Minimum volume of a tephra fallout deposit estimated from a single isopach

A simple method is presented to calculate the minimum volume of an exponentially-thinning fall deposit for which only one isopach is available. This method exploits the fact that the product of the area enclosed within an isopach (A) and its thickness (T) reaches a maximum value (TA)max for a certain isopach. It is shown that the minimum volume is Vmin=3.69TA, which is equal to the actual volume when TA=(TA)max. Calculation of Vmin for 327 isopachs from 74 fallout deposits shows that it is often just a bit lower than the volume calculated from all the available isopachs. It is therefore proposed that Vmin provides a useful minimum volume estimate for fallout deposits for which only one isopach is available.