# Coast Range Arc

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Volcanic arc system in the Pacific Northwest

Image of the Pacific Northwest. Red indicates the inferred boundaries of the Coast Range Arc.

The **Coast Range Arc** was a large [volcanic arc](/source/Volcanic_arc) system, extending from northern [Washington](/source/Washington_(U.S._state)) through [British Columbia](/source/British_Columbia) and the [Alaska Panhandle](/source/Alaska_Panhandle) to southwestern [Yukon](/source/Yukon).[1] The Coast Range Arc lies along the western margin of the [North American Plate](/source/North_American_Plate) in the [Pacific Northwest](/source/Pacific_Northwest) of western [North America](/source/North_America). Although taking its name from the [Coast Mountains](/source/Coast_Mountains), this term is a geologic grouping rather than a geographic one, and the Coast Range Arc extended south into the [High Cascades](/source/Cascade_Range) of the [Cascade Range](/source/Cascade_Range), past the [Fraser River](/source/Fraser_River) which is the northward limit of the Cascade Range proper.

## Geology

### Evolution

Plate tectonics of the Coast Range Arc about 75 million years ago

Volcanism in the arc began during the [Late Cretaceous](/source/Late_Cretaceous) period 100 million years ago based on [andesitic](/source/Andesite) composition of the [Early Cretaceous](/source/Early_Cretaceous) volcanic sections and their close temporal and spatial association with masses of [felsic](/source/Felsic) [intrusive](/source/Intrusive_rock) [igneous rock](/source/Igneous_rock) with [phaneritic](/source/Phaneritic) texture called [tonalite](/source/Tonalite).[1] The basement of the Coast Range Arc was likely Early Cretaceous and [Late Jurassic](/source/Late_Jurassic) intrusions.[1] Stratigraphic and field relations in the arc suggest that the Coast Range Arc was created on [Stikinia](/source/Stikinia), a geologic feature that formed in an older volcanic arc environment during the [Paleozoic](/source/Paleozoic) and [Mesozoic](/source/Mesozoic) periods.[1]

Plate distribution between 64 and 74 million years ago. Arrows represent direction of subduction along North America. Black represents *present-day* land area

One of the major events during the Coast Range Arc was about 85 million years ago when a huge [rift](/source/Rift) developed near the center of the oceanic [Farallon Plate](/source/Farallon_Plate). This rifting event created the oceanic [Kula Plate](/source/Kula_Plate).[2] It is unknown why such a large rupture of the Farallon Plate occurred.[2] Some geologists believe some fundamental change in convection within the Earth's [mantle](/source/Mantle_(geology)) caused the rifting event, while others believe the huge oceanic plate became mechanically unstable as it continued to subduct beneath the Pacific Northwest.[2] The Kula Plate once again continued to subduct beneath the continental margin, supporting the Coast Range Arc.[2]

Volcanism began to decline along the length of the arc about 60 million years ago during the early [Paleogene](/source/Paleogene) period of the [Cenozoic](/source/Cenozoic) era as the rapid northern movement of the Kula Plate became parallel with the Pacific Northwest, creating a [transform fault plate boundary](/source/Transform_fault) similar to the [Queen Charlotte Fault](/source/Queen_Charlotte_Fault).[1][2] During this passive plate boundary, the Kula Plate began subducting underneath [Alaska](/source/Alaska) and southwestern [Yukon](/source/Yukon) at the northern end of the arc during the early [Eocene](/source/Eocene) period.

The Coast Range Arc was home to some of the world's most dangerous and explosive volcanoes. Cataclysmic eruptions at the British Columbia–Yukon border created a huge nested [caldera](/source/Caldera) called the [Bennett Lake Volcanic Complex](/source/Bennett_Lake_Volcanic_Complex) about 50 million years ago during the early [Eocene](/source/Eocene) period. These eruptions were from vents along arcuate fracture systems associated with the caldera, which discharged about 850 cubic kilometres (200 cu mi) of pyroclastic material.[3] This volcanic event occurred shortly before nearly all the Kula Plate had been subducted beneath the North American Plate about 40 million years ago.

View of the [Juneau Icefield](/source/Juneau_Icefield) and granite outcrops in the [Boundary Ranges](/source/Boundary_Ranges) of the Coast Mountains

Since the end of the Coast Range Arc about 50 million years ago, many volcanoes have disappeared from [erosion](/source/Erosion).[2] What remains of the Coast Range Arc to this day are [granitic](/source/Granite) [intrusions](/source/Intrusion), which were formed when magma intruded and cooled at depth beneath the volcanoes.[2] However, remnants of some volcanoes exist in southwestern Yukon, including [Montana Mountain](/source/Montana_Mountain), [Mount Nansen](/source/Mount_Nansen_(Yukon)), and the [Bennett Lake](/source/Bennett_Lake_Volcanic_Complex), [Mount Skukum](/source/Mount_Skukum_Volcanic_Complex) and [Sifton Range volcanic complexes](/source/Sifton_Range_volcanic_complex).

Many granitic rocks of the Coast Range Arc are plentiful in the North Cascades of the Cascade Range, which is the southernmost boundary of the arc.[2] Here, these granites intruded highly deformed ocean rocks and assorted fragments from pre-existing island arcs, largely remnants of the ancient [Bridge River Ocean](/source/Bridge_River_Ocean) which lay between North America and the pre-existing [Insular Islands](/source/Insular_Islands).[2] Massive amounts of molten granite injected over this period, burning the old oceanic sediments into a glittering medium-grade [metamorphic rock](/source/Metamorphic_rock) called [schist](/source/Schist).[2]

The older intrusions of the Coast Range Arc were then deformed under the heat and pressure of later intrusions, turning them into a layered metamorphic rock known as [gneiss](/source/Gneiss).[2] In some places, mixtures of older intrusive rocks and the original oceanic rocks have been distorted and warped under intense heat, weight and stress to create unusual swirled patterns known as [migmatite](/source/Migmatite), appearing to have been nearly melted in the procedure.[2] The remarkable migmatite of the [Chelan](/source/Chelan%2C_Washington) and [Skagit](/source/Skagit_County%2C_Washington) areas in [Washington](/source/Washington_(U.S._state)) are well known in geologic circles.[2] During construction of intrusions 70 and 57 million years ago, the northern motion of the [Kula Plate](/source/Kula_Plate) might have been between 140 and 110 millimetres (5.5–4.3 in) per year.[4] However, other geologic studies determined the Kula Plate moved at a rate as fast as 200 millimetres (7.9 in) per year.[4]

### Geological importance

Plate distribution 55 million years ago. Arrows represent direction of subduction along North America.

Intrusions of the Coast Range Arc are intruded by widespread [basaltic](/source/Basalt) [dikes](/source/Dike_(geology)). These dikes, although not voluminous, provide an important sampling of the post-arc [lithosphere](/source/Lithosphere). Additionally, widespread [volcanic belts](/source/Volcanic_belt), such as the [Anahim Volcanic Belt](/source/Anahim_Volcanic_Belt), lie in the middle of the Coast Range Arc. Volcanics that form the Anahim Volcanic Belt are not strictly related to Coast Range Arc subduction, but might have formed as a result of the North American Plate sliding over a place that has experienced active volcanism for a long period of time which is described as the [Anahim hotspot](/source/Anahim_hotspot).[5] During its formation, it lay beneath granitic intrusions of the Coast Range Arc. The approximately 20-kilometre (12 mi) long Bella Bella and approximately 6-kilometre (4 mi) long Gale Passage [dike swarms](/source/Dike_swarm) lie in granitic intrusions of the Coast Range Arc and are used to calculate the first appearance of the Anahim hotspot about 13 and 12 million years ago.[6]

## See also

- [Coast Mountains](/source/Coast_Mountains)

- [Geology of British Columbia](/source/Geology_of_British_Columbia)

- [Geology of the Pacific Northwest](/source/Geology_of_the_Pacific_Northwest)

- [Cascade Volcanoes](/source/Cascade_Volcanoes)

- [Cascadia subduction zone](/source/Cascadia_subduction_zone)

- [Volcanism in Canada](/source/Volcanism_in_Canada)

- [Pacific Ring of Fire](/source/Pacific_Ring_of_Fire)

## References

1. ^ [***a***](#cite_ref-KO_1-0) [***b***](#cite_ref-KO_1-1) [***c***](#cite_ref-KO_1-2) [***d***](#cite_ref-KO_1-3) [***e***](#cite_ref-KO_1-4) Stowell, Harold H.; McClelland William C. (January 2000). [*Tectonics of the Coast Mountains, Southeastern Alaska and British Columbia*](https://books.google.com/books?id=5SgAthT0MuAC&q=tectonics+of+the+Coast+Mountains+southeastern+Alaska+and+British+Columbia). [Geological Society of America](/source/Geological_Society_of_America). p. 101. [ISBN](/source/ISBN_(identifier)) [0-8137-2343-4](https://en.wikipedia.org/wiki/Special:BookSources/0-8137-2343-4). Retrieved 2008-09-04.

1. ^ [***a***](#cite_ref-SI_2-0) [***b***](#cite_ref-SI_2-1) [***c***](#cite_ref-SI_2-2) [***d***](#cite_ref-SI_2-3) [***e***](#cite_ref-SI_2-4) [***f***](#cite_ref-SI_2-5) [***g***](#cite_ref-SI_2-6) [***h***](#cite_ref-SI_2-7) [***i***](#cite_ref-SI_2-8) [***j***](#cite_ref-SI_2-9) [***k***](#cite_ref-SI_2-10) [***l***](#cite_ref-SI_2-11) [***m***](#cite_ref-SI_2-12) ["The Coast Range Episode (115 to 57 million years ago)"](http://www.washington.edu/burkemuseum/geo_history_wa/Coast%20Range%20Episode.htm). Burke Museum of Natural History and Culture. Retrieved 2008-04-09.

1. **[^](#cite_ref-3)** Lambert, Maurice B. (1978). [*Volcanoes*](https://archive.org/details/volcanoes0000lamb/page/39). [North Vancouver](/source/North_Vancouver_(district_municipality)), [British Columbia](/source/British_Columbia): [Energy, Mines and Resources Canada](/source/Energy%2C_Mines_and_Resources_Canada). pp. [39](https://archive.org/details/volcanoes0000lamb/page/39). [ISBN](/source/ISBN_(identifier)) [0-88894-227-3](https://en.wikipedia.org/wiki/Special:BookSources/0-88894-227-3).

1. ^ [***a***](#cite_ref-KC_4-0) [***b***](#cite_ref-KC_4-1) ["Tectonic overview of the CPC"](https://www.geo.arizona.edu/tectonics/Ducea/Batholiths/Tectonics.htm). [University of Arizona](/source/University_of_Arizona). Retrieved 2008-09-07.

1. **[^](#cite_ref-5)** ["Catalogue of Canadian volcanoes- Anahim volcanic belt"](https://web.archive.org/web/20080615214715/http://gsc.nrcan.gc.ca/volcanoes/cat/belt_anahim_e.php). *Natural Resources Canada*. [Geological Survey of Canada](/source/Geological_Survey_of_Canada). 2008-02-13. Archived from [the original](http://gsc.nrcan.gc.ca/volcanoes/cat/belt_anahim_e.php) on 2008-06-15. Retrieved 2008-06-14.

1. **[^](#cite_ref-QW_6-0)** Ernst, Richard E.; Kenneth L. Buchan (June 2001). [*Mantle Plumes: Their Identification Through Time (Special Paper (Geological Society of America))*](https://books.google.com/books?id=X4W9aGXDa9cC&q=anahim+volcanic+belt&pg=PA261). [Geological Society of America](/source/Geological_Society_of_America). p. 261. [ISBN](/source/ISBN_(identifier)) [978-0-8137-2352-5](https://en.wikipedia.org/wiki/Special:BookSources/978-0-8137-2352-5). Retrieved 2007-11-17.

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Adapted from the Wikipedia article [Coast Range Arc](https://en.wikipedia.org/wiki/Coast_Range_Arc) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Coast_Range_Arc?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.