Subaqueous pyroclastic flows and deep-sea ash layers

Abstract

The modes of formation of subaqueous volcaniclastic rocks can be very diverse (Ch. 3), and recognition of the origins is often not simple. This chapter specifically deals with those volcaniclastics considered to be primary pyroclastic products of highly explosive eruptions. However, one is never far removed from the problems of differentiating primary from redistributed, or epiclastic, volcanigenic sediments. We examine the concept of subaqueous pyroclastic flows and ignimbrites, discussing the types of deposits that have been described, the terminology used and the controversial subjects of subaqueous welding and subaqueous eruption. We suggest that, in general, such deposits are the lateral equivalents of subaerial eruptions from island volcanoes rather than wholly subaqueous deposits of submarine eruptions, and that welding in ignimbrites is a feature found in those deposited on land or in very shallow water. We also consider many examples of widely dispersed deep-sea ash layers, which result from the distal fall-out of large-magnitude continental silicic eruptions. Marine processes which affect their distribution and accumulation are discussed, as well as a method for determining eruption duration from graded bedding. Finally, we briefly speculate on whether subaqueous base surges can form.