Forensic analysis of Cheakamus basalt lavas (BC, Canada): Magnitude, intensity and hazard implications for effusive eruptions in complex mountainous drainage systems

Forensic reconstructions of past volcanic eruptions are a critical component in assessing future volcanic hazards, particularly in regions lacking historical eruptive records. Accurate erupted volume estimates are central to such reconstructions as they underpin assessments of eruption magnitude, intensity, and potential impacts. The Garibaldi Volcanic Belt (GVB) in southwestern British Columbia has erupted episodically over the past ~2 Ma, including several Pleistocene to Holocene valley-filling basaltic effusive eruptions. The Cheakamus basalts (~18–20 ka) represent one such event; sourced from a single high-elevation vent, they inundated over 36 km of the Callaghan and Cheakamus River drainages prior to extensive modification by glacial and post-glacial erosion. Here, we document a series of isolated basalt outcrops located downstream of the previously mapped lava terminus and use petrographic, geochemical, and paleomagnetic data to stratigraphically correlate these remnants with the main Cheakamus basalt sequence. Incorporating distal erosional remnants enables reconstruction of the original lava distribution with a revised areal extent of 38.4 km 2 and a minimum volume of ~2.3 km 3 . This volume corresponds to an eruptive magnitude of ~5.8, suggesting an intensity of ~8.7 for a sustained eruption lasting ~100 days, a scale comparable to recent large, destructive basaltic effusive eruptions (2014–2015 Holuhraun; 2018 Kīlauea; 2021 Cumbre Vieja). These findings demonstrate that erosional processes in glaciated, high-relief terrains can substantially obscure true extents and volumes of lava fields. Consequently, eruption magnitudes, recurrence intervals, and hazard footprints in the GVB and similar mountainous settings may be systematically underestimated.