Basalt eruptions in complex mountain drainages : landscape inundation, response, and hazard implications

This thesis examines the impact of compounding landscape processes in complex terrain on the emplacement, morphology and preservation of volcanic eruptions, using the Late Quaternary Cheakamus basalt eruption in the Garibaldi Volcanic Belt of southwest British Columbia as a case study. I use field, laboratory, and modelling methods to examine how volcanic products like the Cheakamus basalts can be used to discern details of: (i) glacial history; (ii) syn-eruptive lava-drainage system interactions; (iii) post- eruptive landscape recovery and modification; and (iv) hazard implications for similar eruptions. Extensive field mapping, paleomagnetic analysis, and geochemical and geochronological sampling determine the eruptive history of the basalts, including age (~18 ka), eruption duration (<200 years), volume (2.3 km³), estimated eruptive flux (~185 m³s⁻¹), magnitude (5.8) and intensity (8.7). A series of analogue experiments models the emplacement of lavas in mountainous terrain characteristic of the GVB. Results indicate that constrictions in topography lead to lava backup and excess inundation, providing insight into the emplacement processes of the Cheakamus basalts and the resulting volcanic hazards. Analysis of lava distribution, alteration, and textures indicate that the basalts erupted during an ice-free period of the Last Glacial Maximum and interacted extensively with water, attributed to periodic damming and overtopping by the valley’s river system on the timescale of lava-cooling. These textures and morphologies are overprinted by catastrophic post-glacial erosion; mapping and analysis of erosional and depositional landforms indicate a previously unidentified glacial outburst flood which drained a lake of 1.4-8.4 km³, reached velocities up to 30 m s⁻¹, and removed over 0.8 km³ of lava and sediment. This flood occurred at the end of the Pleistocene and fundamentally altered the valley landscape. The valley's mixture of primary eruptive morphologies and secondary erosive features provide a snapshot of a rapidly changing Quaternary landscape. Parsing these compound processes reveals that effusive eruptions in mountainous terrain have a characteristic set of landscape interactions that modify lava emplacement and lead to additional hazards. The glacial history of the area alters and erases evidence of lava emplacement, hindering complete reconstructions of volcanic events and complicating hazard forecasting for similar eruptions which may occur in future.