Chapter 4 Volcanism
4.6 Volcanoes in British Columbia
As shown on the Figure 4.6.1, three types of volcanic environments are represented in British Columbia:
- The Cascade Arc (a.k.a. the Garibaldi Volcanic Belt in Canada) is related to subduction of the Juan de Fuca Plate beneath the North America plate. This extends along the south coast from the U.S. border to the northern end of Vancouver Island.
- The Anahim Volcanic Belt is assumed to be related to a mantle plume. It is in central BC and stretches inland from the coast.
- The Stikine Volcanic Belt (northwestern BC) and the Wells Gray-Clearwater Volcanic Field (central Alberta border area) are assumed to be related to crustal rifting.
Subduction Volcanism
Southwestern British Columbia is at the northern end of the Juan de Fuca (Cascadia) subduction zone, and the volcanism there is related to magma generation by flux melting in the upper mantle above the subducting plate. In general, there has been a much lower rate and volume of volcanism in the B.C. part of this belt than in the U.S. part. One possible reason for this is that the northern part of the Juan de Fuca Plate (i.e., the Explorer Plate) is either not subducting, or is subducting at a slower rate than the rest of the plate. There are several volcanic centres in the Garibaldi Volcanic Belt: the Garibaldi centre (including Mount Garibaldi and the Black Tusk-Mount Price area adjacent to Garibaldi Lake (Figures 4.0.1 and 4.0.2), Mount Cayley, and Mount. Meager (Figure 4.4.3). The most recent volcanic activity in this area was at Mount Meager. Approximately 2,400 years ago, an explosive eruption of about the same magnitude as the 1980 Mount St. Helens eruption took place at Mount Meager. Ash spread as far east as Alberta. There was also significant eruptive activity at Mounts Price and Garibaldi approximately 12,000 and 10,000 years ago during the last glaciation; in both cases, lava and tephra built up against glacial ice in the adjacent valley (Figure 4.6.2). The Table in Figure 4.0.2 at the beginning of this chapter is a tuya, a volcano that formed beneath glacial ice and had its top eroded by the lake that formed around it in the ice.
Mantle Plume Volcanism
The chain of volcanic complexes and cones extending from Milbanke Sound to Nazko Cone is interpreted as being related to a mantle plume currently situated close to the Nazko Cone, just west of Quesnel. The North America Plate is moving in a westerly direction at about 2 cm per year with respect to this plume, and the series of now partly eroded shield volcanoes between Nazco and the coast is interpreted to have been formed by the plume as the continent moved over it.
The Rainbow Range, which formed at approximately 8 Ma, is the largest of these older volcanoes. It has a diameter of about 30 km and an elevation of 2,495 m (Figure 4.6.3). The name “Rainbow” refers to the bright colours displayed by some of the volcanic rocks as they weather.
Rift-Related Volcanism
While B.C. is not