The emplacement of low‐aspect ratio ignimbrites by turbulent parent flows

Abstract

A quantitative model for a radially spreading, suspension‐driven gravity current is used to interpret parent flow conditions of a volumetrically wide range of pyroclastic deposits known from the recent and ancient geologic records of explosive volcanism. The analysis, which requires no freely adjustable parameters, yields values of the solids concentration, time of duration (or, equivalently, a minimal estimate of eruptive flux of solids) and average speed for the ground‐hugging, deposit‐forming gravity currents. A consistent picture emerges in which the initial volumetric concentration of pyroclastic solids in the parent flows is of the order 10−2 or less, and the expanded flows travel with average speeds in the range 25–250 m s−1 to reach their farthest extent in a matter of only minutes. Further application of the model suggests that depositional structures in a widespread and thin ignimbrite are likely to reflect the intensity of mass flux in the parent flow, and thus the intensity and overall volume of the parent eruption. Conditions in small‐magnitude pyroclastic currents of recent history can result in cross‐stratified deposits. Primary sedimentary structures (or their absence) in some regionally widespread and massive deposits known only in the geologic record, in contrast, were probably emplaced under conditions of intense sheet‐flow transport driven by the parent gravity currents.