Kenya Ignimbrites

Abstract

MR. C. M. BRISTOW'S pertinent comments on my communication dealing with froth-flows in Kenya underline a fundamental difference in interpretation of these rocks between me and geologists of the Ministry of Works, Kenya. The latter have attempted to fit the evidence from this field into a framework based on the orthodox concepts of ignimbrite genesis—following Fenner¹, Marshall², Mansfield and Ross³, and Gilbert⁴. I have come to the conclusion that certain features of the flows in question are incompatible with these concepts: and that some departure from the concept of ignimbrites (as clearly defined by Marshall²–products of deposition from immense clouds or showers of intensely heated but generally minute fragments of volcanic magma) is necessary, in this case. Grange⁵, working in the same field as Marshall, remained unconvinced that the process suggested by Marshall had been responsible for the unusual rock types, and preferred to invoke an origin in effusive processes, and Steiner⁶ has lately invoked such processes to explain anomalous features in certain rocks of this field. Even the foremost, active exponents of welding and compaction (litho-static load) processes, Ross and Smith^7,8, have come a long way from Marshall's original concept. The description of an eruption of Cotopaxi quoted by them⁷ as possibly a typical ‘ash flow’ eruption suggests effusion rather than the process envisaged by Marshall. This eruption could equally well have represented vesiculation, expansion and partial or complete rupturing in a rising magma column and effusive flow, quite independent of welding processes—a froth-flow eruption as envisaged by Kennedy⁹ and Boyd¹⁰.