The Canadian Cascade Arc, also called the Canadian Cascades, is the Canadian segment of the North American Cascade Volcanic Arc. Located entirely within the Canadian province of British Columbia, it extends from the Cascade Mountains in the south to the Coast Mountains in the north. Specifically, the southern end of the Canadian Cascades begin at the Canada–United States border. However, the specific boundaries of the northern end are not precisely known and the geology in this part of the volcanic arc is poorly understood. It is widely accepted by geologists that the Canadian Cascade Arc extends through the Pacific Ranges of the Coast Mountains. However, others have expressed concern that the volcanic arc possibly extends further north into the Kitimat Ranges, another subdivision of the Coast Mountains, and even as far north as Haida Gwaii (formerly known as the Queen Charlotte Islands).
Over the last 29 million years, the Canadian Cascade Arc has been erupting a chain of volcanoes along the British Columbia Coast. At least four volcanic zones in British Columbia are related to Cascade Arc volcanism. This includes a large volcanic plateau in The Interior and three linear volcanic belts on The Coast. They were formed during different geological periods, separated by millions of years, and occur in three regions referred to as the back-arc, main-arc and fore-arc. The youngest of the three belts has been sporadically active over the last 4.0–3.0 million years, with the latest eruption having taken place possibly in the last 1,000 years. About 2,350 years ago, a major explosive eruption occurred, sending a massive ash column into the atmosphere. This is recognized as the largest volcanic eruption throughout Canada within the last 10,000 years.
In historical times, the Canadian Cascade Arc has been considerably less active than the American portion of the volcanic arc. It also has no records of historical eruptions. Nevertheless, the volcanic arc poses a threat to the surrounding region. Any volcanic hazard—ranging from landslides to eruptions—could pose a significant risk to humans and wildlife. Even though there are no historical eruptions in the Canadian Cascade Arc, eruptive activity is very likely to resume; if this were to happen, relief efforts would be quickly organized. Teams such as the Interagency Volcanic Event Notification Plan (IVENP) are prepared to notify people threatened by volcanic eruptions.
Geology
Formation
The Cascade Arc was originally created by subduction of the now vanished Farallon Plate at the Cascadia subduction zone. After 28 million years ago, the Farallon Plate segmented to form the Juan de Fuca Plate, which continues to subduct under the Pacific Northwest of North America. In the last few million years, volcanism has declined along the volcanic arc. The probable explanation lies in the rate of convergence between the Juan de Fuca and North American plates. These two tectonic plates currently converge per year; this is only about half the rate of convergence from seven million years ago.
Because of the very large fault area, the Cascadia subduction zone can produce large earthquakes of magnitude 7.0 or greater. The interface between the Juan de Fuca and North American plates remains locked for periods of roughly 500 years. During these periods, stress builds up on the interface between the plates and causes uplift of the North American margin. When the plate finally slips, the 500 years of stored energy are released in a massive earthquake. The most recent, the 1700 Cascadia earthquake, was recorded in the oral traditions of the First Nations people on Vancouver Island. It caused considerable tremors and a massive tsunami that traveled across the Pacific Ocean. The significant shaking associated with this earthquake demolished houses of the Cowichan Tribes on Vancouver Island and caused several landslides. It also made it too difficult for the Cowichan people to stand, and the tremors were so lengthy that they were sickened. The earthquake-generated tsunami ultimately devastated a winter village at Pachena Bay, killing all the people that lived there. The 1700 Cascadia earthquake caused near-shore subsidence, submerging marshes and forests on the coast that were later buried under more recent debris.
Unlike most subduction zones worldwide, there is no deep oceanic trench present along the continental margin in Cascadia. The reason is that the mouth of the Columbia River empties directly into the subduction zone and deposits silt at the bottom of the Pacific Ocean, burying this large depression. Massive floods from prehistoric Glacial Lake Missoula during the Late Pleistocene also deposited large amounts of sediment into the small trench. However, as with other subduction zones, the outer margin is slowly being compressed like a giant spring. When the stored energy is suddenly released by slippage across the fault at irregular intervals, the Cascadia subduction zone can create very large earthquakes, such as the magnitude 9.0 Cascadia earthquake on January 26, 1700.
Main-arc volcanism
Pemberton Volcanic Belt
Volcanic activity of the main-arc began at the southern end of the Pemberton Volcanic Belt about 29 million years ago during the middle Oligocene epoch. It then shifted northwards to the Coquihalla region 22 million years ago, followed by volcanism near the town of Pemberton 16 to 17 million years ago. In the Salal Creek area, volcanic activity occurred 8.0 million years ago and the northernmost Pemberton Belt volcano formed 6.8 million years ago. The northward younging of Pemberton Belt volcanism indicates that a slab window edge was moving northward under British Columbia between at least the onset of arc volcanism 29 million years ago and eruption of the northernmost volcano 6.8 million years ago. The extensive Masset Formation on Haida Gwaii has been suggested by some scientists to be a northern extension of the Pemberton Volcanic Belt. However, the geochemistry and physical volcanology of the Masset Formation indicates that it formed in a rifting environment in contrast to other Pemberton Belt features.
Extensive erosion of the Pemberton Volcanic Belt has removed most of its volcanic peaks, exposing their magma systems. These form several intrusive bodies such as batholiths and stocks. The cessation of volcanism in the Pemberton Belt might have been caused by steepening of the subducted Juan de Fuca slab after the Explorer Plate formed about 6.0 million years ago. This change in tectonics created the modern Canadian Cascade Arc, as well as the Cascade Range and Olympic Mountains.
Chilliwack batholith
The first volcanic event 29 million years ago formed intrusive rocks of the large Chilliwack batholith, which extends south into the U.S. state of Washington. In Canada, the batholith consists of rocks ranging from hypersthene quartz gabbro to albite granite. Three main plutons are present. The oldest and most extensive is an irregularly-zoned pluton composed of quartz diorite at the margins, grading inward through granodiorite to a small core of granite. This pluton is exposed on both sides of Chilliwack Lake, a narrow north–south trending lake in the Cascade Mountains. Two younger plutons, consisting of nearly homogeneous leucocratic biotite quartz monzonite, are situated north of Chilliwack Lake and east of Slesse Mountain.
Coquihalla Volcanic Complex Volcanism 22 to 21 million years ago constructed the Coquihalla Volcanic Complex about northeast of Hope. It comprises volcanic and intrusive rocks that are calc-alkaline felsic to intermediate in composition. Coquihalla Mountain, the highest summit of the Bedded Range with an elevation of , is a major preserved stratovolcano and represents one of the few remaining Miocene volcanoes in southwestern British Columbia. As a result, the Coquihalla Volcanic Complex has been a subject of geological studies to annunciate the remains of what might have been an extensive cover of volcanic rocks during the early Miocene epoch. The Coquihalla Complex also has a different igneous composition than Canadian Cascade Arc volcanoes that formed in the last two million years. Rhyolite tuffs are the primary igneous rocks comprising the Coquihalla Volcanic Complex, with small amounts of basalt or andesite present. This contrasts with modern Canadian Cascade volcanoes in that they are mainly composed of volcanic rocks in the basalt to andesite composition range, with few rocks of more felsic composition than dacite. Changes in magma composition have also occurred in the American portion of the Cascade Arc.
The Coquihalla Volcanic Complex began its formation when large pyroclastic flows were erupted on an erosional surface. Emptying of the shallow magma chamber caused tilting of the unconformity on the Eagle pluton, which is situated to the southeast. Concomitant with faulting, tilting and subsidence, avalanche breccias repeatedly slid off oversteepened exposures of the Pasayten Group and Eagle pluton. After volcanic activity deposited a thick sequence of pyroclastic rocks, a period of dormancy followed. It was during this time that localized conglomerate, sandstone and a large sheet of Pasayten clast breccia was deposited.
Subsequent eruptions produced pyroclastic flows, which were followed by another short break in volcanic activity. Vent clearing eruptions produced pyroclastic breccia, which linger on a mountain ridge north and east of Coquihalla Mountain. Movement along the Jim Kelly Creek fault ceased and subsequent pyroclastic flows filled and overflowed that edge of the basin. Later, numerous subvolcanic intrusions were emplaced and post-Miocene uplift tilted and warped the overlying volcanic rocks. Erosion removed what may have been extensive volcanic cover from the surrounding area and uncovered the buried domes and intrusions. Today, the Coquihalla Volcanic Complex covers an area of about and the volume of pyroclastic material is . A large stock, composed of pyroxene diorite and biotite-pyroxene quartz diorite, forms the present base of Coquihalla Mountain.
Mount Barr Plutonic Complex
South of the Fraser River at Wahleach Lake is the Mount Barr Plutonic Complex. Named after Mount Barr in the Skagit Range of the Cascade Mountains, this plutonic complex ranges in age from 21 to 16 million years old. It consists of at least four plutons. The main pluton, comprising 80% of the complex, comprises felsic and intermediate intrusive rocks ranging from quartz diorite to quartz monzonite. It is roughly circular in shape, forming high Mount Barr. Subsequent magmatism created two younger stocks in the main pluton. The older stock consists of fine-grained biotite hornblende granodiorite while the younger stock comprises leucocratic biotite quartz monzonite. A wide dike, composed of granophyric hornblende plagioclase porphyry, is situated between quartz diorite of the Hope Plutonic Complex and Eocene conglomerate. It represents an offshoot of the main Mount Barr pluton. Just northwest of the main Mount Barr pluton is a biotite hornblende quartz diorite pluton near Hicks Lake. Because of its heterogeneity, it is only occasionally grouped as part of the Mount Barr Plutonic Complex.
Crevasse Crag Volcanic Complex
About southeast of Lillooet Lake is the Crevasse Crag Volcanic Complex. It is about 16 million years old, situated on the summit of a glaciated mountain ridge made of Late Cretaceous and younger intrusive rocks. These form part of the large Coast Plutonic Complex, which extends along the British Columbia Coast. The Crevasse Crag Volcanic Complex comprises volcanic breccias, tuffs and plagioclase-phyric flows. Analyses of major, trace and rare-earth elements indicate that dacite, andesite and basaltic andesite lava flows form its lower flanks.
Salal Creek Pluton