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NatureSpeak Articles

Meager, Garibaldi, and Black Tusk: Three local mountains with a hot past

Note: Since the writing of this article in 2001, new research by Steve Quane at Quest University has a new theory for the formation of Black Tusk - that it formed completely under the ice similar to The Table, and that not much has been eroded away. See Whistler Public Library Facebook page to view Steve's 2018 lecture.

The eruption began with a violent discharge of gas and fine ash from a vent high on the north face of the long-dormant volcano. Within minutes the 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. The plume of airborne ash extended east beyond the Rocky Mountains blanketing thousands of square kilometres with a layer of fine volcanic ash.

No, this was not the 1980 A.D. eruption of Mount St. Helens, but rather the 500 B.C. eruption of Meager Mountain, 60 km northwest of Whistler. Though no one was there to record the event the geological record is clear. 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.

The flare-up of Meager Mountain 2,500 years ago was the most recent of many eruptions in greater Sea-to-Sky country during the Quaternary Period – the last 2 million years of history. It happened long after the Cordilleran Ice Sheet had receded, but others occurred both before and during the period of regional ice cover.

Black Tusk, which predates the Cordilleran Ice Sheet, was formed during two stages. The first cone, built about 1.3 million years ago, was deeply eroded during a long period of dormancy before the second stage cone erupted about 170,000 years ago. The present Tusk owes its shape to prolonged erosion which stripped away the loose pyroclastic cone, leaving only the plug-dome of solid lava that once filled the central conduit and now forms the narrow summit spire.

Mount Garibaldi is a composite cone built during the waning stages of the last major glaciation, when the ice surface stood at more than 1,300 metres above sea level. The outer flanks of the cone initially extended onto the ice and later collapsed when the glaciers receded. Beneath the ice, on what is now the north flank of Mount Garibaldi, extrusion of lava from a satellitic vent thawed a pit in the ice. This filled with successive flows to form the steep-sided, flat topped "Tuya" known as The Table.

Could it happen again? Probably not. Our local volcanoes are seismically quiet and risk assessment is rated low. But if it should I'd like to be around for the show.

Written by: Jack Souther

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