The New Zealand air force has spotted a mass of volcanic rock nearly the size of Belgium afloat in the Pacific about 1,000 kilometers northeast of Auckland.

Witnesses say the strange phenomenon, first spotted Thursday, resembles a polar ice shelf and is made up of golf ball-size lightweight pumice expelled from an underwater volcano. Authorities say the aerial sighting prompted a warning to a New Zealand naval vessel to steer clear of the debris. But officials later said the pumice posed no threat to shipping.

A navy spokesman said a ship carried scientists to the edge of the 26,000 square-kilometer float Thursday night, and said the rocks appeared a brilliant white color under ship spotlights.

Scientists aboard the vessel said the floating debris is unrelated to an eruption this week at Mount Tongariro that sent ash six kilometers into the atmosphere.