Volcanism Of The Mount Edziza Volcanic Complex

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.

The Little Iskut eruptions were much less voluminous than those of the Raspberry eruptive period, depositing only of volcanic material; this makes the Little Iskut Formation the least voluminous geological formation of the first magmatic cycle. Erosional remnants of trachybasalt flows from the Little Iskut eruptive period are exposed in a wide area northeast of the Spectrum Range where they comprise parts of Artifact Ridge and Obsidian Ridge. These flows range in thickness from about near the centre of Artifact Ridge to around the perimeter, suggesting their source was located near Artifact Ridge. This is supported by the existence of dikes along the northern side of Artifact Creek valley which may have been feeders for the overlying trachybasalt flows. A single potassium–argon date of 7.2 ± 0.3 million years has been obtained from Little Iskut trachybasalt.

Armadillo eruptive period

The next eruptive period, the Armadillo period, occurred between 7 and 6 million years ago. It began with explosive activity from a vent at Cartoona Ridge which produced long ash flows and an air-fall pumice deposit that covers an area of several hundred square kilometres. This was followed by the effusion of viscous trachyte and rhyolite lava which piled up around the vent area to produce steep-sided, overlapping domes. As the lava domes continued to grow their slopes became oversteepened, forcing lava to move further away from the vent area. Eventually bulbous mounds of trachyte and rhyolite covered much of the southeastern highlands of the MEVC; these domes were subsequently eroded to form clastic deposits.

Rapid evacuation of a shallow magma chamber nearly south of Cartoona Ridge resulted in the formation of the wide Armadillo Peak caldera. Fractures in the roof of the magma chamber provided passageways for trachyte magma to reach the subsiding caldera floor, resulting in the formation of lava lakes inside the newly-formed depression. Larger volumes of lava eventually spilled over the caldera rim to produce a nearly long, thick sequence of trachyte and rhyolite flows which extends to the west. A thick remnant of trachyte flows that pooled inside the caldera forms the high summit of Armadillo Peak which lies within its southern limit. Erosion in the middle of the caldera has exposed several granite intrusions that are in the form of sills, dikes and irregular subvolcanic masses.

Other volcanic centres were active during the Armadillo eruptive period. Tadeda Peak and the IGC Centre, both satellitic vents of the Armadillo Peak caldera, produced trachyte and rhyolite. Alkali basalt, hawaiite and trachybasalt flows erupted from Sezill Volcano and the Little Iskut shield volcano, many of which are exposed along the Mess Creek Escarpment. The thickest sections of Armadillo basalt flows are exposed in Sezill Creek canyon, Kadeya Creek canyon and near the southwestern end of Raspberry Pass where they reach thicknesses of up to . Individual flows of alkali basalt are thin and voluminous, suggesting they were highly fluid at the time of their eruption.

The Armadillo eruptions deposited of volcanic material, making the Armadillo Formation the most voluminous geological formation of the first magmatic cycle. An anomalously old potassium–argon date of 10.2 ± 1.4 million years has been obtained from Armadillo comendite. Potassium–argon dates more in line with the volcanic stratigraphy include 6.9 ± 0.3 million years and 6.1 ± 0.1 million years from comenditic ash flows, 6.9 ± 0.3 million years from comenditic glass and 6.5 ± 0.2 million years, 6.3 ± 0.5 million years, 6.2 ± 0.1 million years and 6.1 ± 0.2 million years from hawaiite.

Second magmatic cycle

The second magmatic cycle took place between 6 and 1 million years ago during the Pliocene. Like the first magmatic cycle, it is subdivided into three distinct eruptive periods. The first eruptive period is represented by alkali basalt and hawaiite flows of the Nido Formation. They are exposed along the Mess Creek Escarpment and appear to have originated from several separate eruptive centres along the eastern margin of the MEVC. The Spectrum Formation represents the second period of eruptive activity; it is almost entirely underlain by the Nido Formation and consists mostly of trachyte and rhyolite. The third eruptive period is represented by trachyte, comendite and pantellerite of the Pyramid Formation which overlies the Nido Formation.

Nido eruptive period

The Nido eruptive period was a long episode of volcanic activity between 6 and 4 million years ago that involved the effusion of highly mobile, fluid basaltic lava flows from multiple, widely spaced eruptive centres; these eruptive centres included at least six major volcanoes and many more smaller volcanic cones. The lava flows are mineralogically and geomorphologically similar to those of the Raspberry Formation, having buried lag gravels and travelled into valleys where they disrupted the drainage system to form lava-dammed lakes. Volcanism of the Nido eruptive period was limited to the northern and southern ends of the MEVC, such that the lava flows formed two separate lava fields, one at each end of the volcanic complex. The northern lava field is represented by the Tenchen Member while the southern lava field is represented by the Kounugu Member; they are separated by the Armadillo Highlands which acted as a topographic barrier at the time of their eruption. Volcanic activity in both lava fields occurred more or less simultaneously as shown by the existence of Armadillo clasts in glacial deposits that were overridden by lava flows in each field. In 1984, Canadian volcanologist Jack Souther described the lava flows from this period of activity as the remains of composite shield volcanoes.

Three major volcanoes of the Tenchen Member were active during the Nido eruptive period, all of which have since been reduced to eroded remnants. Alpha Peak was the oldest of them; it issued lava flows from both satellitic and central vents which diverted and blocked local streams to form lava-dammed lakes. The second oldest major volcano, Beta Peak, formed south of Alpha Peak. It rose at least above the surrounding landscape and produced lava flows that travelled at least to the north. Remnants of Alpha Peak and Beta Peak basalt are exposed south and east of Mount Edziza. Gamma Peak, the youngest of the three major volcanoes, formed south of Beta Peak on the western flanks of Cartoona Ridge. Lava flows from Gamma Peak buried gently sloping alluvial fans on the northern and western flanks of the Armadillo Highlands. An eroded remnant of this volcano forms a prominent rock pinnacle just southeast of Coffee Crater called Cartoona Peak; Kaia Bluff north of Cartoona Peak is also a remnant of Gamma Peak.

The Kounugu Member contains the eroded remains of at least four volcanoes that were active during Nido time. Swarm Peak, the oldest of the four volcanoes, issued lava flows that travelled down the western and southern flanks of the Little Iskut shield volcano. Vanished Peak further to the south was formed during a major eruption that involved lava fountaining; most of the lava from this eruption flowed to the north and west. Lost Peak consists of volcanic ejecta that was deposited in both subaerial and subaqueous environments; the subaqueous material was deposited in a lake that may have formed between the erupting volcano and a lobe of glacial ice. Exile Hill formed on the southwesternmost edge of the MEVC, most of which was engulfed by younger lava that had flowed to the north and west. Basalt from all four volcanoes is exposed around the perimeter of the younger Spectrum Range.

The Nido eruptions deposited of volcanic material, making the Nido Formation the most voluminous geological formation of the second magmatic cycle. Potassium–argon dating of Nido alkali basalt has given ages of 7.8 ± 0.3 million years, 5.5 ± 1.6 million years, 4.5 ± 0.3 million years and 4.4 ± 0.5 million years. The first age comes from basal basalt of the Kounugu Member overlying basement rocks and, if correct, implies that the Nido eruptions may have initiated during the Raspberry eruptive period.

Spectrum eruptive period

The next eruptive period, the Spectrum period, occurred between 4 and 2 million years ago. A relatively small initial eruption of pumice and ash was followed by the effusion of massive rhyolite flows, each up to thick and long. These rhyolite flows accumulated in rapid succession to form the broad Spectrum Dome which reached a thickness of at least and a width of more than . The predominantly rhyolitic eruptions were followed by the effusion of trachyte lava as deeper parts of the underlying magma chamber were tapped. Formation of the Spectrum Dome was followed by evacuation of the magma chamber, resulting in the creation of a wide caldera which was eventually buried by lava from subsequent eruptions.

Yeda Peak, a high pinnacle in the middle of the Spectrum Range, was the site of an explosive eruption late in Spectrum time that resulted in the formation of a crater. Some of the ejecta accumulated around the vent to form a low volcanic cone whereas the more volatile, pumice-rich phases of the eruption sent ash flows down the slopes of the Spectrum Dome. Renewed volcanism at Exile Hill to the west produced a similar but much smaller eruption that created a roughly wide breccia pipe. Late-stage volcanism of the Spectrum eruptive period also deposited alkali basalt flows of the Kitsu Member which likely issued from multiple eruptive centres on the dome's summit that have since been removed by erosion. These lava flows travelled over a layer of polymict gravel that overlies older volcanic rocks produced during Spectrum time.

The Spectrum eruptions deposited of volcanic material, making the Spectrum Formation the second most voluminous geological formation of the second magmatic cycle. More than 90% of this volcanic material was erupted as lava whereas less than 10% of it was erupted as pumice and pyroclastic flows; trachyte, pantellerite and comendite are the main rocks comprising this volcanic material. An anomalously old potassium–argon date of 5.9 ± 1.1 million years has been obtained from Kitsu Member alkali basalt. Potassium–argon dates more in line with the volcanic stratigraphy include 3.1 ± 0.1 million years and 3.0 ± 0.1 million years from comendite and 3.4 ± 0.1 million years and 2.9 ± 0.1 million years from comenditic glass.

The once continuous Spectrum Dome was substantially eroded to form the current peaks and ridges of the Spectrum Range. Extensive erosion also reduced the size of the dome, leaving behind a few remnants around its northern and southwestern edges. Relatively thin trachyte flows northwest of the Spectrum Range on the Kitsu Plateau are the most distal remnants, although they may have originated from a nearby satellitic vent. Erosional remnants of Kitsu Member alkali basalt flows cap the higher summits of the Spectrum Range where they overlie the unmodified upper surface of the original dome. The original dome was higher in elevation as evidenced by the thick, gently dipping trachyte flows forming the summit of Kitsu Peak, the highest point of the Spectrum Range.

Pyramid eruptive period

The Pyramid eruptive period 1.1 million years ago involved violent explosive eruptions of rock fragments, gas and trachyte pumice from a vent adjacent to the northwestern margin of the MEVC; this explosivity was accompanied by phreatic explosions and pyroclastic surges. Subsequent eruptions sent thin basalt flows into the valley of a north-flowing glacial stream where they formed a small lava-dammed lake. This short period of basaltic volcanism was followed by the extrusion of felsic flows and domes forming The Pyramid, a pyramid-shaped mound on the northeastern flank of Mount Edziza.

Renewed volcanism during this eruptive period produced viscous rhyolite lava and volcanic ejecta of the Sphinx Dome which may have formed subglacially. Some of the ejecta settled in a lake that had formed between the growing dome and an ice field along its southern margin, resulting in the formation of an evenly distributed volcaniclastic deposit on the lake bed. The Sphinx Dome reached a height of and a length of by the time activity ceased.

A third pulse of volcanism constructed the Pharaoh Dome