Circum-crater variability of deposits from discrete, laterally and vertically migrating volcanic explosions: Experimental evidence and field implications

Circumferential variation in sorting, thickness, granulometry, and componentry of tephra ring deposits can result from instabilities in the eruptive jet and interactions with the confining crater. Jet instabilities result in fingers of high particle concentrations that form deposits radiating away from a crater, referred to as rays. Two major types of rayed deposits are described from subsurface explosion experiments: (1) symmetrical rayed deposits with an axisymmetric ejecta blanket, which result from vertically directed eruptive jets and (2) zones of rays that extend out from sectors of a crater, with an asymmetrical proximal ejecta skirt, that result from inclined jets. Variations within each group are also associated with variations in the explosion depth relative to the energy of the explosion. Although the surface morphology of rays is likely to be lost in natural tephra rings due to overlapping deposits of numerous explosions, rayed deposits are expected to be preserved in cross section as lenses of relatively coarse and poorly sorted material compared to surrounding deposits. Asymmetrical deposits of inclined jets are anticipated to be particularly distinctive. The experimental facies associations indicate that these deposits would be easily distinguished, given sufficient exposure, from other heterogeneities caused by wind influence, collapse of the crater rim, or the influence of topography on density currents. These experimental results can also be used to further the discussion of deposits from inclined jets from other explosion scenarios, such as Vulcanian blasts and hydrothermal explosions. The experimental rayed deposits described here indicate that the classic interpretation of clast concentration zones in tephra ring deposits must be reevaluated.