Periodically, the tongue of the Perito Moreno Glacier in Argentina edges forward enough to cut off an arm of Lago Argentino, in essence damming it. It takes anywhere from 10 to 50 months, according to NASA, but eventually the water pressure builds up enough to break through. First it forms a cave, or tunnel. Then, as the water seeps through and infiltrates the ice, it weakens it further, until one day—boom!
The ice tongue is massive, NASA says on its Earth Science Picture of the Day site, covering 97 square miles. When the bridge crashes down it displaces water in the form of a wave.
At 18 miles long, Perito Moreno is one of the largest in Patagonia, NASA says on its Earth Observatory Image of the Day page, which also showcases pictures of the glacier from space. The glacier ranges from 6,825 feet high in the Andes Mountains down to just 600 feet above sea level, at Lago Argentino.
Tourists gather to ogle the scene, and some are lucky enough to be there when the ice bridge collapses. Luckily for the rest of us, one photographer happened to have his video camera handy one midsummer’s day, as he recounts. Here is what he took on January 19, 2013, and the video he shot of the spectacular collapse. It’s a grand sight, not least of all because for once it's an ice phenomena not related to global warming.
More information about and photos of this spectacular glacier are at NASA's Earth Science Division.