DETERMINING VOLUMES OF SUPERERUPTIONS – THE PITFALLS OF “RULES OF THUMB”

Since they are well outside modern experience, our knowledge and understanding of supereruptions rely heavily on studies of past events. A survey of the literature reveals that accurate determinations of the scale of a supereruption are potentially fraught with error. While a host of criteria such as a minimum erupted volume of 450 km^3 or >10^15 kg of magma, and a VEI >8 are suggested for the definition of a super-eruption, use of these criteria requires that we can accurately assess the volumes (and density) of the deposits. The sheer scale of the deposits and the source calderas, coupled with the vagaries of distribution, exposure, and preservation, particularly in older eruptions, makes volume estimation a non-trivial task. Indeed in many cases volume estimations are merely order of magnitude estimates that are often loosely based on three rules of thumb; that magma equivalent intracaldera (I) and outflow (O) deposit volumes are approximately equal; that some outflow deposit volumes are approximately equal to co-ignimbrite ash volumes (A), and that caldera size is a proxy for eruption volume. Drawing from our work on Central Andean super-eruptions I show that all three of these “rules” are untenable and should be discarded when dealing with supereruptions. First, I:O ratios range considerably based on collapse-style and timing. Many of the largest eruptions are sourced from complex trapdoor and piecemeal calderas that yield I:O ratios as high as 4:1 or 5:1. Second, the absence of plinian fall-out precursors to the ignimbrites and minimal crystal-enrichment in the bulk matrix compared to pumice suggests that fallout and ash volumes (A) are less important than generally assumed. Finally, while the largest eruptions certainly come from very large calderas, significant volumes of magma are erupted from ignimbrite shields that show little evidence of collapse.