Gigantic Pumice Raft From Underwater Eruption Is On A Wild Ride Across The Pacific Ocean

Satellite view of the pumice raft on August 13, 2019. (Photo: Joshua Stevens/NASA Earth Observatory)

In early August, sailors in the southwest Pacific Ocean began to see their environment transmogrify. As far as the eye could see, the ocean turned from an azure delight into a colossal gathering of clinking, floating rocks. And then came the foul, sulphurous odours.

These rocks, some as big as a human head, were easy to pick up by hand. They were quickly recognised as pumice, volcanic debris full of holes and pockets of trapped gas that give them buoyancy in water.

Satellite images — plenty of which were shared on Twitter by Simon Carn, a volcanologist at Michigan Technological University — showed a giant pumice raft twisting and warping in the open ocean, pushed around by the winds and waves. The raft covered an area a bit larger than San Francisco.

Unquestionably, a sizeable underwater eruption had taken place, but it wasn’t immediately clear which volcano was responsible. Using satellite imagery, scientists have now found a prime suspect: Unnamed. Seriously, that’s the name of the volcano, at least for now.

This underwater volcano, near the Tongan archipelago, made its confirmed debut in 2001 when a smaller pumice raft was seen emerging in the area. “Erupt once, sure. Erupt twice, ok, I think we better name you,” said Scott Bryan, a geoscientist at the Queensland University of Technology.

This raft is already on the move. It will reach a few islands on the way to the Australia's east coast, perhaps clogging up ports and bays as it piles up inside them, inconveniencing fishers and people simply wanting to hop from island to island. It will get anywhere water flows through, said Ed Venzke, who oversees the database of volcanoes at the Smithsonian Institution’s Global Volcanism Program.

That includes boats still in the region. “There was one report that said the pumice was backing up into their toilet,” Venzke said. For the most part, though, the raft will be pretty harmless to us surface dwellers.

Volcanism, despite its many benefits, is frequently perceived as an agent of destruction. In this case, though, “volcanism is helping life out,” said Janine Krippner of the Global Volcanism Program. Countless marine lifeforms, from algae to mollusks, will do what they always do when objects float by in the ocean: grab on and prepare to invade any land they reach.

It’s not entirely clear when everything kicked off, said Venzke, but reports of some kind of seaward volcanic activity started cropping up sometime after August 7. A few days later, though, boats started to find themselves ominously surrounded by pumice.

Rebecca Carey, a volcanologist at the University of Tasmania, said that this part of the world is decently monitored by certain satellites. Since the satellites can take several images per day, scientists can use them to keep track of the raft’s evolution.

Early on, Bryan explained, the raft was compact, with one boat estimating it was about 30 centimetres thick, and around 50 to 100 square kilometres across. It then thinned and spread out, covering an area of 150 square kilmoetres. An apron of abraded white ash in its wake expanded to an area more than three times that size.

Using observations from seafarers and satellites, Bryan estimated that all that debris would fit into more than 6,000 Olympic-sized swimming pools, a fairly decent amount by any volcano’s standards.

In the grander scheme of things, this boat-consuming raft of Swiss cheese-like debris is a pipsqueak. In 2012, a volcano named Havre, hiding under the waves north of New Zealand, had a colossal upchuck, creating a raft that was 400 square kilometres across.

Not one to be outshone, this Tongan raft did manage to make its own weather. One particularly cohesive clump of it formed an ephemeral island, adorned with its own clouds at one point.

It’s unclear how it managed to do this, but Carey explained that the pumice would have cooled quickly after appearing on the ocean’s surface. That means these clouds are probably unrelated to steam. It’s possible that the raft may have made its own cloud-supporting microclimate, but for now it’s unclear what is going on.

So where did all this pumice come from? Satellite imagery and some telltale seismicity at the onset of the raft’s debut means that it’s very likely to be the Unnamed volcano. A 2007 sonar survey of the area spotted a volcano no more than a couple of hundred feet below the sea surface, according to the Smithsonian’s Global Volcanism Program.

It sits on the Tonga-Kermadec volcanic arc, a line of volcanoes including several that also remain unnamed. The trench, stretching from the eastern North Island of New Zealand to the northern end of the Tongan Islands, is 2,800km long, and marks the point where the Pacific and Australian tectonic plates are colliding, with the Pacific being forced under into hellish depths.

This is a very active zone of volcanism. From the 1970s right to today, there have been a range of pumice rafts reported to have appeared somewhere along it. Our favourite Unnamed volcano, then, is far from alone; it’s a member of a huge family that also includes the champion of deep-ocean eruptions, Havre.

The new pumice, like many of these rafts, is venturing west to Australia’s eastern shores, said Bryan. He reckons this new one will make stops at Fiji, Vanuatu, and New Caledonia on the way, before ending up on the Queensland coast by roughly April 2021.

Not that any beachgoers will recognise it as pumice by then: It will be completely covered in life, Bryan explained. “Out there, there is literally a trillion or so pieces of pumice making up this raft. And each piece of pumice is a vehicle for some organism.”

Millions of mollusks and billions of barnacles and bryozoans — and a frankly ridiculous number of algae — will be heading to Queensland in the coming months. It will be a tremendous influx of biomass. “Each pumice is like its own little island,” said Bryan.

This one is actually perfectly timed, said Bryan, as it will sweep past various islands and reefs by late-November, just in time for major coral spawning events. That means plenty of corals will be brought to the Great Barrier Reef, which has definitely seen better days.

That’s not the only reason Unnamed’s pumice raft is welcome. These rafts also help scientists spot fresh volcanism that they would otherwise miss entirely, giving them an insight into a world they rarely get to see.

Compared to onshore volcano monitoring, looking for and mapping underwater volcanoes, or anything on the seafloor, really, is relatively pricey. Even when it happens, seafloor cartography isn’t the same resolution as maps of the surface world.

“We know so little about the seafloor,” said Jackie Caplan-Auerbach, a seismologist and volcanologist at Western Washington University.

It’s often said that 70 or 80 per cent of the world’s volcanism happens on the seafloor, but this number is sketchy guesstimate, Caplan-Auerbach said. We actually don’t know how much volcanism is going on down there, but our knowledge of plate tectonics and seafloor surveys suggests that it’s a lot. Sadly, unless we stumble on it by chance, it’s often never caught in the act.

That may be changing. Increasingly precise seismometers can sometimes spot a secret eruption, and tech that listens for the sounds of an underwater volcano can be a great way to hear a distant paroxysm. But scientists won’t complain when a submarine volcano decides to be a bit flashy and create a life-supporting raft above it.

Now all that’s left is to give this attention-seeking volcano a moniker. As it falls within the territory of the Kingdom of Tonga, said Carey, “it’ll be up to the kingdom to decide on its name.”

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