The stretch of golf-ball-size pumice rocks was first spotted this week by a New Zealand air force plane about 620 miles northeast of Auckland. The rocks stretch for about 10,000 square miles.
A navy ship took scientists to the rocks Thursday night. Naval Lt. Tim Oscar said the rocks appeared a brilliant white under a spotlight, like a giant ice shelf.
He said it's the "weirdest thing" he's seen in 18 years at sea.
Scientists said the rocks likely spewed up in an eruption by an underwater volcano. They don't believe the eruption is connected to the onshore ash eruption this week of another volcano, Mount Tongariro.
The Defence Force said the mass of rocks stretches 250 nautical miles by 30 nautical miles.
Pumice is made from lava and water and is very lightweight, so it poses no danger to ships. Pumice has a variety of uses: as an ingredient in concrete, polishes and scrubbing cleaners; to stone wash jeans and exfoliate skin.
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