Mapping underwater volcanoes safely
New technology enables scientists to explore Tonga’s caldera and examine the aftermath of the underwater volcano’s unprecedented eruption from January this year.
Some of the most misunderstood volcanoes on the planet are underwater and boast unique marine life that feeds on chemicals found around their hydrothermal vents. Underwater volcanoes are mainly formed along ocean ridges or oceanic plate hotspots or areas of plate subduction. Studying these zones helps us understand what’s happening inside Earth’s interior, and identify the potential to cause hazards like earthquakes and tsunamis.
Tonga, known in full as Hunga Tonga–Hunga Ha'apai, is an underwater volcano in the South Pacific’s Tongan archipelago that has bewildered scientists since its eruption on 15 January 2022. With shockwaves that spread across the globe, this was recently confirmed to be the biggest explosion ever recorded in the atmosphere by modern instruments. As gas-infused magma collided with cold seawater, the pressure released sent a plume of ash soaring into the upper atmosphere, which triggered a tsunami that destroyed homes on Tonga’s nearby islands. Scientists are questioning why it sent a cloud to such heights but emitted less ash than expected for such a magnitude, an event which shows a real dearth of understanding about what’s happening in volcanic zones underwater.
Surveying safely
Maxlimer, a 12-metre long USV, will survey the top of Tonga’s caldera next month (Credit: SEA-KIT International)
Scientists have since set out to understand what happened. Prof. Shane Cronin from New Zealand snorkelled above the underwater volcano to get some rock samples so that he could analyse them back in his lab and then build a detailed timeline for the eruption and tsunami. His analysis will supplement additional research conducted from marine vessels over the volcano.
Hunga Tonga - Here is where Dee Sugar and I went for a spot of snorkel sampling and investigation- no underwater camera unfortunately- went from 30 m at boat to sharp drop off after the rock line - where waves are breaking pic.twitter.com/M962qFipUU
— Shane Cronin (@scronin70) April 10, 2022
Over the months of April and May, a survey was completed around Tonga by a collaboration between The Nippon Foundation and New Zealand’s National Institute of Water and Atmospheric Research (NIWA). From the RV Tangeroa, a team assessed changes in topography, accumulations of volcanic ash, and the state of marine ecosystems, utilising multi-beam sonar, geological surveys, and underwater camera footage. In addition to erosion scars, the survey identified large deposits over 20km from the volcano’s peak, showing that pyroclastic flows reached extensive distances from the volcano. See the technologies and tools being used in this fascinating video shared on Twitter.
More mapping missions ahead
Collecting bathymetric data is invaluable for assessing ocean, climate change and biodiversity issues, but to date just 20% of the world’s seabed has been mapped. That figure will be driven upwards by Seabed 2030, a collaboration between The Nippon Foundation and the General Bathymetric Chart of the Oceans (GEBCO), that has set out to create a definitive map of the entire ocean floor that will be freely accessible. See project director of Seabed 2030, Jamie McMichael-Phillips explain the relevance of this data in a video for World Ocean Day 2022.
The not-for-profit REV Ocean has also been mapping the seafloor and underwater volcanoes - and the images captured from its surveys of deep-sea hydrothermal vents under Arctic ice (along with colleagues from Norway) are mesmerising. REV Ocean’s science director, Alex Rogers, explains how the capture happens: “Many volcanoes are deep and cannot be surveyed using satellites, aircraft or aerial drones because water is a barrier to light and radar. We have to map underwater features like volcanoes using sound. The new REV Ocean ship is equipped with multibeam sonar to do this down to depths of 8,000 meters. This is useful for some purposes but to get very high-resolution maps, for mapping habitats, we now use autonomous underwater vehicles (AUVs) equipped with multibeam sonar.”
Additional studies by AUVs and uncrewed surface vessels (USVs) will bring the workings of underwater volcanoes like the Tonga to light. Scientists are poised to examine the findings revealed by Maxlimer, a 12-metre long USV that is scheduled to arrive in Tonga at the end of this month, and following trials, is expected to survey the top of Tonga’s caldera, in July, to create a map of shape, and establish whether part of the volcano's flank or caldera wall collapsed to trigger the tsunami. Working remotely from SEA-KIT HQ in Essex, Maxlimer’s operators will lower cameras and instruments that will survey the caldera’s surroundings, including the water’s oxygen content, turbidity, and cloudiness. Such pioneering data collected will inform scientists about the potential for underwater volcanoes to set off threatening eruptions, so risks can be better mitigated.
Clarissa Wright is a freelance journalist.