What is the Perito Moreno Glacier, and how do the lake's glaciers work?

What is the Perito Moreno Glacier? Put simply, it is a river of ice. Snow falls on the high Andes, piles up year after year, turns to ice under its own weight and flows slowly downhill toward Lago Argentino. The glacier's walkway map puts it plainly: the movement comes from the push of accumulating snow.⁶ What you see from the boat is the end of that journey, where the ice meets the water and breaks away.

The Perito Moreno is not alone. Parques Nacionales lists the Upsala, Spegazzini, Seco and Onelli among the glaciers of the park's southern zone,⁷ and ice covers almost half of Los Glaciares National Park.⁸ The Perito Moreno and the Upsala are among the glaciers that drain into the arms of Lago Argentino.⁹ A single cruise can show you several of them in a day, each at a different stage of the same process, from steep hanging ice to broad outlets of the field.

Parques Nacionales spells out where its fame comes from: easy access, striking beauty and a singular natural event, the periodic rupture of its front.¹⁰ The road and walkways bring visitors to within sight of the ice, and the boats bring them to its foot. Understanding how the glacier works turns the cruise from a sequence of pretty views into a story you can follow, and helps you know where to look at each stop.

Comparisons make the size of the Perito Moreno Glacier easier to picture. The park leaflet puts its area at 254 km2, a bit larger than Buenos Aires City at 203 km2.³ From its source to the lake it runs for close to 30 km, ending in a front 4 to 5 km wide on the lake's south arm.⁴ From the boat you see only the very end of that length, and still the wall seems to run on in both directions. Pick out a detail on the face, such as a dark band of rock or a deep crack, to give your eye a sense of scale.

Its height above the water depends on the source you read. A 2016 Parques Nacionales note gives the glacier as 60 m high, 5 km wide and 30 km long,²¹ while the park's 2024 leaflet compares the snout to the Obelisk of Buenos Aires, which stands 67.5 m tall.²⁶ Both describe a face the height of a large apartment block, rising straight out of the lake at only 177 m above sea level.²⁰ It is the same water level that laps the shore below El Calafate, on the far side of the same lake.

Big as it is, the Perito Moreno is not the largest glacier in the park. The leaflet gives the Viedma Glacier 975 km2, the largest in Argentina and the second largest in South America.¹⁹ The cruise does not reach the Viedma, which lies further north, but the comparison helps when you see the Upsala and Spegazzini: the glaciers on this lake vary hugely in size and shape, and each fills its valley differently.

The Perito Moreno's most famous trick happens at the Magallanes Peninsula. As Parques Nacionales tells it, once the glacier's advance carries it onto the peninsula, water can no longer pass between Brazo Rico and the main lake.⁵ The glacier becomes a dam. Water builds up behind it on the Brazo Rico side, and the pressure on the ice grows month by month. The ice-front panel of the map on our route page shows how slim the gap between glacier and shore is.

Then the water fights back. In its account of that event, the park describes water first filtering through a small channel under the glacier, which widens as the tunnel walls and floating ice at its base melt.¹¹ The water then starts to pass from Brazo Rico into the Canal de los Tempanos,¹² and the pressure cracks the glacier until an arch forms that finally collapses.¹³ For a while the water rushes beneath an ice bridge, a process that draws thousands of visitors to the walkways.²⁹

Its news pages followed such a cycle over a few days in March 2016, as rangers kept watch.²⁹ On 8 March rangers spotted a filtration in the front of the glacier, the start of the process,²⁹ and on 10 March 2016 the ice bridge broke completely, watched by hundreds of tourists and residents.¹⁴ The walkway map is honest about timing: the rupture is impossible to predict.³⁰ If one is under way during your trip, check the park's news pages and ask at the information center in town.

Glaciers are more than scenery. The park's leaflet explains that they help cool the planet by reflecting between 45 and 85 per cent of the sunlight that reaches them.³⁴ They also store fresh water, which is scarce: only 3% of all the water on the planet is fresh.³⁵ The ice you sail past today is part of that store, slowly released into the lake below. Seeing it up close makes those figures easier to remember.

The colors on the cruise tell the same story. Dense, old glacier ice absorbs the red part of daylight and lets the blue through: the deepest cracks and newly exposed faces glow blue while the weathered surface looks white. The milky turquoise of the lake comes from fine rock flour ground by the ice and carried in by meltwater, which hangs in the water and scatters the light. On a bright day the contrast between blue ice and turquoise water is at its strongest.

Knowing this changes how you watch the day. The Spegazzini, rated by the park as its tallest glacier,²² shows how a glacier squeezed into a steep valley can tower over the water. The wide view of the Upsala shows the scale of an outlet of the ice field. The Perito Moreno, at the end of the route, shows the whole cycle in action: advance, dam, calving and, now and then, rupture. On a clear day you can follow all of it from the deck.