Volcanology of the 2350 B.P. Eruption of Mount Meager Volcanic Complex, British Columbia, Canada: implications for Hazards from Eruptions in Topographically Complex Terrain

Abstract

The Pebble Creek Formation (previously known as the Bridge River Assemblage) comprises the eruptive products of a 2350 calendar year B.P. eruption of the Mount Meager volcanic complex and two rock avalanche deposits. Volcanic rocks of the Pebble Creek Formation are the youngest known volcanic rocks of this complex. They are dacitic in composition and contain phenocrysts of plagioclase, orthopyroxene, amphibole, biotite and minor oxides in a glassy groundmass. The eruption was episodic, and the formation comprises fallout pumice (Bridge River tephra), pyroclastic flows, lahars and a lava flow. It also includes a unique form of welded block and ash breccia derived from collapsing fronts of the lava flow. This Merapi-type breccia dammed the Lillooet River. Collapse of the dam triggered a flood that flowed down the Lillooet Valley. The flood had an estimated total volume of 10 9 m 3 and inundated the Lillooet Valley to a depth of at least 30 m above the paleo-valley floor 5.5 km downstream of the blockage. Rock avalanches comprising mainly blocks of Plinth Assemblage volcanic rocks (an older formation making up part of the Mount Meager volcanic complex) underlie and overlie the primary volcanic units of the Formation. Both rock avalanches are unrelated to the 2350 B.P. eruption, although the posteruption avalanche may have its origins in the oversteepened slopes created by the explosive phase of the eruption. Much of the stratigraphic complexity evident in the Pebble Creek Formation results from deposition in a narrow, steep-sided mountain valley containing a major river.

A 36Cl age determination for Mystery Creek rock avalanche and its implications in the context of hazard assessment, British Columbia, Canada

“…Furthermore, the last volcanic eruption at Mount Meager took place about 2,350 years BP (Hickson et al 1999), which falls within the 36 Cl 2σ age results. The eruption was considered highly explosive (Hickson et al 1999).…”

“…However, it is difficult to make this correlation between the potential seismic shaking caused by the volcanic eruption and the Mystery Creek rock avalanche because of a lack of detailed information. In addition, no volcanic ash was observed on or around the landslide deposit as the ash layer has been traced eastward (Hickson et al 1999), not south.…”

“…The eruption was considered highly explosive (Hickson et al 1999). Volcanic eruptions can record magnitudes as high as M7 (Yokoyama 2001).…”

Welding of pyroclastic conduit infill: A mechanism for cyclical explosive eruptions

“…The Mount Meager volcanic complex is situated at the northern extent of the Garibaldi Volcanic Belt, the northernmost segment of the Cascade Volcanic Belt [ Green et al , ; Read , ; Sherrod and Smith , ]. Its most recent activity produced a range of volcaniclastic deposits and lavas [ Hickson et al , ; Stewart , ]. The youngest eruption of the Mount Meager volcanic complex took place about 2350 years before present and is the most recent eruption within the Garibaldi Volcanic Belt [ Stewart , , and references therein].…”

“…We use a suite of bread‐crust‐textured pyroclasts of welded intravent ignimbrite (Figure a) that occur as accessory lithics in the proximal pyroclastic fallout deposits from the 2350 B.P. eruption of Mount Meager (BC, Canada) [ Hickson et al , ; Michol et al , ; Stasiuk et al , ]. These blocks of welded pyroclastic material (Figure ) display a wide range of compaction/welding intensity and provide a remarkable opportunity to constrain the nature and timescales of mechanical processes operating within explosive volcanic conduits during repose periods between eruptive cycles.…”