A comprehensive study of pumice formation and dispersal: the Cretaio Tephra of Ischia (Italy)

Abstract

The Cretaio Tephra is a small-volume (<0.02 km3) pumice deposit on the island of Ischia in Italy. It was erupted during historical time and is associated with a tuff ring-dome complex extruded along a N-S-trending normal fault system bordering the east side of the Mt. Epomeo resurgent block. 14C ages of the deposit average 1860 a B.P., which is in agreement with the potsherds found in paleosols underlying and overlying the deposit. The eruption sequence consists of 7 members; most of these members are pumice fall deposits, one is an ash and pumice fallout and another is a surge bed. The tephra have a trachytic-phonolitic composition. All chemical characteristics, including major and trace elements, and Sr-isotope composition are homogeneous throughout the sequence. The vesicularity of pumice clasts does not show any systematic change; the erupted magma had the same amount of volatiles, the same flux and vesicle growth behavior throughout the eruption sequence. Grain-size analyses of 90 tephra samples, representative of all the members at variable distance from the supposed vent, have been processed using the Sequential Fragmentation/Transport theory in order to define the effects of eruptive fragmentation and subsequent transport on the particle sizes distribution. Four significant subpopulations have been identified and related to transport mechanisms. © 1992.