An ice island four times the size of Manhattan broke off from one of Greenland’s two main glaciers, scientists said on Friday, in the biggest such event in the Arctic in nearly 50 years.

The new ice island, which broke off on Thursday, will enter a remote place called the Nares Strait, about 620 miles south of the North Pole between Greenland and Canada.

The ice island has an area of 100 square miles (260 square km) and a thickness up to half the height of the Empire State Building, said Andreas Muenchow, professor of ocean science and engineering at the University of Delaware.

Muenchow said he had expected an ice chunk to break off from the Petermann Glacier, one of the two largest remaining ones in Greenland, because it had been growing in size for seven or eight years. But he did not expect it to be so large.