Mechanisms of slope failure on Pyramid Mountain, a subglacial volcano in Wells Gray Provincial Park, British Columbia

Abstract

Pyramid Mountain is a subglacial volcano in Wells Gray Provincial Park in east-central British Columbia. Landslides deform the north and east flanks of the volcano. Field strength testing and rock mass classification designate the hyaloclastite breccia in which the landslides originated as a weak, massive rock mass: uniaxial compressive strengths (UCS) range from 24 to 35 MPa, and geologic strength index (GSI) and rock mass rating (RMR) values are 60–70. The shear strength of fracture surfaces in the hyaloclastite breccia, as measured by laboratory direct shear tests, can be characterized by a friction angle ϕ of 18° and cohesion c of 0.11–0.66 MPa. Limit-equilibrium slope stability analyses show that the landslides were probably triggered by the rapid drawdown of a surrounding englacial lake with no seismic ground acceleration required. Slope measurements and slope stability modeling indicate that Pyramid Mountain was asymmetric prior to failure: the north and east flanks had slope angles of 35°–40°, and the south and west flanks had slope angles of 21°–33°. Slope asymmetry may result from closer ice confinement on up-gradient (north and east) flanks due to higher ice flux in this direction relative to down-gradient (south and west) flanks. At the time of failure, the volcanic edifice was at least partially lithified, with cohesive strengths of 0.19–0.52 MPa. Failures of lithified subglacial and subaqueous volcanic edifices may be triggered by rapid drawdown of surrounding water without seismic loading.