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.
“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.