A crystal concentration method for calculating ignimbrite volume from distal ash-fall deposits and a reappraisal of the magnitude of the Campanian Ignimbrite

Distribution of ignimbrites is controlled chiefly by preexisting topography forming thin veneer deposits on steep-slope relief and thicker, valley-ponding deposits in valley bottoms. The calculation of volumes of ignimbrites is difficult because of the nonlinear dependence of thickness with distance. Calculation using geometrical methods is reviewed and the uncertainty with each method is discussed. Based on the genetic relationship between vitric-enriched, co-ignimbrite air-fall ashes and crystal-enriched ignimbrites, a new method is proposed to calculate ignimbrite volume. A simple equation can be used if the volume of the associate and co-genetic distal ash fall and the ignimbrite vitric loss are known. This simple relation is unaffected by deposit geometry, paleotopography irregularities and post-depositional compaction and erosion. The proposed methodology is used to reassess the controversial volume estimates of the Campanian Ignimbrite (Campi Flegrei, southern Italy). The revised volume of the ignimbrite is 54 km3 (25 km3 DRE). The calculated magnitude of the collapsing phase (sum of the ignimbrite mass and co-ignimbrite ash mass) is 7.2.