Abstract
Estimating the area of tephra fallout and volume of large magnitude eruptions is fundamental to interpretations of the hazards posed by eruptions of this scale. This study uses the tephra from the caldera forming eruption of Mount Mazama (Crater Lake, OR, USA) to demonstrate the challenges faced when working with large prehistoric tephra deposits and outlines the methodologies required to determine eruption volume and magnitude. We combine > 250 Mazama tephra occurrences, reported by a range of disciplines (including archaeology, paleoclimatology and volcanology), with new field studies to better understand the extent of the distal tephra. We find that the Mazama tephra has been remobilised to varying degrees over the past 7000 years, so each tephra locality was appraised for the likelihood that it records primary tephra fallout. We designated 45 of the distal (> 100 km from source) tephra sites as suitable for use in the production of isopachs using a spline fitting method. The new distal isopachs were then integrated with proximal fallout data and estimates of the ignimbrite volume from previous studies to revise the estimated bulk erupted volume from the climactic Mazama eruption to ~ 176 km3 (~ 61 km3 dense-rock equivalent; DRE). This study demonstrates the importance of collating tephra localities from a range of disciplines and that even remobilised deposits provide valuable information about the extent of the deposit. Interpreting remobilised deposits can provide insight into post-eruptive processes that could potentially pose secondary hazards following large magnitude eruptions. We also show that in some circumstances, remobilised deposits preserve important physical properties such as grain size.
Highlights
Large, caldera-forming eruptions pose a potentially devastating hazard to human populations and the environment
The information amassed for each Mazama tephra locality and new field site is available in the database of tephra localities (Supplementary S1)
The locality is situated on a major drainage divide between the Snake River to the east and the John Day and Columbia Rivers to the west; we infer that the deposit at this site provides a minimum estimate of the primary fall deposit thickness, as no tephra has been remobilised from upslope
Summary
Caldera-forming eruptions pose a potentially devastating hazard to human populations and the environment. Carey and Sparks 1986; Pyle 1989, 2000; Bonadonna and Costa 2013) These methods often have limitations for large prehistoric eruptions that have dispersed fine ash over vast areas Isopach maps are the most common way to represent the thickness of a tephra deposit and calculate erupted volume (Pyle 1989; Fierstein and Nathenson 1992; Bonadonna et al 1998). For prehistoric eruptions such as the Mazama, the isopach maps contain a high degree of often unquantified uncertainty (Engwell et al 2015), the effect of which is amplified when isopachs are hand drawn (Klawonn et al 2014). All the isopach maps (Fig. 1) use a relatively small number of ash thickness measurements (less than 55) to reconstruct the deposit over an area that is > 1 million square kilometres, equivalent to ~ 1 measurement per 20,000 km (Lidstrom 1971; Matz 1987; Young 1990)
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