Classification of Pyroclastic Rocks

Abstract

Tables 1 and 2 represent classification of pyroclastic fragments and pyroclastic rocks originally proposed by Wentworth and Williams. The classification is solely based on the nature of the fragments. It is advisable to use classification of pyroclastic deposits based on the mode of emplacement and deposition of the material. Table 3 is a proposed scheme of such classification. Diagnostic characters of each type of deposits are described. Attention is drawn to the importance of recognition of subaqueous pyroclastic deposits which are supposedly very common in geologic formations. Autobrecciated lava, which is a subaqueous lava of viscous magma such as andesite, is also common among the Japanese Tertiary formations and are apt to be misinterpreted as mudflow or as pyroclastic flow deposit. Autobrecciated lava grades to pillow lava with decrease of viscosity of magma.