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A legacy of fire and ice

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

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Meager Mountain, not quite visible from the top of Whistler, is the youngest and also among the oldest of the Garibaldi Belt volcanoes. Best known for its hot springs and geothermal potential, Meager Mountain first erupted about two million years ago. It periodically surged back to life and with each eruption added new bulk to its growing mass. Its most recent eruption, less than 2,500 years ago, began with a violent discharge of gas and fine ash from a vent high on its north face. Within minutes the Lillooet River along the base of the slope was choked with rafts of pumice. The forest was set ablaze and ultimately buried under several metres of volcanic ash. Midway through the eruption a massive glowing avalanche of blocks and ash swept down the slope and smothered the valley under several metres of welded pyroclastic deposits and the plume of airborne ash extended east beyond the Rocky Mountains.

In the years following the eruption the Lillooet River has cut through the thick layer of welded pumice and ash exposing the charred remains of the pre-eruption forest still rooted in the underlying soil. Radiocarbon dates of this material pegged the time of eruption at 2,490 + 50 years before present. The ash deposits, known as the Bridge River Ash, have also been dated and traced across central B.C. into Alberta.

At present the volcanoes of the Garibaldi Belt are quiet, presumed dead but still not completely cold. But the flare-up of Meager Mountain 2,500 years ago raises the question, "Could it happen again?" Was the explosive eruption of Meager Mountain the last gasp of the Garibaldi Volcanic Belt or only the most recent event in its on-going life? The short answer is nobody really knows for sure. There is no seismic activity presently associated with any of the Garibaldi Belt volcanoes. But the Juan de Fuca plate continues to slip under us and the chemistry of hot-spring water and geothermal drilling at both Mount Cayley and Meager Mountain indicates that their deep roots are still very, very hot. So just in case I sometimes do a quick check of the old hot-spots when I get off the Peak Chair.