Mount Meager massif

Group of volcanoes in British Columbia, Canada

The Mount Meager massif is part of the Garibaldi Volcanic Belt (GVB), the northernmost segment of the Cascade Volcanic Arc. This volcanic belt includes cinder cones, calderas, stratovolcanoes and subglacial volcanoes (volcanoes under glaciers or ice sheets) that have been active in the last 10,000 years. The latest explosive eruption in the Garibaldi Volcanic Belt occurred at a crater on the northeastern slope of the massif about 2,400 years ago, which forms a clearly defined depression. Because little is known about the volcanoes north of the massif, such as the Silverthrone and Franklin Glacier Complex, experts disagree about their nature. Some scientists regard the Silverthrone Caldera as the northernmost volcano of the Garibaldi Volcanic Belt, while others contend that the geology of the massif more closely matches that of the GVB. However, there is evidence the Silverthrone and Franklin Glacier complexes are related to activity at the Cascadia subduction zone. Geologically these two volcanoes contain the same rock types as those found elsewhere in the Cascade Arc, including rhyolites, dacites, andesites and basaltic andesites. Such rock types are produced by subduction zone volcanism indicating volcanism at Silverthrone and Franklin Glacier is probably related to subduction. If these two volcanoes are true Cascade Arc volcanoes, the Mount Meager massif is not the northernmost volcano of the Garibaldi Belt or the Cascade Arc.

Volcanism in the Cascade Volcanic Arc is caused by subduction of the Juan de Fuca Plate under the North American Plate at the Cascadia subduction zone. This is a long fault zone lying off the Pacific Northwest from Northern California to southwestern British Columbia. The plates move at a relative rate of more than per year at an oblique angle to the subduction zone. Because of the huge fault area, the Cascadia subduction zone can produce large earthquakes of magnitude 7.0 or greater. The interface between the Juan de Fuca and North American plates remains locked for periods of roughly 500 years. During these periods, stress builds up on the interface between the plates and causes tectonic uplift of the North American margin. When the plate finally slips, it releases 500 years of stored energy in a massive earthquake. The mouth of the Columbia River empties directly into the subduction zone and deposits silt at the bottom of the Pacific Ocean, burying this large depression, or area of sunken land. Massive floods from prehistoric Glacial Lake Missoula during the Late Pleistocene also deposited large amounts of sediment into the trench. However, as with other subduction zones the outer margin is slowly being compressed like a giant spring. When the stored energy is suddenly released by slippage across the fault at irregular intervals, the Cascadia subduction zone can create enormous earthquakes such as the magnitude 9.0 Cascadia earthquake of January 26, 1700. However earthquakes along the Cascadia subduction zone are uncommon, and there is evidence of a decline in volcanic activity over the last few million years. The probable explanation lies in the rate of convergence between the Juan de Fuca and North American plates, which converge at to per year, about half the rate of convergence from seven million years ago. It consists of at least four overlapping stratovolcanoes that are younger from south to north. This includes Mount Rainier (500,000 years old), Lassen Peak (25,000 years old), This eruption ranged in character from explosive to effusive and involved lava dome extrusion, pyroclastic flows, lahars and lava flows. Situated on the northeastern flank of Plinth Peak, the Bridge River Vent has an elevation of .

The name Meager Mountain was adopted on May 6, 1924, as labelled on a 1923 British Columbia map. In 1966 the volcano was renamed Mount Meager. According to a BC Geographical Names letter written in March 1983, "the local name, Cathedral, was duplicated elsewhere, so the mountain was renamed Meager after the creek of that name which lies to the south of it". Meager Creek is named after J. B. Meager who owned timber licences on the creek. The massif's peak names were submitted by Canadian mountaineer Neal M. Carter, who was a member of the British Columbia Mountaineering Club. Devastator Peak was officially named on August 3, 1977 in association with Devastation Glacier. Plinth Peak was officially named on September 6, 1951 as identified in Carter's 1932 sketch map and article "Explorations in the Lillooet River Watershed". Mount Job and Pylon Peak were both officially named on January 17, 1957, from their labels on Carter's 1954 sketch map of the Lillooet River. Capricorn Mountain was originally identified as Mount Capricorn in the 1932 Canadian Alpine Journal, Vol XXI. According to the journal, "the name chosen for the 8440 foot mountain was Mt. Capricorn, a variation of the all too common appellation "Goat Mountain", applied by Bert [Perkins] to the stream which drains the Capricorn glacier at its base". Subsequently, the peak was renamed to Capricorn Mountain on June 22, 1967.

Event Source Years before present Volume Reference Debris flow Capricorn Creek 1850 Jakob (1996); McNeely and McCuaig (1991) Debris flow Capricorn Creek 1903 30,000,000 m3 (1,100,000,000 cu ft) Jakob (1996) Debris flow Devastation Creek 1931 Carter (1931); Decker et al. (1977); Jordan (1994) Rock avalanche Capricorn Creek 1933 Croft (1983) Rock avalanche Devastation Creek 1947 Read (1978) Debris flow Capricorn Creek 1972 Jordan (1994) Rock avalanche Devastation Creek 1975 Mokievsky Zubot (1977); Evans (2001) Debris flow Affliction Creek 1984 Jordan (1994) Rock avalanche Mount Meager 1986 Evans (1987) Debris flow Capricorn Creek 1998 Bovis and Jakob (2000) Debris flow Capricorn Creek 2009 Friele (unpublished data) Rock slide/debris flow Capricorn Creek 2010 48,500,000 m3 (1,710,000,000 cu ft) Guthrie et al. (2012)

A massive rock avalanche occurred at the massif on July 22, 1975. With a volume of , it buried and killed a group of four geologists at the confluence of Devastation Creek and Meager Creek. The landslide originated on the western flank of Pylon Peak and flowed down Devastation Creek for Geologic studies have shown that the landslide was the result of a complex history of glacial erosion, loading and unloading of the toe (a protrusion at the front of the slide mass) caused by the Little Ice Age advance and subsequent retreat of Devastation Glacier due to global warming. Experts initially estimated that the volume of debris totaled , which would make it the second largest landslide on record in Canadian history, behind the 1965 Hope Slide that removed of rock from Johnson Peak, a mountain in the Nicolum Valley near Hope, British Columbia. However, the landslide was later estimated to be more than , which would make it the largest of all time in Canada. The 2010 landslide was wide and long, creating a dam across Meager Creek and the Lillooet River. This created a lake just upstream. Early concerns that the dam might collapse and flood the Lillooet River valley ended a day later, when part of the dam ruptured and slowly released the accumulated water. An evacuation alert was rescinded, and nearly 1,500 residents were allowed to return to their homes on the weekend after the landslide occurred. No injuries were reported.