THE INTERPLAY OF CLAST SIZE, VESICULARITY AND SECONDARY FRAGMENTATION: AN EXAMPLE FROM THE 1.8KA TAUPO ERUPTION

Vesiculation during magma ascent powers magmatic explosive volcanism. Field studies classically use the density of juvenile pyroclasts 16 – 32 mm in diameter to determine magma vesicularity at the point of fragmentation. Larger pyroclasts outside this range may undergo post-fragmentation vesiculation due to inhibited cooling of the interior and smaller pyroclasts may be too small to represent a bulk vesicle distribution accurately. However, the exact thresholds at which these limitations set in have not been previously quantified. The assumptions of this method is that the 16 – 32 mm size range is representative of the fragmented magma.

We explore, in detail, variations in density, and the roles of post-fragmentation vesiculation and secondary fragmentation over a size range of 4 – 128 mm from Unit 2 pyroclasts of the 1.8ka Taupo eruption. Outcomes are i) a clear threshold of post-fragmentation vesiculation is identified at >32 mm, and ii) broken small pieces of such large pyroclasts can artificially skew the density distribution in smaller clasts fractions. We here constrain the uncertainty associated with vesicularity measurements and offer best-practice recommendations in the hope of improving basic physical interpretations from explosive volcanic field studies in the future.