Floating pumice blocks from remote eruption in August ports near Tokyo, Japan

The mixed submarine-subaerial eruption of the volcano in last August, when a new island formed, still shows effects today: large amounts of floating pumice stones washed up on the shores of Greater Tokyo recently.
On other coasts in southwest Japan, pumice rafts from the volcano have been a burden for a month and often impaired fishing and seafaring.

近くで見ると、なかなか衝撃的。改めて、歴史的なできごとだと思う。この先、何百年だっても「昔、与論の海が軽石だらけになったことがあるんだって」って語り継がれてほしい。「昔、与論の砂浜は真っ白だったんだって」にならないことを願う。「軽石を分析する会」とかやってほしいな… pic.twitter.com/996DuhaMYv

— 箱山康子 | ライター･校正者 (@yasukonotes) October 23, 2021

Pumice forms as highly viscous magma erupts with myriads of small gas bubbles trapped within the sticky liquid, which freezes to volcanic glass. It could be described as frozen lava foam. Due to its bubbly structure, it is often lighter than water and thus floats. With time, water enters the cavities by diffusion and through tiny cracks in the glassy walls around the bubbles, but it can take months and even years until the pumice eventually will sink and form a deposit on the sea floor, if it isn't washed up on shores earlier.