The Perito Moreno glacier

It dams a lake until the lake breaks through the ice

The Perito Moreno glacier pours off the Southern Patagonian Ice Field into Lake Argentino in Los Glaciares National Park, in Argentina's Santa Cruz Province, 78 km from the town of El Calafate. It is 30 km long and covers 250 square kilometres, one of 48 glaciers fed by an ice field that holds the world's third-largest reserve of fresh water. Its front is a wall of ice about 5 km wide, standing more than 60 metres above the lake, and it calves bluish icebergs into the milky water in an audiovisual spectacle that draws visitors from all over the world.

The glacier has had three names. The Chilean navy captain Juan Tomás Rogers, the first outsider to see it, named it Francisco Gormaz in 1879; Rudolph Hauthal of the Argentine Boundary Commission later called it Bismarck; and in 1899 Lieutenant Iglesias named it for Francisco Moreno, the explorer who studied the region and helped make Argentina's case in the 1902 boundary arbitration with Chile. Its source stayed in dispute until a 1998 agreement left a small part in Chile and the rest in Argentina.

For decades Perito Moreno was one of the few glaciers on Earth in balance, gaining ice about as fast as it lost it, and glaciologists still argue about why. Since 2020 its northern front has been retreating, possibly because of climate change.

Its fame rests on a dam. As the ice pushes forward it reaches the shore of the Magallanes Peninsula and cuts the lake in two, sealing off the Brazo Rico arm from the Canal de los Témpanos. With no outlet, the water in Brazo Rico can climb as much as 30 metres above the main lake. Eventually the pressure bores through the ice, the bridge collapses, and the trapped water pours out into the main lake and the Santa Cruz River, at up to 8,000 cubic metres a second in a sudden rupture. The cycle runs anywhere from once a year to less than once a decade, with no fixed rhythm; ruptures came in 1917, 1935 and many times since, and the shoreline left behind shows how high the water stood.

The park, protected since 1937 and a World Heritage Site, has a walkway system facing the front, and the single road along the Magallanes Peninsula brings visitors almost to the ice.