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A legacy of fire and ice

Volcanoes of the Garibaldi Belt add dramatic scenery to Whistler's mountain landscape



Only the tops of the highest mountains project above the ice. The summits of Wedge, Blackcomb and Whistler are mere islands of rock – nunatacks – surrounded by a vast expanse of ice. Moraines of broken rock, stripped by glacial erosion from each nunatack, wind across the ice surface and merge like roads entering a superhighway where the glaciers converge at the head of Howe Sound. South of Whistler, beyond the battered remains of the Black Tusk, the steaming dome of Garibaldi towers above the ice surface. A slab spalls off the spine of hot rock projecting from its summit and sends an avalanche of incandescent blocks rushing down the cone and fanning out onto the ash-covered surface of the ice. Off to the west a plume of white vapour rises perpetually from a circular pit thawed into the ice by the lava of Ring Mountain. And a kilometre below the surface the invisible fury of the Cheakamus Valley sub-glacial eruptions sends tremors through the overlying ice.

Of course no one was around to take a video. But given a smattering of geological insight and a pinch of imagination it's possible to recreate a mental image of the time, not that long ago, when the view from the top of Whistler was very different than it is today. The trick is learning to read the clues that are written in the rocks and Bill Mathews, my U.B.C. geology professor, friend, and colleague, was among the first to interpret what happens when molten lava and glacier ice collide. During the Quaternary Period, the last two million years of geological time, the ebb and flow of regional glaciers through Sea to Sky country was punctuated by the episodic eruption of volcanoes, and it was Bill's pioneering work that broke the geological code and first gave us some insight into that turbulent period of our local history.

He did his Ph.D. thesis on Mount Garibaldi back in 1948 and later went on to study several other features in what is now known as the Garibaldi Volcanic Belt. This scattered family of at least 25 young volcanoes strung out between Howe Sound and the Bridge River plateau is a northern extension of the high Cascade volcanoes of western U.S. and part of the Pacific Ring of Fire. In the context of global plate tectonics, it's the result of convergence between North America and the Juan de Fuca plate, a relatively small chunk of the Pacific Ocean crust that takes a nose-dive west of Vancouver Island and slides, very slowly, under the continent. At a critical depth, which just happens to be directly under the Sea to Sky corridor, things get hot enough to melt and magma, a brew of molten rock infused with dissolved gasses, periodically pops up to the surface as lava. Fortunately it only pops up every few thousand years or so but the volcanoes and lava flows left behind by each eruption have become a fascinating and permanent part of Whistler's mountain landscape.