In this photo released by New Zealand Defence Force, pumice rocks are shown after being collected. A mass of small volcanic rocks nearly the size of Belgium was discovered floating off the coast of New Zealand. (Nicole Munro/New Zealand Defence Force/Associated Press)

A mass of small volcanic rocks nearly the size of Belgium has been discovered floating off the coast of New Zealand.

The stretch of golf-ball-size pumice rocks was first spotted this week by a New Zealand air force plane about 1,000 kilometres northeast of Auckland. The rocks stretch for about 26,000 square kilometres.

A navy ship took scientists to the rocks Thursday night. Naval Lt. Tim Oscar says the rocks appeared a brilliant white under a spotlight, like a giant ice shelf.

He says it's the "weirdest thing" he's seen in 18 years at sea.

Scientists say 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 says 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.