Some field and petrographic features of American and New Zealand ignimbrites

Abstract

In a study of some New Zealand and American ignimbrites (welded tuffs or ash-flow deposits), close similarities were found in the lithology and topographic relations of the deposits. Lithic variations in rocks of ignimbrite origin are great, and even in a single sheet gradations from tuff to hard vitrophyre and felsitic rock are common. These gradations form a consistent pattern in the ignimbrites studied. They are illustrated in vertical sections and interpreted as due to differences in cooling environments. Differences in the physical composition or texture and the relation to preexisting topography of some ignimbrites provide clues to the diversity of pyroclastic flows which may exist. © Taylor & Francis Group, LLC.