Thickness variations and volume estimates of tephra fall deposits: the importance of particle Reynolds number

Abstract

Well-preserved tephra fall deposits display thickness variations which are more complex than simple exponential thinning. On plots of log thickness against square root of area enclosed by an isopach contour, many deposits show two or more approximately straight-line segments and in some cases regions of curvature. We show that major changes in thinning rate occur as the particle size decreases with distance from the vent, as a consequence of the change of settling behaviour from high to low Reynolds number as predicted by W.I. Rose. Computer models of sedimentation from laterally spreading plumes predict a steep proximal segment with exponential thinning for coarse ejecta (lapilli and coarse ash) with high Reynolds number ( Re>500). At greater distance finer ejecta are predicted to show power-law thinning. Two distal segments are identified. The most distal segment is composed of low Reynolds number particles and can be approximated by an exponential thinning law, but is better described by a power law. The distal and proximal segments are connected by a curved segment containing mixed populations of intermediate (0.4< Re<500) and low Reynolds number particles ( Re<0.4). Many prehistoric tephra deposits are not preserved beyond the more proximal high Reynolds number segment and deposit volume, using a simple exponential thinning law, would be substantially underestimated if large amounts of ash were erupted. The database (number of defined isopach contours) is usually too sparse to recognise the intermediate region and curvature, so that the data will often be interpreted as two straight-line segments. The model predictions of break-in-slope distances for different column heights show good agreement with observed breaks-in-slope for eruption columns ranging from 12 to 45 km. Data indicate that the decay constant (the thickness half-distance b t) for the proximal segment is also correlated with column height, as predicted by the models, and is only weakly dependent on total grain size distribution. Volcanic plumes rich in very fine ash (such as co-ignimbrite clouds) produce extensive deposits with large values of b t.