Patterns of fine ash dispersal associated with ongoing submarine volcanic eruptions

Abstract:

During the past two decades, water column surveys at an increasing number of actively erupting submarine volcanoes have revealed a common pattern of syneruptive fine ash dispersal associated with arc, forearc and backarc submarine volcanism. The sites have included a variety of lava types, eruptive styles and depths (Table 1). Particle plumes spreading laterally, in all directions, from the flanks of these volcanoes were found in multiple layers of varying thickness downslope of eruptive vents, and were detected tens of kilometers away. In contrast to the magmatic-hydrothermal plumes that rise above the eruptive vents, flank plumes are dominated by fine, sharp, fresh glass shards and devoid of magmatic-hydrothermal tracers. The most intense particle concentrations were often found near the seafloor. When repeat surveys could be made within days, decreased particle concentrations and deepening upper limits for these layers implied rapid settling and/or advection by local currents, and episodic emplacement. Sector collapse scars and large-scale turbidity flow deposits are common on steep slopes of submarine volcanoes, however large landslide events do not appear to be a prerequisite for forming flank plumes; moderate eruption rates, lava-seawater interaction and steep slopes below an eruptive vent are sufficient to initiate the transport of fine ash via smaller sediment gravity flows at, or near, eruptive vents. In conjunction with other evidence (such as hydroacoustic signals and intense magmatic-hydrothermal plumes above summit depths) the presence of flank plumes can be diagnostic of an ongoing eruption. Plume layers that separate from the flanks at intermediate depths can advect fine ash to even greater distances by regional currents (10’s to 100’s of km depending on depth of detachment, surrounding water depth, particle sizes and current velocities) before settling to the seafloor, and may appear as either very thin layers of ash or dispersed fine ash (cryptotephra) in distal marine sediments. These observations can contribute to a better understanding of the contribution of submarine volcanism to fine ash in marine sediments, which is most often attributed to fallout from subaerial eruptions or from eruption columns that rise into the atmosphere from shallow submarine eruptions.