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GNS-Led Voyage To Hunga Volcano Returns With New Insights Into 2022 Eruption

Scientists are now much closer to understanding what triggered the most violent volcanic eruption in more than a hundred years thanks to the return of a successful month-long expedition to Hunga volcano (Tonga) led by GNS Science.

“This has been a remarkably successful expedition, having acquired seven different data sets that will provide us with a holistic overview of Hunga volcano and why it erupted so violently” said voyage leader and Principal Scientist Cornel de Ronde.

“Our various data sets will provide unprecedented insight into the ‘why’, ‘how’ and ‘what’ related to the violent eruption of Hunga volcano in January 2022.”

Multibeam surveys of the bathymetry revealed the rim of Hunga volcano is virtually unchanged from before the eruption with a huge, relatively smooth sided crater in the centre of the volcano 850m below sea level. This is consistent with the eruption being gas-driven, having literally blown the centre of the volcano out.

Water column surveys also reveal a relatively thick chemical ‘soup’ occupying the crater at Hunga which contains noticeable hydrothermal plumes, indicating the ongoing steady release of heat and chemicals more than two years after the major eruption with the periodic injection of larger pulses of fluid and gas.

Seismic data also shows relatively smooth insides to the crater with no evidence for ‘steps’ in the crater walls that might indicate caldera collapse and any associated faulting. The data also indicates that the volcano has had multiple eruption cycles so there is a rich history to dive into to explore the frequency and triggers of volcanoes with enormous eruption power.

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“These results will provide important information as to the mechanism of the eruption which will flow through to better understanding of this once-in-a-lifetime event and so enhance hazard preparedness and resilience for Pacific nations,” Dr de Ronde said.

The R/V Tangaroa arrived in Wellington Port on Wednesday 12 June after completing a month-long voyage led by GNS Science to survey the Hunga volcano to better understand what caused the massive eruption in 2022.

GNS scientists were joined on the expedition by scientists and students from multiple research partners; Natural Resources Tonga, Università di Genova, National Oceanic and Atmospheric Administration (NOAA), Helmholtz Centre for Ocean Research Kiel (GEOMAR), Waipapa Taumata Rau/University of Auckland, Queensland University of Technology, and the University of Toronto. They were ably assisted by technical staff from NIWA and joined by a documentary team based in the UK and Wellington.

Funding for the expedition came from the Tangaroa Reference Group fund, GNS LIEF, Endeavour Fund Beneath the Waves, and SSIF – Offshore Hydrothermal systems.

Surveys

Bathymetry

Surveys showed the volcano rim is mostly unchanged, the Hunga Tonga island however has almost disappeared into the sea with evidence showing that scree has fallen into the crater with up to 40m of infill since the volcano was last mapped by the USV Maxlimer seven months after the eruption. Bathymetry also highlighted nearby adjacent volcanic edifices, several of which are hydrothermally active.

Regional bathymetric map of Hunga volcano (centre) and the surrounding volcanic edifices to the west, SE, SSE and NE, all of which were dredged for volcanic rocks with the western one also hydrothermally active.

Seismics

Data shows the bottom of the Hunga crater is filled with ~250m thickness of layered material, and the submerged flanks of the volcano reveal a multitude of laminated and undulating seafloor bedforms stacked up to 1km thick, many of which have typical signatures for volcaniclastic density-flow deposits. The thick sedimentary deposits suggest that basins surrounding Hunga, and neighbouring volcanoes preserve a rich archive of multiple eruption cycles.

Oblique view of (1) above.

Gravity and magnetics

Throughout the journey, the R/V Tangaroa collected ~1100km of SW-NE oriented gravity and magnetic lines. The quality of gravity anomalies is impressive and has shown that the deep central part of the volcano is characterised by dense rocks. Magnetic anomalies of the surveyed area show that significant change in the magnetic properties of the rocks collected may be related to hydrothermal activity.

Close-up of bathymetry for Hunga volcano showing the large explosion crater in the centre. Scallop-shaped holes in volcano rim in the south possible smaller explosion craters. Black areas are very shallow areas which include the islands of Hunga Ha’apai (left-hand side) and Hunga Tonga (right-hand side).

Petrology

79 samples (roughly 300kg of material) were collected from 32 sites, including from Hunga itself and four neighbouring volcanoes. This diverse suite of samples helps to build up a history of these volcanoes, including lavas and explosively erupted materials. The fresh material likely came from the 2022 eruption and will give insights into Hunga’s hydrothermal system.

Hunga Tonga and Hunga Ha’apai islands from the ship’s deck

Water column

Vertical casts both within the crater and over nearby volcano summits shows there continues to be a robust hydrothermal plume within the crater at Hunga, indicating the ongoing release of heat and chemicals more than two years after the major eruption. CO2 appears to be the dominant gas escaping into the crater.

Raising the CTD (Conductivity/Temperature/Depth) unit.

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