MELBOURNE, Australia — The first thing the sailors noticed was the smell of sulfur.
Then, Larissa Brill and Michael Hoult, a couple sailing in the southwestern Pacific Ocean, saw it: a floating mass of volcanic rocks, with some boulders as large as basketballs, blanketing the ocean as far as their boat’s spotlight could illuminate.
The rocks — a raft of pumice estimated to be as large as 200 football fields — transformed the ocean into an opaque, undulating crust. Scientists say the raft resulted from an underwater volcanic eruption near Tonga this month, and it is slowly floating toward Australia.
A giant raft of floating rock is not all that uncommon. Similar rafts occur every five or so years, scientists said, but such masses are rarely encountered up close by people.
Researchers are curious about whether marine life, hitching a ride on the rocks, might help replenish the Great Barrier Reef, one of the world’s great natural wonders, which has been threatened by consecutive bleaching events in recent years as a result of climate change, and is at risk of eventually dying.
“When the pumice arrives on our shore, it won’t be recognizable,” said Scott Bryan, the geologist who is leading the recovery and analysis of the pumice at the Queensland University of Technology.
The rocks — some no larger than marbles — could be teeming, he said, with marine life including bacteria, algae, barnacles, mollusks, anemones, worms, crabs and it is hoped, corals, along with the organisms that support their growth.
Professor Bryan, who has been studying the effects of underwater volcanic eruptions for two decades, said he hoped corals would arrive in the millions. Even if only 1 percent survive, he said, it could be a blessing for the Great Barrier Reef, which recent studies have shown may not be as resilient as once thought.
The hypothesis that marine life can attach itself to objects — including rocks, floating seaweed and logs — in order to travel great distances while avoiding predators is accepted scientific theory. But some scientists were skeptical about corals’ ability to breed and later colonize over such long distances, as well as their ability to ultimately restore the reef, which is about 2,000 times the size of the pumice raft.
Corals attach themselves to objects on which they grow. In order to colonize the reef, a new generation of baby corals (not those attached to the rock themselves) would need to in effect jump off the rock into the waters off Australia’s east coast, said Terry Hughes, a professor of coral reef science and the director of the ARC Center of Excellence for Coral Reef Studies.
“I’m not aware of any evidence of corals recruiting onto a reef from a piece of pumice,” he said. “If it does happen, it’s very rare.”
In recent years, projects around the world have attempted to help reefs survive the degradations of climate change that include bleaching, invasions by deadly predators and an increased severity of cyclones. But, such projects have yet to be scaled to a degree where they might reasonably save large reefs, scientists said.
Good news stories about the Great Barrier Reef, Professor Hughes added, were dangerous in promoting the idea that silver-bullet solutions exist for what is ultimately a very complex problem.
“The message that we don’t have to worry so much about climate change because the Great Barrier Reef is about to be rescued by pumice is wrong,” he said. “Unfortunately the only fix isn’t an easy one,” Professor Hughes added, “and that’s the greenhouse gas emissions issue.”
Emma Kennedy, a postdoctoral research fellow in marine science at The University of Queensland, agreed that it was misleading to claim the reef could be restored by marine life sailing to Australia via the high seas on a rock.
“Large quantities of pumice are continually being produced by the world’s oceans — so we know pumice rafting is a way that over evolutionary history corals were able to colonize new areas,” Dr. Kennedy wrote by email. But she cautioned against putting too much stock in this raft’s ability to save the reef.
Professor Bryan agreed that while there were challenges for coral in colonizing new areas, there was evidence to show that it was possible for corals to grow to reproductive size on rocks, where they might spawn new larvae into the water. Rocks might also become so heavy with coral growth, he added, that they could sink to the ocean floor and become stuck there.
After the emergence of a similar pumice raft in 2006, Professor Bryan led research that found that more than 80 species had traveled more than 5,000 kilometers from Tonga to the Great Barrier Reef, but just a fraction of them were corals. Still, he said, “you only need a small percentage to take hold to make an impact.”
The pumice raft is expected to reach Australian shores in about seven or eight months, but in the meantime Professor Bryan is collaborating with the sailors, Ms. Brill and Mr. Hoult, who will personally deliver rock samples to him when they reach Brisbane, on Australia’s east coast.