A gigantic floating raft of volcanic rock that emerged from an underwater volcano eruption in the Pacific Ocean is slowly drifting towards the Australian coast, heartening hopes it could benefit the imperilled Great Barrier Reef.

The massive pumice raft – over 20,000 football fields in size and composed of volcanic rock that's light enough to float on the surface of water – appeared only a few weeks ago, after a suspected underwater volcanic eruption near Tonga.

Satellite imagery first revealed the giant formation on the water's surface on August 9, corresponding with reports from sailors at the time.

But the most remarkable observations came from the crew of the Australian adventure catamaran ROAM, who found themselves adrift amidst the huge mass of floating rocks, "completely covering the ocean surface".

(NASA Earth Observatory/Joshua Stevens)

"Rubble slick made up of rocks from marble to basketball size such that water was not visible," the sailors wrote in a Facebook post.

"The rubble slick went as far as we could see in the moonlight and with our spotlight."

The same kind of experience was reported by sailor Shannon Lenz, who posted incredible footage of sailing through the rock raft on YouTube:

"We sailed through a pumice field for 6–8 hours, much of the time there was no visible water," Lenz wrote.

"It was like ploughing through a field. We figured the pumice was at least 6 inches thick."

While the volcanic phenomenon could constitute a sailing hazard for other vessels, news of the raft formation is being welcomed by scientists, especially since the pumice slick is drifting towards the east coast of Australia.

"This is a potential mechanism for restocking the Great Barrier Reef," says geologist Scott Bryan from the Queensland University of Technology (QUT).

"Based on past pumice raft events we have studied over the last 20 years, it's going to bring new healthy corals and other reef dwellers to the Great Barrier Reef."

ROAM sailor Michael Hoult with pumice samples (Sail Surf ROAM/Facebook)

According to Bryan and fellow QUT researchers, the floating rock slick is expected to drift pass New Caledonia and Vanuatu, and may pass through coral reef areas in the eastern Coral Sea.

Importantly, this should happen at about the same time as the region goes through it's main coral spawning later in the year, which could transform the rocky pumice into a travelling ecosystem.

"At the moment the pumice will be bare and barren but over the next few weeks it's going to start getting organisms attached to it," Bryan says.

"It will be able to pick up corals and other reef building organisms, and then bring them into the Great Barrier Reef. Each piece of pumice is a rafting vehicle. It's a home and a vehicle for marine organisms to attach and hitch a ride across the deep ocean to get to Australia."

While the pumice and its payload of algae, barnacles, corals and other marine lifeforms hold the potential to help partially regenerate the Great Barrier Reef's organic matter, others say we need to keep these benefits in perspective.

"Reefs will be gone unless we tackle anthropogenic heating," marine biologist Terry Hughes from James Cook University tweeted in relation to media coverage of the pumice.

"The coral reef crisis won't be solved by a robot, fans, plastic corals or an aquarium - we have to tackle the root causes, especially greenhouse gas emissions."