Submarine Eruption Dynamics, Revealed by Hydroacoustic Data at NW Rota-1, West Mata, and Axial Volcanoes

Abstract:

To date, only two submarine volcanoes have ever been observed in eruption: NW Rota-1 (Marianas), and West Mata (Lau Basin). Both eruptions were also recorded acoustically, with seafloor hydrophones deployed coincident ROV dives, and with moored hydrophones that recorded for durations of >1 yr. Additionally, a 2015 eruption at Axial Seamount, Juan de Fuca ridge, was captured in real time by both seismometers and hydrophones of the Ocean Observatories Initiative. In all cases, the hydrophones recorded two major categories of signals: (1) short duration discrete pulses, and (2) prolonged, diffuse broadband signals with gradual onsets and abrupt ends. Based on simultaneous video and hydroacoustic data at NW Rota-1 and W Mata, we interpret most of the discrete signals as explosions of gas or steam. The prolonged, diffuse broadband signals at both NW Rota-1 and West Mata are observed when sufficient pressure from rising gas slugs allows them to open fractures or break through a vent cap. The duration and amplitude of these signals provide a window into the dynamics of these submarine Strombolian eruptions.

Transitions between signal types over the long term be indicative of changes in gas flux. In December 2009, for example, activity at West Mata was equally composed of discrete and diffuse signals. By April 2010, diffuse signals occurred over 80% of the time. On the short term, dramatic shifts between diffuse and discrete activity were observed at West Mata (see figure), suggesting a transition in the manner in which gas was fed to the vent. Diffuse signals at NW Rota-1 tended to occur in regular bursts of similar duration, although the duration and amplitude varied with time. Hydroacoustic data recorded at Axial reveal clusters of similar explosions indicative of a common source type in the same location, likely reflecting explosions from a single lava flow. Shifts in event similarity imply a new lava flow location. A change in the appearance of discrete explosions on May 1, 2015 coincides with the onset of a series of long diffuse events that bear similarity to those observed at NW Rota-1 and West Mata. Fine ash collected in the caldera confirms that explosive activity occurred at Axial. The dynamics of these eruptions, in terms of source type, source location, and gas flux, are well captured by hydrophones.