Transport of Fine Ash Through the Water Column at Erupting Volcanoes - Monowai Cone, Kermadec-Tonga Arc

Monowai cone is a large, active, basaltic stratovolcano, part of the submarine Monowai volcanic center (MVC) located at ~26°S on the Kermadec-Tonga arc. At other actively erupting submarine volcanoes, magma extrusions and hydrothermal vents have been located only near the summit of the edifice, generating plumes enriched with hydrothermal components and magmatic gasses that disperse into the ocean environment at, or shallower than, the summit depth. Plumes found deeper than summit depths are dominated by fresh volcaniclastic ash particles, devoid of hydrothermal tracers, emplaced episodically by down-slope gravity flows, and transport fine ash to 10’s of km from the active eruptions. A water column survey of the MVC in 2004 mapped intensely hydrothermal-magmatic plumes over the shallow (~130 m) summit of Monowai cone and widespread plumes around its flanks. Due to the more complex multiple parasitic cone and caldera structure of MVC, we analyzed the dissolved and particulate components of the flank plumes for evidence of additional sources. Although hydrothermal plumes exist within the adjacent caldera, none of the parasitic cones on Monowai cone or elsewhere within the MVC were hydrothermally or volcanically active. The combination of an intensely enriched summit plume, sulfur particles and bubbles at the sea surface, and ash-dominated flank plumes indicate Monowai cone was actively erupting at the time of the 2004 survey. Monowai cone is thus the fourth erupting submarine volcano we have encountered, and all have had deep ash plumes distributed around their flanks [the others are: Kavachi (Solomon Island arc), NW Rota-1 (Mariana arc) and W Mata (NE Lau basin)]. These deep ash plumes are a syneruptive phenomenon, but it is unknown how they are related to eruptive style and output, or to the cycles of construction and collapse that occur on the slopes of submarine volcanoes. Repeat multibeam bathymetric surveys have documented two large-scale sector collapse events at Monowai and one at NW Rota-1, as well as constructional deposits extending down the flanks of these volcanoes. Acoustic records at Monowai and NW Rota-1 suggest sector collapse events are infrequent while eruptions, and the resulting supply of depositional material, have been nearly continuous. The sector collapse events occurred at times remote from our plume surveys, so, large landslide events are not a prerequisite for the presence of deep ash plumes. Despite a wide range of summit depths (<10 m at Kavachi to 1500 m at W Mata), lava types (basaltic-andesite, boninite, and basalt), and eruptive styles (Surtseyan, Strombolian, and effusive flows with active pillow formation), the deep particle plumes at each of these volcanoes are remarkably similar in their widespread distribution (to 10’s of km from the summit and at multiple depths down the flanks) and composition (dominantly fresh volcanic ash). Moderate eruption rates, lava-seawater interaction and steep slopes below an eruptive vent may be sufficient to initiate the transport of fine ash into the ocean environment and distal sediments via these types of plumes.