Plinth Peak

 volcano, summit

Stratovolcano
Summit elevation 8783 ft. asl..
British Columbia, Canada

Mount Meager's most recent volcanic eruption 2350 years ago. This eruption was similar in character to the 1980 eruption of Mount St. Helens, erupting from a vent on the north-east side of Plinth Peak. This activity produced a diverse sequence of volcanic deposits, well exposed in the bluffs along the Lillooet River, which is defined as the Pebble Creek Formation. It comprises airfall pumice, pyroclastic flows, welded breccias, lahars and an dacitic lava flow. The eruption was so great that thin, very fine grained, distal deposits of tephra have been identified in Alberta, 530 km (329 mi) east from the vent and sent an ash column at least 20 km (12 mi) high into the stratosphere. An unusual, thick apron of welded vitrophyric breccia may represent the explosive collapse of an early lava dome which deposited ash several meters in thickness near the vent area. Pyroclastic fall deposits up to 80 m (262 ft) thick from the eruption, covers slopes in the area of Mount Meager. About 1–5% of the pumice fragments are banded from white to dark grey. The eruption also blocked the Lillooet River to a height of at least 100 km (62 mi), impounding a lake. The lake reached a maximum elevation of 810 m (2,657 ft) and thus was at least 50 m (164 ft) deep. The dam eventually eroded from water activity, causing a massive outburst flood that sent house-sized boulders down the valley for several kilometers, forming Keyhole Falls. The destructive floodwaters continued much further. In its final stages, the eruption produced a 2 km (1 mi) long glassy, porphyritic dacite lava flow that varies from 15 to 20 m thick. This volcanism is very recent in geological terms, suggesting that the volcano may yet have some ongoing volcanic activity.