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 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 long and 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 . 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 . 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 and comprising about 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 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 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.
Magmatic cycles Volcanism of the MEVC has taken place during five cycles of magmatic activity in the last 8 million years, each of which began with the effusion of alkali basalt and culminated with the eruption of felsic magma. Each cycle was less productive than the previous one, the first magmatic cycle depositing about of volcanic material. Eruptions during the second and third magmatic cycles deposited about and of volcanic material, respectively. Volcanism during the fourth magmatic cycle deposited roughly of volcanic material whereas the fifth cycle has produced an insignificant volume of volcanic material. The fourth and fifth magmatic cycles could possibly be part of a larger cycle that may be ongoing.
First magmatic cycle The first magmatic cycle was restricted to the Late Miocene between 7.5 and 6 million years ago. Three distinct eruptive periods occurred during this magmatic cycle, each producing different types of volcanic rocks. The first eruptive period is represented by alkali basalt and hawaiite flows of the Raspberry Formation. They rest directly on older rocks of the Stikinia terrane and are exposed along the Mess Creek Escarpment. The Little Iskut Formation represents the second period of eruptive activity, consisting mainly of trachybasalt flows and breccia that overlie the Raspberry Formation. Eruptions of the Little Iskut period immediately followed or may have been coeval with those during Raspberry time due to the lack of an erosion surface between the two formations. The third eruptive period is represented by alkali basalt, comendite and trachyte of the Armadillo Formation which overlies the Little Iskut Formation.
Raspberry eruptive period
The Raspberry eruptive period 7.4 million years ago began with the effusion of basaltic lava flows on an erosion surface from near Raspberry Pass. More than of lava flows were extruded in rapid succession, forming a Late Miocene shield volcano. They reached a maximum thickness of more than near their source to only a few metres thick at their terminus. Disruption of the local drainage system by lava flows originating from a cluster of small satellitic cones southeast of the Raspberry shield volcano resulted in the formation of so-named Raspberry Lake in the upper Little Iskut River valley. At least 25 lava flows were extruded during Raspberry time, each reaching thicknesses of . By the time the Raspberry eruptive period had come to an end, the Raspberry shield volcano covered an area of at least and reached an elevation of nearly . Volcanism during Raspberry time did not have long periods of quiescence as suggested by the lack of fluvial layers between individual Raspberry lava flows.
About of volcanic material were deposited by the Raspberry eruptions, making the Raspberry Formation the second most voluminous geological formation of the first magmatic cycle. After the Raspberry eruptive period ceased, Raspberry Lake had already begun to erode a notch along the eastern edge of the lava dam. The Raspberry shield volcano and associated satellitic cones and ash beds had also begun to erode away, but the valleys and lowlands remained filled with thick piles of basaltic lava flows which later were overlain by the much younger Mount Edziza and Spectrum Range volcanoes. In addition to forming the base of the Mess Creek Escarpment, Raspberry basalt is also exposed along the bases of Artifact and Obsidian ridges, as well as south and southeast of Mount Edziza.
Potassium–argon dating of volcanic rocks produced during this eruptive period has yielded a wide variety of ages. This includes 11.4 ± 1.5 million years, 8.4 ± 0.4 million years and 6.4 ± 0.3 million years for Raspberry hawaiite and 6.1 ± 0.4 million years and 5.5 ± 0.1 million years for Raspberry alkali basalt, the first of which is anomalously old and has the largest error. Relatively large atmospheric contents and pervasive carbonate alteration in Raspberry rocks is likely the cause of the large spread in ages. A minimum age for the timing of Raspberry volcanism is 7.4–6.2 million years.
Little Iskut eruptive period
The Little Iskut eruptive period 7.2 million years ago began beneath the waters of Raspberry Lake south of present day Raspberry Pass. Interactions between the lake water and the erupting magma resulted in several violent phreatic explosions, the larger explosions depositing ash and granular particles over much of the lake bed. The phreatic explosions were followed by the eruption of trachybasalt flows which began forming a lava dome on the bed of Raspberry Lake; this lava dome eventually grew above lake level from continued volcanic eruptions to form a small volcanic island. Renewed volcanism then transformed this small island into a broad shield volcano that overlapped with the northern shoreline of Raspberry Lake. By this time much of the original lake had been displaced with shattered rock fragments formed by the quenching and fracturing of lava from thermal shock. Subsequent eruptions of the Little Iskut shield volcano produced thick, irregular, randomly jointed lava flows that travelled down its gentle eastern, southern and western flanks. Lava flowing down the eastern and southern flanks entered the shrinking remnants of Raspberry Lake whereas lava travelling down the western flank merged with the older Raspberry shield volcano.