The Perito Moreno Glacier is one of the largest in Patagonia at 30 kilometers long. The glacier descends from the Southern Patagonian Icefield (image top)—2100 meters elevation (6825 feet) in the Andes Mountains—down into the water and warmer altitudes of Lago Argentino at 180 meters above sea level.
Note: In this photograph from astronauts on the International Space Station, the image is rotated so that north is to the right.
Perito Moreno is perhaps the region’s most famous glacier because it periodically cuts off the major southern arm (known as Brazo Rico) of Lake Argentino. The glacier advances right across the lake until it meets the opposite shoreline, and the ice tongue is “grounded” (not floating) so that it forms a natural dam. The ice dam prevents lake water from circulating from one side to the other, which in turn causes muddier and “milkier” water to concentrate in Brazo Rico. Water flows down under the glacier from the mountains, not only carrying the mud into the lake but also helping lubricate the glacier’s downhill movement.
Because of this natural ice dam, meltwater from the south raises water levels in Brazo Rico by as much as 30 meters above the level of the water in Lago Argentino. The great pressure of this water ultimately causes the ice tongue to rupture catastrophically in a great natural spectacle. The last rupture occurred in March 2012, after this image was taken. The process repeats every four to five years as the glacier grows back towards the opposite shoreline. The repeated ruptures have made the glacier and lake a major tourist attraction in the region.
A more detailed astronaut view of the glacier tongue can be viewed here.