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80-622: The Pelly River (Ts'ekínyäk Chú) is a river in Canada, and is a headstream of the Yukon River . The river originates west of the Mackenzie Mountains and flows 530 km (330 mi) through south-central Yukon . The Pelly has two main tributaries , the Ross and Macmillan rivers. The Selkirk First Nation (Hucha Hudan people) name for the river is Ts'enkínyäk Chú, meaning 'water running between

160-842: A mid-ocean ridge , such as the Mid-Atlantic Ridge , has volcanoes caused by divergent tectonic plates whereas the Pacific Ring of Fire has volcanoes caused by convergent tectonic plates. Volcanoes can also form where there is stretching and thinning of the crust's plates, such as in the East African Rift , the Wells Gray-Clearwater volcanic field , and the Rio Grande rift in North America. Volcanism away from plate boundaries has been postulated to arise from upwelling diapirs from

240-437: A graduated spectrum, with much overlap between categories, and does not always fit neatly into only one of these three separate categories. The USGS defines a volcano as "erupting" whenever the ejection of magma from any point on the volcano is visible, including visible magma still contained within the walls of the summit crater. While there is no international consensus among volcanologists on how to define an active volcano,

320-443: A paper was published suggesting a new definition for the word 'volcano' that includes processes such as cryovolcanism . It suggested that a volcano be defined as 'an opening on a planet or moon's surface from which magma , as defined for that body, and/or magmatic gas is erupted.' This article mainly covers volcanoes on Earth. See § Volcanoes on other celestial bodies and cryovolcano for more information. The word volcano

400-632: A volcano is largely determined by the composition of the lava it erupts. The viscosity (how fluid the lava is) and the amount of dissolved gas are the most important characteristics of magma, and both are largely determined by the amount of silica in the magma. Magma rich in silica is much more viscous than silica-poor magma, and silica-rich magma also tends to contain more dissolved gases. Lava can be broadly classified into four different compositions: Mafic lava flows show two varieties of surface texture: ʻAʻa (pronounced [ˈʔaʔa] ) and pāhoehoe ( [paːˈho.eˈho.e] ), both Hawaiian words. ʻAʻa

480-555: A volcano is of a conical mountain, spewing lava and poisonous gases from a crater at its summit; however, this describes just one of the many types of volcano. The features of volcanoes are varied. The structure and behaviour of volcanoes depend on several factors. Some volcanoes have rugged peaks formed by lava domes rather than a summit crater while others have landscape features such as massive plateaus . Vents that issue volcanic material (including lava and ash ) and gases (mainly steam and magmatic gases) can develop anywhere on

560-465: A volcano that has experienced one or more eruptions that produced over 1,000 cubic kilometres (240 cu mi) of volcanic deposits in a single explosive event. Such eruptions occur when a very large magma chamber full of gas-rich, silicic magma is emptied in a catastrophic caldera -forming eruption. Ash flow tuffs emplaced by such eruptions are the only volcanic product with volumes rivalling those of flood basalts . Supervolcano eruptions, while

640-692: A wide valley, receiving many small tributaries from the east. It passes the Pelly Lakes , out of which flows the Woodside River , and there it turns to the west. The Pelly then assumes a northwesterly course through the Tintina Trench . It flows northwest to receive the Ross River at the town of Ross River where there is a cable ferry . Paralleling the Robert Campbell Highway , it then merges with

720-578: Is a contraction of the words in the Gwich'in phrase chųų gąįį han , which means white water river and refers to "the pale colour" of glacial runoff in the Yukon River. The contraction is Ųųg Han , if the /ųų/ remains nasalized , or Yuk Han , if there is no vowel nasalization. In the 1840s, different tribes had different opinions as to the literal meaning of Yukon . In 1843, the Holikachuks had told

800-727: Is a volcanic field of over 60 cinder cones. Based on satellite images, it has been suggested that cinder cones might occur on other terrestrial bodies in the Solar system too; on the surface of Mars and the Moon. Stratovolcanoes (composite volcanoes) are tall conical mountains composed of lava flows and tephra in alternate layers, the strata that gives rise to the name. They are also known as composite volcanoes because they are created from multiple structures during different kinds of eruptions. Classic examples include Mount Fuji in Japan, Mayon Volcano in

880-503: Is accessible from the Yukon at Pitkas Point), and Mountain Village . After Mountain Village, the main Yukon channel frays into many channels, sprawling across the delta. There are a number of communities after the "head of passes," as the channel division is called locally: Nunum Iqua , Alakanuk , Emmonak , and Kotlik . Of those delta communities, Emmonak is the largest with roughly 760 people in

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960-467: Is also performed along the river in summer by barge , enabling heavy goods, oil, and vehicles to be transported to communities along the Yukon, Tanana, Innoko, and Koyukuk rivers. This service is performed by Ruby Marine and reaches Tanana on the Yukon River and Nenana on the Tanana River. Some of the upper slopes of this watershed (e.g. Nulato Hills ) are forested by Black Spruce . This locale near

1040-412: Is characterized by a rough, clinkery surface and is the typical texture of cooler basalt lava flows. Pāhoehoe is characterized by its smooth and often ropey or wrinkly surface and is generally formed from more fluid lava flows. Pāhoehoe flows are sometimes observed to transition to ʻaʻa flows as they move away from the vent, but never the reverse. More silicic lava flows take the form of block lava, where

1120-554: Is derived from the name of Vulcano , a volcanic island in the Aeolian Islands of Italy whose name in turn comes from Vulcan , the god of fire in Roman mythology . The study of volcanoes is called volcanology , sometimes spelled vulcanology . According to the theory of plate tectonics, Earth's lithosphere , its rigid outer shell, is broken into sixteen larger and several smaller plates. These are in slow motion, due to convection in

1200-567: Is expressed using the volcanic explosivity index (VEI), which ranges from 0 for Hawaiian-type eruptions to 8 for supervolcanic eruptions. As of December 2022 , the Smithsonian Institution 's Global Volcanism Program database of volcanic eruptions in the Holocene Epoch (the last 11,700 years) lists 9,901 confirmed eruptions from 859 volcanoes. The database also lists 1,113 uncertain eruptions and 168 discredited eruptions for

1280-524: Is one of the several tuyas in the area of the Tuya River and Tuya Range in northern British Columbia. Tuya Butte was the first such landform analysed and so its name has entered the geological literature for this kind of volcanic formation. The Tuya Mountains Provincial Park was recently established to protect this unusual landscape, which lies north of Tuya Lake and south of the Jennings River near

1360-466: Is typically low in silica, shield volcanoes are more common in oceanic than continental settings. The Hawaiian volcanic chain is a series of shield cones, and they are common in Iceland , as well. Lava domes are built by slow eruptions of highly viscous lava. They are sometimes formed within the crater of a previous volcanic eruption, as in the case of Mount St. Helens , but can also form independently, as in

1440-600: The Cascade Volcanoes or the Japanese Archipelago , or the eastern islands of Indonesia . Hotspots are volcanic areas thought to be formed by mantle plumes , which are hypothesized to be columns of hot material rising from the core-mantle boundary. As with mid-ocean ridges, the rising mantle rock experiences decompression melting which generates large volumes of magma. Because tectonic plates move across mantle plumes, each volcano becomes inactive as it drifts off

1520-741: The Earn River , also from the right. The river soon exits out of the canyon and winds across a plain, where it receives the Tummel River from the left. It then receives the Macmillan River , its largest tributary, from the right, then turns west, looping around the town of Pelly Crossing and crossing under the Klondike Highway , one of only two bridges on its course (the other is in Faro). The river continues west for about 25 km (16 mi) to merge with

1600-598: The Russian-American Company that their name for the river was Yukkhana and that this name meant big river . However, Yukkhana does not literally correspond to a Holikachuk phrase that means big river . Then, two years later, the Gwich'ins told the Hudson's Bay Company that their name for the river was Yukon and that the name meant white water river . White water river in fact corresponds to Gwich'in words that can be shortened to form Yukon . Because

1680-535: The Seward Peninsula represents the near westernmost limit of the Black Spruce, Picea mariana , one of the most widespread conifers in northern North America. The river flows into several parklands and refuges including: Yukon at Pilot Station (121 miles upstream of mouth) minimum, average and maximum discharge: for statistical calculation The Yukon River is home to one of the longest salmon runs in

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1760-651: The Takhini River ) and Kluane Lake (into the Kluane and then White River). The river passes through the communities of Whitehorse , Carmacks , (just before the Five Finger Rapids ) and Dawson City in Yukon , and crossing Alaska into Eagle , Circle , Fort Yukon , Stevens Village , Rampart , Tanana , Ruby , Galena , Nulato , Grayling , Holy Cross , Russian Mission , Marshall , Pilot Station , St. Marys (which

1840-541: The Teslin River , which has a larger flow when it reaches the Yukon at Hootalinqua. The upper end of the Yukon River was originally known as the Lewes River until it was established that it actually was the Yukon. North of Whitehorse, the Yukon River widens into Lake Laberge , made famous by Robert W. Service 's " The Cremation of Sam McGee ". Other large lakes that are part of the Yukon River system include Kusawa Lake (into

1920-569: The U.S. Geological Survey shows relatively good levels of turbidity, metals, and dissolved oxygen. The Yukon and Mackenzie rivers have much higher suspended sediment concentrations than the great Siberian Arctic rivers. The Yukon River Inter-Tribal Watershed Council , a cooperative effort of 70 First Nations and tribes in Alaska and Canada, has the goal of making the river and its tributaries safe to drink from again by supplementing and scrutinizing government data. The name Yukon , or ųųg han ,

2000-792: The core–mantle boundary , 3,000 kilometres (1,900 mi) deep within Earth. This results in hotspot volcanism , of which the Hawaiian hotspot is an example. Volcanoes are usually not created where two tectonic plates slide past one another. Large eruptions can affect atmospheric temperature as ash and droplets of sulfuric acid obscure the Sun and cool Earth's troposphere . Historically, large volcanic eruptions have been followed by volcanic winters which have caused catastrophic famines. Other planets besides Earth have volcanoes. For example, volcanoes are very numerous on Venus. Mars has significant volcanoes. In 2009,

2080-824: The landform and may give rise to smaller cones such as Puʻu ʻŌʻō on a flank of Kīlauea in Hawaii. Volcanic craters are not always at the top of a mountain or hill and may be filled with lakes such as with Lake Taupō in New Zealand. Some volcanoes can be low-relief landform features, with the potential to be hard to recognize as such and be obscured by geological processes. Other types of volcano include cryovolcanoes (or ice volcanoes), particularly on some moons of Jupiter , Saturn , and Neptune ; and mud volcanoes , which are structures often not associated with known magmatic activity. Active mud volcanoes tend to involve temperatures much lower than those of igneous volcanoes except when

2160-416: The 2000 census. Emmonak's gravel airstrip is the regional hub for flights. Navigational obstacles on the Yukon River are the Five Finger Rapids and Rink Rapids downstream from Carmacks. Despite its length, there are only four vehicle-carrying bridges across the river, listed from upstream to downstream: A car ferry crosses the river at Dawson City in the summer; it is replaced by an ice bridge over

2240-508: The Alaska Department of Fish & Game's Division of Commercial Fisheries banned all fishing of Chinook and chum salmon, including for subsistence. Various organizations are involved to protect healthy salmon runs into the future. The Yukon River Drainage Fisheries Association was formed in 1990 by a consensus of fishers representing the entire drainage in response to recent disaster years. Its organizational goals include giving voice to

2320-678: The Holikachuks had been trading regularly with both the Gwich’ins and the Yup'iks , the Holikachuks were in a position to borrow the Gwich'in contraction and to conflate its meaning with the meaning of Kuig-pak [River-big], which is the Yup'ik name for the same river. For that reason, the documentary evidence suggests that the Holikachuks had borrowed the contraction Ųųg Han [White Water River] from Gwich'in, and erroneously assumed that this contraction had

2400-588: The Lapie River from the left and passes the community of Faro which can be reached by a bridge. After Faro, it runs south of Rose Mountain and receives the Glenlyon River from the left. At the Glenlyon River confluence, the valley narrows and the walls grow higher and steeper, and a short distance later, receives the swift-flowing Tay River from the right. Several kilometres after, it turns north and receives

2480-723: The Pelly in Pelly Crossing (where it crosses the Klondike Highway) and in Faro, as well as a cable ferry at Ross River on the Canol Road . The river's average discharge is about 700 m/s (25,000 cu ft/s) and it drains about 49,000 km (19,000 sq mi) of land. Because the river is fed primarily by glacier melt, the highest average flow is reached around June or July at up to 1,600 m/s (57,000 cu ft/s), and

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2560-612: The Philippines, and Mount Vesuvius and Stromboli in Italy. Ash produced by the explosive eruption of stratovolcanoes has historically posed the greatest volcanic hazard to civilizations. The lavas of stratovolcanoes are higher in silica, and therefore much more viscous, than lavas from shield volcanoes. High-silica lavas also tend to contain more dissolved gas. The combination is deadly, promoting explosive eruptions that produce great quantities of ash, as well as pyroclastic surges like

2640-609: The USGS defines a volcano as active whenever subterranean indicators, such as earthquake swarms , ground inflation, or unusually high levels of carbon dioxide or sulfur dioxide are present. The USGS defines a dormant volcano as any volcano that is not showing any signs of unrest such as earthquake swarms, ground swelling, or excessive noxious gas emissions, but which shows signs that it could yet become active again. Many dormant volcanoes have not erupted for thousands of years, but have still shown signs that they may be likely to erupt again in

2720-584: The Yukon River near old Fort Selkirk . One of two major headwaters of the Yukon River (the other is the Stewart River ), the Pelly River's drainage basin , measured above the town of Pelly Crossing, is 49,000 km (19,000 sq mi) in size. It drains a large part of the sparsely-populated Yukon Plateau of central Yukon west of the Mackenzie Mountains. The Tintina Trench, which the majority of

2800-557: The Yukon include set gillnets , drift nets , dip nets , and fish wheels . The preference of certain gear is largely dependent on the river's varied characteristics in different areas. Some parts of the river do not have eddies to make set-nets successful, whereas in other places the tributaries are small enough to make drifting impractical. Over the last 20 years salmon recruitment, the number of returning adults, has taken several shocks. The late 1980s, 1990s, and 2000s have been marked by radically reduced runs for various salmon species. In

2880-691: The border. The agreement is implemented through the Yukon River Panel, an international body of 12 members, equal-parts American and Canadian, that advises managers of Yukon River fisheries concerning restoration, conservation, and coordinated management. Tribal organizations such as the Association of Village Council Presidents (AVCP), Council of Athabascan Tribal Governments (CATG), and Tanana Chiefs Conference (TCC) work to sustain Yukon River salmon to promote healthy people, cultures, and communities. The main river and tributaries are (sorted in order from

2960-503: The boundary with the Yukon Territory . Mud volcanoes (mud domes) are formations created by geo-excreted liquids and gases, although several processes may cause such activity. The largest structures are 10 kilometres in diameter and reach 700 meters high. The material that is expelled in a volcanic eruption can be classified into three types: The concentrations of different volcanic gases can vary considerably from one volcano to

3040-402: The case of Lassen Peak . Like stratovolcanoes, they can produce violent, explosive eruptions, but the lava generally does not flow far from the originating vent. Cryptodomes are formed when viscous lava is forced upward causing the surface to bulge. The 1980 eruption of Mount St. Helens was an example; lava beneath the surface of the mountain created an upward bulge, which later collapsed down

3120-455: The current, with an estimated 35–50% bound for Canada. As a result, Chinook salmon are noted for their especially rich and fatty meat and are the priciest of all Pacific salmon species. The villages along the Yukon have historically relied on and continue to rely on salmon for their cultural, subsistence, and commercial needs. Salmon are traditionally dried, smoked, and frozen for both human and sled dog consumption. Common methods of fishing on

3200-545: The effects of climate change on ocean food-supply & disease prevalence in returning adults, the methods of fishing used on the river, and the effects of the Bering Sea Pollock trawl fleet on food supply and salmon bycatch. In 2010, the Alaska Department of Fish & Game's Board of Fisheries issued the first-ever restriction for net mesh size on the Yukon, reducing it to 7.5 inches (190 mm). In 2021,

3280-772: The enormous area they cover, and subsequent concealment under vegetation and glacial deposits, supervolcanoes can be difficult to identify in the geologic record without careful geologic mapping . Known examples include Yellowstone Caldera in Yellowstone National Park and Valles Caldera in New Mexico (both western United States); Lake Taupō in New Zealand; Lake Toba in Sumatra , Indonesia; and Ngorongoro Crater in Tanzania. Volcanoes that, though large, are not large enough to be called supervolcanoes, may also form calderas in

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3360-431: The flow is covered with angular, vesicle-poor blocks. Rhyolitic flows typically consist largely of obsidian . Tephra is made when magma inside the volcano is blown apart by the rapid expansion of hot volcanic gases. Magma commonly explodes as the gas dissolved in it comes out of solution as the pressure decreases when it flows to the surface . These violent explosions produce particles of material that can then fly from

3440-592: The formation of a submarine volcano off the coast of Mayotte . Subglacial volcanoes develop underneath ice caps . They are made up of lava plateaus capping extensive pillow lavas and palagonite . These volcanoes are also called table mountains, tuyas , or (in Iceland) mobergs. Very good examples of this type of volcano can be seen in Iceland and in British Columbia . The origin of the term comes from Tuya Butte , which

3520-618: The frozen river during the winter. Plans to build a permanent bridge were announced in March 2004, although they are currently on hold because bids came in much higher than budgeted. There are also two pedestrian-only bridges in Whitehorse, as well as a dam across the river and a hydroelectric generating station. The construction of the dam flooded the White Horse Rapids , which gave the city its name, and created Schwatka Lake . Transportation

3600-568: The future. In an article justifying the re-classification of Alaska's Mount Edgecumbe volcano from "dormant" to "active", volcanologists at the Alaska Volcano Observatory pointed out that the term "dormant" in reference to volcanoes has been deprecated over the past few decades and that "[t]he term "dormant volcano" is so little used and undefined in modern volcanology that the Encyclopedia of Volcanoes (2000) does not contain it in

3680-461: The glossaries or index", however the USGS still widely employs the term. Previously a volcano was often considered to be extinct if there were no written records of its activity. Such a generalization is inconsistent with observation and deeper study, as has occurred recently with the unexpected eruption of the Chaitén volcano in 2008. Modern volcanic activity monitoring techniques, and improvements in

3760-492: The interior of a continent and lead to rifting. Early stages of rifting are characterized by flood basalts and may progress to the point where a tectonic plate is completely split. A divergent plate boundary then develops between the two halves of the split plate. However, rifting often fails to completely split the continental lithosphere (such as in an aulacogen ), and failed rifts are characterized by volcanoes that erupt unusual alkali lava or carbonatites . Examples include

3840-475: The lowest is in December or January as low as 35 m/s (1,200 cu ft/s). Volcanoes near the Pelly River, such as Volcano Mountain , may have once partly blocked or at least altered the Pelly River. Any future activity in this area could disrupt the course of the river and could have a serious impact on people living or working downstream. Yukon River The longest river in Alaska and Yukon, it

3920-419: The mid-oceanic ridge is above sea level, volcanic islands are formed, such as Iceland . Subduction zones are places where two plates, usually an oceanic plate and a continental plate, collide. The oceanic plate subducts (dives beneath the continental plate), forming a deep ocean trench just offshore. In a process called flux melting , water released from the subducting plate lowers the melting temperature of

4000-545: The modelling of the factors that produce eruptions, have helped the understanding of why volcanoes may remain dormant for a long time, and then become unexpectedly active again. The potential for eruptions, and their style, depend mainly upon the state of the magma storage system under the volcano, the eruption trigger mechanism and its timescale. For example, the Yellowstone volcano has a repose/recharge period of around 700,000 years, and Toba of around 380,000 years. Vesuvius

4080-415: The most dangerous type, are very rare; four are known from the last million years , and about 60 historical VEI 8 eruptions have been identified in the geologic record over millions of years. A supervolcano can produce devastation on a continental scale, and severely cool global temperatures for many years after the eruption due to the huge volumes of sulfur and ash released into the atmosphere. Because of

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4160-597: The mountains'. The river was later named by Robert Campbell in honour of Sir John Henry Pelly , governor of the Hudson's Bay Company . The restored Hudson's Bay Company trading post of Fort Selkirk is at the juncture of the Pelly and Yukon Rivers. The Pelly rises in glaciers on the western slopes of the Selwyn Mountains above 1,400 m (4,600 ft) in elevation, close to the Yukon- Northwest Territories boundary. It flows south and west through

4240-653: The mouth heading upstream): tributary tributary (km) (km ) (m /s) (distributary) Teslin Period: 1971–2000 Volcano A volcano is a rupture in the crust of a planetary-mass object , such as Earth , that allows hot lava , volcanic ash , and gases to escape from a magma chamber below the surface. The process that forms volcanoes is called volcanism . On Earth, volcanoes are most often found where tectonic plates are diverging or converging , and because most of Earth's plate boundaries are underwater, most volcanoes are found underwater. For example,

4320-438: The mud volcano is actually a vent of an igneous volcano. Volcanic fissure vents are flat, linear fractures through which lava emerges. Shield volcanoes, so named for their broad, shield-like profiles, are formed by the eruption of low-viscosity lava that can flow a great distance from a vent. They generally do not explode catastrophically but are characterized by relatively gentle effusive eruptions . Since low-viscosity magma

4400-452: The next. Water vapour is typically the most abundant volcanic gas, followed by carbon dioxide and sulfur dioxide . Other principal volcanic gases include hydrogen sulfide , hydrogen chloride , and hydrogen fluoride . A large number of minor and trace gases are also found in volcanic emissions, for example hydrogen , carbon monoxide , halocarbons , organic compounds, and volatile metal chlorides. The form and style of an eruption of

4480-867: The north side of the mountain. Cinder cones result from eruptions of mostly small pieces of scoria and pyroclastics (both resemble cinders, hence the name of this volcano type) that build up around the vent. These can be relatively short-lived eruptions that produce a cone-shaped hill perhaps 30 to 400 metres (100 to 1,300 ft) high. Most cinder cones erupt only once and some may be found in monogenetic volcanic fields that may include other features that form when magma comes into contact with water such as maar explosion craters and tuff rings . Cinder cones may form as flank vents on larger volcanoes, or occur on their own. Parícutin in Mexico and Sunset Crater in Arizona are examples of cinder cones. In New Mexico , Caja del Rio

4560-631: The northern end of the Chilkoot Trail . Either way, Atlin Lake flows into Tagish Lake (via the Atlin River), as eventually does Lake Lindeman after flowing into Bennett Lake . Tagish Lake then flows into Marsh Lake (via the Tagish River). The Yukon River proper starts at the northern end of Marsh Lake, just south of Whitehorse . Some argue that the source of the Yukon River should really be Teslin Lake and

4640-444: The ocean surface as new islands or floating pumice rafts . In May and June 2018, a multitude of seismic signals were detected by earthquake monitoring agencies all over the world. They took the form of unusual humming sounds, and some of the signals detected in November of that year had a duration of up to 20 minutes. An oceanographic research campaign in May 2019 showed that the previously mysterious humming noises were caused by

4720-420: The ocean surface, due to the rapid cooling effect and increased buoyancy in water (as compared to air), which often causes volcanic vents to form steep pillars on the ocean floor. Hydrothermal vents are common near these volcanoes, and some support peculiar ecosystems based on chemotrophs feeding on dissolved minerals. Over time, the formations created by submarine volcanoes may become so large that they break

4800-471: The ocean's surface. In the deep ocean basins, the tremendous weight of the water prevents the explosive release of steam and gases; however, submarine eruptions can be detected by hydrophones and by the discoloration of water because of volcanic gases . Pillow lava is a common eruptive product of submarine volcanoes and is characterized by thick sequences of discontinuous pillow-shaped masses which form underwater. Even large submarine eruptions may not disturb

4880-409: The one that destroyed the city of Saint-Pierre in Martinique in 1902. They are also steeper than shield volcanoes, with slopes of 30–35° compared to slopes of generally 5–10°, and their loose tephra are material for dangerous lahars . Large pieces of tephra are called volcanic bombs . Big bombs can measure more than 1.2 metres (4 ft) across and weigh several tons. A supervolcano is defined as

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4960-425: The overlying mantle wedge, thus creating magma . This magma tends to be extremely viscous because of its high silica content, so it often does not reach the surface but cools and solidifies at depth . When it does reach the surface, however, a volcano is formed. Thus subduction zones are bordered by chains of volcanoes called volcanic arcs . Typical examples are the volcanoes in the Pacific Ring of Fire , such as

5040-418: The plume, and new volcanoes are created where the plate advances over the plume. The Hawaiian Islands are thought to have been formed in such a manner, as has the Snake River Plain , with the Yellowstone Caldera being part of the North American plate currently above the Yellowstone hotspot . However, the mantle plume hypothesis has been questioned. Sustained upwelling of hot mantle rock can develop under

5120-471: The purchase of Alaska by the United States in 1867, the Alaska Commercial Company acquired the assets of the Russian-American Company and constructed several posts at various locations on the Yukon River. The Yukon River has a recent history of pollution from military installations, dumps, wastewater, and other sources. However, the Environmental Protection Agency does not list the Yukon River among its impaired watersheds, and water-quality data from

5200-437: The river's waters flow in, is the northernmost extension of the Rocky Mountain Trench , which stretches well south to British Columbia . The river is navigable by small and medium-sized craft for over 320 km (200 mi), from its mouth to Hoole Rapids, except for a shallow stretch of the river in Bradens Canyon. The Yukon communities of Ross River, Faro and Pelly Crossing are all on the Pelly River. There are bridges across

5280-420: The same literal meaning as the corresponding Yup'ik name Kuig-pak [River-big]. The Lewes River is the former name of the upper course of the Yukon, from Marsh Lake to the confluence of the Pelly River at Fort Selkirk . The generally accepted source of the Yukon River is the Llewellyn Glacier at the southern end of Atlin Lake in British Columbia . Others suggest that the source is Lake Lindeman at

5360-435: The same time interval. Volcanoes vary greatly in their level of activity, with individual volcanic systems having an eruption recurrence ranging from several times a year to once in tens of thousands of years. Volcanoes are informally described as erupting , active , dormant , or extinct , but the definitions of these terms are not entirely uniform among volcanologists. The level of activity of most volcanoes falls upon

5440-429: The same way; they are often described as "caldera volcanoes". Submarine volcanoes are common features of the ocean floor. Volcanic activity during the Holocene Epoch has been documented at only 119 submarine volcanoes, but there may be more than one million geologically young submarine volcanoes on the ocean floor. In shallow water, active volcanoes disclose their presence by blasting steam and rocky debris high above

5520-622: The solidified erupted material that makes up the mantle of a volcano may be stripped away that its inner anatomy becomes apparent. Using the metaphor of biological anatomy , such a process is called "dissection". Cinder Hill , a feature of Mount Bird on Ross Island , Antarctica , is a prominent example of a dissected volcano. Volcanoes that were, on a geological timescale, recently active, such as for example Mount Kaimon in southern Kyūshū , Japan , tend to be undissected. Eruption styles are broadly divided into magmatic, phreatomagmatic, and phreatic eruptions. The intensity of explosive volcanism

5600-446: The summer of 2023, the U.S. Fish and Wildlife Service counted 58,500 Chinook salmon, the second lowest season on record. The United States Department of Commerce issued a Federal Disaster Declaration for the 2008 and 2009 Commercial Chinook Yukon River fisheries, calling for the complete closure of commercial fishing along with restrictions on subsistence fishing. The root cause of these poor returns remains debated, with questions about

5680-406: The surrounding areas, and initially not seismically monitored before its unanticipated and catastrophic eruption of 1991. Two other examples of volcanoes that were once thought to be extinct, before springing back into eruptive activity were the long-dormant Soufrière Hills volcano on the island of Montserrat , thought to be extinct until activity resumed in 1995 (turning its capital Plymouth into

5760-434: The theory of plate tectonics. For example, some volcanoes are polygenetic with more than one period of activity during their history; other volcanoes that become extinct after erupting exactly once are monogenetic (meaning "one life") and such volcanoes are often grouped together in a geographical region. At the mid-ocean ridges , two tectonic plates diverge from one another as hot mantle rock creeps upwards beneath

5840-444: The thinned oceanic crust . The decrease of pressure in the rising mantle rock leads to adiabatic expansion and the partial melting of the rock, causing volcanism and creating new oceanic crust. Most divergent plate boundaries are at the bottom of the oceans, and so most volcanic activity on Earth is submarine, forming new seafloor . Black smokers (also known as deep sea vents) are evidence of this kind of volcanic activity. Where

5920-431: The underlying ductile mantle , and most volcanic activity on Earth takes place along plate boundaries, where plates are converging (and lithosphere is being destroyed) or are diverging (and new lithosphere is being created). During the development of geological theory, certain concepts that allowed the grouping of volcanoes in time, place, structure and composition have developed that ultimately have had to be explained in

6000-525: The village fishers that have traditionally managed these resources, enabling communication between fishers and fishery managers, and helping to preserve the ecological integrity of salmon runs and local cultures' Traditional Ecological Knowledge In March 2001, the U.S. & Canadian governments passed the Yukon River Salmon Agreement to better manage an internationally shared resource and ensure that more Canadian-originated salmon return across

6080-492: The volcano. Solid particles smaller than 2 mm in diameter ( sand-sized or smaller) are called volcanic ash. Tephra and other volcaniclastics (shattered volcanic material) make up more of the volume of many volcanoes than do lava flows. Volcaniclastics may have contributed as much as a third of all sedimentation in the geologic record. The production of large volumes of tephra is characteristic of explosive volcanism. Through natural processes, mainly erosion , so much of

6160-424: The volcanoes of the East African Rift . A volcano needs a reservoir of molten magma (e.g. a magma chamber), a conduit to allow magma to rise through the crust, and a vent to allow the magma to escape above the surface as lava. The erupted volcanic material (lava and tephra) that is deposited around the vent is known as a volcanic edifice , typically a volcanic cone or mountain. The most common perception of

6240-436: The world. Each year Chinook , coho , and chum salmon return to their terminal streams in Alaska, the Yukon Territory , and British Columbia. As salmon do not eat during their spawning migration, Yukon River salmon must have great reserves of fat and energy to fuel their thousands-mile-long journey. The Chinook, which arrive at the mouth of the Yukon River in early June, have the longest journey – as many as 2,000 miles against

6320-421: Was described by Roman writers as having been covered with gardens and vineyards before its unexpected eruption of 79 CE , which destroyed the towns of Herculaneum and Pompeii . Accordingly, it can sometimes be difficult to distinguish between an extinct volcano and a dormant (inactive) one. Long volcano dormancy is known to decrease awareness. Pinatubo was an inconspicuous volcano, unknown to most people in

6400-458: Was one of the principal means of transportation during the 1896–1903 Klondike Gold Rush . A portion of the river in Yukon—"The Thirty Mile" section, from Lake Laberge to the Teslin River —is a national heritage river and a unit of Klondike Gold Rush International Historical Park . Paddle-wheel riverboats continued to ply the river until the 1950s, when the Klondike Highway was completed. After

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