The formation of various ash flows in Indonesia

Abstract

A thorough analysis of the mode of occurrence of various ash flows deposits in Indonesia confirms the present author’s belief in an important difference in mechanism of formation between ash-flow tuff sheets abundantly found in the island of Sumatra and those minor ash flows of nuée ardentes type produced by orogenic volcanic activity such as displayed by Mt Merapi and Mt Agung in Java and Bali.

The present author is more inclined to think that the enormous ash-flow sheets in Sumatra, usually called « welded tuffs » are nothing more than collapsed froth flows emitted from fissures and closely related to the emplacement of granite batholiths in the core of the geanticlines during the third impulse of orogenic uplift in Plio-Pleistocene time, whereas they have nothing to do whatsoever with nuée ardentes in the sense of Lacroix. These nuée ardentes on the other hand are believed to be the result of a delayed action in the formation of crystal nuclei during the magmatic gas phase of a volcanic eruption.