Unstable Mountain Threatens Pemberton Valley

Landslides on Mount Meager have dumped clay and rock several metres deep into the Pemberton Valley at least three times during the last 7300 years.

Researchers warn the mountain could release another massive debris flow over inhabited areas anytime without warning.

The steep, glacier-topped mountain group, located north of Whistler, British Columbia, is built of unstable volcanic rubble and will erupt again.

Recent drilling into the Pemberton Valley bottom encountered remains of a debris flow that had travelled 50 km from Mount Meager shortly before the volcano erupted around 2415 years ago. About 1,000,000,000 m³ of rock and sand slurry spanned the width of the valley.

Two earlier debris flows, around 4450 and 7300 years ago, sent rubble at least 32 km from the mountain, the oldest incident depositing an eight-meter-thick layer of material. There is no sign of volcanic eruptions associated with these events. The area's high rainfall alone is enough to trigger failures of this magnitude.

Lately, the mountain has shed smaller landslides about every ten years, including one in 1975 that killed four geologists near Meager Creek. Logging, mining, tourism and backcountry recreation on surrounding slopes and valleys are vulnerable to the mountain's exceptional geomorphic activity. The probability of Mount Meager covering settled parts of the Pemberton Valley in a debris flow is estimated at about one in 2400 years.