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 northeast of the community of Anahim Lake. With a maximum elevation of , 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.
Geography
Location and terrain The Itcha Range is situated on the northern Chilcotin Plateau, a subdivision of the Fraser Plateau which in turn is one of the main subdivisions of the large Interior Plateau. It is bordered on the west by the Ilgachuz Range, another mountain range on the Chilcotin Plateau. The Itcha Range is located within one of British Columbia's many territorial divisions known as the Range 3 Coast Land District.
Stream erosion has played a significant role in dissecting the mountain range, and many of its peaks are dotted with glaciers. This dissection has resulted in a variety of landforms, such as valleys, crags and domes. Small graveled creeks flow from alpine mountains out onto the regional prairies where there are pale blue rock-bottomed lakes, including the so-named Itcha Lake. Three streams drain the Itcha Range, namely Corkscrew Creek, Downton Creek and Shag Creek. Although the Itcha Range has been dissected by stream erosion and subsequently glaciated, its original shape has been largely preserved. Rocks in the Itcha Range are a variety of colours, including red, white and yellow.
Climate The climate of the Itcha Range is influenced by the presence of the Coast Mountains to the west, which disrupt the flow of the prevailing westerly winds and causes them to drop most of their moisture on the western slopes of the Coast Mountains before reaching the Interior Plateau, casting a rain shadow over the Itcha Range.
In contrast to the Coast Mountains, the Interior Plateau experiences a precipitation peak in the summer months, reflecting the influence of summer convective storms, and most of winter precipitation falls as snow. The mean annual precipitation for the area ranges from about whereas the mean annual temperature is approximately , with a summer mean of and a winter mean of .
Flora and fauna The Itcha Range area is home to a grassland ecosystem that has not been found elsewhere in southern or central British Columbia. It is dominated by communities of Altai fescue and lichen. Extensive and diverse alpine and subalpine vegetation is also present in the area, some species of which are at the northernmost or southernmost of their range.
Several animal species inhabit around the Itcha Range. Among them are cougars, wolves, grizzly bears, black bears, moose, mule deers, mountain goats, beavers, coyotes, red foxes, muskrats, martens, river otters, lynx and wolverines. Also present is the largest herd of woodland caribou in southern British Columbia, as well as the most northerly population of California bighorn sheep in North America.
Geology Background A number of mechanisms have been proposed to interpret the origin of volcanism in the Anahim Volcanic Belt. This includes rift propagation and melting of mantle associated with lithospheric fracturing due to flexuring of the crust along the northern edge of the subducting Juan de Fuca Plate. However, insignificant evidence exists to support these hypotheses. The most common and best mechanism used to explain Anahim Belt volcanic activity is a stationary hotspot. This is supported by a well-defined progression in age of volcanism from west to east along the belt that compares well with the age trend of the Yellowstone hotspot track. The North American Plate moves westward over the hotspot at a rate of between per year. Nazko Cone, a volcanic cone east of the Itcha Range, is centred near the Anahim hotspot.
The Anahim hotspot is underlain by a low-velocity anomaly that extends approximately into the mantle north of the Juan de Fuca slab. However, this low-velocity anomaly may extend deeper southward beneath the Juan de Fuca slab. Coupled with the well documented temporal progression of surface volcanism, this has led to the conclusion that the Anahim hotspot is supplied by a mantle plume over slab edge flow. A small high-velocity anomaly east of Nazko Cone marks the eastern extent of the Anahim hotspot track.
Magmatism in the Anahim Volcanic Belt can be traced as far back as 10–14 million years ago with the emplacement of dike swarms and plutons as well as the eruption of rhyolite flows and breccias on the British Columbia Coast. The slow continuous westward motion of the North American Plate during the late Neogene positioned the Anahim hotspot further east at the Chilcotin Plateau where volcanic activity built the Rainbow Range shield volcano 8.7 to 6.7 million years ago. Volcanism then shifted eastwards, in displacement contrary with the North American Plate's movement, 6.1 million years ago to construct the Ilgachuz Range shield volcano. Renewed volcanic activity southeast of the Ilgachuz Range starting 3.5 million years ago led to the creation of the Itcha Range, the youngest of the three Anahim shield volcanoes. The Itcha Range continued to be an area of Anahim hotspot volcanism well into the Quaternary period (2.58 million years ago to present).
Structure The Itcha Range is the smallest shield volcano in the Anahim Volcanic Belt in terms of area covered. Unlike the Rainbow and Ilgachuz ranges, the Itcha Range is composed of small coalescing volcanic units rather than a stratiform volcanic pile. It is, in many respects, similar to the small alkaline shields found in Kenya and Ethiopia along the East African Rift. About 60% of the shield is exposed whereas roughly 40% of it remains buried under glacial drift deposits. This suggests that the Itcha Range was glaciated repeatedly during the Pleistocene epoch. Glacial striae on polished surfaces of some of the oldest volcanic rocks in the eastern part of the shield and the local presence of drift deposits throughout the stratigraphy indicate that glaciation and volcanism were contemporaneous through much of the volcanic history of the Itcha Range.
With a maximum elevation of , the Itcha Range is the lowest of the three Anahim shield volcanoes. Its highest point is Mount Downton, which is situated in the middle of the shield. Just to the northeast is Itcha Mountain, the second highest peak with an elevation of . These peaks are situated on top of the shield, which has a topographic prominence of about .
The Itcha Range has a broad, gently sloping structure typical of shield volcanoes. It is mainly composed of thick felsic lava flows that were erupted from a central vent. These are overlain by thick mafic alkaline lava flows and at least 30 small cinder cones. Hawaiite is the dominant rock type, but alkali olivine basalt and spinel lherzolite-bearing basanite is also present. They merge laterally with lavas of the much older Chilcotin Group, which surrounds the Anahim Volcanic Belt. However, the exact nature of the relationship between the Anahim Volcanic Belt and the Chilcotin Group is unknown.
Exposed in the middle of the mountain range is an assemblage of deformed andesite to dacite lava flows and volcanoclastic sediments. These basement rocks were created during the late Mesozoic era, long before the Itcha Range formed. They are similar to rocks found in the Hazelton Group, situated north of the Itcha Range, and the Ootsa Lake Group of the Intermontane Belt.
Volcanic history Two stages of volcanic activity constructed the Itcha Range. The first stage, referred to as the felsic shield-building stage, occurred between 3.8 and 3.0 million years ago. Three phases comprise this stage; a pre-explosive phase, an explosive phase and a post-explosive phase. Analysis of the first trachyte magmas to erupt during the pre-explosive phase suggest they were relatively fluid as shown by their areal extent. Eruptions became more viscous during the explosive phase, followed by more viscosity during the post-explosive phase. As a result, the volume of erupted material became smaller over time. The increasing viscousness of felsic lava during the shield-building stage suggests a maturing plumbing system underneath the Itcha Range, which may have consisted of multiple, isolated, cupola-type magma chambers.
A 900,000‑year‑long period of quiescence followed after the felsic shield-building stage, during which time erosion ate away at the shield's gentle slopes. This quiescence was followed by a mafic capping stage between 2.2 and 0.8 million years ago, but renewed activity might have occurred in the last 340,000 years. Alkali olivine basalts of the mafic capping stage were derived from the fractionation of a clinopyroxene, olivine and oxide assemblage. However, the associated hawaiite lavas may have derived from an alkali olivine basalt parent by the fractionation of a clinopyroxene-dominated assemblage at higher pressures. As volcanic activity waned during the mafic capping stage, lava flows became more viscous and decreased in volume. This suggests that parental magmas may have been derived by increasingly smaller degrees of partial melting with time. If volcanic activity of the Itcha Range is related to a hotspot, this temporal and spatial evolution would suggest a waning heat source.
The main body of the shield erupted over an area of about . Volcanic activity associated with the Itcha Range extended south to the Satah Mountain area where lavas erupted along a north-northwest trending fault system and covered an additional area of . Although the Satah Mountain volcanic field is not part of the Itcha Range, it is linked to the range by a volcanic ridge.
Felsic shield-building stage