Life erupts from volcanic 'mud soup'

September 16, 20255 min read

Main points

1. A multinational research team, led by the University of Tasmania, has been investigating the aftermath of the 2022 eruption of Mount Hunga in Tonga.
2. Researchers have discovered a devastated seascape around the giant underwater volcano, but signs of marine life are slowly returning, including inside the volcano's caldera.
3. The knowledge gained from this voyage will be important in helping to strengthen the resilience of Tonga and the Pacific region to the impacts of natural disasters.

In 2022, Tonga's Hunga volcano erupted, causing devastating impacts on Tongan island communities and the surrounding marine environment. It was the planet's most powerful eruption in over 100 years.

After the eruption, the seafloor around this underwater volcano, located 65 kilometers north of Tonga's main island, was covered in volcanic ash. The ash is so fine that, when stirred, it can behave more like soup than sediment. It can even be liquefied and re-moved by earthquakes, continually reshaping the seafloor.

A lone batfish adds a splash of colour as it patrols the devastated seabed around the volcano. Photo: CSIRO.

Ash is as fine as powder, and these fine volcanic deposits are easily redistributed by seafloor currents. When left undisturbed, they settle as soft mud and silt, uncompressed into the seafloor depressions. They also settle into depressions where thick mudflats can form. Scattered boulders, ejected from the volcano during eruptions and carried on seafloor sediment currents, sit atop these mudflats like islands in a dark gray sea.

But what appears barren is actually slowly coming back to life. This is what the scientific team on the CSIRO research vessel (RV) Investigator crossed the Pacific Ocean to investigate.

Putting seafloor mud under a microscope

Led by Associate Professor Rebecca Carey from the University of Tasmania, an international team of researchers, including a member of Tonga Geological Services, is mapping the erupted volcano structure and studying life on the seafloor. To do this, they are using the comprehensive suite of scientific instruments on board RV Investigator .

Voyage Chief Scientist, Associate Professor Rebecca Carey, UTAS (left) with Australian High Commissioner to the Kingdom of Tonga, Mr Brek Batley, and Professor Vanessa Lucieer, UTAS. Photo: Australian High Commission to Tonga-Kelepi Baba.

The researchers deployed a multicorer, a device that looks a bit like a lunar lander, to take short, one-meter samples of seafloor sediment and test them for life. To drill deeper into the mud, they used the ship's giant piston corer. This impressive device sits in a cradle that runs the length of the ship's aft deck and can collect sediment cores up to 24 meters long.

A much smaller benthic (seafloor) sled was also used by the scientific team to collect biological samples from the seafloor.

Physical sampling is combined with a photographic survey of the seafloor using an underwater camera system called a deep towed camera or DTC. DTCs are a great tool for ocean exploration and can capture high-resolution images and videos of the seafloor at depths of up to 4,000 meters.

Landing! The Multicorer returns to deck with core tubes filled with volcanic sediment. Photo: CSIRO-Fraser Johnston.The Giant Piston Corer, which can collect 24-metre sediment cores, stretches across the back deck of the RV Investigator . Photo: CSIRO-Margot Hind.

Sediment cores and videos from deep-towed cameras revealed two types of communities: those that cling to hard surfaces, and opportunistic species that grow in mud.

Here, scattered rocks become essential.

A stone that does not roll will gather moss.

Sitting atop soupy layers of muddy sediment, the boulders act as oases and anchor points for colonization. Bryozoans, often called 'moss animals' (because their colonies resemble moss), and other bottom-dwelling species take advantage of these hard surfaces, forming small reefs. These scattered boulders are the stepping stones of the restoration process, providing the first anchor points for life in the muddy sea.

Rocks ejected by the volcano during the eruption provided vital anchors for life, including bryozoans and shrimp, on the shifting seafloor. Photo: CSIRO.A sea cucumber sucks food from the muddy seafloor around the volcano. Photo: CSIRO.Stalked hydroids are a common sign of life returning to the seafloor around the volcano. Photo: CSIRO.

Yet, despite more than three years having passed since the eruption, marine life on the desert-like volcanic mud landscape remains sparse. Perhaps this is limited by the challenge of growing on a seafloor that is too soft to provide firm footholds or is constantly being renewed, leading to cycles of burial and rebirth.

Digging a little deeper reveals a different story for some very small sea pioneers.

Mud soup for you

In the first few centimeters of the mud, the scientists saw signs of re-colonization. Here they found colonies of microscopic single-celled organisms called porcelaneous miliolid foraminifera, or 'forams', which thrive in soft, muddy sediment. They were accompanied by agglutinated (glued together) benthic foraminifera, tubular foraminifera. These made small shells armored in fine sediment and stood upright in the mud, with pseudopodia (hand-like structures) protruding from the surface.

As the saying goes, life finds a way.

Importantly, the fragile shell remains of drifting and free-swimming microfossils, including planktonic foraminifera and pteropod molluscs, are observed in greater abundance in the first few centimetres of the mud layer. This suggests that the sediment 'soup' begins to settle and becomes more stable and habitable as it is compacted.

Mele Manu, Senior Geologist with Tonga Geological Services, examines a sediment core from the volcano. Photo: CSIRO-Fraser Johnston. Microfossils of tiny single-celled forams were collected from a sample of volcanic sediment. Photo: CSIRO-Fraser Johnston.

Mele Manu, Senior Geologist with Tonga Geological Services, said the voyage would provide Tonga with important new data on seafloor changes and hazards following the 2022 eruption, enhancing its monitoring and risk prevention capabilities.

'The research also improves our understanding of ecosystem restoration and the return of marine life, supporting Tonga's biodiversity and the livelihoods that depend on it,' Ms Manu said.

Importantly, the observations and data collected on the voyage will increase understanding of how long it takes marine ecosystems to recover from large-scale disturbances. This knowledge is essential for Tonga and other Pacific island communities that rely on the ocean for food security and prosperity, helping to support the region's resilience to the impacts of natural disasters.

CSIRO research vessel (RV) Investigator conducts a survey inside the caldera of Hunga volcano, Tonga. Photo: CSIRO-Karl Forcey.

This research was supported by a sea time grant on RV Investigator from CSIRO Marine National Facility (MNF). Supported by the National Collaborative Research Infrastructure Strategy (NCRIS), MNF is one of Australia's most productive research infrastructures, delivering research to help ensure the security, protection and prosperity of our marine environment.

A team of CSIRO scientists is also working in the Kingdom of Tonga as part of an ACIAR-funded soil management project to improve the resilience of farming systems in Tonga through better soil management.