Cooling of tephra during fallout from eruption columns

Abstract

Models are presented for the cooling of tephra during fallout from explosive eruption columns. All tephra particles are assumed to be spherical and heat loss is considered to occur by radiation and forced convection. Grainsize is the most important control on the cooling. Clasts larger than 25 cm diameter suffer little heat loss, whereas clasts smaller than 1.6 cm diameter are completely cold on deposition. Large clasts form a well-developed chilled margin during fallout and a breadcrust texture can result if vesiculation of the hot interior occurs. The results of these calculations are combined with a model for fallout from the margins of an eruption column to predict the proximal temperature variation with distance from the vent in the deposits. Temperatures high enough for dense welding in proximal fallout deposits can extend from a few hundred metres to nearly 2 km. Extent of the welded facies increases with column height, mean grainsize and magmatic temperature. Welded fallout deposits are only predicted to occur for high temperature silicic and intermediate magmas with temperatures >850°C. These predictions are in good agreement with observations, in that welded fallout deposits have only been documented in high temperature dacites, rhyolites and panellerites. A postulated fallout origin for welded rocks that can be traced significantly further than 2 km from vent must be suspect.