Anahim hotspot

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The Anahim hotspot is marked 45 on map.

The Anahim hotspot is a Miocene-to-Holocene hotspot which is responsible for the volcanic activity which forms the volcanoes in central British Columbia, Canada.

1 Nature of the hotspot
2 Future
3 See also
4 References
5 External links

[edit] Nature of the hotspot

The plume, of which the Anahim hotspot is thought to be the surface expression, is thought to be similar to the hotspot that feeds the volcanoes of the Hawaiian Islands. About 13 and 12 million years ago, the Anahim hotspot created the ~20 km (12 mi) long Bella Bella and ~6 km (4 mi) long Gale Passage dike swarms. Individual dikes are as thick as 20 m (66 ft) and have basalt, trachyte, and comendite compositions. The converging dikes may mark the arrival of the Anahim hotspot.^[1] The small size of the swarm suggests that a miniplume may be responsible.^[1] During the early to middle Miocene, the Coast Mountains were favored thermally-driven uplift arising from the Miocene passage of the Anahim hotspot beneath the range, and response to convergence in late Miocene-Pliocene time.^[2] About 8 million years ago, as the North American Plate moved westward, it formed the Rainbow Range. About 5 million years ago it created the Ilgachuz Range. Finally, about 2.5 million years ago, it created the Itcha Range. Throughout the Pleistocene and Holocene epochs, the Anahim hotspot has fed activity to numerous volcanoes. Nazko Cone is the youngest of these, which is less then a million years old.^[3] The Rainbow Range is the largest Anahim volcano, although Nazko Cone is now the site of the most intense volcanic activity,^[4] located directly on top of the Anahim hotspot at 52°54′0″N, 123°44′0″W.^[5]^[6]

Image:Anahim Volcanic Belt-en.svg

The Anahim Volcanic Belt

Over the past 13 million years, the Anahim hotspot has created a belt of volcanoes called the Anahim Volcanic Belt, which stretches from the coast of British Columbia, to the Interior Plateau near Quesnel. The volcanoes generally get younger along a line from the coast to the interior. Each of the volcanoes was once over the hotspot, and migration of the North American Plate over the hotspot has pulled the volcanoes away from the hotspot's magmatic source at a rate of about 4.7 centimetres per year. As a result, the volcanoes are progressively older to the west. The supply and composition of magma to the volcanoes changes with time as the volcanoes grow over the hotspot and migrate away.

Analysis of the chemical composition of the lavas gives important clues about the source and dynamics of the hotspot plume. It has been erupting low-viscosity magmas to form the broad shield volcanoes of the Ilgachuz, Itcha and Rainbow ranges. 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. These rhyolites can be quite hot and form violent eruptions, despite their low water content. The Anahim hotspot appears to have a shield-forming event approximately every 3 million years, the last such event formed the Itcha Range about 2.5 million years ago.

The cycle of the eruption at Nazko Cone 7,200 years ago, started with an eruption of two different progressions of runny lava flows and an older, grey basalt overlain by a younger, darker black basaltic lava flow. The passive eruptions were followed by a period of explosive eruptions.^[3] This explosive activity built three overlying cinder cones that broke by the two lava flows near the end of the explosive phase of activity. The last phase of explosive activity spread tephra to the north and east of the cones. The deepest deposits near the cones is (>3 m) and thin to less than a few centimetres only a few kilometres away,^[3] which suggests that the explosive eruptions at Nazko Cone were fairly small. However, the last eruption from Nazko Cone could have started forest fires, since there is charcoal inside the tephra layer.^[3]

[edit] Future

Volcanism appears to have ceased in the western parts of the Anahim Volcanic Belt, but if the suggestion is correct, future volcanic activity can be expected in the area of Nazko Cone and east of it. Future activity from the Anahim hotspot is most likely in the form of basaltic cinder cones, but eruptions of less mafic magma, typical of the eastern sections of the Anahim Volcanic Belt, cannot be ruled out.

A series of <3.0 Magnitude earthquakes began October 9th, 2007 in the vicinity of Nazko Cone which could signal the resumption of intense subterraenean volcanic activity in the area. 34 such <3.0 Magnitude earthquakes were observed on October 10th, 2007 alone. Since than more than 1000 small earthquakes have been recorded.^[7] These earthquakes are thought to have originated 25 kilometers below the surface, but none of these earthquakes have been felt by people. The cause of this seismic activity is believed to be the upwelling of magma because the area is not close to any faults or tectonic plate boundaries.^[8] This is the first indication of potential volcanic activity in Canada in almost two centuries, the last of which happened around 1830 to 1850 in northwestern British Columbia.^[9] If an eruption were to occur, it would probably be similar to the small cinder cone building event 7,200 years ago at Nazko Cone.^[10]