The Canadian Cordillera hosts numerous Pleistocene and Holocene volcanoes and volcanic deposits, including a number of volcanoes that have erupted within the last several hundred years. The nature and composition of volcanic edifices and deposits are diverse and dictated by the complex configuration of tectonic plates along the western margin of British Columbia and the thermal structure of the underlying mantle. Our modern knowledge of these is built upon more than a century of field- and increasingly, laboratory-based studies. We recognize five distinct volcanic domains within the Cordillera that are distributed across British Columbia, the Yukon Territory, and easternmost Alaska. These include the Wrangell Volcanic Belt, the Northern Cordilleran Volcanic Province, the Anahim Volcanic Belt, the Wells Grey-Clearwater Volcanic Field, and the Garibaldi Volcanic Belt representing the northern extension of the Cascade Volcanic Arc. Volcanism in the Canadian Cordillera spans the full range of explosive to effusive behaviours, encompasses the suite of common volcanic chemical compositions (alkaline to calc-alkaline and nephelinite to peralkaline rhyolite), and is expressed by long-lived stratovolcanoes, shield volcanoes, and calderas, as well as shorter-lived tephra cones and associated lava flows. The range in tectonic settings (subduction to extension), eruption environments (subaerial–subaqueous–cryospheric), and topographic variability make volcanism within the Canadian Cordillera as diverse as anywhere on Earth, yet it is also the least studied. Here, we summarize the current state of knowledge concerning volcanism within the Canadian Cordillera and conclude with thoughts on research areas that merit further effort, namely glaciovolcanism and volcanic hazards.
The western margin of North America hosts a diverse assemblage of large Cenozoic igneous provinces. The temporal, spatial, and petrological characteristics of these provinces are a consequence of the variety of tectonic environments and magmatic processes, including lithospheric extension (e.g., Basin and Range), migration of mantle plumes (e.g., Snake River Plain), and subduction (e.g., Cascade Volcanic Arc, CVA). The Canadian Cordillera situated in British Columbia and the Yukon Territory also hosts a diverse array of numerous (≳300) Pleistocene (∼2.6 m.y. to 11 700 years) and Holocene (<11 700 years) volcanoes (Fig. 1). This includes a minimum of 54 Holocene eruptions in 10 different regions of the Canadian Cordillera that span the length and breadth of the entire region (Hickson and Edwards 2001). This volcanism includes explosive to effusive eruptive styles and is expressed by long-lived stratovolcanoes and shield volcanoes (e.g., Hamilton and Evans 1983; Souther 1990; Edwards et al. 2002), shorter-lived tephra cones, and associated lavas (e.g., Le Moigne et al. 2020; Williams-Jones et al. 2020) as well as isolated tephra and crypto-tephra beds (e.g., Jensen et al. 2014, 2019). Pleistocene and Holocene volcanic rocks within the Canadian Cordillera span a wide range of petrological characteristics, including basic to silicic chemical compositions, common (e.g., basalt, andesite, and dacite) to less common (e.g., nephelinite to peralkaline rhyolite) rock types, and alkaline, subalkaline, and calc-alkaline rock suites.
Historically, Canadian volcanism and volcanoes have been understudied relative to their global counterparts leading to a general lack of awareness of the extent of Pleistocene–Holocene volcanism within the Canadian Cordillera (cf. Hildreth 2007). For example, a topical volume published in 1977 by the Geological Association of Canada entitled “Volcanic Regimes in Canada” comprised 23 papers of which only 2 concerned Cenozoic volcanic rocks within the Canadian Cordillera (i.e., Fiesinger and Nicholls 1977; Souther 1977). This is largely due to their relative inaccessibility, as many of the edifices and deposits in Canada require expensive transportation (e.g., helicopters and float planes) and present other logistical complexities (e.g., extended remote camping, wildlife issues, and technically challenging terrains). Up to and including the 1970s, our knowledge of Cordilleran volcanism was derived mainly from regional mapping wherein the reporting and descriptions of volcanic occurrences, and the data accrued, came almost exclusively from the Geological Survey of Canada.
Over the past 40 years, there have been significant advances in understanding the nature, causes, implications, and consequences of Pleistocene and Holocene volcanism in Canada. Our intent, here, is to provide a “state-of-the-science review” on volcanism within the Cordillera to help celebrate the 60th anniversary of the Canadian Journal of Earth Sciences. Below, we provide an overview of Cordilleran volcanism that includes historical contributions, the current state-of-the-science, and mileposts to outstanding research issues.
2. Setting the stage: historical volcanology
We benefit significantly from the fundamental mapping and foundational ideas that developed over the first 70 years of the 20th century. Months-long field seasons by H. Bostock, G.M. Dawson, H. Gabrielse, F.A. Kerr, W. Matthews, J. Souther, H. Tipper, and others produced regional-scale geological maps that also identified and highlighted Cenozoic and Recent volcanic deposits. Many of these extensive field expeditions in the 1920s–1940s were funded by Canada’s Federal government with the aim of identifying strategic resources.
While not tasked with explaining the origins of these largely non-economic deposits, early workers understood the importance of being inclusive in their mapping and stratigraphic work. One example of an early and remarkable scientific insight was that many of the youngest Cordilleran volcanic deposits were closely associated with glacial deposits. Kerr (1925, 1948), for example, defined the Tuya Formation, recognized the cotemporal nature of volcanism and glaciation in the Dease Lake area (1925), and advanced the idea of damming of lavas by masses of ice at Hoodoo Mountain (1948).
The works of William (Bill) Mathews and Jack Souther deserve specific mention because they both contributed substantial knowledge of the volcanology of the Canadian Cordillera. In addition to geological mapping, they also produced new datasets and ideas relevant to physical volcanology and the petrological–geochemical sciences. Early work by Mathews (1947) in the Tuya-Teslin area of northern British Columbia developed ideas about entire volcanic edifices being formed under and within now extinct ice sheets. There, he first recognized and defined “tuya” volcanoes and provided our first understanding of their paleoenvironmental import (Watson and Mathews 1944; Mathews 1947). After his paradigm-shifting work in northern British Columbia, he focused on mapping and describing volcanic deposits of the northern CVA in southwestern British Columbia (i.e., Garibaldi Volcanic Belt, GVB) (Fig. 2; Mathews 1951, 1952, 1958).
Souther’s career spanned the length and breadth of Canadian Cordillera volcanism, as well as studies of Mesozoic volcanism on Axel Heiberg Island during Operation Franklin in the Canadian High Arctic. He mapped and described volcanic rocks from the Wrangell Volcanic Belt (WVB) near the northern limits of the Canadian Cordillera to its southern limits expressed as the GVB. He also spent the later part of his career leading the exploration for geothermal resources in Canada (Souther 1980) and was one of the first to propose crustal-scale tectonic features to explain the anomalous origins of non-subduction volcanism throughout British Columbia (Souther 1970). His two largest projects were focused on Mount Edziza (Fig. 2), within the central Northern Cordilleran Volcanic Province (NCVP), and the Ilgachuz Range in the centre of the Anahim Volcanic Belt (AVB). Souther spent more than two decades mapping the enormous (>650 km3) Mount Edziza volcanic complex (Figs. 2A–2C; Souther and Symons 1974; Souther et al. 1984; Souther 1992), as well as the Ilgachuz volcanic complex (∼600 km3; Souther 1986; Souther and Souther 1994). For each of these edifices, Souther provided extensive petrographic, geochemical, and geochronological datasets that he used to document their tectonic and petrological origins, range of eruption styles, and petrological–geochemical evolution (Souther and Hickson 1984).
3. Distribution of volcanic fields
Volcanism localized along the western margin of North America is closely related to the current tectonic configuration between the North American, Pacific, and Gorda–Juan de Fuca–Explorer plates (Fig. 1). Where plate boundaries are convergent (e.g., Washington, Alaska), magmatism is driven by subduction and dominated by calc-alkaline stratovolcanoes (e.g., WVB, CVA, and Aleutian Volcanic Arc, AVA; Figs. 1, 2D, and 2E). In northern British Columbia and the Yukon Territory, volcanism is unrelated to subduction as the boundary between the Pacific plate and North America transitions to continent-parallel strike slip. Here, magmatism is dominated by mafic-to-silicic alkaline volcanoes ascribed to extensional forces that potentially reflect Miocene to Recent changes in relative plate motions (e.g., Fig. 1; Souther 1970; Edwards and Russell 2000).
Canada’s young volcanoes have historically been grouped or divided into five domains mainly based on their geographic distributions that are related to the regional tectonics of northwestern North America. These are the WVB, the NCVP, the AVB, the Wells Grey-Clearwater Volcanic Field (WGCVF), and the GVB that is essentially the northern extension and termination of the CVA (Fig. 3). The Chilcotin Group basalts (CGb) represent another important suite of volcanic rocks covering a large portion of the Interior Plateau in central and south-central British Columbia. These Neogene–Paleogene plateau basalt lavas include basalt and associated pyroclastic and volcaniclastic sedimentary rocks of Late Oligocene to Early Pleistocene age extending from the Okanagan north to the Nechako Plateau but exclude AVB volcanoes and the youngest Pleistocene to Holocene valley-filling basalts (Bevier 1983; Dostal and Hamilton 1996; Anderson et al. 2001; Dohaney et al. 2010).
3.1. Wrangell Volcanic Belt
The WVB comprises a volcanic arc (WVA) and several individual volcanic fields, has a length more than 450 km and a width of up to 190 km, and is distributed across eastern Alaska, southwestern Yukon, and northwestern British Columbia (Souther 1991; Skulski et al. 1992; Thorkelson et al. 2011; Trop et al. 2022; Fig. 3). The WVB is situated largely within the Wrangell–St. Elias Mountains and comprises six volcanic fields (Wrangell, Sonya Creek, St. Clare, Nines Creek, Alsek, and Stanley Creek; Trop et al. 2022). The main volcanoes within the WVA are situated within and immediately outboard (south) of a Cretaceous suture zone that separates the accreted Insular terranes from older terranes exposed inboard (northeast) of the Denali Fault (Figs. 3 and 4A).
Volcanism was initiated ∼30 Ma and extends to the Holocene; Mount Churchill erupted explosively <2000 ya (i.e., White River ash; Downes 1985; Jensen et al. 2014) and Mount Wrangell has had historic eruptions (Global Volcanism Program 2017). Volcanic landforms include shield volcanoes (e.g., Mount Wrangell), stratovolcanoes (e.g., Mount Churchill), caldera complexes, lava domes, and scoria cones, including some of the largest and highest (>4600 m above sea level) Quaternary volcanoes on Earth (Fig. 4A).
Preece and Hart (2004) provided a synthesis of the geochemical variations found in <5 Ma volcanic deposits documenting the complex interplay between magma generation and evolution processes and the tectonic elements of the arc. The WVB volcanic rocks are dominated by calc-alkaline, medium-K basaltic andesites and andesites but range from basalt to rhyolite and from calc-alkaline to transitional tholeiitic (Fig. 4B). Preece and Hart (2004) identified three geochemical suites (Fig. 4B) based on TiO2 contents in samples with SiO2 < 60 wt.%. Volcanic rocks having TiO2 > 1.15 wt.% define Trend 1, while Trend 2 is defined by volcanic rocks with <1.15 wt.% TiO2. Trend 1 is a high-TiO2 transitional tholeiitic suite related to localized intra-arc extension over at least the last 1 Ma. Trend 2 includes an arc-wide low TiO2, calc-alkaline suite (Trend 2a), as well as a low-TiO2 calc-alkaline suite (Trend 2b) representing volcanoes spanning the northwest to southeast limits to the arc system (including adakitic, amphibole dacite magmas). All three suites contain mafic parental compositions consistent with derivation from a MORB-like mantle wedge variably enriched via the addition of subduction components (Preece and Hart 2004).
Volcanism in the WVB largely reflects subduction-related arc magmatism and slab-edge upwelling of asthenosphere (e.g., Thorkelson et al. 2011) beneath southwestern Alaska and northeastern British Columbia. Wrangell arc magmatism was initiated ∼30–17 Ma along the north flank of the Wrangell–St. Elias Mountains in Alaska and subsequently progressed southeastward ∼17–10 Ma along the Duke River fault in the Yukon (Trop et al. 2022) before progressing back westward ∼6–3 Ma along the southern flank of the Wrangell–St. Elias Mountains in Alaska and then northwestward from ca. 6 Ma to the present in the western Wrangell Mountains (Fig. 4A). The voluminous and sustained nature of WVB magmatism has been interpreted to result from shallow subduction-related flux melting combined with slab edge melting driven by asthenospheric upwelling along the lateral edge of the slab (Thorkelson et al. 2011; Trop et al. 2022, and references therein).
The portion of the Wrangell field situated in Alaska is larger than that in Canada and features Quaternary shield and stratovolcanoes located above a northeastward-dipping subducting slab along the northeastern edge of the Yakutat microplate. The eastern part of the Wrangell field comprises 13–5 Ma calc-alkaline to tholeiitic lavas, pyroclastic deposits, shallow intrusions, and sedimentary strata. The Sonya Creek field, located in the US immediately west of the Yukon–Alaska border, represents the earliest (ca. 30–19 Ma) phase of arc magmatism related to subduction of Yakutat microplate oceanic lithosphere beneath North America (Trop et al. 2022, and references therein).
In Canada, the WVB is represented by four volcanic fields of Miocene (18–11 Ma) lavas (Fig. 4A), pyroclastic rocks, and shallow intrusions including the Alsek, St. Clare, Sonya Creek, and Nines Creek fields (Trop et al. 2022). The volcanic fields are mainly distributed for >300 km along the Duke River fault. The volcanic deposits are locally faulted and folded by late tectonism and highly eroded due to uplift leaving remnants of more extensive Miocene lavas and pyroclastic rocks. The youngest volcanic deposits found in this part of Canada are those of the White River ash resulting from the Holocene explosive eruption(s) of Mount Churchill (Downes 1985; Lerbekmo 2008; Jensen et al. 2014) situated in the Wrangell field within Alaska (Fig. 4). Wrangell volcanic fields in Canada are interpreted as being derived from both subduction-related arc magmatism (calc-alkaline) and intraplate-type magmatism having no arc-like chemistry (alkaline). The close spatial association of volcanoes and strike-slip faults and their geochemical compositions (i.e., primitive alkaline mafic volcanics) suggest that eruptions in both Canada and Alaska occurred along “leaky” strike-slip faults (Skulski et al. 1992; Thorkelson et al. 2011; Trop et al. 2022).
3.2. Northern Cordillera Volcanic Province
The NCVP, as defined by Edwards and Russell (1999, 2000), includes more than 100 mapped occurrences of Pleistocene and younger volcanic rocks distributed across northwestern British Columbia, the Yukon Territory, and adjacent easternmost Alaska (Fig. 5). The NCVP encompasses a broad range of volcanic styles, including large volcanic plateaus with mafic and felsic eruption products, isolated volcanic cones, lavas, and glaciovolcanic eruption products (e.g., Figs. 2A–2C). Volcanism across the NCVP has been studied by many more research groups relative to other volcanic domains in the Canadian Cordillera. Research themes have varied but include studies of edifice stratigraphy and petrology (Hamilton and Evans 1983; Eiché et al. 1987; Souther 1992; Edwards et al. 2002, 2011), focused studies on glaciovolcanism (e.g., Mathews 1947; Allen et al. 1982; Moore et al. 1995; Edwards and Russell 2002; Russell et al. 2021), analysis of primitive lava chemistries to characterize mantle source regions (Nicholls et al. 1982; Francis and Ludden 1990, 1995; Cousens and Bevier 1995; Edwards and Russell 2000; Abraham et al. 2005; Hyndman and Canil 2021; Canil and Hyndman 2023), and studies of peridotite xenoliths to characterize the underlying Cordilleran lithosphere (Ross 1983; Brealey et al. 1984; Canil and Scarfe 1989; Peslier et al. 2002; Harder and Russell 2006; Ghent et al. 2019; Francis et al. 2010, 2019, and references therein; Canil and Russell 2022).
The Northern Cordilleran Volcanic Province subsumes the volcanic deposits previously referred to as the Stikine Volcanic Belt (e.g., Souther 1992) and includes volcanism north of latitude 55°N and west of longitude 126°W and those volcanic centres west of the Tintina fault system and east of the Denali-Coast fault system (Fig. 5A). The Tintina-Northern Rocky Mountain Trench fault system separates rocks having an affinity with the North American plate (i.e., autochthonous) to the east from the accreted (i.e., allochthonous or paratochthonous) tectonostratigraphic terranes to the west. The Denali-Coast fault system extends almost 2000 km from central Alaska to northwestern British Columbia and is an approximate boundary between Wrangellia terrane to the west and more coherent allochthonous terranes to the east (e.g., Stikinia; Fig. 5A). Recent geophysical work by Miller et al. (2018) and Gama et al. (2022) has shown that the Denali Fault System demarcates a fundamental lithospheric boundary, with colder and thicker lithosphere to the north of the boundary. Consequently, NCVP volcanism is spread across four major tectonostratigraphic terranes: Stikinia, Cache Creek, Yukon-Tanana, and Cassiar. Edwards and Russell (2000) used phase equilibria calculations based on lava compositions and geothermometry of mantle-derived xenoliths to construct a north–south cross-section of the NCVP lithosphere. The model cross-section predicted that the lithosphere beneath the Northern Cordilleran Volcanic Province thickens from the north to the south and that it is thicker beneath Stikinia (southern NCVP) than beneath the Cache Creek and Yukon-Tanana terranes (northern NCVP; Fig. 5A).
The southern limit of the NCVP is defined by isolated volcanic vents and eroded lava remnants north of the town of Terrace and the Skeena Arch (Fig. 5A), which marks the start of a gap in magmatism between the NCVP and the three Neogene magmatic provinces to the south, the AVB, the Chilcotin Group basalts, and the Wells Gray-Clearwater Volcanic Field (Fig. 3).
Eruption ages for NCVP volcanism span more than 10 Ma, with older magmatism focused at large complexes (Level Mountain and Edziza; Hamilton and Evans 1983; Souther et al. 1984) in the centre of the NCVP. Many individual centres show evidence of interactions with ice and are assumed to be Pleistocene; this has been confirmed recently for more than 20 centres in the Tuya-Kawdy area (Edwards et al. 2020), where tuya volcanoes range in age from 2.8 to 0.06 Ma. The NCVP also hosts the largest number of Holocene vents that span most of the length of the province from Volcano Mountain in the central Yukon southward to the Tseax tephra cone and lavas (erupted in the 1700s; Williams-Jones et al. 2020) in British Columbia. It also hosts the youngest eruption in Canada just south of the Iskut River at Lava Fork, which may have erupted in the 1800s (Russell and Hauksdottir 2001). Oral histories of local First Nation’s record the eruption of Tseax (1675–1778 CE), but no evidence has been found in oral histories for the eruption of Lava Fork.
NCVP volcanism has produced mainly alkali olivine basalt and Na-rich trachybasalt (i.e., hawaiite) (Souther 1991; Edwards and Russell 2000); however, more strongly alkaline rock types, including nephelinite, basanite, and peralkaline phonolite, trachyte, and Na-rich rhyolite (i.e., comendite), are locally abundant (Fig. 5B). The more evolved rock types (e.g., phonolite to Na-rich rhyolite) are found at four longer-lived volcanic systems: Level Mountain (Hamilton and Evans 1983), Edziza (Souther 1992), Maitland (Souther 1977), and Hoodoo (Edwards et al. 2002). The most MgO-rich nephelinites, basanites, and alkaline basalts from throughout the NCVP show trace element abundances and isotopic compositions that are consistent with an asthenospheric source region similar to average oceanic island basalt (e.g., Edwards and Russell 2000, and references therein).
Souther (1977) ascribed magmatism within the northern Cordillera to rifting based on the geochemical compositions of the volcanic rocks and on normal faulting expressed by the Mess Lake graben near Mount Edziza. Edwards and Russell (1999, 2000) provided a temporal link between this magmatism and relative plate motions supporting a modified incipient rift model. The presence of a slab window from 20 Ma onwards (Thorkelson and Taylor 1989) facilitated access to renewed asthenosphere not modified by subduction beneath the northwestern edge of North America. Therefore, when changes in relative Pacific–North American plate motions generated extensional stresses (∼11 Ma), fresh, hot asthenosphere beneath the relatively thick (45–60 km; 1–2 GPa) lithosphere was available to produce alkaline magmas. Abundant east–west faults in the northern Cordillera may also provide corridors of local extension facilitating magma transport and localizing volcanism. It is also possible that crustal loading and unloading by continental ice sheets during the Quaternary exploited a pre-existing, magma-charged lithosphere to produce subordinate magmatism in a rhythm sympathetic to ice fluctuations (Grove 1974; Edwards et al. 2002).
3.3. Anahim Volcanic Belt
The AVB comprises an ∼330 km long west–east-trending linear array of volcanic and plutonic centres at ∼52°N latitude (Fig. 6). It is a regionally distinct structure oriented perpendicular to the major geomorphological, structural, and tectonic elements of the Canadian Cordillera. As a group, the edifices of the AVB define a pronounced trans-Cordilleran zone of alkaline (Na + K + Al)/Si > 2/3) and peralkaline ((Na + K)/Al > 1) magmatism that has propagated eastward from the western margin of the Coast belt over the past 14–12 Ma (Fig. 5; Souther 1986; Kuehn et al. 2015).
The AVB volcanic centres include Miocene-aged intrusive rocks exposed on the western edges of the Coast belt where uplift rates have been substantial, a series of large, complex, partially dissected shield volcanoes (e.g., Rainbow, Ilgachuz, and Itcha) ranging in age from 8 to 1.5 Ma, and the Holocene tephra cone(s) and lavas at Nazko (Fig. 6C). Canada’s only documented volcanic seismic swarm occurred in the vicinity of Nazko cone in 2007 (Cassidy et al. 2011; Cassidy and Mulder in press).
The chemical compositions of AVB volcanic rocks are alkaline to peralkaline and diverse, including alkaline basalts, basanites, nephelinites, trachytes, and phonolite (Fig. 6B). Kuehn et al. (2015) provided additional data on many smaller volcanic centres situated near Satah and Baldface mountains and immediately south and east of the Itcha Range, respectively. These volcanic centres are chemically varied, exhibit alkaline to peralkaline affinities, and range in composition from undersaturated basanites to more highly evolved trachytes and phonolites. In this regard, they are very similar to other AVB volcanic centres. 40Ar/39Ar age determinations for these volcanic centres indicate Early Pleistocene eruption ages contemporaneous with Itcha volcanism spanning ∼1.6 Ma from ∼2.52 to 0.91 Ma (excluding one outlier, Fig. 6C).
The unique compositional character of AVB volcanic rocks, and their trans-Cordilleran distribution, combined with the apparent linear age progression with distance from the margins of the Cordillera have driven hypotheses concerning their origin. In continental settings, alkaline to peralkaline volcanic rocks are commonly interpreted to result from rifting (e.g., East African Rift, Africa; Rhine Graben, Europe; Basin and Range, USA). While the NCVP also has extension-related, long-lived alkaline to peralkaline volcanism (Level Mountain: Hamilton and Evans 1983; Mount Edziza: Souther et al. 1984; Hoodoo Mountain: Edwards et al. 2002), the volcanism expressed by individual AVB volcanic centres appears to be different. First, the volcanism at each volcanic centre was relatively short-lived (<2 Ma; cf. Fig. 6C), unlike the long-lived volcanism at other Cordilleran peralkaline volcanoes (e.g., Level Mtn or Edziza) or in the East African Rift (cf. Kuehn et al. 2015). Furthermore, rift-related volcanism rarely creates linear, time-correlated geographic trends.
The apparent linear decrease in ages from west to east (Fig. 6C) is consistent with the possibility that AVB magmatism is related to the passage of the North American plate over a relatively stationary mantle plume (Souther 1986). Velocities for the North American plate during the Neogene are 2–3 cm/year, which is permissive of the distance–time distribution of AVB volcanoes (Kuehn et al. 2015, and references therein). Euler pole calculations and modelling of a potential AVB hotspot trend are also permissive of the mantle plume/hotspot hypothesis (Hickson 1987). Furthermore, the calculated North American plate motion vector constrained by the AVB volcanism (∼14.5 Ma to present) is ∼2.1 cm/year with an azimuth of ∼250° that is within error of values for the Yellowstone hotspot track (azimuth of 249.5 ± 10.7° and 2.68 ± 0.78 cm/year, Gripp and Gordon 2002; 2.38 ± 0.21 cm/year, Anders et al. 2014). However, recent mapping and dating of alkaline volcanic centres on the western margin of the Cordillera near Milbanke Sound (i.e., Kitasu Hill and MacGregor cone; Hamilton et al. in press) returned Pleistocene to Holocene ages indicating simultaneous volcanism at both the leading and trailing (i.e., Nazko cone; Fig. 6C) ends of the hotspot track. This suggests that, as the geochronometry database expands, the current age–distance trend upon which the original “hot-spot” hypothesis relies may become more diffuse.
An additional possibility is that AVB volcanism is related to the configuration of the subducted slab (i.e., slab window; Thorkelson and Taylor 1989; Thorkelson et al. 2011) beneath the western Canadian Cordillera. The AVB is situated immediately north of the edge of the subducting Juan de Fuca/Explorer slab and at an acute angle to that subducted plate at depth (Audet et al. 2008). Upwelling of asthenospheric material along the edge of the subducted plate could initiate sustained partial melting and alkaline magma production and promote crustal attenuation and rifting (Souther et al. 1987; Audet et al. 2008; Thorkelson et al. 2011). Sustained mantle magma production would also support crustal lithospheric storage combined with crystallization and chemical assimilation leading to the production of more silicic peralkaline magmas. The progressive westward migration of the North American plate over the edge of the slab window could also explain the linear west-to-east decrease in magmatic ages for this narrow volcanic belt oriented perpendicular to the subduction-transform margin (Kuehn et al. 2015). These competing hypotheses (i.e., hotspot vs. slab edge) might be resolved using more recent seismic tomographic data (e.g., Sigloch and Mihalynuk 2017).
3.4. Garibaldi Volcanic Belt (northern Cascade Volcanic Arc)
The CVA is a chain of volcanoes extending ∼1250 km from southwest British Columbia, Canada, to northern California in the United States (Figs. 1 and 7). Quaternary magmatism is a response to north-easterly subduction of remnants of the Farallon plate (Juan de Fuca, Explorer, and Gorda plates) beneath the North American plate (Fig. 7A; Hildreth 2007). Volcanism has produced more than 2300 individual vents, of which 22 are major volcanic structures (Hildreth 2007; Wilson and Russell 2018). The volcanoes define a band of predominantly calc-alkaline, intermediate-composition stratovolcanoes that are situated ∼200 km inland from the coast.
The GVB is the northern segment of the CVA and results from subduction of the Juan de Fuca plate (the largest fragment of the Farallon plate) beneath the North American Plate (Fig. 7B; Mathews 1958; Green 1981; Hildreth 2007). The GVB centres stretch from Glacier Peak in northern Washington State (USA) northwards ∼340 km to the Bridge River volcanic field (Fig. 7B; Venugopal et al. 2020). Some classifications also include the Silverthrone and Franklin Glacier volcanic fields, which are situated off the main trend of the GVB (Fig. 7A; e.g., Green 1981; Green et al. 1988; Souther 1991).
Volcanism in the belt spans the entire Quaternary period (Pleistocene to Holocene in age) and is expressed by more than 100 eruptive centres, with deposits ranging in composition from alkaline basalt to rhyolite (Mathews 1958; Green 1981; Lawrence et al. 1984; Green et al. 1988; Brooks and Friele 1992; Wilson and Russell 2018, and references therein). The belt hosts three major long-lived stratovolcanoes (Mount Garibaldi, Mount Cayley, and Mount Meager), in addition to a plethora of smaller, ancillary volcanic centres (Souther 1991; Hildreth 2007). Each of the three stratovolcanoes comprises porphyritic andesite to rhyolite and the group contains some of the oldest Quaternary volcanic rocks within the belt (∼3.5 Ma; Green et al. 1988). The most recent volcanic activity was a late Holocene (2360 calendar years B.P.) subplinian to Vulcanian eruption at Mount Meager (Read 1990; Hickson et al. 1999, and references therein). Oral history of the local First Nations living in vicinity of the Mount Meager records the eruption history (Wilson et al. in press).
The smaller (<1 km3), ancillary centres are well distributed throughout the belt and are predominantly basaltic in composition, monogenetic, or short-lived features, and have ages spanning the entire mid-late Pleistocene (Harris et al. 2023). Moderate volumes of alkaline to transitional basalt are restricted to the northern end of the belt in the Mount Meager, Bridge River, and Salal Glacier volcanic fields (Lawrence et al. 1984; Mullen and Weis 2015; Wilson and Russell 2017; Venugopal et al. 2020; Harris et al. 2023). The origin of these basaltic rocks is widely debated, with studies suggesting that they derived from either low-degree partial melting of the mantle wedge or from mantle upwelling along the margin of the subducting Juan de Fuca and Explorer plates.
One unique aspect of the GVB is its protracted history of multiple glaciations, with cycles of advancing and retreating Cordilleran ice sheets (CIS) throughout the Pleistocene (Clague et al. 2009; Clague and Ward 2011). Consequently, GVB hosts numerous (∼50%) glaciovolcanic landforms resulting from eruptions and interactions with ice—many of these derive from eruptions within the CIS or earlier manifestations of the CIS (Wilson and Russell 2018; Harris et al. 2023; see “Discussion”).
3.5. Wells Grey-Clearwater Volcanic Field
The WGCVF is a Quaternary-age volcanic field in east-central British Columbia (52°N, 120°W; Fig. 8) that is uniquely situated at the boundary between the Intermontane and Omineca tectono-morphological belts. Thus, the volcanic field is underlain by both allocthonous terranes of Quesnellia and Cache Creeks (Fig. 5A) and crystalline basement rocks belonging to the ancestral continental margin (Fig. 8). Souther’s original synopsis of Canadian Cordilleran volcanism included the WGCVF as part of the east–west-trending AVB (Fig. 8A) before being recognized as a separate volcanic domain by Hickson et al. (1995). Additionally, plate modelling showed that volcanism in the WGCVF was not consistent with the WGCVF being an eastern extension of the AVB (Hickson 1987).
The WGCVF comprises numerous, small-volume (<1 km3) mafic volcanic centres (Fig. 8B) that erupted from 3.5 Ma to Holocene (∼7600 years) and combined, comprise ∼25 km3 (Hickson 1987; Hickson et al. 1995). The volcanic rocks are commonly olivine porphyritic and less commonly carry plagioclase and clinopyroxene phenocrysts. Compositionally, most WGCVF volcanoes are alkali olivine basalt to transitional basalt but there are subordinate volumes of nephelinite and basanite (Fig. 8C). There is an apparent general increase in alkalinity (i.e., trachybasalts to basanites) with decreasing age across the WGCVF (Hickson and Vigouroux 2014).
Previous workers have suggested that this intraplate volcanism results from regional extension allowing asthenospheric magmas to exploit pre-existing, deep faults related to the late stages of terrane (e.g., Quesnellia) accretion to ancestral North America (e.g., miogeoclinal and pericratonic rocks of the Kootenay terrane) (Hickson et al. 1995; Hickson and Vigouroux 2014). The WGCVF volcanic rocks, themselves, are enriched in incompatible elements, especially large-ion lithophile elements, display similar trace element patterns to ocean island basalts (i.e. OIB), and are thought to have originated from partial melting of an upwelling asthenospheric mantle source. Melting of the asthenospheric mantle might have stemmed from extension of the overlying lithosphere in response to the early stages of back-arc basin opening in the Omineca and Intermontane belts (Hickson et al. 1995).
Many of the WGCVF centres feature deposits enriched in crusted and mantle-derived xenoliths. Friedman et al. (2016) recently published a comprehensive petrochemical study of the mantle-derived peridotitic xenoliths hosted by the Tasse basalts near Quesnel Lake ∼50 km west of the WGCVF. Their work showed the mantle xenoliths derived from a compositionally heterogeneous, variably metasomatised, subcontinental lithospheric mantle source. They suggest that the underlying mantle lithosphere originated as subarc mantle wedge peridotite at a convergent plate margin (Friedman et al. 2016; Polat et al. 2018).
The WGCVF has a diverse range of volcanic deposits and landforms, including tuyas and ice-marginal valley-edge deposits, volcanoclastic-lacustrine deposits, and associated pillow lavas and hyaloclastites (Hickson and Vigouroux 2014). Volcanism within the WGCVF records evidence for at least four periods of glaciation since 3 Ma, including the most recent continental glaciation (i.e., Fraser Glaciation). That record is expressed by distinct edifice morphologies (flat-topped and conical tuyas) and distinctive lithofacies indicative of eruptions in a cryospheric environment (e.g., pillow lavas, pillow breccias, hyaloclastite, and palagonite) (Hickson et al. 1995; Hickson 2000). Locally, sedimentary deposits with characteristics of ice deposition are interlayered with the volcanic deposits (Hickson and Vigouroux 2014).
In several instances the glaciovoclanic edifices within the WGCVF document major glacial events and allow for estimations of minimum ice thicknesses associated with these glaciations (Hickson 1987). For example, the McLeod Hill tuya rises 250–350 m above the surrounding plateau and, thereby, constrains the minimum syn-eruptive thickness of the enclosing ice sheet at ∼3.5 Ma. The Jack’s Jump edifice is dated at 1.9 Ma and features ice-marginal deposits that record a subaqueous to subaerial transition in depositional environment at ∼1700 masl. That elevation is ∼700 m above the present-day valley floor and constrains the thickness of this pre-Fraser valley-filling ice sheet. Similarly, the younger glaciovolcanic deposits at Sheep Track Bench (∼270 ka) preserve a transition between subaqueous and subaerial lavas at 1400 m elevation that is also ∼700 m above surrounding drainage system.
3.6. Chilcotin Group basalts (CGB)
One of the more enigmatic expressions of volcanism in the Canadian Cordillera is the Chilcotin Group. It comprises mostly basaltic lavas spread across the central and southern parts of British Columbia, covering an area of ∼30 000 km2 and with a roughly estimated volume of up to 3500 km3 (Bevier 1983) (Fig. 8A). The lavas also have a broad range in eruption ages (28–1 Ma) and overlap in space and time with the volcanic rocks of the AVB and the WGCVF (Figs. 8A and 8B; Bevier 1983; Hickson and Souther 1984; Dohaney et al. 2010; Andrews et al. 2011). The flat to shallowly dipping CGb lavas form moderately dissected, valley-incised plateaus, mainly comprising basaltic lithofacies varying from 5 to 200 m in thickness (e.g., Bevier 1983). The CGb lavas are typically massive to columnar jointed and olivine-phyric basalt with locally associated pillow basalt and hyaloclastite, occasional red-weathering paleosols, and rare, intercalated silicic tephra. While most of the lavas do not have identified vents, locally small tephra cones, volcanic necks, and gabbroic feeders mark vent locations. These locations are summarized in Dohaney et al. (2010).
The CGb have a transitional geochemistry that is distinct from most of the other extension-related areas of alkaline volcanism within the Canadian Cordillera (e.g., NCVP, AVA, and WGCVF; Fig. 8C). Bevier (1983) proposed a back-arc petrotectonic setting for the CGb based in part on its geographic position mainly inboard of the subducting Juan de Fuca–Gorda–Explorer plates.
Much of the area underlain by CGb lavas is itself covered by a thick blanket of till. This till blanket, with depths of several tens of metres in some locations, limits our understanding of the actual distribution, thickness, and extent of the CGB. Andrews et al. (2011) used water-well records to estimate the thicknesses of CGb beneath glacial drift. They showed that the Chilcotin Group is typically thin (<50 m) and perhaps averaging <25 m thick across most of its mapped distribution. In addition, they suggested that its actual subdrift distribution is very patchy. The thickest sections (>50 m) recorded were restricted to areas where several of the water wells were located outside the mapped extent of the CGb and where the wells may have intersected older basalt (Eocene Endako Group; Anderson et al. 2001; Andrews et al. 2011).
The revised distribution map for the CGb of Dohaney et al. (2010) suggests that the distribution of the CGb is less extensive (by ∼48%) than previously assumed. They argued that the CGb is thickest where lavas ponded in paleo-valleys and could be used to map the distributions of Neogene channels in the Fraser Basin drainage. They also suggested that the geochronology, morphology, and distribution of the basalts were indicative of the CGb resulting from episodic volcanism distributed across the region producing many, small, low-volume, low-profile volcanoes (or rifts) rather than large, sustained outpourings of basalt from large, long-lived, high-volume vents.
4. Discussion: quo vadimus?
The Canadian Cordillera hosts numerous Pleistocene and Holocene volcanoes that are diverse in origins (e.g., subduction to extension), composition (e.g., calc-alkaline to peralkaline), landforms (e.g., stratovolcanoes to shield volcanoes), as well as eruptive style (e.g., explosive vs. effusive) and environment (subaerial–subaqueous–cryospheric). Volcanism within the Canadian Cordillera is as diverse as anywhere on the planet, yet among the least studied.
Future research programmes are bound to provide exciting new observations, datasets, and insights informing on Cordilleran volcanism. Field-based petrological and geochemical studies of these volcanoes are critical for understanding (and refining) our ideas on their magmatic origins, the nature and diversity of their eruptive history, as well as the architecture of the lithosphere with respect to magma storage depths and timescales (e.g., Le Moigne et al. 2022). In particular, many more radiometric (e.g., 40Ar/39Ar) eruption ages for volcanoes across the Canadian Cordillera are needed. These data can constrain magma production rates and fluxes, magma residence times, recurrence intervals for effusive and explosive events, stratigraphic correlations, and issues of causality (e.g., glacial pumping; Grove 1974; Edwards et al. 2002; Wilson and Russell 2020). Paleomagnetic studies are a tremendous complement to volcanological studies in that they can sometimes constrain the age of eruption, but they also inform on thermal history and eruption duration (e.g., Williams-Jones et al. 2020; Harris et al. 2022b; Borch et al. in press).
Alkaline mafic volcanic centres are ubiquitous in the Canadian Cordillera, and they commonly host samples of the underlying mantle lithosphere (i.e., xenoliths). Direct studies of these volcanic-hosted mantle-derived xenoliths (i.e., peridotite, pyroxenite, and dunite) provide insights on the mineralogical and geochemical heterogeneity, thermal state (e.g., Nicholls et al. 1982; Brearley et al. 1984; Canil and Scarfe 1989; Harder and Russell 2006; Ghent et al. 2019), structural properties (Ross 1983), age, and origins of the subcontinental mantle lithosphere (e.g., Peslier et al. 2002; Francis et al. 2010, and references therein; Canil and Russell 2022). These studies provide direct petrologic constraints on the mantle lithosphere (Edwards and Russell 2000; Hyndman and Canil 2021; Canil and Hyndman 2023, and references therein).
Below, we restrict our discussion to two critical and emerging lines of scientific investigation: (1) glaciovolcanism and (2) volcanic hazard and risk assessment.
Glaciovolcanism occurs wherever volcanoes and glaciers coincide in space and time and includes “volcano interactions with ice in all its forms (including snow and firn) and, by implication, any meltwater created by volcanic heating of that ice” (Kelman et al. 2002; Smellie and Edwards 2016). Glaciovolcanic edifices have distinctive morphologies and lithofacies that reflect their contact with, or impoundment by, ice (e.g., Watson and Mathews 1944; Mathews 1947; Hickson 2000; Edwards and Russell 2002; Kelman et al. 2002) and are records of paleoenvironmental conditions during eruptions.
The Canadian Cordillera of British Columbia and the Yukon Territory has been inundated multiple times by incarnations of the CIS during the Pleistocene (Fig. 9; Clague et al. 2009; Clague and Ward 2011; Edwards et al. 2020). Consequently, a significant amount of Cordilleran volcanism is glaciovolcanic in nature. The Last Glacial Maximum, expressed as Fraser Glaciation (∼30 000 calendar years B.P.) in the Cordillera and coinciding with Marine Isotope Stage 2 (Lisiecki and Raymo 2005), is the latest example of a major Cordilleran ice advance manifest as the CIS (Fig. 9; Booth et al. 2003; Clague et al. 2009; Clague and Ward 2011).
The coincidence of volcanism and glaciation was recognized 100–70 years ago by geologists mapping parts of the Canadian Cordillera (e.g., Kerr 1925, 1948; Mathews 1947, 1951; Grove 1974). Watson and Mathews (1944) in the Tuya–Teslin region of NW British Columbia documented numerous, small, apparently young (Pleistocene) volcanoes (Figs. 10A–10D) and provided the first descriptions of morphologies and lithofacies associations that became diagnostic of glaciovolcanic edifices:
" … flat-topped volcanic mountains of somewhat circular plan. The lower parts of these mountains are composed essentially of beds of black basaltic agglomerate and tuff having dips, probably initial, of 15 to 30 degrees. In some mountains these rocks dip radially outward from the centre, suggesting that they form the flanks of a cone. Near the tops of most of these mountains the beds of agglomerate and tuff are truncated by remarkably level surfaces, presumably formed by erosion, and are capped with flat-lying lavas which commonly reach 300-400 feet in thickness" (page 29).
In his landmark paper, Mathews (1947) proposed the term “tuya” for these flat-topped, steep-sided volcanoes (Figs. 10A–10C) and interpreted their morphology and volcanic lithofacies as resulting from volcanic eruptions beneath and within late Pleistocene glacial ice sheets. He also noted similar aged, cone-shaped volcanoes lacking flat tops (Fig. 10D) and comprising only pillow basalt, dykes, and fragmental deposits. He postulated that these non-flat-topped volcanic edifices were also glaciovolcanic in origin but that they had not breached the surface of the enclosing englacial lake. Later workers have referred to these edifices as subglacial mounds (e.g., Hickson et al. 1995; Hickson 2000) or more recently are classified as conical tuyas (Russell et al. 2014; Fig. 11).
Mathews also worked in the GVB (Fig. 5) and made unique advances in intermediate-composition (i.e., non-basaltic) glaciovolcanism. For example, Mathews (1951) first described “the Table” that is a steep-sided, flat-topped, elliptically shaped (330 m long and 150 m wide), vertical column of coherent, porphyritic andesite (cf. Wilson et al. 2019a). He proposed that the Table was a new type of tuya and was a glaciovolcanic edifice constructed almost entirely of lava and lacking the subaqueous and volcaniclastic lithofacies that typify the classical (mafic) tuyas (e.g., Mathews 1947; Russell et al. 2014; Smellie and Edwards 2016). It is now considered the type locality for lava-dominated tuyas, which are one of nine categories of glaciovolcanoes recognized globally (Fig. 11; Russell et al. 2014). At least five other lava-dominated tuyas have been found within the GVB, including Ring Mountain (Fig. 10E), Little Ring Mountain, Cauldron Dome, and Slag Hill Tuya in the Mount Cayley Volcanic Field (Kelman et al. 2002) and the Black Tusk in the Garibaldi Lake Volcanic Field (Wilson et al. 2019a).
Russell et al. (2014) devised a descriptive genetic classification scheme for glaciovolcanic edifices (i.e., tuyas). Their classification consolidated the diverse nomenclature that developed mainly in Canada, Iceland, and Antarctica since the 1940s. The morphology and lithofacies associations of individual tuyas inform directly on the style(s) of eruption (i.e., explosive vs. effusive) and the local glaciohydraulic environment (Fig. 11). On that basis, Russell et al. (2014) identified nine distinct glaciovolcanic model edifices that reflect the interplay between volcanism and glaciohydrology (panels 1–9, Fig. 11). The glaciohydraulic environment reflects how well melt water is sealed by the enclosing ice sheet (i.e., rows, Fig. 11) and the style of eruption largely controls the types (e.g., lava vs. tephra) and proportions of lithofacies (i.e., columns, Fig. 11).
4.1.1. Land-based paleo-ice distributions
Glaciovolcanoes are a means to constrain paleo-environments at the time of eruption, and this has import for paleoclimate studies. Glaciovolcanic edifices and their deposits provide direct evidence for the presence of ice and associated bodies of meltwater at the time of eruption. Subaqueous lithofacies, for example, indicate the presence and the minimum depth of a standing body of water (i.e., englacial lake). The subaqueous lithofacies (e.g., pillow lavas and palagonitized tephra) point to glaciovolcanism in situations where no physical means of supporting a deep body of water presently exists.
Some glaciovolcanoes comprise only subaqueous deposits (e.g., Figs. 10D and 11, panels 7–9) in which case they can only constrain the minimum depth of the water body during the eruption. Many glaciovolcanoes feature subaqueously deposited stratigraphic units overlain by subaerial deposits creating passage zones (Jones 1968; Edwards et al. 2011; Russell et al. 2013, 2014): diachronous surfaces marking transitions between subaqueous and subaerial depositional environments (Figs. 10B, 10C, and 11, panels 4–6). In glaciovolcanic settings, the elevation of passage-zone surfaces unequivocally records the height and depth of a paleo-englacial lake. The depth of the water body also fixes the minimum thickness of the enclosing ice sheet needed to provide enough pressure to balance the hydrostat pressure and prevent leakage. In several instances, glaciovolcanoes host multiple passage zones at different elevations recording the dynamic interplay of edifice growth and englacial lake dynamics (i.e., rises and falls in lake level) throughout the eruption and even recognition of lake-level drops that may imply outburst flood events (e.g., Jones 1968; Russell et al. 2013, 2014, 2021).
In summary, forensic analysis of stratigraphic successions and precise age dating of glaciovolcanic edifices (tuyas) represent a powerful resource as the volcanic deposits themselves record direct interaction between volcanism and ice masses. Such analysis can establish the presence of glacial ice, its thickness, the depths of englacial lakes of meltwater, and the distribution of ice sheets in space and time. This information is critical as it provides a terrestrial-based proxy for Earth’s paleoclimates by constraining waxing and waning of glaciations in space and time (e.g., Mathews 1947; Smellie et al. 2008; Wilson and Russell 2018; Wilson et al. 2019b).
Marine records of Quaternary climate change are essentially continuous and complete, while the terrestrial record for glaciations over the same period is sparse due to the low preservation potential. However, marine records do not provide direct evidence for the land-based distributions and thicknesses of continental ice sheets. While the correlations between physical land-based glaciations and global oceanic records remain a challenge (e.g., McGarvie 2009; Edwards et al. 2020), glaciovolcanic deposits can be directly dated, thereby tracking the presence and thicknesses of ice in space and time. In this regard, tuyas define the paleoenvironmental conditions extant at the time of eruption and provide critical constraints on continental ice distributions and thicknesses on Earth through time (e.g., Smellie 2000; Edwards et al. 2020; Harris et al. 2022b).
4.1.2. Causal linkages
Glacial processes may modulate volcanism in several ways. Understanding that potential causality (i.e., volcanism and glacial processes) is important for improving eruption forecasting, managing volcanic hazards, and for using volcanoes as an unbiased record of paleoenvironment. In Iceland, studies have suggested that the isostatic influence of large, ice sheets may alter mantle magma productivity in regions where magmatism is driven by decompression melting (e.g., Maclennan et al. 2002). A recent study suggested that large-scale glacial erosion may be as important as deglaciation in influencing decompression-driven global magma productivity (Sternai et al. 2016). Near to the surface, glacial erosion may cause rapid stress changes within a volcanic structure by de-buttressing slopes and weakening the rocks. Glacial loading/unloading influences crustal stress, and these stresses may cause instabilities in magma storage zones and affect magma transport by restricting or enhancing dike propagation (e.g., Grove 1974; Edwards and Russell 2002; Edwards et al. 2002; Jellinek et al. 2004; Mora and Tassara 2019).
In Canada, speculations into volcano–glacier causality began in the mid 20th century (cf. Wilson and Russell 2020). Mathews (1958) suggested that the large, post-glacial outpourings of dacite from Mount Garibaldi may have been in response to retreat of the last CIS. Grove (1974) was one of the first researchers, after Mathews (1947, 1958), to explicitly link the timing of volcanism to glacial advances (loading) and retreats (unloading). He suggested that basaltic volcanism in northeastern British Columbia was triggered by decompression of crustal fissures by ice retreat during deglaciation. Hoodoo Mountain volcano is a 17 km3 phonolitic Quaternary volcano in north-central British Columbia that has been largely shaped, if not controlled, by the proximity of large regional sheets of ice that have periodically enclosed the entire edifice. Edwards et al. (2002) suggested the potential for coupling between crustally stored magmas undergoing differentiation and glacial loading and unloading. They proposed that slight changes in lithostatic pressure accompanying fluctuations in CIS could amplify volcanism by “glacial pumping” of magmas stored in the mid-crust (∼10–12 km).
In Iceland, where the lithosphere is thin and the mantle is near to its melting point, glacial unloading has been shown to increase magma productivity and initiate eruptions by facilitating inward flow and melting of hot asthenosphere (Maclennan et al. 2002). However, in continental settings and especially in volcanic arcs, the lithosphere is substantially thicker, and magmatism is controlled by deep-seated processes that operate on timescales that are vastly longer than the timescales of glaciations (i.e., 104–6 ka vs. 102–3 ka, respectively). Rates of magmatism in volcanic arcs are steady relative to glacial cycles and unlikely to be affected by the nominal loading and unloading effect (Δ10 to 30 MPa) of rapid glacial changes. Causal linkages between volcanism and glaciation in continental arcs most likely reflect processes that modulate transport and storage of the magmas through the crust (Glazner et al. 1999; Wilson and Russell 2020).
Recently, Wilson and Russell (2020) used stochastic (Monte Carlo) simulations to investigate to what extent the glacial loading and unloading of southwest British Columbia during the last glaciation (i.e., Fraser, ∼30 kyr) could control volcanism in the GVB. Their approach was to model the deformation of an elastic crustal lithosphere by glacial loading/unloading over the last glacial cycle and explore its potential effects on dike propagation and rates of eruption (cf. Nakada and Yokose 1992; Glazner et al. 1999). An average background flux of magma was supplied to the lithosphere, and they tracked magma ascent in response to the lithospheric stress distributions (compression vs. extension) induced by glacial loading and unloading using a Monte Carlo simulation that randomly varied magma driving pressure.
The Monte Carlo model results showed how glacial loading can suppress volcanism as migrating magmas are trapped by compressive stresses in the upper lithosphere. For the GVB, Wilson and Russell (2020) predicted that the ice loading by the last CIS could have caused a 50%–60% reduction in eruption rate. However, relaxation of these stresses during deglaciation (i.e., unloading) releases accumulated magma and may have caused a transient ∼4-fold increase in eruption rate. The peak volcanic output lagged the glacial maxima by 6 kyr before returning to background levels. They also found that the asymmetry of the model glacial loading events greatly controlled the timing and magnitude of post-glacial volcanism (Jellinek et al. 2004). Rapid unloading (relative to loading) led to a higher peak volcanic output with a shorter lag time relative to peak glaciation. Conversely, a rapid loading and prolonged unloading promoted only a minor increase in volcanic output observed long after peak glaciation. Accordingly, in arc environments, longer and more intense glacial periods facilitate extended periods of crustal magma charging and will result in many more eruptions if followed by rapid deglaciation. Wilson and Russell (2020) noted that by modulating crustal magma residence times, glacial pumping could extend magmatic differentiation that should leave a petrographic/geochemical imprint on the modulated magmas.
4.2. Active volcanism: monitoring for hazard and risk assessment
Due to the inherent complexity of magmatic–volcanic systems, accurately determining whether one is active or extinct can be challenging, especially with the glacial influence on magma production and lag times (cf. Wilson and Russell 2020). Although somewhat arbitrary, the most widely accepted distinction of an “active vs. extinct” volcanic system is that an “active” system has the potential to erupt again in the future, and an “extinct” system does not. Generally, a volcanic system is considered active if it has had an eruption in the Holocene and/or it is showing signs of unrest such as surface deformation, fumarolic degassing, seismicity, etc. and thus has the potential to erupt in the future. The term “dormant” is also often used to refer to a volcano that is potentially active but not currently erupting (e.g., Szakács 1994).
Despite the abundance of active volcanic systems in western Canada (i.e., ∼54 eruptions from 28 volcano groups during the Holocene; Kelman and Wilson in press), their eruptive histories remain poorly constrained. Of the four Holocene and relatively well-dated eruptions, including Lava Fork (1800s; e.g., Russell and Hauksdottir 2001), Tseax (1700s; e.g., Williams-Jones et al. 2020), Mount Meager (2360 calendar years B.P.; e.g., Hickson et al. 1999), and Nazko cone (7200 calendar years B.P.; e.g., Souther et al. 1987), only Tseax and Mount Meager are known to have an oral history of eruption among First Nations peoples (Wilson et al. in press). The eruptions of Lava Fork, Tseax (LeMoigne et al. 2020), and Nazko were characterized by basaltic tephra cones, airfall, and lava flows. Mount Meager’s recent activity comprised a Plinian rhyodacite explosive eruption having a Volcanic Explosivity Index of 4 and produced widespread airfall deposits, pumiceous pyroclastic density current deposits, and block and ash flow (welded) deposits. It also resulted in a catastrophic outburst flood following the subsequent failure of a temporary volcanic dam in the Lillooet River (Andrews et al. 2014).
The events at Mount Meager are notable for their style and magnitude and provide some indication of what Canada’s next eruption might entail should it occur at one of the major stratovolcanoes. However, due to the overall low level of eruptive activity and commonly remote nature of Canadian volcanic systems, there is only very limited continuous monitoring of Canadian volcanoes via the regional Canadian National Seismograph Network (Natural Resources Canada). Furthermore, the network is not fully optimized for volcano seismology (Cassidy and Mulder in press). Nevertheless, during a 25 h period between 9 and 10 October 2007, eight ML 2.3–2.9 earthquakes were detected within ∼20 km of Nazko cone. This led to the temporary deployment of a seismoacoustic array that detected a swarm of microearthquakes (>1000 events) with foci at depths of 25–31 km; however, seismic activity ceased after ∼2 months (Cassidy et al. 2011; Cassidy and Mulder in press). A small passive CO2 soil gas survey carried out during this period detected no evidence of shallow magmatic degassing (Hickson et al. 2009) and the event is interpreted as evidence of magma injection into the lower crust beneath Nazko and the AVB (Cassidy et al. 2011).
In June 2016, serendipitous visual observations (during a helicopter-supported Mountain goat population survey; pers. comm. D. Canil, July 2016) noted fumarolic activity from glaciovolcanic caves on Job glacier at Mount Meager. Along with renewed interest in the potential for geothermal energy production, these observations led to several integrated multidisciplinary geological and geophysical surveys on the Mount Meager massif (Grasby et al. 2020). The new geophysical imaging (e.g., Hanneson and Unworth 2023) suggests the presence of an 8–9 km deep magma body (at least 2000 km3 of 18–32% dacitic-to-trachydacitic melt at 800–900 °C), while a retrospective study of deep long-period seismicity identified seismic activity, and thus potential magma pathways, between 4 and 45 km depth (Lu and Bostock 2022). While still under development, local monitoring at Mount Meager currently consists of two static optical cameras (daily image acquisition), a single broadband seismometer (on the southwest flank, operated by the Meager Creek Development Corporation), and regular optical satellite imagery acquisition (Planet Labs) with a focus on surface change detection. While most of the monitoring at Mount Meager is currently carried out by university teams and corporate partners, Natural Resources Canada are developing an operational synthetic aperture radar interferometry (InSAR) volcano monitoring system that covers Mount Meager and seven other active volcanoes of interest (Mount Garibaldi, Mount Cayley, Tseax, Lava Fork, Mount Edziza, Nazko, and Hoodoo; Kelman and Wilson in press).
In addition to the expanding volcano monitoring program, risk and hazard assessments have been undertaken as part on an ongoing effort to expand our understanding of potential volcanic threats to population and infrastructure. Foremost is the recent threat assessment ranking for Canadian volcanoes that had the goal of determining those systems of highest priority for expanded monitoring and further research. Mount Meager and Mount Garibaldi had the highest Hazard scores, followed by Mount Cayley, Mount Price, and Mount Edziza due in part to their level of activity, existing knowledge of past activity, and proximity to population centres (Kelman and Wilson in press). Importantly, Canadian volcanic systems have and continue to pose a wide range of volcanic hazards, including widespread tephra fall, extensive lava flows, and catastrophic flooding with the potential to significantly impact population and infrastructure. While this study was informed in part by three volcano hazard assessments for Nazko (Hickson et al. 2009), Mount Meager (Warwick et al. 2022), and Mount Garibaldi (Morison and Hickson 2023), the high number of active, yet understudied, volcanic systems in Canada clearly argues for more volcano hazard assessments, particularly in areas where populations and infrastructure are at risk.
Furthermore, the extensive history of glacier–volcano interaction has in many cases resulted in volcanic structures that are heavily eroded and/or altered such that the remaining edifice is highly susceptible to non-volcanically triggered failures. For example, on 6 August 2010, a 53 Mm3 debris avalanche (the largest failure in Canadian history) occurred on the southwest flank of Mount Meager, resulting in the temporary damming of the Lillooet River and evacuation of 1500 residents from nearby Pemberton (Roberti et al. 2018). With changing climate affecting the Canadian Cordillera, the dynamic interplay between rapidly degrading alpine glaciers, which originally buttressed mountain slopes, and volcanic systems is leading to more and more intense hazardous events. This climate-induced challenge necessitates expanded research and monitoring to assess the potential for such events and to mitigate their impact on our communities.
The Canadian Cordillera comprises a rich landscape hosting numerous Pleistocene and Holocene volcanoes that have produced historic eruptions. The volcanoes are diverse in origins, compositions, and eruptive styles and environments (i.e., subaerial–subaqueous–cryospheric) reflecting the Canadian Cordillera’s tectonic complexity. The knowledge base for volcanism in the Canadian Cordillera has grown substantially over the last 50 years but there is much room for making new significant scientific contributions. In addition to studies that refine our petrological understanding of volcanism across the Cordillera or inform on the nature and origins of its mantle lithosphere, new forensic studies are warranted to address issues of volcanic hazards and risks. Such knowledge will become ever more critical as Canada’s population increases and our communities and infrastructure expand to encroach on these volcanic landscapes. Lastly, the Canadian Cordillera offers a unique laboratory for studies of glaciovolcanism in diverse physiographic settings (i.e., from plateaus to mountain systems). The coincidence of major glacial events and volcanism over the last ∼4 m.y. offers the opportunities for physical volcanological studies of glaciovolcanic eruptions (e.g., Russell et al. 2013; Wilson et al. 2019a, 2021; Rowell et al. 2022), for studies of paleo-ice distributions in space and time (e.g., Wilson et al. 2019b), or for investigating volcano–glacier causality (Wilson and Russell 2020). These studies are critical for continued refinement of the Quaternary record of ice ages in the Northern Hemisphere and will create knowledge and tools that will support studies of glaciovolcanism on other planets and moons.
The manuscript was initiated in response to conversations with Brendan Murphy, who suggested that the topic would fit well into the special collection of papers celebrating the Canadian Journal of Earth Sciences’ 60th Anniversary. The manuscript benefitted substantially from reviews provided by an anonymous referee and, especially, D. Milidragovic.
All data used here are from the published literature.
Formal analysis: JKR, BRE, GWJ
Validation: JKR, BRE, GWJ, CJH
Visualization: BRE, JKR
Writing – original draft: JKR, GWJ, BRE, CJH
Writing – review & editing: JKR, GWJ, BRE
This research was supported by Natural Sciences and Engineering Research Council of Canada Discovery Grants program (JKR: RGPIN-2018-03841; GWJ: RGPIN-2016-03953), United States National Science Foundation (BRE EAR 1220403, ARRA EAR-0910712, and EAR-0439707), and Dickinson College Office of the Provost (BRE). Natural Resources Canada (Geological Survey of Canada) supported the work of CJH and many students and researchers.