{{Short description|Volcanic arc system in the Pacific Northwest}} [[File:Coast Range Arc map.jpg|thumb|right|300px|Image of the Pacific Northwest. Red indicates the inferred boundaries of the Coast Range Arc.]] The '''Coast Range Arc''' was a large [[volcanic arc]] system, extending from northern [[Washington (U.S. state)|Washington]] through [[British Columbia]] and the [[Alaska Panhandle]] to southwestern [[Yukon]].<ref name="KO">{{cite book | last = Stowell | first = Harold H. |author2=McClelland William C. | title = Tectonics of the Coast Mountains, Southeastern Alaska and British Columbia | date = January 2000 |publisher = [[Geological Society of America]] |pages = 101 |url = https://books.google.com/books?id=5SgAthT0MuAC&q=tectonics+of+the+Coast+Mountains+southeastern+Alaska+and+British+Columbia |isbn = 0-8137-2343-4 |accessdate =2008-09-04}}</ref> The Coast Range Arc lies along the western margin of the [[North American Plate]] in the [[Pacific Northwest]] of western [[North America]]. Although taking its name from the [[Coast Mountains]], this term is a geologic grouping rather than a geographic one, and the Coast Range Arc extended south into the [[Cascade Range|High Cascades]] of the [[Cascade Range]], past the [[Fraser River]] which is the northward limit of the Cascade Range proper.
==Geology== ===Evolution=== [[File:Late Cretaceous Coast Mountains plate tectonics.jpg|thumb|right|300px|Plate tectonics of the Coast Range Arc about 75 million years ago]] Volcanism in the arc began during the [[Late Cretaceous]] period 100 million years ago based on [[andesite|andesitic]] composition of the [[Early Cretaceous]] volcanic sections and their close temporal and spatial association with masses of [[felsic]] [[Intrusive rock|intrusive]] [[igneous rock]] with [[phaneritic]] texture called [[tonalite]].<ref name="KO"/> The basement of the Coast Range Arc was likely Early Cretaceous and [[Late Jurassic]] intrusions.<ref name="KO"/> Stratigraphic and field relations in the arc suggest that the Coast Range Arc was created on [[Stikinia]], a geologic feature that formed in an older volcanic arc environment during the [[Paleozoic]] and [[Mesozoic]] periods.<ref name="KO"/>
[[File:North America subduction.jpg|thumb|left|250px|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]] developed near the center of the oceanic [[Farallon Plate]]. This rifting event created the oceanic [[Kula Plate]].<ref name="SI"/> It is unknown why such a large rupture of the Farallon Plate occurred.<ref name="SI"/> Some geologists believe some fundamental change in convection within the Earth's [[mantle (geology)|mantle]] caused the rifting event, while others believe the huge oceanic plate became mechanically unstable as it continued to subduct beneath the Pacific Northwest.<ref name="SI"/> The Kula Plate once again continued to subduct beneath the continental margin, supporting the Coast Range Arc.<ref name="SI"/>
Volcanism began to decline along the length of the arc about 60 million years ago during the early [[Paleogene]] period of the [[Cenozoic]] era as the rapid northern movement of the Kula Plate became parallel with the Pacific Northwest, creating a [[transform fault|transform fault plate boundary]] similar to the [[Queen Charlotte Fault]].<ref name="KO"/><ref name="SI"/> During this passive plate boundary, the Kula Plate began subducting underneath [[Alaska]] and southwestern [[Yukon]] at the northern end of the arc during the early [[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]] called the [[Bennett Lake Volcanic Complex]] about 50 million years ago during the early [[Eocene]] period. These eruptions were from vents along arcuate fracture systems associated with the caldera, which discharged about {{convert|850|km3}} of pyroclastic material.<ref>{{cite book | last = Lambert | first = Maurice B. | pages = [https://archive.org/details/volcanoes0000lamb/page/39 39] | title = Volcanoes | year = 1978 | publisher = [[Energy, Mines and Resources Canada]] | location = [[North Vancouver (district municipality)|North Vancouver]], [[British Columbia]] | isbn = 0-88894-227-3 | url = https://archive.org/details/volcanoes0000lamb/page/39 }}</ref> This volcanic event occurred shortly before nearly all the Kula Plate had been subducted beneath the North American Plate about 40 million years ago.
[[File:Juneau Icefield.jpg|thumb|right|250px|View of the [[Juneau Icefield]] and granite outcrops in the [[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]].<ref name="SI"/> What remains of the Coast Range Arc to this day are [[granite|granitic]] [[intrusion]]s, which were formed when magma intruded and cooled at depth beneath the volcanoes.<ref name="SI">{{cite web|publisher=Burke Museum of Natural History and Culture|url=http://www.washington.edu/burkemuseum/geo_history_wa/Coast%20Range%20Episode.htm |title=The Coast Range Episode (115 to 57 million years ago)|accessdate=2008-04-09}}</ref> However, remnants of some volcanoes exist in southwestern Yukon, including [[Montana Mountain]], [[Mount Nansen (Yukon)|Mount Nansen]], and the [[Bennett Lake Volcanic Complex|Bennett Lake]], [[Mount Skukum Volcanic Complex|Mount Skukum]] and [[Sifton Range volcanic complex]]es.
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.<ref name="SI"/> Here, these granites intruded highly deformed ocean rocks and assorted fragments from pre-existing island arcs, largely remnants of the ancient [[Bridge River Ocean]] which lay between North America and the pre-existing [[Insular Islands]].<ref name="SI"/> Massive amounts of molten granite injected over this period, burning the old oceanic sediments into a glittering medium-grade [[metamorphic rock]] called [[schist]].<ref name="SI"/>
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]].<ref name="SI"/> 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]], appearing to have been nearly melted in the procedure.<ref name="SI"/> The remarkable migmatite of the [[Chelan, Washington|Chelan]] and [[Skagit County, Washington|Skagit]] areas in [[Washington (U.S. state)|Washington]] are well known in geologic circles.<ref name="SI"/> During construction of intrusions 70 and 57 million years ago, the northern motion of the [[Kula Plate]] might have been between {{convert|140|and(-)|110|mm}} per year.<ref name="KC">{{cite web|publisher=[[University of Arizona]]|url=https://www.geo.arizona.edu/tectonics/Ducea/Batholiths/Tectonics.htm|title=Tectonic overview of the CPC|accessdate=2008-09-07}}</ref> However, other geologic studies determined the Kula Plate moved at a rate as fast as {{convert|200|mm}} per year.<ref name="KC"/>
===Geological importance=== [[File:North America subduction1.jpg|thumb|right|250px|Plate distribution 55 million years ago. Arrows represent direction of subduction along North America.]] Intrusions of the Coast Range Arc are intruded by widespread [[basalt]]ic [[dike (geology)|dikes]]. These dikes, although not voluminous, provide an important sampling of the post-arc [[lithosphere]]. Additionally, widespread [[volcanic belt]]s, such as the [[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]].<ref>{{cite web|title=Catalogue of Canadian volcanoes- Anahim volcanic belt |work=Natural Resources Canada |publisher=[[Geological Survey of Canada]] |date=2008-02-13 |url=http://gsc.nrcan.gc.ca/volcanoes/cat/belt_anahim_e.php |accessdate=2008-06-14 |url-status=dead |archiveurl=https://web.archive.org/web/20080615214715/http://gsc.nrcan.gc.ca/volcanoes/cat/belt_anahim_e.php |archivedate=2008-06-15 }}</ref> During its formation, it lay beneath granitic intrusions of the Coast Range Arc. The approximately {{convert|20|km|0|adj=on}} long Bella Bella and approximately {{convert|6|km|0|adj=on}} long Gale Passage [[dike swarm]]s 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.<ref name=QW>{{cite book | last = Ernst | first = Richard E.|author2=Kenneth L. Buchan | title = Mantle Plumes: Their Identification Through Time (Special Paper (Geological Society of America))|publisher= [[Geological Society of America]]|date=June 2001|pages= 261 | url = https://books.google.com/books?id=X4W9aGXDa9cC&q=anahim+volcanic+belt&pg=PA261 | isbn = 978-0-8137-2352-5|accessdate= 2007-11-17}}</ref>
==See also== *[[Coast Mountains]] *[[Geology of British Columbia]] *[[Geology of the Pacific Northwest]] *[[Cascade Volcanoes]] *[[Cascadia subduction zone]] *[[Volcanism in Canada]] *[[Pacific Ring of Fire]]
==References== {{reflist}}
[[Category:Continental arcs]] [[Category:Cascade Range]] [[Category:Coast Mountains]] [[Category:Volcanism of British Columbia]] [[Category:Volcanism of Washington (state)]] [[Category:Volcanism of Yukon]] [[Category:Volcanism of Alaska]] [[Category:Cretaceous System of North America]] [[Category:Cretaceous volcanism]] [[Category:Eocene volcanism]]