The most remarkable thing about the six volcanoes we can see from Whistler and Blackcomb is how little they resemble the classic image of a volcano and how little they resemble one another. Although they have all been active within the last million years and all are part of the same volcanic belt, each one is the product of a unique history – a history of both violent and passive eruptions interwoven with the ebb and flow of continental ice sheets.
The Black Tusk and Mount Fee may once have had the conical symmetry of a diminutive Mount Fuji but glacial erosion has stripped away their outer mantle of pyroclastic bombs and ash, leaving only the solidified plugs of lava that once filled their central vents. Mount Garibaldi, built on a foundation of glacier ice, slumped into its present misshapen form when the ice melted, and Ring Mountain, confined during its birth to a pit thawed in the ice, never did develop a cone. Much of Powder Mountain's history remains hidden, except for a few scraps of lava protruding from beneath its present ice cap. But for me the most intriguing volcano is Mount Cayley, not only because of its past but because of its possible future.
From the top of Whistler it is 25 km to Mount Cayley but even at that distance its incredibly steep serrated profile is an imposing sight against the western skyline. Too precipitous to hold snow, the bare rock of its jagged upper slopes contrasts with the rounded, snow-covered summits of surrounding granitic peaks.
The present form of Mount Cayley is probably fairly close to its original shape. The eruption of thick viscous lava flows was accompanied by the extrusion of near solid domes that shouldered aside the surrounding granite and expanded like spring mushrooms forcing their way through the forest litter. The first hot, semi-rigid rock broke through the surface about 3 million years ago and grew into the crumbling spine of rock that now forms the summit of Mount Cayley. Other eruptions followed, forming the precipitous spine of Vulcan's Thumb and two smaller domes, the youngest being only 300,000 years old.
Mount Cayley has been the focus of several geological studies. In 1982 John Clague and I published a study of debris flows triggered by landslides from the steep unstable slopes in upper Turbid Creek. In 1984 the camp we occupied for that work was wiped out by yet another massive debris flow that ran all the way to Squamish River. Relatively high subsurface temperatures, predicted by geophysical work and the analysis of hot spring waters, has been confirmed by temperature measurements in diamond drill holes. But most exciting of all – deep seismic profiling has identified a large, "bright spot," a reflector about 10 km below Mount Cayley interpreted to be a mid-crustal magma chamber or body of very hot rock.
Best we keep a seismic ear to the ground!
Written by: Jack Souther