Ilgachuz Range

The Ilgachuz Range is a name given to an extinct shield volcano in British Columbia, Canada. It is not a mountain range in the normal sense, because it was formed as a single volcano that has been eroded for the past 5 million years. It lies on the Chilcotin Plateau, located some 350 kilometres (220 mi) north-northwest of Vancouver and 30 km north of Anahim Lake. The highest peak of the range is Far Mountain. The range supports a unique grassland ecosystem. This type of grassland has not been seen anywhere else in central and southern British Columbia. The climate is cool and dry; typical of higher elevations of the Interior Plateau.

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 km2 and loses over 900 m elevation before joining the Fraser.

Ilgachuz Range
Ilgachuz Range
Satellite image of the Ilgachuz Range
Highest point
PeakFar Mountain
Elevation2,410 m (7,910 ft)
Coordinates52°47′12.1″N 125°19′23.9″W / 52.786694°N 125.323306°W
Geography
CountryCanada
ProvinceBritish Columbia
DistrictRange 3 Coast Land District
Range coordinates52°46′N 125°18′W / 52.767°N 125.300°WCoordinates: 52°46′N 125°18′W / 52.767°N 125.300°W
Parent rangeChilcotin Plateau
Borders onRainbow and Itcha Range
Geology
Formed byShield volcano
OrogenyAnahim hotspot volcanism
Age of rock6.1 to 4.0 million years

Geology and history

Origins

Anahim Volcanic Belt-en
Location of Anahim hotspot in millions of years ago, including the Anahim Volcanic Belt

The Ilgachuz Range began erupting 6.1 million years ago and has grown steadily since then. Like all of the Anahim volcanoes, 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 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 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 is the largest of all volcanoes in the Anahim Volcanic Belt.

First Nations

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 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 the Coast from Bella Coola. Red ochre used in paint and decoration was also obtained from this area.

Structure

Ilgachuz Range southern
Southern flank of the Ilgachuz Range

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 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.[1] 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 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 is recognizable in several areas.[1]

Ilgachuz Range northern
Smooth northern flanks of the Ilgachuz Range are remnants of the original shield volcano

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 a lower unit of microsyenite with red and green glassy zenocrysts.[1]

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 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.[1]

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 the calderas. Brick red cinder deposits are considered to be a late phase of this assemblage.[1]

Provincial Park

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 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.

Subsidiary peaks

Peaks within the Ilgachuz Range include:

  • Calliope Mountain
  • Camlick Mountain
  • Campanula Peak
  • Cindercone Peak
  • Crepis Peak
  • Far Mountain
  • Go-around Mountain
  • Hump Mountain
  • Mizzen Mountain
  • Mount Scot
  • Phacelia Peak
  • Pipe Organ Mountain
  • Saxifraga Mountain
  • Tundra Mountain

See also

References

  1. ^ a b c d e Report on Preliminary Geology and Geochemistry of the Ilga Claim group Archived 2008-09-10 at the Wayback Machine Retrieved on 2007-10-11
  • "Ilgachuz Range". BC Geographical Names.

External links

Anahim Peak

Anahim Peak, sometimes mistakenly called Anaheim, is a volcanic cone in the Anahim Volcanic Belt in British Columbia, Canada, located 39 km (24 mi) northwest of Anahim Lake and 11 km (7 mi) east of Tsitsutl Peak. It was formed when the North American Plate moved over a hotspot, similar to the one feeding the Hawaiian Islands, called the Anahim hotspot. It is one of the several volcanoes in the Anahim Volcanic Belt that stands out all by itself, rising from the Chilcotin Plateau, between the Rainbow Range and the Ilgachuz Range and near the headwaters of the Dean River.

Anahim Volcanic Belt

The Anahim Volcanic Belt is a 600 km (373 mi) long volcanic belt, stretching from just north of Vancouver Island to near Quesnel, British Columbia, Canada. The Anahim Volcanic Belt has had three main magmatic episodes: 15–13 Ma, 9–6 Ma, and 3–1 Ma. The volcanoes generally become younger eastward at a rate of 2 cm (0.8 in) to 3.3 cm (1.3 in) a year. The Nazko Cone, which last erupted only 7,200 years ago, is the youngest Anahim volcano. These volcanoes are thought to have formed as a result of the North American Plate sliding westward over a long-lived center of upwelling magma called the Anahim hotspot. The hotspot is thought to be similar to the one feeding the Hawaiian Islands.

Future volcanism is most likely in the form of basaltic cinder cones, but eruptions of less mafic magma, typical of the eastern portions of the belt, cannot be ruled out. A series of earthquakes began October 9, 2007 in the vicinity of Nazko Cone which was related to intense subterranean volcanic activity in the area.

The volcanic belt is defined by 37 Quaternary basalt centers and three large shield volcanoes called the Rainbow Range, Ilgachuz Range and the Itcha Range. These three large volcanoes have built up dome-like piles of lava and fragmental rocks to a height of 8,130 feet (2,478 m) at Tsitsutl Peak in the Rainbow Range, 7,873 feet (2,400 m) at Far Mountain in the Ilgachuz Range, and 7,760 feet (2,365 m) at Mount Downton in the Itcha Range. The Rainbow Range is a low dome-like cone about 20 miles (32 km) diameter, with Anahim Peak an obsidian plug on its north-east flank. The Ilgachuz Range is 15 miles (24 km) or more in diameter, and the Itcha Range is 10 miles (16 km) wide and about 40 miles (64 km) long. All have been dissected by late Tertiary, pre-Pleistocene stream erosion.

Major volcanoes of the Anahim Volcanic Belt include:

Ilgachuz Range, 2,410 metres (7,907 ft)

Itcha Range, 2,368 metres (7,769 ft)

Rainbow Range, 2,478 metres (8,130 ft)

Baldface Mountain

Baldface Mountain is a conical butte-like summit in the West-Central Interior of British Columbia, Canada. It is east of Itcha Lake and northeast of the community of Anahim Lake.This mountain is a volcanic cone consisting of trachyte and phonolite. Polygenetic in nature, it was active 2.52 to 2.37 million years ago.

Bella Bella and Gale Passage dike swarms

The Bella Bella and Gale Passage dike swarms are two parallel dike swarms on the Central Coast of British Columbia, Canada. They range in age from 14.5 to 12.5 million years old. They are both chemically bimodal, consisting of rocks such as basalt, trachyte and comendite. They form the westernmost extent of the Anahim Volcanic Belt on Athlone Island, Dufferin Island and Denny Island.The Bella Bella and Gale Passage dike swarms are petrographically similar to the shield complexes in the central Anahim Volcanic Belt. As a result, the swarms are thought to represent the roots of a peralkaline magma system in which they are the magma conduits connecting the underlying magma chamber to the volcanic centre at the surface, which has been extensively eroded to remnants of eruptive breccia.

Chilcotin Plateau

The Chilcotin Plateau is part of the Fraser Plateau, a major subdivision of the Interior Plateau of British Columbia. The Chilcotin Plateau is physically near-identical with the region of the same name, i.e. "the Chilcotin", which lies between the Fraser River and the southern Coast Mountains and is defined by the basin of the Chilcotin River and so includes montane areas beyond the plateau. East of the Chilcotin Plateau, across the Fraser River, is the Cariboo Plateau, while to the north beyond the West Road (Blackwater) River is the Nechako Plateau. West and south of the Chilcotin Plateau are various subdivisions of the Coast Mountains, including the Chilcotin Ranges which lie along the plateau's southwest.

Included within the definition of the Chilcotin Plateau are the Rainbow Range, near Bella Coola and the similarly volcanic Ilgachuz Range and Itcha Range both of which are major shield volcanoes. The Camelsfoot Range, north of Lillooet, is included in the Chilcotin Plateau by some systems of classification.

The plateau is nearly entirely drained by the Chilcotin River and its tributaries, the largest of which are the Chilanko and Chilko Rivers. Also draining the plateau on its eastern edge is Churn Creek, which forms the east flank of the Camelsfoot Range and enters the Fraser directly. On the west side of the plateau, the basins of the Dean, Homathko and Atnarko Rivers penetrate the massifs of the Coast Mountains and have their beginnings, or the early part of their courses, on the Chilcotin Plateau.

The Chilcotin Plateau consists of basaltic lava of the Chilcotin Group, a group of related volcanic rocks that is nearly parallel with the Fraser Plateau. It extends along the adjacent Garibaldi Volcanic Belt in the Coast Mountains. Volcanism of the Chilcotin Plateau is considered to be a result of extension of the crust behind the coastal Cascadia subduction zone.

Far Mountain

Far Mountain is the highest of over 13 peaks in the Ilgachuz Range in the Anahim Volcanic Belt in British Columbia, Canada. The Ilgachuz Range is one of the three major shield volcanoes that formed the Anahim Volcanic Belt when the North American Plate moved over a hotspot (the Anahim hotspot). This is similar to the one which feeds the Hawaiian Islands. The mountain is located in the western part of Itcha Ilgachuz Provincial Park.

Festuca Pass

Festuca Pass is a mountain pass through the Ilgachuz Range in the West-Central Interior of British Columbia, Canada. Located in a valley west of Carnlick Creek between Calliope Mountain and Pipe Organ Mountain, Festuca Pass is the only named pass in the Ilgachuz Range. It was named by Roy Taylor of UBC Botanical Gardens after Festuca altaica, a grass species that is present in the pass.

Itcha Ilgachuz Provincial Park

Itcha Ilgachuz Provincial Park is a provincial park in the Chilcotin Country of British Columbia, Canada. The park is 111,977 hectares in size and contains Far Mountain and Mount Downton, its two most prominent peaks.

Itcha Mountain

Itcha Mountain is one of the two named volcanic peaks of the Itcha Range, which is located in the Chilcotin District of the Central Interior of British Columbia, Canada. It is in the Anahim Volcanic Belt, which formed when the North American Plate moved over a hotspot, similar to the one feeding the Hawaiian Islands. The Anahim Volcanic Belt includes other immediately nearby ranges, the Rainbow and Ilgachuz Ranges. Itcha Mountain is located 42 km (26 mi) northeast of Anahim Lake and 2 km (1 mi) northeast of Mount Downton, another peak of the Itcha Range. Both of these peaks are located within Itcha Ilgachuz Provincial Park, as is Far Mountain, the park's highest peak.

This mountain is the namesake for the Itcha Range, which is one of the several large shield volcanoes that stands all by itself.

Itcha Range

The Itcha Range, also known as the Itchas, is a small isolated mountain range in the West-Central Interior of British Columbia, Canada. It is located 40 km (25 mi) northeast of the community of Anahim Lake. With a maximum elevation of 2,375 m (7,792 ft), it is the lowest of three mountain ranges on the Chilcotin Plateau extending east from the Coast Mountains. Two mountains are named in the Itcha Range; Mount Downton and Itcha Mountain. A large provincial park surrounds the Itcha Range and other features in its vicinity. More than 15 animal species are known to exist in the Itcha Range area, as well as a grassland community that is limited only to this location of British Columbia. The Itcha Range is within territory which has been occupied by aboriginal peoples for millennia. This area has a relatively dry environment compared to the Coast Mountains in the west.

In contrast to most mountain ranges in British Columbia, the Itcha Range represents an inactive shield volcano. This highly dissected volcanic edifice consists of a variety of rock types, including basanite, hawaiite, trachyte, rhyolite, phonolite and alkali olivine basalt. They were deposited by different types of volcanic eruptions characterized by passive lava flows and explosivity. Two stages of eruptive activity have been identified at the volcano along with three sub-phases that are limited only to the first stage of development. The main body of the Itcha Range is between 3.8 and 3.0 million years old and thus over two million years ago it passed the most active shield stage of life. A period of dormancy lasting for almost a million years followed, which was interrupted by the post-shield stage of volcanism 2.2 to 0.8 million years ago. More recent volcanic activity in and around the Itcha Range might have occurred in the last 340,000 years to produce cinder cones.

The Itcha Range is part of an east-west trending volcanic zone called the Anahim Volcanic Belt. This consists of large shield volcanoes, small cinder cones, lava domes and lava flows that become progressively younger from west to east. Several explanations have been made regarding the creation of this feature, each citing a different geologic process. If volcanic activity were to resume at the Itcha Range, Canada's Interagency Volcanic Event Notification Plan (IVENP) is prepared to notify people threatened by eruptions.

King Island Pluton

The King Island Pluton is a mid-to-late Miocene intrusion of syenite and alkali granite on the Central Coast of British Columbia, Canada. It is over 20 km (12 mi) long and 2.5 km (1.6 mi) wide, extending from King Island in the west to the mainland in the east. The pluton is exposed in 1,000 m (3,300 ft) cliffs along the Dean and Burke channels west of Bella Coola.The King Island Pluton is petrographically similar to the shield complexes in the central Anahim Volcanic Belt. As a result, the pluton is thought to represent the magma chamber of an extinct volcanic centre that has since eroded away. At the time of its formation, the pluton was emplaced 2 to 5 km (1.2 to 3.1 mi) below the surface.

Mount Downton

Mount Downton is the highest summit of the 10 km (6 mi) diameter Itcha Range, located 40 km (25 mi) northeast of Anahim Lake and 33 km (21 mi) east of Far Mountain in the Chilcotin District of the Central Interior of British Columbia, Canada. It lies within Itcha Ilgachuz Provincial Park.

Nazko Cone

Nazko Cone is a small potentially active basaltic cinder cone in central British Columbia, Canada, located 75 km west of Quesnel and 150 kilometers southwest of Prince George. It is considered the easternmost volcano in the Anahim Volcanic Belt. The small tree-covered cone rises 120 m above the Chilcotin-Nechako Plateau and rests on glacial till. It was formed in three episodes of activity, the first of which took place during the Pleistocene interglacial stage about 340,000 years ago. The second stage produced a large hyaloclastite scoria mound erupted beneath the Cordilleran Ice Sheet during the Pleistocene. Its last eruption produced two small lava flows that traveled 1 km to the west, along with a blanket of volcanic ash that extends several km to the north and east of the cone.

Rainbow Range (Chilcotin Plateau)

The Rainbow Range, formerly known as the Rainbow Mountains, is a mountain range in British Columbia, Canada, located 40 kilometres (25 mi) northwest of Anahim Lake. Located on the western edge of the Chilcotin Plateau, the range adjoins the Coast Mountains Pacific Ranges to the south, and the Kitimat Ranges to the north. In some classification systems it is considered part of the Coast Mountains. It lies north of the Bella Coola and Atnarko Rivers and south and west of the Dean River, which curves around its north flank, and is relatively drier in climate and easier of terrain than more mountainous areas immediately west.

Once called Tsitsutl, meaning "rainbow mountains" in the Ulkatcho dialect of the Carrier language, that name is now the name of the range's highest peak.

Satah Mountain

Satah Mountain is a cinder cone, located 35 km (22 mi) east-northeast of Nimpo Lake in central British Columbia, Canada. It areas to the south form a N-S-trending chain of cinder cones of Pleistocene and Holocene age. It lies on the Chilcotin Plateau.

Satah Mountain occupies the high point of the Satah Mountain volcanic field. This consists of a long ridge of trachytic lava domes and flows and basaltic and trachybasaltic cinder cones that extend south from the felsic Itcha Range volcanic complex.

Tsitsutl Peak

Tsitsutl Peak is the highest volcanic peak of the Rainbow Range in British Columbia, Canada, located within Tweedsmuir South Provincial Park, 43 km (27 mi) northwest of Anahim Lake and 44 km (27 mi) northeast of Thunder Mountain.

West Road River

The West Road River or Blackwater River is an important tributary of the Fraser River, flowing generally north-eastward from 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 the Fraser Plateau.

The river is 280 km (170 mi) long, draining an area of approximately 12,000 km2 (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 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 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 only major settlements along the river are the Indian reserves of the Kluskus First Nation in the area of the Kluskus Lakes, which are a chain of lakes midway along its length. Near the river to its south, on the Nazko, is the settlement of Nazko, a ranching community, the focus of which is the reserve of the Nazko First Nation.

Whitetop Mountain (British Columbia)

Whitetop Mountain is a forested hill in the West-Central Interior of British Columbia, Canada. It is on the northwest side of junction of the Chilcotin River and Downton Creek. Whitetop is a volcanic cone of the Chilcotin Plateau and Anahim Volcanic Belt.

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