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Dacite block and ash avalanche hazards in mountainous terrain : 2360 yr. BP eruption of Mount Maeger, British Columbia Stewart, Martin L.


The Mount Meager volcanic complex hosts deposits from the youngest known explosive volcanic eruption in Canada (2360 yr. BP). These deposits reflect the consequences of erupting dacite magmas into a region of extreme topographic relief. Regions of this kind represent one of the most hazardous and, potentially, high risk natural environments on the planet. Mapping of the Pebble Creek Formation deposits has elucidated a unique distribution of hazardous events of varying intensity, timing, and frequency associated with the 2360 yr. BP eruption event. For example, lavas erupted onto the steep slopes of the volcano failed under gravitational stresses producing hot block and ash avalanches. During the later stages of this extrusive activity, cold rock avalanches were produced on the oversteepened slopes of the volcano, in one case mixing with a hot block and ash avalanche. Both volcanic and rock avalanches were highly mobile, travelling down the slopes of the volcano, fdling the valley and running up the opposite valley wall. Deposits from these events preserve features that are diagnostic of their hot volcanic or cold mass wasting origins and allow these two deposit types to be distinguished in the field. Discrimination is essential because these deposits are superficially very similar in appearance and may be complexly interlayered, but they have different hazard implications. The steep narrow valleys surrounding the Mount Meager volcano provide an efficient catchment for volcanic and rock avalanches. During the 2360 yr. BP eruption cycle thick block and ash avalanche deposits formed a natural dam against the flow of the Lillooet River, a major drainage system. Volume estimates and flow rates of the current Lillooet River suggests there was an ongoing competition between building of the pyroclastic dam and filling of the lake. Gradual buildup of the dam was periodically interrupted by overtopping and catastrophic failure of the dam by the encroaching lake. This catastrophic failure released high volumes of water, flooding the Lillooet valley as far as Pemberton, B.C. and produced highly destructive debris flows. Properly assessing the distribution and nature of volcanic and associated hazards in mountainous regions features is requisite for formulating a risk model for areas such as Mount Meager. Mapping of the deposits elucidates the distribution, and frequency of past hazardous events. The detailed stratigraphic analysis presented in this study demonstrates the destructive potential of these events based on the intensity and time of onset for analogous modern events.

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