A ‘raft’ of pumice rock the size of Birmingham has been spotted in the Pacific ocean.

The floating sheet is more than 150 square kilometres large and was caused by an underwater volcano which erupted near Tonga.

Experts believe that the rocks – some of which are the size of basketballs – will reach the Australian coast in seven to 10 months where they could replenish the Great Barrier Reef, which is being destroyed by coral bleaching.

Michael and Larissa Hoult, who are crew of the Australian adventure catamaran ROAM, found themselves sailing through the mass of rocks earlier this month while on their way to Fiji.

“We entered a total rock rubble slick made up of pumice stones from marble to basketball size,” they wrote on a Facebook post on 15 August.

Coral reefs: Trying to combat climate change damage Show all 25 1 /25 Coral reefs: Trying to combat climate change damage Coral reefs: Trying to combat climate change damage Masked Butterflyfish (Chaetodon semilarvatus) swimming over a bommie reef in the Egyptian Red Sea marine reserve of Ras Mohamed, off the southern tip of the Sinai peninsula AFP/Getty Coral reefs: Trying to combat climate change damage Rising sea temperatures cause corals to bleach (go white) and die Getty/iStock Coral reefs: Trying to combat climate change damage A giant clam is seen nestled among coral reefs at the Obhor coast, 30 kms north of the Red Sea city of Jeddah AFP/Gett Coral reefs: Trying to combat climate change damage Coral reef in seychelles that has degraded After the reef has died they break up and become rubble. On this reef there is some regrowth of young corals so there is hope for recovery Getty Coral reefs: Trying to combat climate change damage Coral gardening A rabbitfish in a net H Goehlich Coral reefs: Trying to combat climate change damage A school of fish and a sea can in a healthy coral reef off the coast of Isla Mujeres, Mexico Getty/Lumix Coral reefs: Trying to combat climate change damage Sky views of great barrier reef in Australia Getty/iStock Coral reefs: Trying to combat climate change damage A fish swims among coral reefs at the Obhor coast AFP/Getty Coral reefs: Trying to combat climate change damage Researchers from the Interuniversity Institute for Marine Sciences in the southern Israeli resort city Eilat monitor coral growth while scuba diving in the Red Sea AFP/Getty Coral reefs: Trying to combat climate change damage Coral gardening A rope nursery Nature Seychelles Coral reefs: Trying to combat climate change damage Fish swimming off the coast of Egypt's Red Sea resort of Hurghada. The rebounding tourism sector is worrisome for the fragile marine ecosystem AFP/Getty Coral reefs: Trying to combat climate change damage Coral gardening A parrotfish on the reef C Reveret Coral reefs: Trying to combat climate change damage Gorgonian sea fan on a a coral reef in the Egyptian Red Sea marine reserve of Ras Mohamed AFP/Getty Coral reefs: Trying to combat climate change damage A diver swims during a Great Barrier Reef experience on Lady Elliot Island, Australia Getty/Tourism Queensland Coral reefs: Trying to combat climate change damage Jessica Bellsworthy, a PhD student conducting research on the coral reefs of the Gulf of Eilat, holds a coral in an aquarium at the Interuniversity Institute for Marine Sciences in Eilat AFP/Getty Coral reefs: Trying to combat climate change damage Coral reefs in the water off the Obhor coast, 30 kms north of the Red Sea city of Jeddah in 2008 AFP/Getty Coral reefs: Trying to combat climate change damage A diver photographs golden anthias (Pseudanthias aurulentus) on a coral reef in the Egyptian Red Sea marine reserve of Ras Mohamed AFP/Getty Coral reefs: Trying to combat climate change damage FUNAFUTI, TUVALU - AUGUST 15: From the air the ocean (L) and the logoon (R) and separated by a thin stip of land on August 15, 2018 in Funafuti, Tuvalu. The small South Pacific island nation of Tuvalu is striving to mitigate the effects of climate change. Rising sea levels of 5mm per year since 1993, well above the global average, are damaging vital crops and causing flooding in the low lying nation at high tides. Sea water rises through the coral atoll on the mainland of Funafuti and inundates taro plantations, floods either side of the airport runway and affects peoples homes. The nation of 8 inhabited islands with an average elevation of only 2m above sea level is focusing on projects to help it and its people have a future. Four of the outer islands are 97% solar energy dependent and the Tuvalu Government is working to achieve 100% renewable energy from wind and solar by 2025. Tuvalu's 11,000 inhabitants see the effects of climate change in their daily life. (Photo by Fiona Goodall/Getty Images for Lumix) Fiona Goodall Getty/Lumix Coral reefs: Trying to combat climate change damage A photo taken on April 4, 2019 shows fish swimming off the coast of Egypt's Red Sea resort of Hurghada. - In dazzling turquoise waters off Egypt's Red Sea coast, scuba divers swim among delicate pink jellyfish and admire coral -- but the rebounding tourism sector is worrisome for the fragile marine ecosystem. (Photo by Mohamed el-Shahed / AFP) (Photo credit should read MOHAMED EL-SHAHED/AFP/Getty Images) MOHAMED EL-SHAHED AFP/Getty Coral reefs: Trying to combat climate change damage Coral gardening A damselfish Sarah Frias-Torres Coral reefs: Trying to combat climate change damage Divers swim past a coral reef in the Egyptian Red Sea marine reserve AFP/Getty Coral reefs: Trying to combat climate change damage A puffer fish hovering above coral in the Egyptian Red Sea marine reserve AFP/Getty Coral reefs: Trying to combat climate change damage Researchers from the Interuniversity Institute for Marine Sciences in the southern Israeli resort city Eilat monitor coral growth while scuba diving on June 12, 2017 in the Red Sea off Eilat. Global warming has in recent years caused colourful coral reefs to bleach and die around the world -- but not in the Gulf of Eilat, or Aqaba, part of the northern Red Sea. At the Interuniversity Institute for Marine Sciences in southern Israeli resort city Eilat, dozens of aquariums have been lined up in rows just off the Red Sea shore containing samples of local corals AFP/Getty Coral reefs: Trying to combat climate change damage This photo taken on April 21, 2017 shows an aerial shot of part of mischief reef in the disputed Spratly islands on April 21, 2017. Philippine Defence Secretary Delfin Lorenzana flew to a disputed South China Sea island on April 21, brushing off a challenge by the Chinese military while asserting Manila's territorial claim to the strategic region. / AFP PHOTO / TED ALJIBE (Photo credit should read TED ALJIBE/AFP/Getty Images) TED ALJIBE AFP/Getty Coral reefs: Trying to combat climate change damage fish swimming off the coast of Egypt's Red Sea resort of Hurghada AFP/Getty

“The waves were knocked back to almost calm and the boat was slowed to one knot. The rubble slick went as far as we could see in the moonlight.”

The pumice – which forms when molten rocks cool rapidly – comes from an unnamed volcano that is believed to have erupted on 7 August. It last erupted in 2001 and the summit is thought to be 40 metres below the surface.

Ninety per cent of the pumice is underwater and experts have welcomed the discovery, saying the raft could be a home for billions of marine organisms.

The pumice – which forms when molten rocks cool rapidly -–comes from a volcano that probably erupted on 7 August. Pictured is a Nasa image of the raft (NASA)

“This is some of the newest rock on our planet, only being a few days old,” said Associate Professor Scott Bryan, a geologist from the Queensland University of Technology (QUT).

“The pumice is from an eruption that no one really witnessed. This is a potential mechanism for restocking the Great Barrier Reef.

“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,” he said.

Submarine volcanic eruptions such as this are believed to happen once every five years are often completely missed by experts as there is no monitoring equipment to catch the earthquakes before the blast.

Dirty water threatens the Great Barrier Reef Show all 20 1 /20 Dirty water threatens the Great Barrier Reef Dirty water threatens the Great Barrier Reef Months after scientists warned that it is at greater risk than ever, the Great Barrier Reef is facing a new challenge: dirty water. AFP/Getty Dirty water threatens the Great Barrier Reef Run-off from the recent floods in Australia is spreading out from the coast with the potential to smother the fragile reef. Matt Curnock / Reuters Dirty water threatens the Great Barrier Reef The floodwater is thick with sediment from the shore and scientists fear that this may block out the sun, preventing photosynthesis for the algae that the coral relies on to survive. AFP/Getty Dirty water threatens the Great Barrier Reef Dr Frederieke Koon from the Australian Institute of Marine Science however, has told the BBC that the floodwater contains nutrients that have boosted algae growth in some areas, turning the water "a thick blanket of green". Matt Curnock / Reuters Dirty water threatens the Great Barrier Reef Despite such benefits in some areas, the fear is that the floodwater will contain pesticides that could prove extremely damaging and due to a lack of wind, it isn't dispersing. Matt Curnock / Reuters Dirty water threatens the Great Barrier Reef Dr Koon added that "a bit of wind and wave action can break the plumes up quite quickly, but we have literally had no wind so they're just sitting there hanging". Matt Curnock / Reuters Dirty water threatens the Great Barrier Reef Fears over the future of the reef have been building in recent years, with the US National Oceanographic and Atmospheric Administration forecasting in October 2018 that the entire 130,000 miles square system is at high risk of bleaching. Matt Curnock / Reuters Dirty water threatens the Great Barrier Reef These aerial photos show the murky water spreading across the reef. Matt Curnock / Reuters Dirty water threatens the Great Barrier Reef A handout photo taken by Matt Curnock on February 13, off Townsville. 2019 and released on February 15 shows sediment (R) approaching Central Great Barrier Reef from recent unprecedented rain and flooding around Cape Cleveland, off Townsville. - Runoff from recent floods in northern Australia is flowing onto parts of the Barrier Reef, scientists said on February 14, starving coral of light and providing fodder for the predatory crown-of-thorns starfish. (Photo by MATT CURNOCK / Matt Curnock / AFP) / --EDITORS NOTE--- RESTRICTED TO EDITORIAL USE - MANDATORY CREDIT "AFP PHOTO / Matt Curnock" - NO MARKETING - NO ADVERTISING CAMPAIGNS - DISTRIBUTED AS A SERVICE TO CLIENTS - NO ARCHIVESMATT CURNOCK/AFP/Getty Images AFP/Getty Dirty water threatens the Great Barrier Reef Sediment-filled water is seen in the Great Barrier Reef, Australia February 13, 2019 in this picture obtained from social media on February 15, 2019. MATT CURNOCK/TROPWATER JCU/via REUTERS THIS IMAGE HAS BEEN SUPPLIED BY A THIRD PARTY. MANDATORY CREDIT. NO RESALES. NO ARCHIVES. Reuters Dirty water threatens the Great Barrier Reef Sediment-filled water is seen in the Great Barrier Reef, Australia February 13, 2019 in this picture obtained from social media on February 15, 2019. MATT CURNOCK/TROPWATER JCU/via REUTERS THIS IMAGE HAS BEEN SUPPLIED BY A THIRD PARTY. MANDATORY CREDIT. NO RESALES. NO ARCHIVES. Matt Curnock / Reuters Dirty water threatens the Great Barrier Reef Sediment-filled water is seen in the Great Barrier Reef, Australia February 13, 2019 in this picture obtained from social media on February 15, 2019. MATT CURNOCK/TROPWATER JCU/via REUTERS THIS IMAGE HAS BEEN SUPPLIED BY A THIRD PARTY. MANDATORY CREDIT. NO RESALES. NO ARCHIVES. Matt Curnock / Reuters Dirty water threatens the Great Barrier Reef Sediment-filled water is seen in the Great Barrier Reef, Australia February 13, 2019 in this picture obtained from social media on February 15, 2019. MATT CURNOCK/TROPWATER JCU/via REUTERS THIS IMAGE HAS BEEN SUPPLIED BY A THIRD PARTY. MANDATORY CREDIT. NO RESALES. NO ARCHIVES. Matt Curnock / Reuters Dirty water threatens the Great Barrier Reef A handout photo taken by Matt Curnock on February 13, off Townsville. 2019 and released on February 15 shows sediment (bottom) approaching Central Great Barrier Reef from recent unprecedented rain and flooding around Cape Cleveland, off Townsville. - Runoff from recent floods in northern Australia is flowing onto parts of the Barrier Reef, scientists said on February 14, starving coral of light and providing fodder for the predatory crown-of-thorns starfish. (Photo by MATT CURNOCK / Matt Curnock / AFP) / --EDITORS NOTE--- RESTRICTED TO EDITORIAL USE - MANDATORY CREDIT "AFP PHOTO / Matt Curnock" - NO MARKETING - NO ADVERTISING CAMPAIGNS - DISTRIBUTED AS A SERVICE TO CLIENTS - NO ARCHIVESMATT CURNOCK/AFP/Getty Images AFP/Getty Dirty water threatens the Great Barrier Reef Sediment-filled water is seen in the Great Barrier Reef, Australia February 13, 2019 in this picture obtained from social media on February 15, 2019. MATT CURNOCK/TROPWATER JCU/via REUTERS THIS IMAGE HAS BEEN SUPPLIED BY A THIRD PARTY. MANDATORY CREDIT. NO RESALES. NO ARCHIVES. TPX IMAGES OF THE DAY Matt Curnock / Reuters Dirty water threatens the Great Barrier Reef Sediment-filled water is seen in the Great Barrier Reef, Australia February 13, 2019 in this picture obtained from social media on February 15, 2019. MATT CURNOCK/TROPWATER JCU/via REUTERS THIS IMAGE HAS BEEN SUPPLIED BY A THIRD PARTY. MANDATORY CREDIT. NO RESALES. NO ARCHIVES. Matt Curnock / Reuters Dirty water threatens the Great Barrier Reef Sediment-filled water is seen in the Great Barrier Reef, Australia February 13, 2019 in this picture obtained from social media on February 15, 2019. MATT CURNOCK/TROPWATER JCU/via REUTERS THIS IMAGE HAS BEEN SUPPLIED BY A THIRD PARTY. MANDATORY CREDIT. NO RESALES. NO ARCHIVES. Matt Curnock / Reuters Dirty water threatens the Great Barrier Reef Sediment-filled water is seen in the Great Barrier Reef, Australia February 13, 2019 in this picture obtained from social media on February 15, 2019. MATT CURNOCK/TROPWATER JCU/via REUTERS THIS IMAGE HAS BEEN SUPPLIED BY A THIRD PARTY. MANDATORY CREDIT. NO RESALES. NO ARCHIVES. Matt Curnock / Reuters Dirty water threatens the Great Barrier Reef Sediment-filled water is seen in the Great Barrier Reef, Australia February 13, 2019 in this picture obtained from social media on February 15, 2019. MATT CURNOCK/TROPWATER JCU/via REUTERS THIS IMAGE HAS BEEN SUPPLIED BY A THIRD PARTY. MANDATORY CREDIT. NO RESALES. NO ARCHIVES. Matt Curnock / Reuters Dirty water threatens the Great Barrier Reef Sediment-filled water is seen in the Great Barrier Reef, Australia February 13, 2019 in this picture obtained from social media on February 15, 2019. MATT CURNOCK/TROPWATER JCU/via REUTERS THIS IMAGE HAS BEEN SUPPLIED BY A THIRD PARTY. MANDATORY CREDIT. NO RESALES. NO ARCHIVES. Matt Curnock / Reuters

This one would probably have gone undetected if it was not for the pumice found in the sea.

Although the pumice is currently bare, Professor Bryan says organisms like crabs and corals will start growing on it in the coming weeks.

“So 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,” he said.

This could prove to be a lifeline for the Great Barrier Reef.

Marine heatwaves caused by climate change have caused mass bleaching on around half the corals. A major heatwave in 2016 wiped out 30 per cent of its shallow-water corals in the nine months between March and November that year.

Coral bleaching is linked to global warming and occurs when stresses from higher water temperatures cause the corals to expel symbiotic photosynthetic algae, draining them of all colour and eventually causing them to die.