| Ilgachuz Range | |
|---|---|
 Satellite image of the Ilgachuz Range | |
| Highest point | |
| Peak | Far Mountain |
| Elevation | 2,410 m (7,910 ft) |
| Coordinates | 52°47′12.1″N125°19′23.9″W / 52.786694°N 125.323306°W /52.786694; -125.323306 |
| Geography | |
 | |
| Country | Canada |
| Province | British Columbia |
| District | Range 3 Coast Land District |
| Range coordinates | 52°46′N125°18′W / 52.767°N 125.300°W /52.767; -125.300 |
| Parent range | Chilcotin Plateau |
| Borders on | Rainbow and Itcha Range |
| Geology | |
| Formed by | Shield volcano |
| Orogeny | Anahim hotspotvolcanism |
| Rock age | 6.1 to 4.0 million years |
TheIlgachuz Range is a name given to anextinctshield volcano inBritish Columbia, Canada. It is not amountain range in the normal sense, because it was formed as a single volcano that has beeneroded for the past 5 million years. It lies on theChilcotin Plateau, located some 350 kilometres (220 mi) north-northwest ofVancouver and 30 km north ofAnahim Lake. The highest peak of the range isFar Mountain. The range supports a uniquegrassland 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 theInterior Plateau.
The 280 kilometres (174 mi) longWest Road River rises in the Ilgachuz Range and flows east to its confluence with theFraser River betweenPrince George andQuesnel. It drains an area of approximately 12,000 km2 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, the Ilgachuz Range has its origins in theAnahim hotspot—a plume ofmagma rising from the Earth'smantle in centralBritish Columbia. The hotspot remains in a fixed position, while theNorth 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 undercontinental crust, basaltic magma is trapped in the less dense continental crust, which is heated and melts to formrhyolites. 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 theItcha Range andNazko Cone, acinder cone east of the Ilgachuz Range and the youngest Anahim volcano. The Ilgachuz Range is the largest of these, although theRainbow Range is the largest of all volcanoes in the Anahim Volcanic Belt.
The first recorded ascent of the Ilgachuz Range was by theSouth Carrier andChilcotin tribes. They have lived in the area for hundreds of years, travelling when necessary to hunt and trap animals such asbeaver,caribou,moose, and to gatherplants androots. Fishing camps were also established in the area.
The Ilgachuz Range is, or was, an important source ofobsidian for the South Carrier and Chilcotin tribes. Obsidian was greatly desired because very sharparrowheads and cuttingknives could be made from it. Like allglass and some other types of naturally occurring rocks, obsidian breaks with a characteristicconchoidal fracture, creating razorlike edges. It was also used forjewelry. Anahim obsidian was traded extensively throughout the BC Interior and up and down the Coast fromBella Coola.Red ochre used inpaint and decoration was also obtained from this area.
The Ilgachuz Range is the second largestshield volcano in theAnahim Volcanic Belt which includes other immediately nearby ranges, theRainbow Range andItcha 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 oftrachyte andrhyolite eruptions, and late extrusion of a sequence ofbasaltic lava flows.[1] Evacuation of the volcano'smagma chamber resulted in the failure of one or more centrally locatedcalderas. It is divided into Precaldera, Dome Forming, Intra Caldera and Shield Forming assemblages.
The Precaldera Assemblage is best exposed on the east side ofPipe 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 mottledgreen togrey,yellow,ochre,red andwhite. A greentuffbreccia composed ofpumice fragments,feldsparcrystals and minordebris is recognizable in several areas.[1]
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 minorobsidian. 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 possiblylahars, has been uncertainly known in the gap betweenPhacelia Peak and Calliope Mountain suggesting this area is the southern edge of the caldera. Alternatively, largely unsortedbreccia and debris deposits exist on the ridge north ofSaxifraga Peak, possibly indicating the main, or a subsidiary, caldera edge.[1]
The Shield Forming Assemblage contains a series ofbasalt and minor comendite eruptions, and is best exposed onFar Mountain andMount Scot. The basalts issued fromfissure vents primarily located peripheral to the calderas. Brick red cinder deposits are considered to be a late phase of this assemblage.[1]
Surrounding and including the range isItcha Ilgachuz Provincial Park, a 112,000-hectare park of unique landscape in the West Chilcotin Uplands while theRainbow Range lies partly in theTweedsmuir South Provincial Park. The park includes volcanic landforms, alpine environments, andforest sites scattered withwetlands.
Itcha Ilgachuz Provincial Park is extremely remote and unroaded; the closest communities areAnahim Lake,Alexis Creek,Nimpo Lake,Redstone, andNazko. The closest major centre isQuesnel, located approximately 200 km east of the park.
Peaks within the Ilgachuz Range include: