An unusually fast glacial pace

The Muldrow Glacier on the north side of Denali in Alaska is undergoing a rare surge.

In the past few months the 39-mile-long river of ice has been moving as much as 90 feet a day, 100 times its usual speed.

Most surges, which typically last only a few months and occur on only about 1% of glaciers worldwide, occur on remote glaciers and are detected only after they've ended.

Because of that relative rarity, scientists haven't been able to study surges enough to explain why they happen or to gauge how climate change may be affecting them.