Subscribe Today! Please read: Readers of local content on the Herald and News website – – will require a subscription beginning today. For the first few months, non-subscribers will still be able to view 10 articles for free. If you are not already a subscriber, now is a great time to join for as little as $10/month!

A “pumice raft” is seen floating in the South Pacific Ocean after an underwater volcano eruption produced the porous rocks.

Michael and Larissa Hoult were sailing from the Vava’u islands of Tonga to Fiji in the South Pacific Ocean when they saw it — a “total rock rubble slick” extending for miles around them. The rocks, some the size of marbles and others as big as basketballs, bumped against their boat and clogged their rudders.

“It was quite eerie, actually,” Larissa Hoult told CNN. “The whole ocean was matte.”

The couple had sailed upon what scientists call a “pumice raft,” a mass of floating, porous rock that forms when a volcano erupts from the ocean floor. The hot lava, full of gas bubbles, cools in the ocean water and produces pumice, which floats to the surface above the eruption.

The raft spotted by the Hoults has been compared in size to Manhattan, Washington and 20,000 football fields.

Sailors’ warning

The Hoults had received an Aug. 14 email warning of “pumice fields,” which had been spotted by other sailors and detected from space by NASA satellites. By the evening of Aug. 15, they were surrounded by the rocks and their sulfur smell, they said in a post on the Facebook page that tracks their sailing and surfing adventures.

“The rocks were kind of closing in around us, so we couldn’t see our trail or our wake at all,” Michael Hoult told CNN. “We could just see the edge where it went back to regular water — shiny water — at night.”

They posted photos of their discovery to Facebook, and in the week since have received homework assignments from scientists and university professors looking for photos and specimens of the pumice the Hoults collected.

Among them was Scott Bryan, a geologist and associate professor at Queensland University of Technology, who has been studying underwater volcanic pumice for two decades, he told the Australian Broadcasting Corporation.

Australia bound

This mass, he told ABC, is slowly floating toward Australia’s coastline — where in seven to 12 months it could arrive with a host of marine life ready to potentially revive the badly damaged Great Barrier Reef.

“In this 150-odd square kilometers of pumice out there right now, there’s probably billions to trillions of pieces of pumice all floating together, and each piece of pumice is a vehicle for some marine organism,” Bryan told ABC.

Organisms such as algae, barnacles, snails, crabs and possibly even corals could attach themselves to the pumice raft, serving as a “natural mechanism for species to colonize, restock and grow in a new environment,” Bryan said.

Martin Jutzeler, a lecturer at the University of Tasmania who studies underwater volcanoes, told the BBC that one downside is that pumice rafts can also transport nonnative, invasive species from one ocean habitat to another.

It’s possible the raft might not even make it to Australia, he said. Over time, it will probably break apart and disperse.

Terry Hughes, director of the ARC Center of Excellence for Coral Reef Studies at James Cook University in Australia.

He also noted that the pumice raft is just one two-thousandth the size of the Great Barrier Reef, and that coral rarely settles on pumice.

Shannon Lenz and Tom Whitehead encountered the pumice raft as well, telling the BBC in a video interview that the floating stones had stretched “to the horizon on all sides.”

“First we thought it was whale poop,” Whitehead told BBC. “But then as it became obvious there was way more of it than any normal-sized pod of whales could come up with.”