The Silverthrone volcanic complex (Green et al., 1988) lies to the north of the Garibaldi Volcanic Belt. Its affinity is unclear because it has been only minimally studied. It is considered to be part of the Garibaldi Volcanic Belt, although it also lies on the overlapping trend of the much older Pemberton Volcanic Belt (which formed as a result of subduction of the now-vanished Farallon plate). Silverthrone’s nearest prominent neighbour is the much older Franklin Glacier complex, about 55 km east-southeast. However, several small remnants of andesite and rhyolite lava are present on ridge crests and in valleys between the two complexes. The Silverthrone complex is roughly circular and about 20 km in diameter, consisting of a pile of lava domes, lava flows, and breccia. Terrain is extremely rugged, with steep slopes extending from near sea level to elevations greater than 3000 m. Very little is known about Silverthrone’s eruptive history.

The oldest volcanic rocks at Silverthrone comprise volcanic breccias, some of which are welded (cemented together by volcanic heat). These are overlain by flows of rhyolite, dacite, and andesite totalling about 900 m in thickness. Material in the lower portion of this pile of lava flows was potassium-argon dated at 750,000 years old, while material from slightly higher in the pile was dated at approximately 400,000 years old. All of these ancient volcanic deposits were deeply dissected by erosion, creating the topography onto which the younger succession erupted.

Unlike its neighbour to the southeast, Franklin Glacier, Silverthrone possesses young volcanic rocks that are probably of similar age to those of the rest of the Garibaldi Volcanic Belt. These youngest volcanic rocks are mostly andesite and basaltic andesite flows and pyroclastic breccia deposits that issued from vents on the periphery of the complex. Many of these deposits are currently covered by glacial ice, but pyroclastic cone remnants protrude through some glaciers, and lava flows are exposed at lower elevations. Lava flows from several vents on the north side of the complex extend down valleys and coalesce in the Pashleth Creek valley, then flow into the Machmel River Valley. This flow is continuously exposed for more than 25 km over an elevation change from 2000 m above sea level to within 100 m of sea level; its minimal degree of erosion suggests that it may be very young, probably less than a thousand years old. Other lava flows extend south into the headwaters of Kingcome River.

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