The thickness,volume and grainsize of tephra fall deposits

An improved empirical method for the plotting of field data and the calculation of tephra fall volumes is presented. The widely used area plots of ln(thickness) against ln(isopach area) are curved, implying an exponential thinning law. Use of ln(thickness)–(area)^1/2 diagrams confirm the exponential dependence of many parameters (e.g. thickness, maximum and median clast size) with distance from source, producing linear graphs and allowing volumes to be calculated without undue extrapolation of field data. The agreement between theoretical models of clast dispersion and observation is better than previously thought. Two new quantitative parameters are proposed which describe the rates of thinning of the deposit (b_t the thickness half-distance) and the maximum clast size (b_c the clast half-distance). Many deposits exhibit different grainsize and thickness thinning rates, with the maximum clast size diminishing 1–3 times slower than the thickness. This implies that the entrained grainsize population influences the morphologic and granulometric patterns of the resulting deposit, in addition to the effects of column height and wind-speed. The grainsize characteristics of a deposit are best described by reference to the half-distance ratio (b_c/b_t). A new classification scheme is proposed which plots the half-distance ratio against the thickness half-distance and may be contoured in terms of the column height.