Most of us first heard about the "Pacific Ring of Fire" in a junior high-school geography class. If the name conjured up visions of glowing lava or other terrestrial pyrotechnics the image soon faded into just another abstract idea – a map of the Pacific surrounded by dots representing geologically "young" volcanoes. Even worse "geologically young", we were told, could mean anything less than a couple million years old.
Waiting for a volcano to erupt is not recommended for thrill-seekers. In fact most volcanoes, even those classified as "active", spend relatively little time in actual eruption. The time between is spent, like any other mountain – quietly being eroded, growing forests, or providing a good ski run. Such is the present, passive state of our local "young" volcanoes.
Mount Garibaldi, Black Tusk, Mount Fee, Mount Cayley, Powder Mountain, and Ring Mountain are all visible from the top of Whistler and Blackcomb while Meager Mountain, at the head of Lillooet valley, is barely hidden behind older mountains. Except for the dramatic, and much photographed spire of Black Tusk none of these volcanoes present a threatening or even very remarkable presence among the surrounding granitic mountains of the Coast Range. But between Mount Garibaldi in the south and Meager Mountain in the north more than 20 additional, but even less obvious, "young" volcanoes define our local, 100 kilometre-long, segment of the "Pacific Ring of Fire". Collectively they form the Garibaldi Volcanic Belt, a northern extension of the high Cascade volcanoes of western United States.
Historic records of eruptions in the Cascades include the emission of steam and ash from Mount Baker in the early 1840s, a moderately violent eruption of Mount Lassen in 1915, and most recently the catastrophic eruption that decapitated Mount St. Helens in 1980. Although there are no historic accounts of eruptions in the Garibaldi Volcanic Belt, geological studies reveal a long and complex history of volcanic activity beginning with an eruption of Mount Cayley about 4 million years ago and culminating with an explosive eruption of Meager Mountain only 2,500 years ago.
Underthrusting of the continent by the easterly spreading oceanic crust of the Juan de Fuca Crustal Plate is the driving force behind the generation of magma along our portion of the "Ring of Fire". Each time magma breaks out of the earth's crust onto the surface a new volcano is born or an existing volcano is altered. Of the many volcanoes in the Garibaldi Belt no two record the same history. Some erupted before the last regional glaciation, others were built on top of or under the ice, while the most recent erupted after the ice had receded. Some of the eruptions were benign, others explosive. But that is a story for next week’s column.
Written by: Jack Souther