The Mount Meager massif in the Garibaldi Volcanic Belt of southwestern British Columbia was the source for a massive (VEI-5) Plinian eruption 2,350 years ago similar in character to the 1980 eruption of Mount St. Helens in the U.S. state of Washington. The eruption originated from a vent on the northeast flank of Plinth Peak, the highest and one of four overlapping stratovolcanoes which together form the Mount Meager massif. This activity produced a diverse sequence of volcanic deposits, well exposed in bluffs along the long Lillooet River, which are grouped as part of the Pebble Creek Formation. The explosive power associated with this Plinian eruption sent an ash column estimated to have risen at least above Meager, indicating it entered the second major layer of the Earth's atmosphere. As prevailing winds sent ash and dust as far as to the east, it created the large Bridge River Ash deposit, extending from Mount Meager to central Alberta. Pyroclastic flows travelled downstream from the vent and buried trees along Meager's forested slopes, which were burned in place. An unusual, thick apron of welded vitrophyric breccia may represent the explosive collapse of a former lava dome which deposited ash several meters in thickness near the vent area. This collapse blocked the Lillooet River to a height of at least , forming a lake. The lake reached a maximum elevation of and thus was at least deep. The pyroclastic deposits blocking the Lillooet River eventually eroded from water activity, causing a massive outburst flood that sent small house-sized boulders down the Lillooet River valley, and formed high Keyhole Falls. The final phase of activity produced a long glassy dacite lava flow that varies from thick. This is the largest known explosive eruption in Canada in the past 10,000 years. Two clusters of hot springs are found at the Mount Meager massif, suggesting magmatic heat is still present and volcanic activity continues.