Discrimination of hot versus cold avalanche deposits: Implications for hazard assessment at Mount Meager, B.C

The surficial deposits surrounding the Mount Meager volcanic complex include numerous avalanche deposits. These deposits share many attributes: (a) they are nearly monolithologic and comprise mainly intermediate volcanic rock clasts, (b) they lack internal structure, and (c) they are very poorly sorted. Despite these similarities, the avalanche deposits represent two distinct processes. Mass wasting of the Mount Meager volcanic edifice has produced cold rock avalanche deposits, whereas gravitational collapse of active lava domes and flows has produced hot block and ash avalanche deposits. The ability to discriminate between these “hot” and “cold” avalanche deposits is a critical component in the assessment of hazards in volcanic terranes. Hot block and ash avalanche deposits can be distinguished by the presence of radially-oriented joints, breadcrust textures, and incipient welding, which are features indicative of high emplacement temperatures. Conversely, rock avalanche deposits resulting from mass wasting events may be distinguished by the presence of clasts that preserve pre-depositional weathering and jointing surfaces. Volcanic avalanches are mechanically similar to rock avalanches but pose a greater hazard due to high temperatures, increased fluidization from degassing and the potential to decouple highly mobile elutriated ash clouds. The increasing use of hazardous regions such as the Lillooet River valley requires more reliable risk assessment in order to minimize losses from future hazardous events.