900-685: The Mid-Atlantic Ridge , a divergent tectonic plate boundary in the middle of the Atlantic Ocean, Iceland is the site of about 130 volcanoes of various types. Icelandic shield volcanoes are generally of Holocene age, between 5,000 and 10,000 years old. The volcanoes are also very narrow in distribution, occurring in two bands in the West and North Volcanic Zones. Like Hawaiian volcanoes, their formation initially begins with several eruptive centers before centralizing and concentrating at

1976-651: The Mount Edziza volcanic complex , a group of late Cenozoic volcanic rocks in northwestern British Columbia , Canada. In 1988, Jack Souther mapped the Edziza and Pillow Ridge formations separately and the number of geological formations comprising the Mount Edziza volcanic complex had dropped to 13; the Sheep Track and Kounugu formations had been reassigned as members of the Big Raven and Nido formations, respectively, and are no longer recognized. Nearly all of

2052-587: The Tahltan language , referring to the deep volcanic ash deposits or pumice -like sand covering large portions of the Big Raven Plateau around Mount Edziza . According to David Stevenson of University of Victoria 's Anthropology Department, "sand" or "dust" is instead translated as "kutlves" in the Tahltan language. An explanation listed in the BC Parks brochure is that Edziza means "cinders" in

2128-421: The 1976–1979 Viking mission . The principal difference between the volcanoes of Mars and those on Earth is in terms of size; Martian volcanoes range in size up to 14 mi (23 km) high and 370 mi (595 km) in diameter, far larger than the 6 mi (10 km) high, 74 mi (119 km) wide Hawaiian shields. The highest of these, Olympus Mons , is the tallest known mountain on any planet in

2204-419: The Edziza Formation is trachyte which was deposited by volcanic eruptions at the end of the third magmatic cycle of the Mount Edziza volcanic complex 0.9 million years ago. Trachyte of the Edziza Formation is in the form of lava flows and pyroclastic rocks that comprise the central stratovolcano of Mount Edziza , as well as lava domes on its summit and flanks. At least four lava lakes ponded inside

2280-576: The Edziza Formation is located on the east-central edge of the Big Raven Plateau, a barren plateau in Cassiar Land District bounded on the west by Mess Valley , on the north by Klastline Valley , on the east by Kakiddi Valley and on the south by Chakima and Walkout valleys, the latter two of which are separated by mountainous terrain . It lies at the northern end of the Mount Edziza volcanic complex which also includes

2356-443: The Edziza Formation. The main feature consisting of Edziza Formation trachyte is the central stratovolcano of Mount Edziza. It comprises explosion breccias, landslide or lahar deposits and thick, steeply-dipping flows that were erupted as highly viscous lava. These rocks are well-exposed in 850-metre-high (2,790-foot) cliffs on the eastern flank of Mount Edziza where they occur along the north side of Tenchen Valley. This section of

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2432-568: The Galapagos Islands. La Cumbre is an active shield volcano on Fernandina Island that has been erupting since April 11, 2009. The Galápagos islands are geologically young for such a big chain, and the pattern of their rift zones follows one of two trends, one north-northwest, and one east–west. The composition of the lavas of the Galápagos shields are strikingly similar to those of the Hawaiian volcanoes. Curiously, they do not form

2508-535: The Galápagos Platform. This platform creates a shallow water depth of 360 to 900 m (1,181 to 2,953 ft) at the base of the islands, which stretch over a 174 mi (280 km) diameter. Since Charles Darwin 's visit to the islands in 1835 during the second voyage of HMS Beagle , there have been over 60 recorded eruptions in the islands, from six different shield volcanoes. Of the 21 emergent volcanoes, 13 are considered active. Cerro Azul

2584-468: The Glacier Dome eruption is exposed for 2 kilometres (1.2 miles) north of Pyramid Creek; its distal end thins to about 15 metres (49 feet). South of Pillow Ridge on the northwestern flank of Mount Edziza is a rounded pyroclastic cone with a small central crater that probably formed during the latter stages of Edziza Formation volcanism . This pile of agglutinated trachyte spatter, pumice and bombs

2660-556: The Tahltan language. Another explanation proposed by Canadian volcanologist Jack Souther is that Edziza is a corruption of Edzerza, the name of a local Tahltan family. The Edziza Formation was first described by Jack Souther, Richard Lee Armstrong and J. Harakal in 1984 who grouped it together with the Pillow Ridge Formation in their descriptions and mapping. It was mapped as one of 15 geological formations of

2736-782: The accumulation of fragmentary material from particularly powerful explosive eruptions, and rarer felsic lava shields formed by unusually fluid felsic magmas. Examples of pyroclastic shields include Billy Mitchell volcano in Papua New Guinea and the Purico complex in Chile ; an example of a felsic shield is the Ilgachuz Range in British Columbia , Canada. Shield volcanoes are similar in origin to vast lava plateaus and flood basalts present in various parts of

2812-415: The breccia has eroded away. The Tenchen cirque headwall exposes dikes, sills and breccias within the central conduit that have been intensely altered by hydrothermal solutions, resulting in the rocks being bright yellow and ochre -weathered. Complete alteration occurs in the middle of the altered zone where rock has been reconstituted to a pure white, amorphous material; the only original mineral present

2888-508: The central Mexican Michoacán–Guanajuato volcanic field average 340 m (1,100 ft) in height and 4,100 m (13,500 ft) in width, with an average slope angle of 9.4° and an average volume of 1.7 km (0.4 cu mi). Rift zones are a prevalent feature on shield volcanoes that is rare on other volcanic types. The large, decentralized shape of Hawaiian volcanoes as compared to their smaller, symmetrical Icelandic cousins can be attributed to rift eruptions. Fissure venting

2964-479: The contact need not be particularly distinct. For instance, a unit may be defined by terms such as "when the sandstone component exceeds 75%". Sequences of sedimentary and volcanic rocks are subdivided on the basis of their shared or associated lithology . Formally identified lithostratigraphic units are structured in a hierarchy of lithostratigraphic rank , higher rank units generally comprising two or more units of lower rank. Going from smaller to larger in rank,

3040-520: The crater where the eastern side of the summit ridge has been breached by active cirques. They overlie breccia of the central conduit and are in the form of at least four cooling units. Trachyte with well-developed columnar jointing forms the lower two units, each of which is about 30 metres (98 feet) thick. The two upper units consist of lighter green trachyte; the lower unit has a thickness of nearly 90 metres (300 feet). Well-developed, vertical and rectangularly jointed trachyte forms this unit and comprises

3116-451: The extents of the volcano have not been confirmed. Although Tamu Massif was initially believed to be a shield volcano, Sanger and his colleagues acknowledged in 2019 that Tamu Massif is not a shield volcano. Shield volcanoes feature a gentle (usually 2° to 3°) slope that gradually steepens with elevation (reaching approximately 10°) before flattening near the summit, forming an overall upwardly convex shape. These slope characteristics have

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3192-437: The formation. A member need not be mappable at the same scale as a formation. Formations are the primary units used in the subdivision of a sequence and may vary in scale from tens of centimetres to kilometres. They should be distinct lithologically from other formations, although the boundaries do not need to be sharp. To be formally recognised, a formation must have sufficient extent to be useful in mapping an area. A group

3268-410: The general mass behind it moves forward. Though the top of the flow quickly cools down, the molten underbelly of the flow is buffered by the solidifying rock above it, and by this mechanism, ʻaʻā flows can sustain movement for long periods of time. Pāhoehoe flows, in contrast, move in more conventional sheets, or by the advancement of lava "toes" in snaking lava columns. Increasing viscosity on the part of

3344-573: The height of the volcano above the sea floor , the "true" height of Mauna Loa from the start of its eruptive history is about 17,170 m (56,000 ft). Mount Everest , by comparison, is 8,848 m (29,029 ft) in height. In 2013, a team led by the University of Houston 's William Sager announced the discovery of Tamu Massif , an enormous extinct submarine volcano, approximately 450 by 650 km (280 by 400 mi) in area, which dwarfs all previously known volcanoes on Earth. However,

3420-468: The largest volcanic features on Earth. The summit of the largest subaerial volcano in the world, Mauna Loa , lies 4,169 m (13,678 ft) above sea level , and the volcano, over 60 mi (100 km) wide at its base, is estimated to contain about 80,000 km (19,000 cu mi) of basalt. The mass of the volcano is so great that it has slumped the crust beneath it a further 8 km (5 mi). Accounting for this subsidence and for

3496-641: The lava or shear stress on the part of local topography can morph a pāhoehoe flow into an ʻaʻā one, but the reverse never occurs. Although most shield volcanoes are by volume almost entirely Hawaiian and basaltic in origin, they are rarely exclusively so. Some volcanoes, such as Mount Wrangell in Alaska and Cofre de Perote in Mexico, exhibit large enough swings in their historical magmatic eruptive characteristics to cast strict categorical assignment in doubt; one geological study of de Perote went so far as to suggest

3572-543: The least viscous lavas. Whereas stratovolcanoes and lava domes are the product of highly viscous flows, and cinder cones are constructed of explosively eruptive tephra , shield volcanoes are the product of gentle effusive eruptions of highly fluid lavas that produce, over time, a broad, gently sloped eponymous "shield". Although the term is generally applied to basaltic shields, it has also at times been applied to rarer scutiform volcanoes of differing magmatic composition—principally pyroclastic shields , formed by

3648-415: The main lithostratigraphic ranks are bed, member, formation, group and supergroup. Formal names of lithostratigraphic units are assigned by geological surveys . Units of formation or higher rank are usually named for the unit's type location , and the formal name usually also states the unit's rank or lithology. A lithostratigraphic unit may have a change in rank over a some distance; a group may thin to

3724-778: The most prominent ones are situated along the Snake River in Idaho and the Columbia River in Washington and Oregon, where they have been measured to be over 1 mi (2 km) in thickness. Calderas are a common feature on shield volcanoes. They are formed and reformed over the volcano's lifespan. Long eruptive periods form cinder cones, which then collapse over time to form calderas. The calderas are often filled up by progressive eruptions, or formed elsewhere, and this cycle of collapse and regeneration takes place throughout

3800-549: The picritic shields. They do not follow the pattern of caldera growth and destruction that other shield volcanoes do; caldera may form, but they generally do not disappear. Bingöl Mountains are one of the shield volcanoes in Turkey . In East Africa, volcanic activity is generated by the development of the East African Rift and from nearby hotspots. Some volcanoes interact with both. Shield volcanoes are found near

3876-506: The rift and off the coast of Africa, although stratovolcanoes are more common. Although sparsely studied, the fact that all of its volcanoes are of Holocene age reflects how young the volcanic center is. One interesting characteristic of East African volcanism is a penchant for the formation of lava lakes ; these semi-permanent lava bodies, extremely rare elsewhere, form in about 9% of African eruptions. The most active shield volcano in Africa

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3952-570: The same volcanic "line" associated with most hotspots. They are not alone in this regard; the Cobb–Eickelberg Seamount chain in the North Pacific is another example of such a delineated chain. In addition, there is no clear pattern of age between the volcanoes, suggesting a complicated, irregular pattern of creation. How the islands were formed remains a geological mystery, although several theories have been proposed. Located over

4028-424: The seafloor. The Hawaiian and Galápagos shields, and other hotspot shields like them, are constructed of oceanic island basalt. Their lavas are characterized by high levels of sodium , potassium , and aluminium . Features common in shield volcanism include lava tubes . Lava tubes are cave-like volcanic straights formed by the hardening of overlaying lava. These structures help further the propagation of lava, as

4104-402: The site of one of the longest-lasting rift eruptions in known history. The Hawaiian shield volcanoes are not located near any plate boundaries ; the volcanic activity of this island chain is distributed by the movement of the oceanic plate over an upwelling of magma known as a hotspot . Over millions of years, the tectonic movement that moves continents also creates long volcanic trails across

4180-569: The smaller Arctic Lake and Kitsu plateaus to the south. This complex of shield volcanoes , stratovolcanoes , lava domes , calderas and cinder cones forms a broad, intermontane plateau at the eastern edge of the Tahltan Highland , a southeast-trending upland area extending along the western side of the Stikine Plateau . Small portions of the Edziza Formation occur north, northwest and southwest of Nuttlude Lake which

4256-429: The southern end of the ridge are the highest and consist of greenish grey, sparsely porphyritic trachyte. They comprise well-formed, small diameter rock columns that rise nearly vertically for more than 90 metres (300 feet) above the ice cap. Nunataks elsewhere on the summit ridge are more subdued, consisting of pyroclastic debris that has been glacially reworked. The remnants of several lava lakes are exposed inside

4332-409: The steep Tenchen cirque headwall are large volcanic blocks and small breccia fragments within the central conduit that accompanied with collapse of a narrower summit with a much smaller crater . The collapse may have been caused by a violent, climactic eruption, effusion of the Kakiddi lava flows or the outpouring of voluminous lava during dome formation. Prior to collapse, the summit of Mount Edziza

4408-527: The stratovolcano contains a lower assemblage of chaotic explosion breccias and an upper assemblage of lava that overlies the breccias. Lava of the upper assemblage is highly irregular or lenticular in cross section , but it contains individual cooling units as much as 150 metres (490 feet) thick. The lower assemblage encloses up to 120-metre-long (390-foot) and 15-metre-thick (49-foot) lavas that occur as relatively small, crudely-jointed, irregular masses and discontinuous lenses. Extremely coarse breccias occur in

4484-521: The summit crater which cooled into four rock units that are exposed in the breached eastern crater rim. These lava lakes overlie rock fragments inside the central volcanic conduit which accompanied with collapse of a narrower and higher summit. A number of explanations have been made regarding the origin of the name Edziza. A 1927 report by J. Davidson of the British Columbia Land Surveyors claims that Edziza means "sand" in

4560-483: The summit crater as they consist of trachyte similar to that of Nanook Dome. Glacier Dome and Triangle Dome are two elliptical masses of trachyte with concentric flow layering on the northwestern and western flanks of Mount Edziza, respectively. Triangle Dome displays a pattern of columnar cooling joints that indicates it was formed by volcanic activity in a subglacial environment. A 24-metre-thick (79-foot) trachyte flow that may have issued during an early, fluid phase of

4636-463: The surface of a rocky planet. However, they are most characteristic of ocean island volcanism associated with hot spots or with continental rift volcanism. They include the largest active volcanoes on Earth, such as Mauna Loa . Giant shield volcanoes are found on other planets of the Solar System , including Olympus Mons on Mars and Sapas Mons on Venus . The term 'shield volcano'

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4712-1082: The surface wells up), such as the Hawaiian–Emperor seamount chain and the Galápagos Islands , or over more conventional rift zones, such as the Icelandic shields and the shield volcanoes of East Africa. Although shield volcanoes are not usually associated with subduction , they can occur over subduction zones. Many examples are found in California and Oregon, including Prospect Peak in Lassen Volcanic National Park , as well as Pelican Butte and Belknap Crater in Oregon. Many shield volcanoes are found in ocean basins , such as Kīlauea in Hawaii, although they can be found inland as well—East Africa being one example of this. The largest and most prominent shield volcano chain in

4788-440: The surrounding heat, and hit the ground still hot, accumulating into spatter cones . If eruptive rates are high enough, they may even form splatter-fed lava flows. Hawaiian eruptions are often extremely long-lived; Puʻu ʻŌʻō , a cinder cone of Kīlauea , erupted continuously from January 3, 1983, until April 2018. Flows from Hawaiian eruptions can be divided into two types by their structural characteristics: pāhoehoe lava which

4864-420: The term "compound shield-like volcano" instead. Most mature shield volcanoes have multiple cinder cones on their flanks, the results of tephra ejections common during incessant activity and markers of currently and formerly active sites on the volcano. An example of these parasitic cones is at Puʻu ʻŌʻō on Kīlauea —continuous activity ongoing since 1983 has built up a 2,290 ft (698 m) tall cone at

4940-472: The volcano's lifespan. Interactions between water and lava at shield volcanoes can cause some eruptions to become hydrovolcanic . These explosive eruptions are drastically different from the usual shield volcanic activity and are especially prevalent at the waterbound volcanoes of the Hawaiian Isles . Shield volcanoes are found worldwide. They can form over hotspots (points where magma from below

5016-442: The volcano's rift zones. Central-vent eruptions, meanwhile, often take the form of large lava fountains (both continuous and sporadic), which can reach heights of hundreds of meters or more. The particles from lava fountains usually cool in the air before hitting the ground, resulting in the accumulation of cindery scoria fragments; however, when the air is especially thick with pyroclasts , they cannot cool off fast enough because of

5092-433: The volcanoes of Hawaiʻi Island , by far the most intensively studied of all shields because of their scientific accessibility; the island lends its name to the slow-moving, effusive eruptions typical of shield volcanism, known as Hawaiian eruptions . These eruptions, the least explosive of volcanic events, are characterized by the effusive emission of highly fluid basaltic lavas with low gaseous content . These lavas travel

5168-534: The walls of the tube insulate the lava within. Lava tubes can account for a large portion of shield volcano activity; for example, an estimated 58% of the lava forming Kīlauea comes from lava tubes. In some shield volcano eruptions, basaltic lava pours out of a long fissure instead of a central vent, and shrouds the countryside with a long band of volcanic material in the form of a broad plateau . Plateaus of this type exist in Iceland, Washington, Oregon, and Idaho;

5244-422: The western proximity and contain massive, up to 7.5-metre-wide (25-foot) volcanic blocks that were probably deposited directly from the central conduit during eruption. At the eastern end of Tenchen Valley are more distal breccias that rarely contain volcanic blocks more than 1.5 metres (4.9 feet) wide. Local rounding and stratification of these breccias is indicative of lahar or landslide deposition. Exposed in

5320-423: The westernmost portion of Sphinx Dome , a rhyolite dome of the 1.1 million-year-old Pyramid Formation; two thick remnants of the Edziza Formation also occur on the steep northern and southern flanks of this dome. Basalt flows of the roughly one million-year-old Ice Peak Formation are overlain by the Edziza Formation which overlaps with the northern flank of Ice Peak. The 0.9 million-year-old Pillow Ridge Formation

5396-724: The world is the Hawaiian–Emperor seamount chain, a chain of hotspot volcanoes in the Pacific Ocean. The volcanoes follow a distinct evolutionary pattern of growth and death. The chain contains at least 43 major volcanoes, and Meiji Seamount at its terminus near the Kuril–Kamchatka Trench is 85 million years old. The youngest part of the chain is Hawaii, where the volcanoes are characterized by frequent rift eruptions, their large size (thousands of km in volume), and their rough, decentralized shape. Rift zones are

5472-686: The world with a permanent lava lake, which has been active since at least 1967, and possibly since 1906. Other volcanic centers include Menengai , a massive shield caldera, and Mount Marsabit in Kenya. Shield volcanoes are not limited to Earth; they have been found on Mars , Venus , and Jupiter's moon, Io . The shield volcanoes of Mars are very similar to the shield volcanoes on Earth. On both planets, they have gently sloping flanks, collapse craters along their central structure, and are built of highly fluid lavas. Volcanic features on Mars were observed long before they were first studied in detail during

5548-436: The world. These are eruptive features which occur along linear fissure vents and are distinguished from shield volcanoes by the lack of an identifiable primary eruptive center. Active shield volcanoes experience near-continuous eruptive activity over extremely long periods of time, resulting in the gradual build-up of edifices that can reach extremely large dimensions. With the exclusion of flood basalts, mature shields are

5624-422: Was at least 610 metres (2,000 feet) higher than its current elevation of 2,786 metres (9,140 feet). The broad, nearly flat summit of Mount Edziza is truncated by a 3-kilometre-wide (1.9-mile) crater that formed after collapse of the original summit. A circular ridge surrounding this ice-filled crater is partially exposed above the ice cap as a discontinuous series of spires and serrated nunataks . Spires forming

5700-421: Was erupted 0.9 million years ago at the end of the third magmatic cycle of the Mount Edziza volcanic complex. Its composition straddles near the pantelleritic trachyte/ comenditic trachyte boundary and is in the form of explosion breccias , lava flows and lava domes. The trachyte contains a matrix of sodic plagioclase alkali feldspar , katophorite , arfvedsonite , aenigmatite and opaque oxides that

5776-480: Was the source of at least two separate lobes of lava that flowed onto the adjacent plateau surface. The northwesterly lobe partially encircles Tsekone Ridge and borders as well as overlies breccia of Pillow Ridge. Extending west of the pyroclastic cone is the second lobe of lava which, together with the northwesterly lobe, is largely overlain by volcanic ash and colluvial material . Stratigraphic unit Units must be mappable and distinct from one another, but

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