Subaqueous, intermediate to silicic-composition explosive volcanism: a review

Abstract

Abstract Mass flow deposits of pyroclastic debris commonly consist of two parts, a lower coarse-grained basal section and an upper finer-grained bedded section. Subaqueous pyroclastic flow deposits, particularly those derived from a vertically-directed eruption, appear better-sorted than their subaerial counterparts because of the efficient sorting characteristics of water compared to air. Subaqueous deposits frequently are depleted in bubble-wall pumice that floats off and vitric ash that remains in suspension. Hence, the deposits show a relative enrichment of crystals, especially at the bases, from (1) flotation and suspension of the vitric material and (2) ingestion of water at the head of the flow. A subaqueous volcaniclastic flow deposit may contain evidence indicative of both turbulent and non-turbulent movement. The flow may have had a turbulent head and non-turbulent body, or flow transformations may have occurred from (1) collapse of the eruption column; (2) changes in slope and in the velocity of the flow; and/or (3) the amount of water mixed into the flow. Retention of heat in the subaqueous environment may be enhanced by lateral eruptions, high eruption rates, large eruptive volumes, and little or no mixing of pyroclastic debris with water. The lack of welded subaqueous deposits in the geologic record may be a result of: (1) effusive rather than explosive eruptions in a deep-water environment; (2) explosive phreatic disintegration of pyroclastic flows in a shallow subaqueous environment; and/or (3) the inability of subaerial pumiceous pyroclastic flows to penetrate the land/water interface. Characteristics of explosive, intermediate to silicic-composition, subaqueous volcanic eruptions also have been summarized in terms of: (1) environment of the eruption; (2) orientation of the eruption; and (3) composition of the juvenile debris. These three aspects of explosive subaqueous volcanism are ideal end-members, and gradations no doubt occur.