Two of the major pumice-fall deposits of Somma-Vesuvius are described. One, the Pompei Pumice, resulted from the classic eruption of A.D. 79, which is taken as the type example of a Plinian eruption. The Pompei Pumice is dispersed south and southeast of the volcano and has been traced to a distance of 72 km. The other, the Avellino Pumice, is probably a thousand years older. It is dispersed to the east and northeast and has been traced to a distance of 50 km. The two deposits are remarkably similar in character, each consisting of a lower part of white pumice and an upper part of mafic gray pumice having a similar grain-size distribution. The distinction between the two deposits is based on (a) the differing proportions of felsic crystals, mafic crystals, and lithic fragments in selected sieve grades; (b) the abundance of nepheline crystals in the Avellino Pumice and their absence from the Pompei; and (c) the greater abundance of crystals in the Avellino Pumice and of lithic debris in the Pompei. The change in composition of the erupting magmas was accompanied by an increase in the vigor of each eruption, as shown by an increase in the density and size of the pumice, in the content and size of lithic fragments, and in the area of dispersal. This is correlated with an increasing depth of origin in a compositionally zoned magma chamber. Granulometric analyses of some ninety samples show that the deposits are well sorted, the sorting improving slightly with increasing distance from the source. Maps showing the median terminal fall velocity, based on the grain-size data, enable limiting values to be derived for the height of the eruptive column and the wind strength. The volume of each deposit has been estimated from the isopach maps, namely, 2.6 km3 for the Pompei Pumice and 2.1 km2 for the Avellino Pumice.
- Geological Society of America