A mostly green and brown map with labels depicting the locations of several geographical features.
Map of the MEVC showing many of its features

The Mount Edziza volcanic complex (MEVC) in British Columbia, Canada, has a long history of volcanism that spans more than six million years. It occurred during five cycles of magmatic activity which were characterized by 13 periods of eruptive activity. This volcanism has led to the formation of several types of volcanic landforms, including cinder cones, stratovolcanoes, subglacial volcanoes, shield volcanoes, lava domes and lava fields. The 1,000-square-kilometre (390-square-mile) plateau comprising the MEVC owes its origin to successive eruptions of highly mobile lava flows. Eruptions of the MEVC issued a wide variety of volcanic rocks. Among them were alkali basalt, hawaiite, trachybasalt, benmoreite, tristanite, mugearite, trachyte and comendite. The latter seven volcanic rocks were produced by varying degrees of magmatic differentiation in underground magma reservoirs.

The first magmatic cycle between 12 and 5.3 million years ago is represented by the Raspberry, Little Iskut and Armadillo geological formations, each of which is the product of a distinct eruptive period. Three distinct periods of eruptive activity also characterized the second magmatic cycle between 6.0 and 1.0 million years ago. They are represented by the Nido, Spectrum and Pyramid geological formations. The third magmatic cycle about one 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. They are represented by the Arctic Lake, Klastline and Kakiddi geological formations. The fifth magmatic cycle began in the last 20,000 years and may continue to the present. A 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.[1][2] It is 75 kilometres (47 miles) long and 20 kilometres (12 miles) wide, consisting of several stratovolcanoes, shield volcanoes, subglacial volcanoes, lava domes and cinder cones.[1][3][4] This volcanic complex of Miocene-to-Holocene age comprises a broad, steep-sided, intermontane plateau that rises from a base elevation of 760 metres (2,500 feet).[1][5][6] A northerly-trending, elliptical, composite shield volcano consisting of multiple flat-lying lava flows forms the plateau. Four central volcanoes of felsic[lower-alpha 1] composition dominate the plateau, the highest of which is Mount Edziza with an elevation of 2,786 metres (9,140 feet).[1]

The MEVC has a volume of 670 cubic kilometres (160 cubic miles) and an area of 1,000 square kilometres (390 square miles), making it the second largest eruptive centre in the Northern Cordilleran Volcanic Province after Level Mountain.[8] The Northern Cordilleran Volcanic Province is a broad area of volcanoes extending from northwestern British Columbia northwards through Yukon into easternmost Alaska.[9] It is the most volcanically active area in Canada, experiencing an eruption roughly every 100 years.[10] Volcanism of the Northern Cordilleran Volcanic Province is thought to result from rifting of the North American Cordillera driven by changes in relative plate motion between the North American and Pacific plates.[11]

Eruption rate and composition

A yellow and green map with contour lines depicting a mountainous environment.
Topographic map of the MEVC with Mount Edziza Provincial Park in green

The eruption rate of the MEVC has varied throughout its long volcanic history. Volcanism about seven million years ago occurred at a higher rate than it does today, having increased the rate of magmatism in the Northern Cordilleran Volcanic Province from 100,000 cubic metres (3,500,000 cubic feet) per year to 300,000 cubic metres (11,000,000 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 four and three million years ago.[8] 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.[12] An eruption recurrence interval of 379 years has been calculated for the MEVC by dividing 11,000 by the number of demonstrable Holocene eruptions. This would make the MEVC the most active eruptive centre in Canada throughout the Holocene.[13]

Volcanism of the MEVC took place during five magmatic cycles, each of which began with the effusion of alkali basalt and culminated with the eruption of felsic magma. The most voluminous rocks produced during these magmatic cycles were mafic[lower-alpha 2] alkali basalts and hawaiites. MEVC hawaiites are thought to have formed as a result of partial fractional crystallization and the accumulation of feldspar inside rising columns of mantle-derived alkali basalt. Felsic peralkaline rocks[lower-alpha 3] such as comendite and trachyte were also produced during these magmatic cycles. They resulted from prolonged fractional crystallization of mantle-derived basalt in magma chambers. Volcanic rocks of intermediate composition such as benmoreite, trachybasalt, mugearite and tristanite were produced in relatively small volumes. They were the result of alkali basalt having pooled in large subterranean magma chambers on a shorter timespan.[15] The chemistry and petrography of MEVC rocks is indicative of bimodal volcanism, a phenomenon associated with continental rifting.[16]

First magmatic cycle

The first magmatic cycle was restricted to the Late Miocene between 12 and 5.3 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.[17] They rest directly on older rocks of the Stikinia terrane and are exposed along the Mess Creek Escarpment.[18] The Little Iskut Formation represents the second period of eruptive activity.[17] It conformably overlies the Raspberry Formation and consists mainly of trachybasalt flows and breccia.[19] The third eruptive period is represented by alkali basalt, comendite and trachyte of the Armadillo Formation which conformably overlies the Little Iskut Formation.[20]

Raspberry eruptive period

The Raspberry eruptive period between 12 and 5.4 million years ago began with the effusion of basaltic lava flows from near Raspberry Pass.[21] More than 83 cubic kilometres (20 cubic miles) of lava flows were extruded in rapid succession, forming a Late Miocene shield volcano.[22] They reached a maximum thickness of more than 300 metres (980 feet) near their source to only a few metres thick at their terminus.[23] Disruption of the local drainage system by lava flows during this eruptive period resulted in the formation of so-named Raspberry Lake in the upper Little Iskut River valley.[24] The lava flows damming Raspberry Lake originated from a cluster of small satellitic cones southeast of the Raspberry shield volcano.[25] By the time the Raspberry eruptive period had come to an end, the Raspberry shield volcano covered an area of at least 775 square kilometres (299 square miles) and reached an elevation of nearly 2,100 metres (6,900 feet).[23]

About 119 cubic kilometres (29 cubic miles) of volcanic material was deposited by the Raspberry eruptions, making the Raspberry Formation the most voluminous geological formation of the first magmatic cycle.[17][26] After the Raspberry eruptive period ceased, Raspberry Lake had already begun to erode a notch along the eastern edge of the lava dam.[24] The Raspberry shield volcano and associated satellitic cones and ash beds had also begun to erode away, but the valleys and lowlands would remain filled with thick piles of basaltic lava flows.[27] These lava flows would later be overlain by the much younger Mount Edziza and Spectrum Range.[24]

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.[28] The first date 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.[29] A minimum age for the timing of Raspberry volcanism is 7.4–6.2 million years.[30]

Little Iskut eruptive period

The Little Iskut eruptive period 7.2 million years ago began beneath the waters of Raspberry Lake.[31] Interactions between the lake water and the erupting magma resulted in several violent phreatic explosions, the larger explosions having deposited ash and granular particles over much of the lake bed.[24] The phreatic explosions were followed by the eruption of trachybasalt flows which began forming a lava dome on the bed of Raspberry Lake.[32] This lava dome eventually grew above lake level from continued volcanic eruptions to form a small volcanic island.[33] Renewed volcanism then transformed this small island into a broad shield volcano that overlapped with the northern shoreline of Raspberry Lake.[34] By this time much of the original lake had been displaced with shattered rock fragments formed by the quenching and fracturing of lava.[32] Subsequent eruptions of the Little Iskut shield volcano produced 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 while lava travelling down the western flank merged with the older Raspberry shield volcano.[33]

The Little Iskut eruptions were much less voluminous than those of the Raspberry eruptive period, having deposited only 14.6 cubic kilometres (3.5 cubic miles) of volcanic material.[26] This makes the Little Iskut Formation the least voluminous geological formation of the first magmatic cycle.[17] The lack of an erosion surface between the Raspberry and Little Iskut formations suggests that the Little Iskut eruptions immediately followed or may have been coeval with eruptions of the Raspberry period.[30] Erosional remnants of trachybasalt flows from the Little Iskut eruptive period are exposed in a 10-kilometre-wide (6.2-mile) area northeast of the Spectrum Range. They range in thickness from about 300 metres (980 feet) near the centre of Artifact Ridge to 90 metres (300 feet) around the parameter, suggesting that their source was located near Artifact Ridge. This is supported by the existence of dikes[lower-alpha 4] along the northern side of Artifact Creek valley which may have been feeders for the overlying trachybasalt flows.[19] A single potassium–argon date of 7.2 ± 0.3 million years has been obtained from Little Iskut trachybasalt.[28]

Armadillo eruptive period

An overhead view of snow-covered mountains with intervening river valleys.
Mount Edziza appears at the top of this image west of snow-covered Nuttlude and Kakiddi lakes. The snow-covered mountains at the right-centre, lying between the Mess Creek (left) and Little Iskut River (right) drainages, are in the Spectrum Range.

The next eruptive period, the Armadillo period, occurred between 7.0 and 6.0 million years ago.[1][17] It began with explosive activity from a vent at Cartoona Ridge which produced 10-kilometre-long (6.2-mile) ash flows and an air-fall pumice deposit that covers an area of several hundred square kilometres.[36] 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.[37]

Rapid evacuation of a shallow magma chamber nearly 8 kilometres (5.0 miles) south of Cartoona Ridge resulted in the formation of the 3-kilometre-wide (1.9-mile) Armadillo Peak caldera.[1][38] 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 13-kilometre-long (8.1-mile) sequence of trachyte and rhyolite flows. The sequence is up to 460 metres (1,510 feet) thick and extends to the west.[37]

A number of other volcanic centres were active during the Armadillo eruptive period.[37] Tadeda Peak and the IGC Centre, both satellitic vents of the Armadillo Peak caldera, produced trachyte and rhyolite.[39] Alkali basalt, hawaiite and trachybasalt flows issued from Sezill Volcano and the Little Iskut shield volcano, many of which are exposed along the Mess Creek Escarpment.[40] 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 180 metres (590 feet).[41]

The Armadillo eruptions deposited 159 cubic kilometres (38 cubic miles) of volcanic material, making the Armadillo Formation the most voluminous geological formation of the first magmatic cycle.[17][26] An anomalously old potassium–argon date of 10.2 ± 1.4 million years has been obtained from Armadillo comendite.[42] 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.[28]

Second magmatic cycle

The second magmatic cycle took place between 6.0 and 1.0 million years ago during the Pliocene and Early Pleistocene. 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.[17] They are exposed along the Mess Creek Escarpment and appear to have issued from several separate eruptive centres along the eastern margin of the MEVC.[43] The Spectrum Formation represents the second period of eruptive activity.[17] It is almost entirely underlain by the Nido Formation and consists mostly of trachyte and rhyolite.[44] The third eruptive period is represented by trachyte, comendite and pantellerite of the Pyramid Formation which overlies the Nido Formation.[45]

Nido eruptive period

The Nido eruptive period was a long episode of volcanic activity that took place between 6.0 and 4.0 million years ago.[46] It 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 buried lag gravels and travelled into valleys where they disrupted the drainage system to form lava-dammed lakes.[47] 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 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. Both lava fields 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.[43]

Three major volcanoes of the Tenchen Member were active during the Nido eruptive period, all of which have since been reduced to eroded remnants.[43] Alpha Peak was the oldest of the three major volcanoes. 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 12 kilometres (7.5 miles) south of Alpha Peak. It rose at least 365 metres (1,198 feet) above the surrounding landscape and produced lava flows that travelled at least 13 kilometres (8.1 miles) to the north. 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.[48] An eroded remnant of Gamma Peak forms a prominent spire just southeast of Coffee Crater called Cartoona Peak.[49][50]

The Kounugu Member contains the eroded remains of at least four volcanoes that were active during Nido time.[51] 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.[52] Lost Peak consists of volcanic ejecta that deposited in both subaerial and subaqueous environments.[53] The subaqueous material was deposited in a lake that may have ponded between the erupting volcano and a lobe of glacial ice.[53] Exile Hill formed on the extreme western edge of the MEVC and was almost completely inundated by younger lava flows.[52]

The Nido eruptions deposited 127 cubic kilometres (30 cubic miles) of volcanic material, making the Nido Formation the second most voluminous geological formation of the second magmatic cycle.[17][26] 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.[28] The first age comes from basalt of the Kounugu Member and, if correct, implies that Nido eruptions may have spanned from Raspberry to post-Armadillo time.[54]

Spectrum eruptive period

The next eruptive period, the Spectrum period, occurred between 4.0 and 2.0 million years ago.[17] A relatively small initial eruption of pumice and ash was followed by the effusion of massive rhyolite flows, each up to 150 metres (490 feet) thick and 13 kilometres (8.1 miles) long.[55] These rhyolite flows accumulated in rapid succession to form the broad Spectrum Range dome which reached a thickness of at least 750 metres (2,460 feet) and a width of more than 20 kilometres (12 miles). The predominantly rhyolitic eruptions were later replaced by the effusion of trachyte lava as deeper parts of the underlying magma chamber were tapped.[56] Formation of the Spectrum Range dome was followed by evacuation of the magma chamber, resulting in the creation of a 4.5-kilometre-wide (2.8-mile) summit caldera.[1][56] This caldera was eventually buried by lava from subsequent eruptions.[57]

Orange, red and dark-colored rocks exposed on the side of a mountain.
Oxidized lava flow near the Raspberry Pass area

Yeda Peak was the site of an explosive eruption near the end of the Spectrum period. This eruption resulted in the formation of a crater near the crest of the Spectrum Range dome. Some of the ejecta accumulated around the vent to form a low volcanic cone while the more volatile, pumice-rich phases of the eruption sent ash flows down the slopes of the dome.[56] Renewed volcanism at Exile Hill 8 kilometres (5.0 miles) west of the Yeda Peak vent produced a similar but much smaller eruption.[58] Late-stage volcanism of the Spectrum eruptive period produced alkali basalt flows of the Kitsu Member.[59] These flows likely issued from several eruptive centres that have since been destroyed by erosion.[60]

The Spectrum eruptions deposited 119 cubic kilometres (29 cubic miles) of volcanic material, making the Spectrum Formation the second most voluminous geological formation of the second magmatic cycle.[17][26] More than 90% of this volcanic material was erupted as lava while less than 10% of it was erupted as pyroclastic flows and pumice.[61] 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.[28][62]

The once continuous Spectrum Range 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.[44] 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.[63] The original dome was much higher in elevation as evidenced by the thick, gently dipping trachyte flows forming the summit of Kitsu Peak.[44]

Pyramid eruptive period

The Pyramid eruptive period took place 1.1 million years ago.[17][64] It involved violent explosive eruptions of rock fragments, gas and trachyte pumice from a vent adjacent to the northwestern margin of the MEVC.[65] 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.[66] This short period of basaltic volcanism was followed by the extrusion of felsic flows and domes forming The Pyramid.[64][66]

Renewed volcanism during this eruptive period produced the Sphinx Dome which may have formed subglacially.[64][67] Growth of the Sphinx Dome involved the eruption of viscous rhyolite and volcanic ejecta. 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[lower-alpha 5] deposit on the lake bed. The Sphinx Dome reached a height of 800 metres (2,600 feet) and a length of 5 kilometres (3.1 miles) by the time activity ceased.[68]

A third pulse of volcanism constructed the Pharaoh Dome just south of the lake that ponded during Sphinx Dome activity.[68] Eruptions were at first subglacial which led to a series of phreatic steam explosions and the quenching of rhyolite lava by meltwater. Pharaoh Dome eventually built above the level of the surrounding ice as flows of rhyolite continued to enlarge the dome.[64][68] By the time activity ceased, Pharaoh Dome had risen above the surface of a large ice field as a nunatak; it was subsequently buried under glacial ice.[68]

The Pyramid eruptions were much less voluminous than those of the Nido and Spectrum eruptive periods, having deposited only 11.4 cubic kilometres (2.7 cubic miles) of volcanic material.[26] This makes the Pyramid Formation the least voluminous geological formation of the second magmatic cycle.[17] Potassium–argon dating of comenditic glass produced during the Pyramid eruptive period has yielded ages of 1.2 ± 0.4 million years and 1.20 ± 0.03 million years.[69] Trachyte produced during this eruptive period has yielded potassium–argon dates of 0.94 ± 0.12 million years and 0.94 ± 0.05 million years.[70]

Third magmatic cycle

The third magmatic cycle occurred between about 1.0 and 0.8 million years ago during the Early Pleistocene.[71] It was characterized by three distinct eruptive periods, each represented by a geological formation.[17][64] The first eruptive period created the Ice Peak Formation which overlies the Armadillo, Nido and Pyramid formations.[72] A wide variety of volcanic rocks comprise the Ice Peak Formation, including alkali basalt, hawaiite, trachybasalt, tristanite, mugearite, benmoreite and trachyte.[73] The second eruptive period resulted in the creation of the Pillow Ridge Formation which consists mainly of alkali basalt.[17] This geological formation is confined to Pillow Ridge and Tsekone Ridge at the northern end of the MEVC.[64][74] The third eruptive period produced the Edziza Formation which consists mainly of trachyte that overlies the Ice Peak Formation.[75][76]

Ice Peak eruptive period

The Ice Peak eruptive period began at a time when the MEVC was covered by a receding regional ice sheet. Volcanism initially began on the southern flank of Sphinx Dome where pyroclastic material mixed with meltwater from residual ice to produce highly mobile debris flows and lahars. Lava flows advanced across the glaciated surface as successive eruptions built Ice Peak, resulting in the formation of narrow meltwater lakes. These lakes were displaced as the lava flows continued to advance down slope.[77] Basaltic lava travelled further down slope onto the MEVC plateau while more viscous trachybasalt, tristanite, mugearite, benmoreite and trachyte lava accumulated around the vent area to form the steep, upper part of Ice Peak.[78] At its climax, Ice Peak was a symmetrical stratovolcano containing a small crater at its summit; its symmetrical structure was later destroyed by glacial erosion.[79] Potassium–argon dating of massive trachyte flows in the upper part of Ice Peak has yielded ages of 1.5 ± 0.4 million years and 1.5 ± 0.1 million years.[80] These dates being older than those of the Pyramid eruptive period may be due to excess argon.[81]

An overhead view of an oval-shaped alpine plateau covered with snow.
Satellite imagery of Mount Edziza (middle) and the surrounding snow-covered Big Raven Plateau

Two thick lobes of trachyte lava issued from satellitic domes on the western flank of Ice Peak during this eruptive period, both of which were deposited onto the MEVC plateau.[82] The southern lobe, Koosick Bluff, ranges in elevation from 1,890 to 2,010 metres (6,200 to 6,590 feet). It is bounded by cliffs that rise 60–90 metres (200–300 feet) to a nearly flat surface. With a length of nearly 2 kilometres (1.2 miles) and a width of more than 1 kilometre (0.62 miles), Koosick Bluff is the largest of the two lava lobes. The northern and smaller lobe, Ornostay Bluff, is similar in composition and structure to Koosick Bluff.[83] It has a potassium–argon date of 1.5 ± 0.4 million years which may be due to excess argon.[81][83] The steep sides and unusually large thicknesses of these two lava lobes is attributed to them having been extruded through glacial ice.[64]

Volcanic activity during Ice Peak time created two volcanoes west of the Armadillo Highlands. The northern volcano, Camp Hill, began forming when the MEVC was still partially covered by glacial ice. Eruptions under the glacial ice formed a circular meltwater pond which quenched the erupting lava and caused phreatic explosions. The resulting fractured and churned debris accumulated around the erupting vent to create a broad tuff ring.[77] This feature eventually grew above the level of the meltwater pond to produce subaerial lava fountains which formed a relatively steep-sided pyroclastic cone on top of the tuff ring.[84] By this time the surrounding glacial ice had retreated, allowing basalt flows to spread over the Big Raven Plateau.[85] The southern volcano, Cache Hill, formed during a period of eruptions on the western side of the Armadillo Highlands. Basalt flows blocked a northwesterly flowing river in a broad valley to form a small lava-dammed lake. Subsequent basalt flows travelled to the southeast and northwest, the southeasterly flows having entered the lava-dammed lake to create pillow lava.[85]

A circular volcanic plug called The Neck formed on the eastern side of the MEVC during the Ice Peak eruptive period. It consists of an older outer ring of fine grained trachyte and a younger inner core of coarse grained trachyte, suggesting that The Neck was the source of more than one trachyte eruption.[86] This roughly 300-metre (980-foot) in diameter volcanic conduit has a potassium–argon date of 1.6 ± 0.2 million years which may be due to excess argon.[81][86]

The eruptions during Ice Peak time deposited 76.7 cubic kilometres (18.4 cubic miles) of volcanic material, making the Ice Peak Formation the most voluminous geological formation of the third magmatic cycle.[17][26] This is the latest MEVC eruptive period involving the outpouring of more than 20 cubic kilometres (4.8 cubic miles) of lava.[64] It is also the only eruptive period of the MEVC involving the eruption of benmoreite and mugearite lavas.[87]

Pillow Ridge eruptive period

The next eruptive period, the Pillow Ridge period, occurred when the MEVC was still overlain by an ice sheet.[64][88] Subglacial volcanism injected basaltic lava into the base of the ice sheet where the molten basalt was quenched and then shattered by phreatic explosions. Accumulation of this fragmented debris around the erupting vent created a subglacial pile of tuff, breccia and pillow lava inside a meltwater cavity. The overlying ice sheet sagged as the volcanic pile and enclosing meltwater cavity grew larger, resulting in the formation of a meltwater lake inside a depression on the surface of the ice sheet. This meltwater lake was churned by phreatic explosions and probably reached a length of more than 4 kilometres (2.5 miles). Successive eruptions eventually built the volcanic pile above lake level to form a small temporary island that issued subaerial lava flows and lava fountains. The volcanic pile that formed as a result of this activity comprises Pillow Ridge.[89] Fission track dating of Pillow Ridge alkali basalt has yielded ages of 0.9 ± 0.3 million years and 0.8 ± 0.25 million years.[90]

Another pulse of subglacial volcanism during the Pillow Ridge period created nearby Tsekone Ridge. Although it formed in a similar environment to that of Pillow Ridge, there is no evidence the Tsekone Ridge eruption was large enough to penetrate the overlying ice sheet. The basaltic magma that issued during this eruption may have been leftover from the series of eruptions that formed Pillow Ridge.[89]

The eruptions during Pillow Ridge time were much less voluminous than those of the Ice Peak eruptive period, having deposited only 2.9 cubic kilometres (0.70 cubic miles) of volcanic material.[26] This makes the Pillow Ridge Formation the least voluminous geological formation of the third magmatic cycle.[17]

Edziza eruptive period

The flat-topped summit of a snow-covered mountain with a rocky and snow-covered cliff in the foreground.
The snow-covered summit crater of Mount Edziza

The Edziza eruptive period constructed the symmetrical stratovolcano of Mount Edziza after the regional ice sheet had retreated from the MEVC.[64][91] Growth began on the upper northern flank of Ice Peak with the eruption of viscous trachyte flows and steep-sided lava domes.[91] The lava domes were punctuated by vent-clearing explosions which ejected volcanic blocks and lava bombs onto the slopes of the growing stratovolcano.[92] Formation of the stratovolcano was followed by collapse of the original summit, creating the 2-kilometre (1.2-mile) in diameter crater that truncates it.[1][92] The cause of this collapse may have been a violent, climactic eruption that deposited parts of the original summit onto the flanks of the volcano. Prior to collapse, the summit of Mount Edziza was at least 610 metres (2,000 feet) higher than it is today. Part of the eastern crater rim was destroyed by a small phreatic explosion which provided a new passageway for the venting of volcanic gases.[92]

The Edziza eruptions deposited 18 cubic kilometres (4.3 cubic miles) of volcanic material.[26] This makes the Edziza Formation the second most voluminous geological formation of the third magmatic cycle.[17] Most of the volcanic activity during this eruptive period was concentrated in the summit area of Mount Edziza but at least a few vents were active near the base of the volcano. Volcanism on the southeastern rim of the summit crater created Nanook Dome. Lava from this dome flowed onto the outer surface of the stratovolcano and into the summit crater to form lava lakes.[92] Triangle Dome and Glacier Dome formed on the western and northeastern flanks of Mount Edziza, respectively.[93] A trachyte flow from the latter dome travelled onto the gently sloping surface of the Big Raven Plateau. Lava from a small pyroclastic cone on the northwestern flank of Mount Edziza nearly engulfed both Pillow Ridge and Tsekone Ridge on the surrounding plateau.[92] Pantelleritic trachyte produced during the Edziza eruptive period has yielded a potassium–argon date of 0.9 ± 0.3 million years.[70]

Fourth magmatic cycle

The fourth magmatic cycle took place between 0.8 and 0.2 million years ago during the Pleistocene.[17] Like the previous three magmatic cycles, it was characterized by three distinct eruptive periods.[17][64] The first eruptive period created the Arctic Lake Formation which underlies much of the Arctic Lake Plateau.[94] Alkali basalt flows and related pyroclastic rocks comprise the Arctic Lake Formation.[95] The second eruptive period is represented by the Klastline Formation along the Kakiddi and Klastline valleys.[96] Thick alkali basalt flows are the main features of the Klastline Formation.[97] The third eruptive period produced thick trachyte flows and pyroclastic rocks of the Kakiddi Formation which occupy valleys on the eastern flank of Ice Peak.[98]

Arctic Lake eruptive period

The Arctic Lake eruptive period occurred 0.71 million years ago on and adjacent to the Arctic Lake Plateau.[94] At least seven volcanoes formed during this period, all of which issued basaltic lava.[64][95] Lava fountaining at the extreme northern end of the Arctic Lake Plateau created Outcast Hill which blocked westerly flowing streams. The damming of these streams resulted in the formation of a temporary lake against the eastern side of Outcast Hill. Lava from this volcano flowed into the lake but most of it travelled westward towards the Mess Creek Escarpment.[99] Tadekho Hill 4 kilometres (2.5 miles) to the south formed on top of a 180-metre-high (590-foot) remnant of Spectrum trachyte. Lava from Tadekho Hill spread onto the surrounding plateau surface to form a small shield volcano. Outcast Hill and Tadekho Hill both formed when the Arctic Lake Plateau was relatively free of glacial ice.[100]

The Arctic Lake Plateau was subsequently covered with ice as glaciers advanced from the neighbouring Spectrum Range. Subglacial volcanism at the height of this glacial advance created Wetalth Ridge near the middle of the plateau.[100] This was followed by the eruption of four other volcanoes on the Arctic Lake Plateau during the waning stages of glaciation. Two small mounds of quenched pillow lava informally called Knob 1 and Knob 2 formed subglacially south of Wetalth Ridge.[101] The third volcano, Source Hill, was created during a massive lava eruption when only the central part of the Arctic Lake Plateau contained a thin lobe of glacial ice. Late-stage volcanism of the Arctic Lake eruptive period formed Thaw Hill on the eastern side of the Arctic Lake Plateau.[100]

The Arctic Lake eruptions were much less voluminous than those of the Edziza eruptive period, having deposited only 2 cubic kilometres (0.48 cubic miles) of volcanic material.[26] This makes the Arctic Lake Formation the least voluminous geological formation of the fourth magmatic cycle.[17] Alkali basalt from this eruptive period has yielded a potassium–argon date of 0.71 ± 0.05 million years.[70]

Klastline eruptive period

The Klastline eruptive period 0.62 million years ago was characterized by minor lava fountaining and the effusion of massive basalt flows from at least three vents along the northern flank of Mount Edziza.[64][102] The basalt flows travelled adjacent to Buckley Lake and into the Klastline and Kakiddi valleys, the most extensive ones having reached 25 kilometres (16 miles) long. Pyroclastic cones developed on the lower slopes of the MEVC where eruptions were subaerial. Explosive interaction between lava and meltwater from an alpine glacier formed the Klastline tuff cone higher up on the plateau. Lava from Klastline Cone entered Kakiddi Valley where it blocked the flow of Kakiddi Creek. This lava then flowed north across dry gravel bars to the confluence with Klastline Valley, temporarily damming the Klastline River to form a large shallow lake. Most of the lava continued to flow westward through Klastline Valley for at least another 19 kilometres (12 miles).[99]

Black and white photo of a river with rocky cliffs in the background.
Lava flows overlying sediment at the mouth of the Tahltan River

Lava of the Klastline eruptive period flowed 55 kilometres (34 miles) downstream along the Stikine River from its confluence with the Klastline River.[103] As the lava advanced it buried glacial and nonglacial sediment along the Stikine and Tahltan rivers.[104] Isolated remnants of this lava are preserved along the river canyon walls and are subdivided into two geological members. The Junction Member is characterized by swirly jointed basalt while the overlying Village Member consists of regular columnar basalt jointing. At least five distinct lava flows comprise the Village Member. They collectively reach a maximum thickness of 100 metres (330 feet) and are vesicular in texture.[103] The Klastline lava along the Stikine River had travelled some 83 kilometres (52 miles) from the MEVC.[105]

The Klastline eruptive period deposited 5.4 cubic kilometres (1.3 cubic miles) of volcanic material, making the Klastline Formation the second most voluminous geological formation of the fourth magmatic cycle.[17][26] Potassium–argon dating of Klastline alkali basalt has yielded ages of 0.62 ± 0.04 million years and 0.33 ± 0.03 million years.[70][106] The first date is from a lava flow remnant in Klastline Valley while the second date is from a Village Member basalt flow on the Tahltan River.[106][107] Argon–argon dating of Village Member basalt about 2 kilometres (1.2 miles) downstream from the mouth of the Tahltan River on the east bank of the Stikine River has yielded an age of 0.30 ± 0.10 million years.[108]

Kakiddi eruptive period

The Kakiddi eruptive period 0.3 million years ago involved the eruption of a massive trachyte flow that reaches almost 1 kilometre (0.62 miles) wide and 60–90 metres (200–300 feet) thick.[64][109] It advanced 7 kilometres (4.3 miles) down the eastern flank of the MEVC into Kakiddi Valley where it spread out into a more than 20-square-kilometre (7.7-square-mile) terminal lobe. The source of this lava flow remains unknown but it may have issued from Ice Peak and possibly Nanook Dome at the summit of Mount Edziza.[110] A relatively small lava flow issued from a vent on the western flank of Ice Peak and advanced onto the Big Raven Plateau.[111]

The Kakiddi eruptions deposited 8.3 cubic kilometres (2.0 cubic miles) of volcanic material.[26] This makes the Kakiddi Formation the most voluminous geological formation of the fourth magmatic cycle.[17] Potassium–argon dating has yielded ages of 0.31 ± 0.07 million years for Kakiddi mugearite and 0.30 ± 0.02 million years, 0.29 ± 0.02 million years and 0.28 ± 0.02 million years for Kakiddi trachyte.[112] This suggests that the Kakiddi eruptions may have been coeval with eruptions of the Klastline period.[103] The Kakiddi eruptive period was short-lived as evidenced by the small error and close clustering of the potassium–argon dates.[113]

Fifth magmatic cycle

A black cone-shaped mountain rising over glacial ice.
Tennena Cone from the northwest

The fifth magmatic cycle, which may still be ongoing, commenced in the last 20,000 years with the onset of the Big Raven eruptive period.[17][64] It was marked by the eruption of subglacial volcanoes, cinder cones and lava flows along the entire length of the MEVC, as well as a single eruption of pumice from the southwestern flank of Ice Peak.[114] Most of the Big Raven eruptions took place on the western flank of Ice Peak and on the northern flank of Mount Edziza where lava flows from several vents accumulated to form the Desolation and Snowshoe lava fields.[115] About 1.7 cubic kilometres (0.41 cubic miles) of volcanic material was deposited by the Big Raven eruptions.[26]

The rocks deposited during the Big Raven eruptive period comprise the Big Raven Formation.[17] They are mainly alkali basalts and hawaittes, although a small volume of comenditic trachyte belonging to the Sheep Track Member was also produced.[116] More than 29 eruptions took place during this eruptive period, most of which resulted in the creation of cinder cones.[13][115] These cones are of Holocene age and occur in the Snowshoe Lava Field, the Desolation Lava Field and adjacent to the Spectrum Range.[117] Eruptions during Big Raven time continued within the last 2,000 years, but the precise age of the latest one is unknown.[117][118]

Snowshoe Lava Field

One of the first volcanoes to erupt during the Big Raven eruptive period was Tennena Cone which formed high on the western flank of Ice Peak.[119] It issued basaltic magma under the summit ice cap during the height of the neoglaciation when the ice cap was much larger in area than it is now.[64][119] As the molten basalt accumulated around the erupting vent, it was quenched by the overlying ice cap to form the steep-sided, pyramid-shaped pile of pillow lava that comprises Tennena Cone. A meltwater channel thawed from the base of the cone provided the pathway for a thin lava flow. As the lava flow reached the western edge of the ice cap, it caused a violent interaction with meltwater which spread onto the plateau.[119] Two unnamed volcanoes also in the Snowshoe Lava Field formed subglacially south of Tennena Cone.[115]

After the summit ice cap retreated from lower elevations, renewed volcanism in the Snowshoe Lava Field constructed Cocoa Crater, Coffee Crater and other subaerial cinder cones by lava fountaining. Their construction was accompanied by the eruption of massive lava flows that travelled into the valleys of Sezill Creek and Taweh Creek. Lava from a fissure eruption south of Tencho Glacier flowed west into Taweh Creek and east into Shaman Creek. The result of this eruption was the formation of The Saucer which is one of the youngest volcanic features in the Snowshoe Lava Field.[120]

Arctic Lake Plateau and east slope centres

A dark-coloured volcanic cone rising above a sparsely snow-covered plateau.
Nahta Cone from the east

Two Big Raven centres occur at the southern end of the MEVC.[115] Nahta Cone is the southernmost and lies near the northern edge of the Arctic Lake Plateau.[121] It was the source of a narrow, 3-kilometre-long (1.9-mile) basaltic lava flow that travelled northwards and then westwards into the head of Nahta Creek.[122] The other eruptive centre is a now-destroyed cinder cone that formed on the unstable southern flank of Kuno Peak in the Spectrum Range.[123] It produced a basaltic lava flow that travelled onto the Arctic Lake Plateau. Subsequent landsliding on Kuno Peak removed much of the original cone and buried the associated lava flow.[124]

At least three eruptive centres were active on the deeply eroded eastern flank of Mount Edziza during the Big Raven eruptive period.[115] Cinder Cliff in the north fork of Tenchen Creek valley formed when an eruption of basaltic lava engulfed loose debris and ponded against stagnant ice.[119] The other two eruptive centres, Icefall Cone and Ridge Cone, have been glaciated and are poorly exposed. Both cones produced lava flows but they are also poorly exposed, having been almost completely buried under gravel, moraine, talus and glacial ice. A 6.5-kilometre-long (4.0-mile) lava flow occupying a narrow, wedge-shaped valley on the east slope of Mount Edziza may have issued from Icefall Cone, Ridge Cone or an undiscovered vent inside the valley. Its terminus lies near Kakiddi Lake where it is well exposed for 2 kilometres (1.2 miles).[125]

Desolation and Mess Lake lava fields

An overhead view of the edge of a cliff-bounded plateau.
False colour image of air-fall tephra from The Ash Pit

The first cones to form in the Desolation Lava Field were Sleet Cone and Storm Cone, both of which produced lava flows that travelled over glacial till.[126] Subsequent volcanism created the three Triplex Cones 3 kilometres (1.9 miles) north of Storm Cone. They issued a 12-kilometre-long (7.5-mile) sequence of lava flows that extends northwesterly to near the south shore of Buckley Lake. Renewed eruptive activity formed Sidas Cone and Twin Cone, both of which are products of simultaneous lava fountaining from more than one vent.[127] Lava flows from both cones travelled to the northwest and northeast, respectively.[128] The subsequent eruption of Moraine Cone produced a roughly 14-kilometre-long (8.7-mile) lava flow that travelled northeast into the Kakiddi Creek and Klastline River valleys; both streams were temporarily dammed by the lava flow. Eve Cone and Williams Cone were created by the latest eruptions in the Desolation Lava Field, both of which produced lava flows more than 10 kilometres (6.2 miles) long.[129] Willow twigs preserved in ejecta from Williams Cone have yielded a radiocarbon date of 610 CE ± 150 years.[130][131]

The Mess Lake Lava Field northwest of the Spectrum Range issued from three cinder cones on the edge of the Mess Creek Escarpment.[118][122] Lava flows from the two oldest cones travelled to the west and most likely cascaded down the escarpment into Mess Creek valley. However, any remnants of this lava on the escarpment or in Mess Creek valley have been removed by erosion. The youngest cinder cone, The Ash Pit, formed at the south end of the Mess Lake Lava Field. The Ash Pit eruption produced mainly pyroclastic ejecta in the form of ash and cinders.[122] Much of this material was blown to the northeast by a strong, uniform wind during eruption and deposited onto the Kitsu Plateau.[132]

Kana Cone and Walkout Creek centres

The Kana Cone eruption was characterized by the effusion of basaltic lava flows and the build up of volcanic ejecta around the erupting vent. Several lobes of lava were produced during this eruption.[130] They flowed around eroded remnants of lava produced during the Klastline eruptive period and engulfed the Klastline River valley, temporarily damming the river. The Klastline River was forced to establish a new route along the northern valley wall where it still flows to this day. Several pulses of lava took place during the Kana Cone eruption, each resulting in the formation of new lava channels.[133]

Two small cinder cones formed in Walkout Creek valley during the Big Raven eruptive period, both of which produced basaltic lava flows.[134] The largest cone is about 120 metres (390 feet) high and was constructed on top of a slow moving landslide originating from the northern side of the valley. Both cones have been deeply dissected, the larger cone having been segmented into arcuate, step-like slices from continued movement of the landslide.[122]

Sheep Track Member

A small but violent VEI-3 eruption burst from the southwestern flank of Ice Peak near the end of the Big Raven eruptive period.[120][131] It deposited granular trachyte pumice of the Sheep Track Member which fell over an area of about 40 square kilometres (15 square miles).[135] Larger, snowball-sized chunks of this pumice fell near the vent area while smaller, pea-sized fragments landed around the parameter of the deposit.[136] All of the Snowshoe Lava Field flows and cones are covered by Sheep Track pumice with the exception of The Saucer which likely postdates the Sheep Track eruption.[137] The location of the vent that issued the pumice is unknown but it may lie under Tencho Glacier.[64][138] Fission track dating has yielded an age of 950 CE ± 6,000 years for the Sheep Track pumice.[131]

See also

Notes

  1. ↑ Felsic pertains to magmatic rocks that are enriched in silicon, oxygen, aluminum, sodium and potassium.[7]
  2. ↑ Mafic pertains to magmatic rocks that are relatively rich in iron and magnesium, relative to silicium.[7]
  3. ↑ Peralkaline rocks are magmatic rocks that have a higher ratio of sodium and potassium to aluminum.[14]
  4. ↑ A dike is a sheet-shaped intrusion of magma into pre-existing rock.[35]
  5. ↑ Volcaniclastic rocks are broken fragments (clasts) of volcanic rock.[14]

References

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  3. ↑ Souther 1992, p. 1.
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Sources

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