The Perito Moreno Glacier () is a glacier located in the Los Glaciares National Park in the south west of Santa Cruz province, Argentina. It is one of the most important tourist attractions in the Argentine Patagonia. The 250 km² ice formation, of 30 km in length, is one of 48 glaciers fed by the Southern Patagonian Ice Field located in the Andes system shared with Chile. This icefield is the world's third largest reserve of fresh water.
The Perito Moreno Glacier is one of only three Patagonian glaciers that are not retreating. Periodically the glacier advances over the L-shaped "Lago Argentino" ("Argentine Lake") forming a natural dam which separates the two halves of the lake when it reaches the opposite shore. With no escape route, the water-level on the Brazo Rico side of the lake can rise by up to 30 meters above the level of the main lake. The enormous pressure produced by this mass of waters finally breaks the ice barrier holding it back, in a spectacular rupture event. This dam/rupture cycle is not regular and it naturally recurs at any frequency between once a year to less than once a decade.
The terminus of the Perito Moreno Glacier is 5 km wide, with an average height of 60 meters above the surface of the water, with a total ice depth of 170 meters. It advances at a speed of up to 2 m per day (around 700 m per year), although it loses mass at approximately the same rate, meaning that aside from small variations, its terminus has not advanced or receded in the past 90 years. At its deepest part, the glacier has a depth of approximately 700 m.
The glacier first ruptured in 1917, taking with it an ancient forest of arrayán (Luma apiculata) trees. The last rupture occurred in March 2006, and previously in 2004, 1988, 1984, 1980, 1977, 1975, 1972, 1970, 1966, 1963, 1960, 1956, 1953, 1952, 1947, 1940, 1934 and 1917. It ruptures, on average, about every four to five years. The Perito Moreno glacier, located 78 km from El Calafate, was named after the explorer Francisco Moreno, a pioneer who studied the region in the 19th century and played a major role in defending the territory of Argentina in the conflict surrounding the international border dispute with Chile.
