An explosive past
The Meager Creek Volcanic Complex holds the title for the most recent major explosive eruption in Canada. Around 2,400 years ago (specifically, about 2360 B.P.), the Bridge River Vent on the complex's northeastern flank erupted in a sequence of violent events. The eruption, similar in scale to the 1980 eruption of Mount St. Helens, registered a 5 on the Volcanic Explosivity Index (VEI).
The event began with a massive collapse of the volcano's flank, followed by a Plinian eruption that sent a column of ash and gas at least 20 kilometers into the atmosphere. As the column collapsed, it generated devastating pyroclastic flows—superheated avalanches of ash, pumice, and gas—that traveled 7 kilometers down the Lillooet River valley. The eruption ejected rhyodacite pumice, and the resulting ash layer, known as the Bridge River Ash, can be found as far away as Alberta, over 500 kilometers from the source. This ash deposit is now a critical time marker for geologists and archaeologists studying the last few thousand years of history in the region.
A mountain falling apart
The danger from the Meager complex is not only eruptive. The same magma that powered its last eruption continues to heat groundwater, creating an active hydrothermal system. This system vents acidic steam and fluids through the mountain's rock, a process called hydrothermal alteration. This process gradually turns hard volcanic rock into soft, weak clay, severely destabilizing the volcano's slopes.
This instability has a dramatic history of failures. On August 6, 2010, the south flank of Mount Meager collapsed, triggering one of the largest landslides in Canadian history. An estimated 53 million cubic meters of rock and debris roared down Capricorn Creek and into the Meager Creek valley below. The debris flow traveled 12.7 kilometers, temporarily dammed the Lillooet River, and registered as a magnitude 2.6 earthquake. The slopes of the complex are in a constant state of slow deformation, and climate change, which accelerates glacier and snow melt, may increase the frequency of such events. The combination of weak rock, steep slopes, and potential for meltwater from glaciers makes future landslides a near certainty.