Constraining explosive volcanism: subjective choices during estimates of eruption magnitude
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When estimating the magnitude of explosive eruptions from their deposits, individuals make three sets of critical choices with respect to input data: the spacing of sampling sites, the selection of contour intervals to constrain the field measurements, and the hand contouring of thickness/isomass data, respectively. Volcanologists make subjective calls, as there are no accepted published protocols and few accounts of how these choices will impact estimates of eruption magnitude. Here, for the first time, we took a set of unpublished thickness measurements from the 1959 Kīlauea Iki pyroclastic fall deposit and asked 101 volcanologists worldwide to hand contour the data. First, there were surprisingly consistent volume estimates across maps with three different sampling densities. Second, the variability in volume calculations imparted by individuals’ choices of contours is also surprisingly low and lies between s = 5 and 8 %. Third, volume estimation is insensitive to the extent to which different individuals “smooth” the raw data in constructing contour lines. Finally, large uncertainty is associated with the construction of the thinnest isopachs, which is likely to underestimate the actual trend of deposit thinning. The net result is that researchers can have considerable confidence in using volume or dispersal data from multiple authors and different deposits for comparative studies. These insights should help volcanologists around the world to optimize design and execution of field-based studies to characterize accurately the volume of pyroclastic deposits.
KeywordsExplosive volcanism Pyroclastic deposit Eruption size Kīlauea Iki 1959
This work was supported by the Fred M. Bullard Fellowship and NSF awards EAR-0810332 and EAR-1145159. We are very grateful to all participants who took the time to produce isopach maps for this study. We also thank Wendy Cockshell and Isaac Ishihara for their invaluable help in digitizing the hand-drawn maps.
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