The best known and most documented eruption of the Mount Meager massif is a large explosive eruption that occurred about 2,400 years ago . This eruption , which likely reached 5 on the , was similar to the 1980 eruption of Mount St. Helens . It sent a massive Plinian column at least high into the atmosphere . Prevailing westerly winds carried volcanic ash from this explosion eastwards to as far as Alberta . Nearby areas were devastated by heavy pyroclastic fall when parts of the Plinian column collapsed . Later , a series of pyroclastic flows were erupted and travelled downstream . After this , a lava flow was erupted that repeatedly collapsed on the steep slopes of Plinth Peak , creating a thick , welded breccia deposit that blocked the Lillooet River . This created a lake just upstream which later collapsed to produce a massive . Large boulders were carried downstream for more than , but the destructive floodwaters continued further . Later , a small dacite lava flow was erupted , which cooled into well-preserved columnar joints . The entire eruption cycle originated from the Bridge River Vent on the northeastern flank of Plinth Peak . This is the latest known eruption of the Mount Meager massif , as well as the largest known Holocene explosive eruption in Canada . However , it is unknown when this eruption ended
1980 eruption of Mount St. Helens
1980 eruption of Mount St. Helens
Mount Meager massif