Origin of Ignimbrites

Abstract

F. J. FITCH¹ has criticized a new hypothesis proposed by me² on the origin of ignimbrites of Pliocene to Pleistocene Age, from the North Island of New Zealand. The new petrogenetic concept is based on the presence of two contrasting glasses and their textural relationship, which suggest liquid immiscibility. It is emphasized that the presence of the two glasses can be observed in fresh ignimbrites only because both glasses undergo early selective crystallization. It is because of this crystallization of the glassy constituents that liquid immiscibility has not been recognized earlier. When both glasses have crystallized the ignimbrite contains no glass at all, although two glasses were present immediately on solidification of the rock. Many ignimbrites, particularly ancient ones, therefore, do not afford any evidence concerning the presence of two contrasting glasses and their textural relationship.