Okinawa is full of pumice.
Pumice floating along the coastline.
When and where it shows up depends on the tide and the currents. Here a current is depositing more pumice along the Onna coast. The gray rocky streaks on the beach is more pumice deposited during earlier high tides.
Pumice is volcanic rock that's created during eruptions. Magma deep underground is under high pressure and can hold a lot of dissolved gases, like a bottle of soda. During an eruption, the rapid pressure drop forms gas bubbles in the lava, and it creates lava foam. When that foamy lava hits cold water, it solidifies rapidly with all those bubbles still trapped inside, and creates a sponge-like rock that's light enough to float on water.
A piece of pumice, about 2cm across.
I cut through it with a hobby knife (carefully; it breaks easily), then smoothed the surface with a file. I tried using sandpaper, but the pumice removed the sandpaper, not the other way around :)
A macro shot of part of the surface. The smallest visible holes are very roughly 10µm across. It's like a sponge of silica glass.
An underwater eruption near Ogasawara created a lot of pumice in August. That pumice has been drifting along the ocean currents until it started washing ashore along the Okinawan coastline two weeks or so ago. These pictures are from around Onna beach, just next to OIST (Okinawa Institute of Science and Technology) where I work(*).
As you might imagine, a harbor full of floating rocks means fishermen can't take their boats out to sea, and a beach full of grey, abrasive pebbles is no fun for the tourists coming to swim and relax in the sun. Even if you clear the beach, the next high tide just brings in more of the stuff.
Pumice deposited on a beach by the waves.
Not everybody has a problem with pumice-strewn beaches.
It's not all bad, though. Pumice is light and breakable, so it will turn to sand fairly quickly. And it can be useful; large chunks are popular for skin care — they're an excellent natural file — and smaller pebbles are great for improving drainage in potted plants and the like. So many people have tried to sell this pumice online that Mercari — the most popular online marketplace here — have a temporary ban on pumice sales on its platform.
The video below is really crappy quality, but you can hear the sound of the rocks rubbing against each other as they're rocked back and forth by the waves:
* I mention OIST — Okinawa Institute of Science and Technology — because I've realized that they, like many organizations, subscribe to a service that alerts them for any mention of them or their research. Furthermore, the public relations department has to actually read anything that crops up, in case it's important.
So, if you have a blog, Youtube channel or Twitch stream and would like a few more viewers, just gratuitously mention a few big, public organizations. Hi Micheal!