Specific Gravity Characteristics of Recent Volcaniclastic Sediment: Implications for Sorting and Grain Size Analysis

Abstract

Specific gravity of water-saturated, surface-dried individual grains and bulk, sieved samples of pumice lapilli, ash, and slightly vesiculated rock fragments produced by recent eruptions at Mount St. Helens, Washington, as well as the composition of sediment derived from this material, indicate two major departures in the character of volcaniclastics from more familiar quartzo-feldspathic sediment. These are: (1) a three-to-five-fold variation in the specific gravity of volumetrically important detrital grains, as opposed to a less than 5% variation in 99% of the volume of average quartzo-feldspathic sandstone; and (2) significant variation in specific gravity that is inversely related to grain size within clast populations of the same composition. This latter observation is attributed to the larger volume of vesicles, particularly non-interconnected vesicles, with increasing grain size. Sorting in volcaniclastic sediment, therefore, is not only a function of depositional process, environment, and post-depositional modification but also of sediment composition. Statistical analysis of sieve grain size data on weight-percent basis is inappropriate for evaluation of volcaniclastic sediments.