pumice

ニュージーランド海軍は同国の沖、太平洋上に長さ約460キロ、幅約56キロにわたって大量の軽石が漂流しているのを発見したと発表しました。軽石に覆われた海面の広さはベルギーの面積に匹敵するそうです。何かの前兆ではないことを願います。

pumice、正式にはpumice stoneは「軽石」のことです。Webster（Online）は"a volcanic glass full of cavities and very light in weight used especially in powder form for smoothing and polishing"と定義しています。ちなみに数えるときにはa piece of を使います。（GP）

The Defence Force says crew on board HMNZS Canterbury spotted the pumice about 85 nautical miles south-west of Raoul Island on Thursday night.

It says the area of pumice was about 250 nautical miles long and 30 nautical miles wide. The Defence Force says the ship was brought to a halt so GNS scientists could take samples and then analyse them to see what volcano the pumice came from.

GNS Science says an underwater volcano, Monowai, has been active along the Kermadec Arc and the pumice could be from there.
Scientists on board HMNZS Canterbury say the volcanic activity of Tongariro, White Island and the Kermadec Arc is unrelated.

http://www.radionz.co.nz/news/national/112950/sea-of-pumice-north-of-new-zealand

http://edition.cnn.com/2012/08/10/world/asia/floating-pumice/index.html