Rocks composed of volcanic fragments and their classification

Abstract

Summary Wentworth and Williams ' (1932) and Blythe 's (1940) reviews of pyroclastic nomenclature are excellent accounts of pre-1940 terminology. Changes or additions to pyroclastic names since that time have come mainly from vigorous research on processes of pyroclastic flow and their resulting depositional product, ignimbrite. It has long been realized that pyroclastic fragments become mixed with other types of fragments, but only recently has this been recognized in classifications. Russian workers ( Blokhina et al., 1959 ; Vlodavets et al., 1962 ), Panto (1959) , Fisher (1961) and Torok (1962) have attempted to unify pyroclastic and epiclastic rock terminology. Russian authors do this by classifying rock mixtures composed of pyroclastic and non-volcanic (“sedimentary”) mixtures. Fisher (1961) attempts to unify the terminology by naming genetic terms for processes of fragmentation, viz., pyroclastic, epiclastic and autoclastic, to which may be added alloclastic ( Wright and Bowes , 1963 ) and hyaloclastic ( Rittmann , 1960 ), and by using the standard clast-size limits of the Wentworth Scale for all of the genetic types. Perhaps the most fundamental disagreements in volcaniclastic names arise from disagreement or misconceptions about some common terms such as “sedimentary”, “volcanic”, “epiclastic” and “pyroclastic”. For example, the terms pyroclastic and epiclastic refer to different processes of fragmentation, not to different processes of deposition. Both types of particles may be deposited by streams or wind in any physiographic environment, but this does not alter their original mode of fragmentation. The terms sedimentary and volcanic are much broader in meaning than epiclastic and pyroclastic. International agreement on nomenclature of volcaniclastic sediments and rocks will very likely never come about until agreement is reached on the meaning and use of these four terms.