The floating raft of golf ball-sized pumice rocks, covering almost 10,000 square miles, was spotted by a New Zealand air force plane about 620 miles northwest of Auckland.
Scientists taken to the to the rocks Thursday night by a New Zealand navy ship said they were likely spewed up in an eruption of an underwater volcano, a release from the navy said.
The floating rocks are a brilliant white, like a giant ice shelf, when seen under a spotlight, navy Lt. Tim Oscar said.
Scientist said they don't believe the floating pumice is related to the onshore ash eruption this week of a Mount Tongariro, a volcano on New Zealand's North Island.
Officials say the island of floating small rocks poses no danger to shipping.