EVALUATION OF MEASUREMENT STYLES FOR USE IN TEPHRA GRAIN VOLUME ESTIMATION

Field measurements of the diameter of individual tephra grains are used to construct isopleth maps of volcanic airfall deposits and to back-calculate plume and wind conditions at the time of deposition. Ideally, the estimated diameter should yield correct pyroclasts volumes for settling speed calculations.

In this work, three measurement methods are evaluated against each other with regard to accurately calculating clast volumes. The first style of measurement, the Traditional method, requires that the three axes be mutually perpendicular, with the largest measured axis coinciding with the literal longest axis of the clast. The Geometric method requires that all axes are mutually perpendicular but allows the main axis to be chosen relative to the shape of the clast. The third method, Freeform, places no restrictions on axis selection, allowing the measurer to make a “best-guess” approximation. Following axis measurement by caliper, the volume of the clast is determined through volume displacement. The clast volume is compared to volumes calculated from the axes measurements assuming spherical and prismatic geometries. For spherical calculations, the arithmetic mean of the three measured axes is taken to be the diameter of a sphere whose volume approximates that of the clast. In prismatic calculations the three measured axes are multiplied together, approximating the clast as a rectangular prism.

The Geometric method produces the best volume approximations, closely followed by the Freeform method. Both of those methods strongly outperform the Traditional method. For all methods, the spherical volume approximation yielded more accurate volumes than the prismatic approximation.