Sources of uncertainty in the Mazama isopachs and the implications for interpreting distal tephra deposits from large magnitude eruptions

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 km 3 (~61 km 3 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.

The radius of the umbrella cloud helps characterize large explosive volcanic eruptions

“…However, statistical methods used to estimate eruption volume and column height are sensitive to deposit erosion 14 . In particular, older deposits are often eroded, reworked, or sparsely sampled, leading to uncertainty and bias in volume and column height estimates 15,16 , which may result in misclassification of older eruptions 17 .…”

How do the grain size characteristics of a tephra deposit change over time?

“…Geochemical alteration of the buried tephra layer could continue indefinitely. However, despite these observations, temporal changes in tephra GSD are not well understood (Buckland et al 2020;Dugmore et al 2020). We addressed this knowledge gap by comparing recent measurements of tephra grain size with similar measurements made decades earlier, shortly after the tephra was deposited.…”

Fifty years of limnology (1969-2019) at Mahoney Lake, British Columbia, Canada

“…Deposition of Mazama ash was noted as significant in this lake, with nearly one metre of tephra preserved in the sediments. This contrasts with only eight centimetres of Mazama tephra at nearby Lake of the Woods, British Columbia (Heinrichs et al 2004), which is more typical of most Mazama tephra layers in lakes within the region (Buckland et al 2020). Okenone pigments decreased significantly during tephra deposition in Mahoney Lake, but reached their highest concentrations during the last few thousand years.…”