The spectacular eruption of a volcano on land is a display worthy of the big screen (which may explain why the fiery phenomenon often crops up in disaster movies.). Billowing smoke pours into the sky, molten rock explodes upwards like fireworks and angry orange lava streams down the side of the mountain.
A stranger sight – but one that still happens – is the mighty rafts of volcanic rock that can float across the surface of the ocean for thousands of miles after an underwater eruption.
In fact, it’s easy to forget that volcanoes erupt underwater too. “Although 70% of volcanism occurs underwater, submarine eruptions are seldom witnessed owing to their remoteness and absence of technologies enabling their detection,” write the authors of a 2020 paper in Geophysical Research Letters.
In August 2019, close to Tonga in the Pacific Ocean, an unnamed volcano erupted 200 metres underwater – if you placed London’s Gherkin building at the bottom, it’s tip wouldn’t reach the water’s surface.
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Magma contains gases, including water vapour, carbon dioxide and sulphur dioxide. These gases stay dissolved in the molten rock because of the intense pressure deep inside the Earth, like a genie trapped inside a bottle.
As the magma gets closer to the surface, where there is lower pressure, these gases start to expand and form bubbles inside. When it gushes into the ocean and slams into the cold seawater, it cools in an instant.
This can create pumice: hardened rock chock-a-block with bubbles like an Aero chocolate bar. “Such pumice rocks are full of holes and cavities, and they easily float,” explains NASA Science on its website.
And it can make a lot of it. The explosion that August created a vast raft of pumice that spread for 195 km2. That’s bigger than the Caribbean island of Aruba.
Sailors came across the strange phenomenon in the open ocean: the stench of sulphur invaded their nostrils as the rocks drifted past them like cork, bumping into the sides of the boat.
“It was certainly pretty extraordinary,” Rachel Mackie wrote in a comment on Facebook when sharing a video of the field of pumice, adding that the rocks “cleaned the weed off the hull but took the anti-foul and paint as well!”
In the video, there’s rubble all the way to the horizon and it’s hard to believe you’re looking at the ocean. The floating layer of pumice was said to be 30cm deep with some of the stones measuring up to 80cm in diameter.
These fields of buoyant rock can bob along the surface for thousands of kilometres before they eventually soak up enough water that they are dragged back down into the ocean. Until then, the current and winds push them along, sometimes carrying them across the ocean and other times washing them up on shore. This raft made it to Fiji the following month.
By the following April, pumice from the raft had travelled around more than 3,000 kilometres (more than 1,860 miles) and found its way to Australian waters.
Scientists were hopeful that it might bring some helpful hitchhikers along with it. Marine organisms can use the floating rocks like a taxi to make their way long distances across the ocean. "Overall, we've identified more than one hundred different species attached to the pumice – a tremendous diversity of plants and animals," explains professor Scott Bryan, a pumice expert at Queensland University of Technology (QUT) in a statement.
"The sheer numbers of individuals and this diversity of species [...] being transported thousands of kilometres in only a matter of months is really quite phenomenal."
According to his PLOS One study, this can include corals, barnacles, mussels and oysters. "Each piece of pumice is a home, and a vehicle for an organism," he says, describing the arrival of new creatures as being "almost like a vitamin shot for the Great Barrier Reef."
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