Eruption source parameters for a Canadian mafic explosive eruption: an example from Sii Aks (Tseax) volcano

Abstract

Abstract Observations from tephra fall deposits are needed to constrain input parameters used to model tephra fall hazard, but this is challenging for mafic Canadian volcanoes, which are often in remote locations. Here, we present eruption source parameters (ESPs) for the approximately 1700 CE eruption of Sii Aks (Tseax) volcano, British Columbia, based on laboratory analysis of field samples, Indigenous knowledge and tephra dispersion modelling. We analysed tephra samples from 30 locations and found the total grain size distribution (TGSD) to be unimodal, poorly sorted and positively skewed with a median grain size of −1.1 ϕ (2.1 mm). This provides the first TGSD, grounded by field observations, for a Canadian eruption. We used our TGSD and erupted mass estimates as inputs for tephra dispersion modelling and further constrained the plume height based on locations where the Adaawak (Nisga'a histories) indicate the ash cloud was visible. The best-fit source parameters for the eruption indicate a 4–6 km high plume lasting for 1–6 hours. Our results provide a range of ESPs that can be used to model future tephra fall hazard from small, mafic eruptions within the distributed volcanic fields that comprise Canada's Northern Cordillera Volcanic Province.