A beach covered in pumice after an underwater volcanic eruption.

A beach covered in pumice after an underwater volcanic eruption.
The biggest underwater volcanic eruption Japan has seen since the end of WWII has left shores covered in pumice stone. In one video, stones are seen covering the waves, while images show fishing ports inundated with volcanic rock.

The volcano, Fukutoku-Okanoba, is situated on the Ogasawara island chain, about 800 miles from Tokyo. It sits about 25 meters below the surface of the sea.

It exploded on August 13, with images showing a huge plume of gas rising out of the sea, into the air. Satellite images from NASA showed a huge bright plume streaming from the vent, stretching for miles.

In a NASA statement, Andrew Tupper, a meteorologist with Natural Hazards Consulting, said: "What was remarkable about this eruption is that it went straight from being a submarine event to an eruption cloud reaching the lower boundary of the stratosphere. That is not very common for this type of volcano. We normally see lower-level plumes from submarine eruptions."

Huge quantities of pumice stone ejected from the volcano have now started washing up along coastlines in the Okinawa Prefecture.

Greg Valentine, Professor of Geology and Director of the Center for Geohazards Studies at the University at Buffalo College of Arts and Sciences, told Newsweek large amounts of pumice appearing after eruptions is fairly common: "Especially in the western Pacific where the Ring of Fire, caused by subduction of the ocean crust, includes numerous volcanoes that have not yet reached the surface of the sea. Also, sometimes volcanoes that are on land but near the sea will produce large volumes of pumice that end up in the ocean."

In terms of the damage it could cause, Valentine said: "Pumice is very abrasive, so when combined with wave action it can cause quite a bit of damage to boats and any infrastructure (for example, piers) that are on the surface of the water or that penetrate the surface. It can also clog pipes and engines."

According to The Asahi Shimbun, tens of millions of cubic meters of pumice were ejected during the eruption. The pumice has now washed up at 10 coastal locations, causing huge disruption to fishing vessels. Okinawa officials said large numbers of fish being held in Hentona port had died from swallowing the stone.

So much pumice was ejected during the eruption that the floating rock could be seen from space.

Japanese meteorologist Sayaka Mori tweeted images of the pumice, saying: "Sea snot? Sea foam? No, they are pumice stones floating over the ocean around Okinawa. The stones are debris from the huge August undersea volcanic eruption 1,300 km [a little over 800 miles] from Okinawa. Experts say the stones might reach the coast near Tokyo in late November."

Rafts of pumice are often produced after underwater volcanic eruptions. In 2019, a Paris-sized raft was spotted heading towards the Great Barrier Reef in Australia. It was thought to have come from a huge underwater eruption of a volcano near the island of Tonga. In 2012, a pumice raft 300 miles long and 30 miles wide was observed near New Zealand. This was produced by the deep, submarine eruption of the Havre caldera volcano in the Southwest Pacific.

The Japan coast guard said that eventually the pumice will reach parts of the Japan mainland soon, with the north-flowing current transporting the floating rock.

"If the pumice floats towards land it will eventually become part of the coastal deposits," Valentine said. "If it does not encounter land, it might drift around for months, possibly years, before breaking up and/or sinking."

The plume from the volcanic eruption
The plume from the volcanic eruption that has produced the huge amounts of pumice now washing up on Japanese shores.