Pumice stones, which have drifted ashore in Okinawa Prefecture after an undersea eruption last summer, have become something of a fixture of the coastal landscape. But luckily, they won’t be around forever.

A cleaning vessel removes pumice stones off the coast of Nago, Okinawa Prefecture.

Experts say that the drifting of pumice stones from the Fukutoku-Okanoba underwater volcano in the Ogasawara Islands has already passed its peak and is likely to stop entirely between April and June.

Meanwhile, pumice stones from an undersea volcanic eruption off Tonga in the South Pacific in January are unlikely to hit Okinawa, they say.

“The peak has passed,” said Yuzo Kato, 82, professor emeritus at the University of the Ryukyus. He predicts that the amount of drifting pumice stones will continue to decrease until they fully disappear around the end of April.

The eruption of Fukutoku-Okanoba in August last year also created a temporary island from the accumulated pumice stones and volcanic ash spewed from the submerged caldera. The island formation is quickly eroding away, however, adding to the total volume of pumice stones that will wash ashore in Okinawa.

It is estimated that the pumice stones from this newborn island will arrive in Okinawa in about four months, and after that they will most likely completely disappear, Kato said.

Satoshi Mitarai, 51, associate professor at the Okinawa Institute of Science and Technology Graduate University (OIST), largely agrees with Kato’s estimates, saying that the oceanic detritus from of the eruption will continue to be visible until around June, when he believes the pumice stones that have already drifted ashore will be circulated around the Okinawan island chain.

According to Mitarai’s simulation based on 282 drifting buoys placed in the sea around Okinawa and data on winds and ocean currents for the past five years, the pumice stones will hang around near Okinawa until around May due to north winds, but that they are expected to move toward Honshu on the Black Current when the south wind gets stronger from around June, he said.

When it comes to pumice from the Jan. 15 volcanic eruption off the coast of Tonga, the amount of pumice stones floating on the sea surface from the eruption is small, but the reason for this is not entirely clear, according to Teruki Oikawa, 51, a senior researcher at the National Institute of Advanced Industrial Science and Technology (AIST).

“It is unlikely that large amounts of pumice (originating from the Tonga eruption) will drift near Japan, including Okinawa,” he said.

Kato of the University of the Ryukyus also believes that Tongan pumice stones will not drift ashore in Japan.

OIST’s Mitarai explained that such stones are unlikely to reach Japan as a result of the North Equatorial Currents and South Equatorial Currents, named for their proximity to the equator, which he believes are difficult for the stones to cross.