Western North America

Glaciovolcanism in western North America occurs in the Garibaldi Volcanic Belt (GVB), the Wells Gray-Clearwater volcanic field (WG), the Northern Cordilleran Volcanic Province (NCVP), and the Cascade Range (northern California, Oregon, and Washington). The Garibaldi Volcanic belt (GVB) (Figure 1) is the northern extent of the United States Cascades (Green et al. 1988; Guffanti & Weaver 1988; Read 1990; Sherrod & Smith 1990; Hickson 1994). Stretching from Watts Point, which is located near the head of Howe Sound, northward through the Salal Glacier volcanic complex, to Silverthrone Mountain, the GVB formed as a result of the subduction of the Juan de Fuca plate beneath the North American plate (Green et al. 1988; Rohr et al. 1996). There are at least eight separate volcanic complexes in the GVB ranging in composition from high-alumina basalt to rhyolite, although intermediate compositions dominate. Due to intermittent glaciations, there are numerous glaciovolcanic features, including tuyas, flow-dominated tuyas, subglacial domes, subglacial mounds, ice-marginal flows, subglacial esker-like flows, as well as the probable erosional remnants of supraglacial eruptions (Mathews, 1952b). The most recent eruption occurred at Mount Meager at 2360 B.P. (Clague et al. 1995; Leonard 1995); volcanic products range from Miocene to Holocene in age.

Figure 1. Neogene and Quaternary volcanic centres in western Canada. (Modified from Hickson [2000].)

Glaciovolcanic features of the GVB are substantially different from those described in Iceland (Jones 1966, 1969, 1970; Furnes 1980; Tuffen et al. 2001), Antarctica (Smellie et al. 1993; Smellie & Skilling 1994; Smellie 2000), and other parts of British Columbia (Mathews 1947; Souther 1992; Hickson 1987; Hickson et al. 1995; Edwards et al. in press). At intermediate composition centres in the GVB, tuyas are flow-dominated (e.g. the Table, Ring Mountain, Little Ring Mountain), and there are many subglacial domes (e.g. Ember Ridge, Slag Hill). Basaltic centres (e.g. Tuber Hill), however, have more typical tuyas (containing sequences of pillows, hyaloclastite, and flows), similar to those described for Iceland and other parts of British Columbia. There are also numerous impoundment features (e.g. the Barrier, Pali Dome). There is an apparent scarcity of primary fragmental deposits (e.g. hyaloclastite), particularly for intermediate compositions. These unique features are probably due to lava composition, minimization of direct lava-water contact during eruptions, and topography. Composition affects volcano morphology because eruption temperature decreases, and viscosity and glass transition temperature increase, as silica content increases. The result of this is that silicic subglacial volcanoes can melt less water and are less able to trap it near their vents, leading to the formation of structures which are shaped by the surrounding ice, rather than structures formed by eruption into water-filled chambers. The GVB's steep topography also enhances meltwater drainage from vent areas, favours lava flow impoundment in ice-filled valleys, and may, due to the associated high erosion rates, influence the observed distribution of primary fragmental glaciovolcanic deposits.

The Wells Gray-Clearwater volcanic field lies in east-central British Columbia at the eastern end of the Anahim Volcanic Belt, which is hypothesized to be the trace of an easterly-migrating mantle hot spot (Bevier et al., 1979; Rogers, 1981). It includes an unusually complete record of late Cenozoic deposits. Volcanic activity in the region continued through at least two episodes of glacial advance during the late Cenozoic, resulting in subaerial, subaqueous, and subglacial voclanic deposits. Compositions erupted are mostly alkali olivine basalt. Glaciovolcanic features include low-profile tuyas (e.g. McLeod Hill, Mosquito Mound, and Gage Hill), subglacial mounds (e.g. Pyramid Mountain), and ice-marginal deposits (e.g. Trophy Mountain, Jack's Jump). Volcano-ice interaction in the Wells Gray-Clearwater volcanic field has been studied in detail by Hickson (1987), Hickson and Souther (1984), and Hickson et al. (1995).

The Northern Cordilleran Volcanic Province (NCVP)covers a large area (approximately 1200 by 400 km) of northwestern British Columbia, the Yukon Territory, and the Alaskan Panhandle. Neogene to Quaternary deposits there are alkaline and chemically bimodal, and range from large basaltic plateaux to stratovolcanoes to individual cinder cones. It is hypothesized that the principal region for this magmatism is crustal-scale extension generated by far-field forces between the Pacific and North American plates (Edwards and Russell, 1999). The NCVP is geologically similar to other extensional continental volcanic provinces, such as the Basin and Range and the East African Rift system, although pervasive extensional faulting has not yet been recognized in the NCVP.

Like other parts of British Columbia, the NCVP has experienced repeated glacial episodes, resulting in numerous glaciovolcanic deposits. These include the Tuya Buttes volcanic field, one of the first glaciovolcanic regions to be studied in detail, from which the term tuya is derived (Mathews, 1947). In addition to the tuyas and subglacial mounds of the Tuya Buttes region, there are also complex polygenetic edifices with evidence for volcano-ice interaction at Mount Edziza (e.g. Souther and Hickson, 1984; Souther et al., 1984) and Hoodoo Mountain (e.g. Edwards and Russell, 1994, 1995, 1997; Edwards et al., in press).