Subaqueous eruption-fed density currents and their deposits

Density currents fed directly from subaqueous eruptions can be divided conceptually into three groups based on modes of fragmentation and transport: (I) explosive fragmentation, with deposition from a gas-supported current; (II) explosive fragmentation, with deposition from a water-supported current; (III) fragmentation of flowing lava, with deposition from a water-supported current (Fig. 1). Group I products include subaqueously emplaced welded ignimbrite and other high-temperature emplaced subaqueous pyroclastic flow deposits. Group II products are the most varied, and include representatives of both high- and low-concentration turbidity currents, grainflows and debris flows, and are termed eruption-fed aqueous density currents. Some clasts in such currents are transported and even deposited at high temperature, but the transporting currents, ranging from grain flows to dilute turbidity currents, are water dominated even though steam may be developed along large clasts' margins. Group III products formed from lava flow-fed density currents tend to be weakly dispersed down-gradient along the seafloor, and generally consist largely of fragments formed by dynamo-thermal quenching and spalling. Bursting of bubbles formed by vapor expansion probably contributes to some Group III beds. Distinctive column-margin fall deposits may form in water-excluded zones that developed very locally around vents in association with Group I and II deposits, and are distinguished by heat retention structures, indicators of gas-phase transport and absence of current-formed depositional features. (C) 2000 Elsevier Science B.V. All rights reserved.