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28-644: The 280 kilometres (174 mi) long West Road River rises in the Ilgachuz Range and flows east to its confluence with the Fraser River between Prince George and Quesnel . It drains an area of approximately 12,000 km and loses over 900 m elevation before joining the Fraser. The Ilgachuz Range began erupting 6.1 million years ago and has grown steadily since then. Like all of the Anahim volcanoes,

56-448: A concentrated dispersion of interacting pyroclasts and partly trapped gas). The former type are sometimes called pyroclastic surges (even though they may be sustained rather than "surging") and lower parts of the latter are sometimes termed pyroclastic flows (these, also, can be sustained and quasi steady or surging). As they travel, pyroclastic density currents deposit particles on the ground, and they entrain cold atmospheric air, which

84-469: A diameter of 25 km (16 mi). The Ilgachuz Range was created by two chemically separate magmatic periods; an early complex series of trachyte and rhyolite eruptions, and late extrusion of a sequence of basaltic lava flows. Evacuation of the volcano's magma chamber resulted in the failure of one or more centrally located calderas . It is divided into Precaldera, Dome Forming, Intra Caldera and Shield Forming assemblages. The Precaldera Assemblage

112-466: A lower unit of microsyenite with red and green glassy zenocrysts. The Intra Caldera Assemblage is best exposed on the north edge of the caldera. The lower unit, indicative of caldera formation, is an epiclastic boulder-landslide deposit, roughly bedded and dipping into the caldera. Similar material, grading up into finer debris flows and possibly lahars , has been uncertainly known in the gap between Phacelia Peak and Calliope Mountain suggesting this area

140-410: A pyroclastic rock by cementation or chemical reactions as the result of the passage of hot gases ( fumarolic alteration) or groundwater (e.g. hydrothermal alteration and diagenesis ) and burial, or, if it is emplaced at temperatures so hot that the soft glassy pyroclasts stick together at point contacts, and deform: this is known as welding . One of the most spectacular types of pyroclastic deposit

168-606: A temperature of over 1200 °C). The hotspot has existed for at least 13 million years, and the Anahim Volcanic Belt stretches almost 600 kilometres (400 mi) away from the hotspot. More recently, the hotspot has formed the Itcha Range and Nazko Cone , a cinder cone east of the Ilgachuz Range and the youngest Anahim volcano. The Ilgachuz Range is the largest of these, although the Rainbow Range

196-490: A variety of pyroclastic rock that forms from volcanic steam explosions and they are entirely made of accidental clasts. 'Phreatomagmatic' pyroclastic deposits are formed from explosive interaction of magma with groundwater . The word pyroclastic is derived from the Greek πῦρ , meaning fire; and κλαστός , meaning broken. Unconsolidated accumulations of pyroclasts are described as tephra . Tephra may become lithified to

224-591: A volcanic caldera. Pyroclasts include juvenile pyroclasts derived from chilled magma, mixed with accidental pyroclasts, which are fragments of country rock . Pyroclasts of different sizes are classified (from smallest to largest) as volcanic ash , lapilli , or volcanic blocks (or, if they exhibit evidence of having been hot and molten during emplacement, volcanic bombs ). All are considered to be pyroclastic because they were formed (fragmented) by volcanic explosivity, for example during explosive decompression, shear, thermal decrepitation , or by attrition and abrasion in

252-585: A volcanic conduit, volcanic jet, or pyroclastic density current. Pyroclasts are transported in two main ways: in atmospheric eruption plumes, from which pyroclasts settle to form topography-draping pyroclastic fall layers, and by pyroclastic density currents (PDCs) (including pyroclastic flows and pyroclastic surges ), from which pyroclasts are deposited as pyroclastic density current deposits, which tend to thicken and coarsen in valleys, and thin and fine over topographic highs. During Plinian eruptions , pumice and ash are formed when foaming silicic magma

280-414: Is an ignimbrite , which is the deposit of a ground-hugging pumiceous pyroclastic density current (a rapidly flowing hot suspension of pyroclasts in gas). Ignimbrites may be loose deposits or solid rock, and they can bury entire landscapes. An individual ignimbrite can exceed 1000 km in volume, can cover 20,000 km of land, and may exceed 1 km in thickness, for example where it is ponded within

308-399: Is best exposed on the east side of Pipe Organ Mountain where it contains a bedded pile over 300 m thick of weakly consolidated, moderately to extremely different, pyroclastics , flows and deposits of uncertain origin. Colours range from mottled green to grey , yellow , ochre , red and white . A green tuffbreccia composed of pumice fragments, feldspar crystals and minor debris

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336-440: Is extremely remote and unroaded; the closest communities are Anahim Lake , Alexis Creek , Nimpo Lake , Redstone , and Nazko . The closest major centre is Quesnel , located approximately 200 km east of the park. Peaks within the Ilgachuz Range include: West Road River The West Road River or Blackwater River or Tiyakoh is an important tributary of the Fraser River , flowing generally north-eastward from

364-467: Is fragmented in the volcanic conduit, because of rapid shear driven by decompression and the growth of microscopic bubbles. The pyroclasts are then entrained with hot gases to form a supersonic jet that exits the volcano, admixes and heats cold atmospheric air to form a vigorously buoyant eruption column that rises several kilometers into the stratosphere and cause aviation hazards . Particles fall from atmospheric eruption plumes and accumulate as layers on

392-720: Is recognizable in several areas. The Dome Forming Assemblages include most of the rhyolite domes, related flows and the Ilgachuz Comendite. The northerly domes are subcircular talus mounds of plate sized pieces of light to dark grey, slightly porphyritic, flowbanded rhyolite with minor obsidian . Massive to banded chalcedony blobs and veinlets are related with these domes. The southern domes are somewhat different in nature, comprising intrusive and extrusive phases of cream colored porphyries. The Sax Dome contains an upper portion of cream coloured, aphanitic to fine quartz porphyry felsite with abnormal green glass filled fractures, and

420-519: Is the largest of all volcanoes in the Anahim Volcanic Belt. The first recorded ascent of the Ilgachuz Range was by the South Carrier and Chilcotin tribes. They have lived in the area for hundreds of years, travelling when necessary to hunt and trap animals such as beaver , caribou , moose , and to gather plants and roots . Fishing camps were also established in the area. The Ilgachuz Range is, or was, an important source of obsidian for

448-474: Is the reserve of the Nazko First Nation . Pyroclastic rock Pyroclastic rocks are clastic rocks composed of rock fragments produced and ejected by explosive volcanic eruptions. The individual rock fragments are known as pyroclasts . Pyroclastic rocks are a type of volcaniclastic deposit, which are deposits made predominantly of volcanic particles. 'Phreatic' pyroclastic deposits are

476-426: Is the southern edge of the caldera. Alternatively, largely unsorted breccia and debris deposits exist on the ridge north of Saxifraga Peak , possibly indicating the main, or a subsidiary, caldera edge. The Shield Forming Assemblage contains a series of basalt and minor comendite eruptions, and is best exposed on Far Mountain and Mount Scot . The basalts issued from fissure vents primarily located peripheral to

504-427: Is then heated and thermally expands. Where the density current becomes sufficiently dilute to loft, it rises into the atmosphere as a 'phoenix plume' (or 'co-PDC plume'). These phoenix plumes typically deposit thin ashfall layers that may contain little pellets of aggregated fine ash. Hawaiian eruptions such as those at Kīlauea produce an upward-directed jet of hot droplets and clots of magma suspended in gas; this

532-524: The Coast from Bella Coola . Red ochre used in paint and decoration was also obtained from this area. The Ilgachuz Range is the second largest shield volcano in the Anahim Volcanic Belt which includes other immediately nearby ranges, the Rainbow Range and Itcha Range . It stands at 2,410 metres (7,907 ft) above sea level - slightly shorter than its neighbor, Rainbow Range. It has

560-545: The Fraser Plateau. The river is 280 km (170 mi) long, draining an area of approximately 12,000 km (4,600 sq mi), and dropping over 900 m (3,000 ft) before joining with the Fraser. The river is of significant historical importance to both First Nations and Canadian history. For centuries, the Dakelh (Carrier) and Tsilhqot'in peoples used a trail—the so-called " Grease Trail "—on

588-587: The Ilgachuz Range has its origins in the Anahim hotspot —a plume of magma rising from the Earth's mantle in central British Columbia . The hotspot remains in a fixed position, while the North American Plate drifts over it at a rate of 2 to 3.3 centimetres per year. The upwelling of the hot magma creates volcanoes, and each individual volcano erupts for a few million years before the movement of

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616-536: The South Carrier and Chilcotin tribes. Obsidian was greatly desired because very sharp arrowheads and cutting knives could be made from it. Like all glass and some other types of naturally occurring rocks, obsidian breaks with a characteristic conchoidal fracture , creating razorlike edges. It was also used for jewelry . Anahim obsidian was traded extensively throughout the BC Interior and up and down

644-622: The calderas. Brick red cinder deposits are considered to be a late phase of this assemblage. Surrounding and including the range is Itcha Ilgachuz Provincial Park , a 112,000-hectare park of unique landscape in the West Chilcotin Uplands while the Rainbow Range lies partly in the Tweedsmuir South Provincial Park . The park includes volcanic landforms, alpine environments, and forest sites scattered with wetlands . Itcha Ilgachuz Provincial Park

672-450: The ground, which are described as fallout deposits. Pyroclastic density currents arise when the mixture of hot pyroclasts and gases is denser than the atmosphere and so, instead of rising buoyantly, it spreads out across the landscape. They are one of the greatest hazards at a volcano, and may be either 'fully dilute' (dilute, turbulent ash clouds, right down to their lower levels) or 'granular fluid based' (the lower levels of which comprise

700-479: The northern side of the river in their trade with coastal First Nations communities. The name Grease Trail refers to one of the main commodities transported along the route— eulachon grease, a highly prized staple, traces of which coated parts of the route after centuries of use. It was this trail that Sir Alexander Mackenzie used in his historic overland journey west to the Pacific Ocean in 1793, traversing

728-877: The northern slopes of the Ilgachuz Range and across the Fraser Plateau in the Chilcotin region of central British Columbia , Canada . With only one major tributary, the Nazko River ("river flowing from the south" in the Carrier language ), its confluence with the Fraser is approximately 40 km northwest of Quesnel . It forms the division between the Chilcotin Plateau (S) and the Nechako Plateau (N), which are subdivisions of

756-442: The plate carries it away from the rising magma. However, where hotspots occur under continental crust , basaltic magma is trapped in the less dense continental crust, which is heated and melts to form rhyolites . Due to their high content of crystals and gasses, rhyolites initiate violent eruptions, though their water content is low and they have a relatively low temperature (around 800-900 °C when erupted, whereas basalt can have

784-531: The river itself on his return. He named the river in 1793. The West Road (Blackwater) River has been designated as a heritage river by the government of British Columbia. The two major settlements along the river are the Kluskus First Nation and Ulkatcho First Nation (Alexis Family). Near the river to its south, on the Nazko, is the settlement of Nazko , a ranching community, the focus of which

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