Monowai is an active submarine volcanic center in the Kermadec Arc, Southwest Pacific Ocean. During May 2011, it erupted over a period of 5 days, with explosive activity directly linked to the generation of seismoacoustic T phases. We show, using cross‐correlation and time‐difference‐of‐arrival techniques, that the eruption is detected as far as Ascension Island, equatorial South Atlantic Ocean, where a bottom moored hydrophone array is operated as part of the International Monitoring System of the Comprehensive Nuclear‐Test‐Ban Treaty Organization. Hydroacoustic phases from the volcanic center must therefore have propagated through the Sound Fixing and Ranging channel in the South Pacific and South Atlantic Oceans, a source‐receiver distance of ~15,800 km. We believe this to be the furthest documented range of a naturally occurring underwater signal above 1 Hz. Our findings, which are consistent with observations at regional broadband stations and long‐range, acoustic parabolic equation modeling, have implications for submarine volcano monitoring.

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Monowai is a submarine volcanic center in the Kermadec Arc, Southwest Pacific Ocean. In the past, activity at the volcano had been intermittently observed in the form of fallout at the sea surface, discolored water, changes in seafloor topography, and T phase seismicity, but there is no continuous record for more recent years. In this study, we investigated 3.5 years of recordings at a hydrophone array of the International Monitoring System, located near Juan Fernández Islands, for long‐range underwater sound waves from Monowai. Results from direction‐of‐arrival calculations and density‐based spatial clustering indicate that 82 discrete episodes of activity occurred between July 2003 and March 2004 and from April 2014 to January 2017. Volcanic episodes are typically spaced days to weeks apart, range from hours to days in length, and amount to a cumulative sum of 137 days of arrivals in total, making Monowai one of the most active submarine arc volcanoes on Earth. The resolution of the hydrophone recordings surpasses broadband network data by at least 1 order of magnitude, identifying seismic events as low as 2.2 mb in the Kermadec Arc region. Further observations suggest volcanic activity at a location approximately 400 km north of Monowai in the Tonga Arc and at Healy or Brothers volcano in the southern Kermadec Arc. Our findings are consistent with previous studies and highlight the exceptional capabilities of the International Monitoring System network for the scientific study of active volcanism in the global ocean.

Late’iki (previously known as Metis Shoal) is a highly active volcano in the Tofua arc with at least four temporary island-building eruptions and one submarine eruption in the last 55 years. The most recent eruption, commencing in October 2019, resulted in lava effusion and subsequent phreatic explosions, the construction of a short-lived island that was quickly eroded by wave action and possibly further phreatic activity that continued into January 2020. The two-pyroxene dacite from the 2019 eruption is similar to the 1967/8 eruptions suggesting the magma is residual from earlier eruptions and has not undergone further differentiation in the last 50 years. New observations of the 2019 eruption site confirm the lava-dominant character of the volcano summit but a thin veneer of wave-reworked, finely fragmented lava material remains that is interpreted to have been produced by phreatic explosions from hot rock-water interactions during the effusive eruption. A notable absence of quench-fragmented hyaloclastite breccias suggests that non-explosive quench fragmentation processes were minimal at these shallow depths or that hyaloclastite debris has resedimented to greater depths beyond our summit survey area.