Satellite image of a pumice raft floating in the ocean
This satellite image of a pumice raft floating near the Kingdom of Tonga gave scientists a clue that an underwater volcano had erupted in the Pacific Ocean. Credit: Joshua Stevens/NASA Earth Observatory

Sailing through rocks is anything but quiet. Last month, vessels in the South Pacific clinked and clanked their way through pumice spewed out from an undersea volcano. These temporary islands of volcanic rock, shaped and propelled by ocean currents, wind, and waves, provide a literal toehold for marine life like barnacles, coral, macroalgae, and mollusks.

“We were in a large area surrounded as far as the eye could see.”

In early August, an unnamed volcano near the Kingdom of Tonga erupted roughly 40 meters underwater. The eruption sent pieces of gray pumice—porous rock filled with gas bubbles—floating to the surface. This volcanic debris, some fragments as large as beach balls, then aggregated into pumice “rafts” spanning roughly 200 square kilometers.

Several sailing crews have encountered the rocks.

“We were in a large area surrounded as far as the eye could see,” said Rachel Mackie, the purser and chef of Olive, a private vessel that sailed into a raft on 9 August near Late Island. There was a strong smell of sulfur, said Mackie, and Olive took a beating. “When the larger rocks hit the steel hull, it reverberated.”

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Pumice rafts aren’t that common, said Martin Jutzeler, a volcanologist at the University of Tasmania in Hobart. “We see about two per decade.”

Not all undersea eruptions produce them, but the rafts that do form tend to stick around. They can last for months or years until the pumice abrades itself into dust or finally sinks. And floating pumice can traverse long distances—when the same unnamed volcano near Tonga erupted in 2001, the pumice raft it created eventually arrived in Queensland, Australia, said Jutzeler.

These transient, movable islands play an important role in marine ecosystems, scientists agree. Barnacles, coral, and macroalgae have all been found clinging to pumice, riding the waves en route to a new home.

“It’s a perfect little substrate,” said Jutzeler.

In 2012, Scott Bryan, a geologist at the Queensland University of Technology in Australia, and his colleagues showed that pumice rafts can significantly increase the dispersal of marine organisms. Bryan and his team found that more than 80 species traveled thousands of kilometers aboard pumice following the 2006 eruption of Home Reef Volcano in Tonga. “Pumice is an extremely effective rafting agent that can…connect isolated shallow marine and coastal ecosystems,” the researchers wrote in PLoS ONE.

Pumice rafts are definitely a way to get organisms to disperse widely.

The long-distance journeys of pumice rafts are “definitely a way to get organisms to disperse widely,” said Erik Klemetti, a volcanologist at Denison University in Granville, Ohio, not involved in the research. But the idea that the stowaways aboard pumice rafts might replenish the Great Barrier Reef’s corals is wishful thinking, said Klemetti. “That’s probably an oversell.”

Jutzeler and his colleagues are planning to study pumice from last month’s eruption. They’ve been in touch with several vessels that passed through the rafts, and they’ve arranged to analyze some of the rocks. (But the samples they’ve been promised are currently stuck in transit in Fiji, said Jutzeler.)

By analyzing the chemistry of the pumice, Jutzeler and his colleagues hope to learn about the properties of the underwater volcanic eruption. For instance, was it eruptive or effusive?

Studying the rocks’ surfaces will also reveal how quickly they’re being abraded, which will shed light on how rapidly volcanic dust is being deposited into the ocean. That’s important because some plankton feed on this volcanic debris, which can result in phytoplankton blooms, said Jutzeler.

Jutzeler and other researchers are keeping a close watch on how the rafts are moving. Satellite imagery—from Terra, Aqua, Sentinel, and Landsat satellites, for instance—provides nearly daily updates. Ocean currents, wind, and waves sculpt and power the rafts, which now number in the hundreds.

They’ll likely arrive in Fiji in a few weeks, Jutzeler predicts.

—Katherine Kornei (@katherinekornei), Freelance Science Journalist

6 September 2019: This story has been updated to correct the distance a previous pumice raft traveled.


Kornei, K. (2019), Volcanic eruption creates temporary islands of pumice, Eos, 100, Published on 06 September 2019.

Text © 2019. The authors. CC BY-NC-ND 3.0
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