The Mount Edziza volcanic complex (MEVC) in British Columbia, Canada, has a history of volcanism that spans more than 7 million years. It has taken place during five cycles of magmatic activity, each producing less volcanic material than the previous one. Volcanism during these cycles has created several types of volcanoes, including cinder cones, stratovolcanoes, subglacial volcanoes, shield volcanoes and lava domes. The roughly 1,000 km2 (400 mi2) volcanic plateau forming the base of the MEVC originated from the successive eruptions of highly mobile lava flows. Volcanic rocks such as basalt, trachybasalt, benmoreite, tristanite, mugearite, trachyte and rhyolite were deposited by multiple eruptions of the MEVC; the latter six rock types are products of varying degrees of magmatic differentiation in underground magma reservoirs. Renewed effusive volcanism could block local streams with lava flows whereas renewed explosive volcanism could disrupt air traffic with volcanic ash across parts of northwestern Canada. At least 10 distinct flows of obsidian were produced by volcanism of the MEVC, some of which were exploited by indigenous peoples in prehistoric times to make tools and weaponry. The first magmatic cycle took place between 7.5 and 6 million years ago and is represented by the Raspberry, Little Iskut and Armadillo formations, each of which is the product of a different eruptive period. Three distinct periods of eruptive activity also characterized the second magmatic cycle between 6 and 1 million years ago; they are represented by the Nido, Spectrum and Pyramid geological formations. The third magmatic cycle about 1 million years ago is represented by the Ice Peak, Pillow Ridge and Edziza geological formations, each of which is also the product of a distinct eruptive period. Three distinct periods of eruptive activity also characterized the fourth magmatic cycle between 0.8 and 0.2 million years ago which are represented by the Arctic Lake, Klastline and Kakiddi geological formations. The fifth magmatic cycle began at least 20,000 years ago and may be ongoing; the single distinct eruptive period of this magmatic cycle is represented by the Big Raven Formation.
Background The Mount Edziza volcanic complex is a linear group of volcanoes in northwestern British Columbia, Canada. It is about 65 km (40 mi) long and 20 km (12 mi) wide, consisting of several stratovolcanoes, shield volcanoes, subglacial volcanoes, lava domes and cinder cones. This volcanic complex includes a broad, steep-sided, intermontane plateau that rises from a base elevation of 760 or 816 metres (2,500 or 2,675 feet). A northerly-trending, elliptical, shield volcano consisting of multiple flat-lying lava flows forms the plateau which is capped with several smaller volcanic structures of the complex. Four central volcanoes of felsic composition overlie the plateau, the highest of which is Mount Edziza with an elevation of 2,786 m (9,140 ft). The plateau is subdivided into three smaller plateaus; from north to south they are the Big Raven, Kitsu and Arctic Lake plateaus. The MEVC is one of the largest volcanic complexes in North America, covering about 1,000 km2 (390 mi2) and comprising about 665 cubic kilometres (160 cubic miles) of volcanic material. After Level Mountain, the MEVC is the largest eruptive centre in the Northern Cordilleran Volcanic Province which extends from northwestern British Columbia northwards through Yukon into easternmost Alaska. This volcanic province is the most volcanically active area in Canada, having undergone at least three eruptions in the last 500 years. Volcanism of the Northern Cordilleran Volcanic Province began 20 million years ago, resulting from rifting of the North American Cordillera driven by changes in relative plate motion between the North American and Pacific plates.
Eruption rate and composition
The eruption rate of the MEVC has varied throughout its long volcanic history. When the volcanic complex started erupting at least 7.4 million years ago, it increased the rate of magmatism in the Northern Cordilleran Volcanic Province from 100,000 to 300,000 cubic metres (3,500,000 to 10,600,000 cubic feet) per year. A period of quiescence appears to have followed at the MEVC and elsewhere in the Northern Cordilleran Volcanic Province between about 4 and 3 million years ago. Magmatism of the Northern Cordilleran Volcanic Province has since rebounded to a relatively constant rate of 100,000 cubic metres (3,500,000 cubic feet) per year, significantly less than that estimated for the Cascade Volcanic Arc of western North America. The MEVC has undergone an eruption roughly every 379 years throughout the current Holocene epoch based on the number of demonstrable Holocene eruptions in the last 11,000 years, of which there are at least 29. This makes the MEVC the most active eruptive centre in Canada throughout the Holocene; its relatively frequent eruptions also make it one of the most hazardous volcanic complexes in Canada. The most voluminous rocks produced by volcanism of the MEVC are mafic alkali basalts and hawaiites which represent about 60% of the total eruptive volume. Felsic peralkaline rocks such as trachyte, comendite and pantellerite were also produced by volcanism of the MEVC and represent about 40% of the total eruptive volume, resulting from prolonged fractional crystallization of mantle-derived basaltic magma in magma chambers. Peralkaline volcanism with similar chemistry, mineralogy and isotopic composition has also occurred at the Rainbow Range in central British Columbia, as well as at the Afar Triangle in East Africa and in the Great Basin of western North America. Volcanic rocks of intermediate composition such as benmoreite, trachybasalt, mugearite and tristanite were produced in relatively small volumes; they were the result of alkali basaltic magma having pooled in large subterranean magma chambers on a shorter timespan. The chemistry and petrography of MEVC rocks is indicative of bimodal volcanism, a phenomenon associated with continental rifting.
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