| Summary: | 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. === Science, Faculty of === Earth, Ocean and Atmospheric Sciences, Department of === 1 Cd-rom === Graduate
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