(CBS NEWS) -- A massive iceberg about four times the size of Manhattan broke off Antarctica on Saturday, the U.S. National Ice Center confirmed to CBS News.

The iceberg break, an event known as calving, involved a chunk measuring close to 100 square miles, the Ice Center said. However, the iceberg broke into smaller bergs that the center isn't tracking. The berg that is being tracked measured 71.5 square miles.

It drifted from Pine Island Glacier -- one of the largest glaciers within the West Antarctic Ice Sheet, accounting for about 20 percent of the ice sheet's total ice flow to the ocean, according to NASA scientists. Pine Island loses an estimated 45 billion tons of ice each year. Scientists watch this glacier closely because evidence has been pointing to even faster loss of ice in the future, NASA says.

Saturday's enormous break comes just two months after a 2,200 square-mile piece of ice, nearly the size of the state of Delaware, detached from the ice shelf known as Larsen C in July. It was one of the largest icebergs ever recorded.

But the size of the icebergs is not what has scientists so concerned.

"It's not the size of the iceberg that matters. It's the frequency," Ian Howat, a glaciologist at The Ohio State University, told CBS News. "Infrequent big icebergs are normal -- lots of little icebergs are abnormal."

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