Shelf Ice

In 1991 Will Steger’s international team of explorers spent weeks crossing the Larsen ice shelf on the eastern side of the Antarctic Peninsula. The team’s photographs from the Larsen show a flat plain of snow-covered ice extending to the horizon, similar to images from the salt flats. You would guess, looking at the photographs, the team members, their sleds and dogs were standing on solid ground. Below their feet, however, was approximately 720 feet (220 meters) of ice floating on top of the ocean.

How do shelves of ice form? As you might imagine, it’s a long process. First a cold spring season allows fast ice, a layer of sea ice that is frozen fast to the shore, to remain in place through the next year. Fast ice that survives more than one year is called bay ice. Bay ice that stays in place for many years is called shelf ice. Shelf ice gets thicker not only from freezing seawater, but also from snow and from glaciers moving towards the coasts. Ice shelves can be up to 1000 ft (300 m) thick. That’s almost a fifth of a mile thick.

While crossing the Larsen, the ice shelf felt very stable to Steger and his team. Scientists at Queen’s University estimate the shelf could have been stable for as long as 12,000 years—that many years ago there were still mastodons, mammoths, and saber-toothed cats roaming the earth. Over the course of three days in 2002, however, a chunk of ice the size of the New England state of Rhode Island broke free from the Larsen B ice shelf. The speed of the collapse surprised even the scientists who were monitoring the shelf. Scientists link the collapse with global climate change.


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