Pumice Island as large as Israel found in South Pacific

[link to www.msnbc.msn.com]

An "island" of floating pumice rocks bigger in area than Israel has been spotted in the South Pacific, New Zealand's Royal Navy said.
Officers on a Royal New Zealand Air Force ship saw the rock raft southwest of Raoul Island Thursday. It measures an astounding 300 miles (482 kilometers) in length and more than 30 miles (48 km) in width, the Navy said.
Lt. Tim Oscar of the Royal Australian Navy described the rocks as "the weirdest thing I've seen in 18 years at sea," according to the Australian Associated Press......


Any ideas as to what caused this?