The August 2019 Tongan 18.325oS/174.365oW submarine volcanic eruption: eruptive processes and pumice raft formation and evolution

The August 7th 2019 eruption of Volcano 0403-091, a shallow submarine volcano near Tonga, produced a 200 km2 raft of pumice - a highly porous, buoyant volcanic rock that can float in water for many weeks, during which time it is dispersed around the oceans as it drifts in the direction of winds and currents. This floating pumice raft has already reached Fiji and is expected to reach Vanuatu, New Caledonia and ultimately Australia in the coming months. Pumice rafts provide a home for numerous marine species that begin to colonise the drifting pumice, and this raft has sparked global interest because of its potential to transport species, including corals, as far as the Australian Great Barrier Reef. Pumice rafts are also of interest to scientists and society because of their potential to damage ships: past rafts have demonstrated that they can abrade hulls, jam or damage propellers and rudders, and block water intakes (causing engine failure) - even for large cargo ships. A future pumice raft that enters a major shipping lane could therefore cause severe economic disruption, just like the 2010 European aviation ash crisis. To prepare hazard mitigation strategies for such an event, we need to better understand what kinds of eruptions (hence which volcanoes) can create floating pumice, and improve our ability to forecast where rafts will travel to via ocean currents.

This eruption has provided us with a unique opportunity to gain crucial data that will help us to understand how pumice rafts are formed, how their physical characteristics change as they drift across the ocean, and how pumice raft-producing eruptions impact ecosystems and biogeochemical processes both at the eruption site and in distant regions reached by the raft. We propose to visit the volcano and perform a seafloor survey of the eruptive vent and shallow volcano summit using a Remotely Operated Vehicle (ROV) that records video and collects samples, as well as additional sample collection from deeper locations using a dredge system with attached camera. This will enable us to map the new morphology and seafloor deposits of the volcano, and document the species living on the volcano and how they have been affected by the eruption and hydrothermal activity. Samples of seafloor eruptive products from near the vent (which never floated) and up to 6 km away (which floated some way before sinking) will be compared with samples of floating raft pumice collected by sailors within one week of the eruption. By analysing the chemistry (magma composition, gas contents) and comparing the physical characteristics (clast size, bubble connectivity) of these different samples we can investigate the eruption processes that control whether a volcano can produce a floating pumice raft. We also intend to sample raft pumice when it washes up on nearby coastlines (e.g. Fiji, New Caledonia) after several weeks of floating. This will reveal how pumice characteristics (e.g. size, shape, buoyancy) change during dispersal, which will help us improve models of pumice raft dispersal and hazard assessment. It will also reveal the temporal change in the number and type of marine species that colonise the raft, allowing us to assess which species may be transported to the Australian Great Barrier Reef.

It is important to undertake this survey and sampling as soon as possible after the eruption, before the shallow vent can be altered by wave/cyclonic action (which can erode and redistribute eruption products) and while the seafloor ecosystem is still in a state of responding to the eruption and hydrothermal activity. This eruption and its raft have been unusually well documented e.g. we have clear satellite images of raft dispersal, whereas past rafts have been hidden by clouds. It is therefore an extremely rare and valuable opportunity to obtain both seafloor and floating samples with excellent constraints; a quality of opportunity that is unlikely to occur again in coming decades.

Our study will investigate shallow submarine pumice raft-forming eruptions in general, and Volcano 0403-091 near Tonga in particular, both of which have a wide range of potential economic and societal impacts. The research outputs of this study will therefore have a wide range of potential beneficiaries.

Floating pumice rafts are a considerable hazard to maritime vessels and infrastructure. Damage to steering, blocking of water intakes (resulting in engine failure) and hull abrasion have all been documented during past pumice rafting events and it is only a matter of time before a raft intersects a major shipping route. The global economy is reliant on shipping (for example, the maritime sector added £37 bn of gross value to the UK economy in 2015) and so is vulnerable to economic shocks caused by supply chain disruption. As was the case for the 2010 European ash crisis for aviation, safety thresholds and reliable dispersal models for pumice rafts are currently lacking. This means that in the event of a major pumice raft event today, shipping operations would likely have to stop entirely or undergo lengthy re-routing. Our research will produce fundamental information for hazard mitigation that will therefore directly benefit shipping and insurance industries, as well as governmental and other policy makers responsible for hazard management and response. On a smaller scale, this will also benefit members of the general public who use small craft for leisure or fishing activities.

The potential for pumice rafts to damage reefs and coastal areas (by choking/blocking of shallow waterways) or to introduce new species to distant regions has implications for conservationists, particularly those working to preserve areas of 'pristine' reefs (e.g. New Caledonia) and those working in areas under environmental stress (e.g. the Great Barrier Reef), where new species could either rejuvenate or harm the reef. Our study of the temporal evolution of biological colonisation of the raft will be an important step towards constraining and preparing for these potential impacts.

More specifically to Volcano 0403-091, our study will reveal its new post-eruptive morphology and whether it poses a navigational hazard, benefiting local sailors and maritime safety boards, who may need to define exclusion zones. The volcano has erupted at least twice in the last two decades, is part of a large submarine caldera system, and lies only a few km away from inhabited islands. Our study will be the first survey of the submarine edifice, and is anticipated to lay the foundation for future studies of the wider system that will assess its potential for future large magnitude caldera-forming (and potentially tsunamigenic) eruptions that would devastate nearby coastal communities. Our study will also lead to improved science outreach in the western Pacific region, with collaborations with local researchers and the development of citizen science campaigns designed to inform the general public of the potential hazards and impacts of pumice rafts, and to encourage them to participate in science by contributing their observations and samples of pumice rafts that reach coastal areas.