Perito Moreno is not growing: it was anchored to a rock ridge, and since 2019 it has been letting go

There is a sentence repeated on the walkways, on the boats and in the guidebooks: Perito Moreno is «one of the few glaciers in the world that doesn't retreat». Sometimes, by simplification, it becomes «one of the few that is growing». Anyone who hears it while a block of ice the size of a building falls tends to accept it as one more fact of the landscape, on the same list as the front's 5 kilometres of width or its 70 metres of height above the lake.

The sentence was true for a long time, but the reason is almost never told, and without the reason it is misunderstood. This article explains why a glacier in the province of Santa Cruz could hold its front while the ice field that feeds it lost mass, what exactly the rupture cycle so many people travel to see is, and why glaciologists now say that stability was a window, not a property.

The first surprise is not the size but the closeness. At most of the world's large glaciers the front is viewed from a boat or from a distant lookout. Here the walkways are built on the Magallanes Peninsula, a tongue of land that pushes into Lago Argentino, and the glacier ends a few hundred metres from the railing. You hear it creak. Every so often a column of ice leans, falls, and the roar arrives a second after the image.

The second is the shape. The front is not a straight wall: it is an arc that opens into two faces, one facing north over the Canal de los Témpanos and another facing south over Brazo Rico. Between them, the ice points directly at the peninsula where you are standing. That arrangement, which looks like a scenic detail, is the key to everything else: the glacier does not flow into an open lake; it wedges itself against a shore.

The third is the speed. The ice in front of you moves between one and two metres a day near the front. A glacier that does not retreat is not a still glacier: it is a conveyor belt that never stops advancing and that loses at the front, through calving, roughly what it receives at the back. That is the first correction worth making to the walkway sentence.

Perito Moreno is not an isolated glacier. It is one of the dozens of tongues that descend from the Southern Patagonian Ice Field, one of the largest ice masses on the planet outside the polar regions, shared between Argentina and Chile along the cordillera. On the Argentine side, Los Glaciares National Park, created in 1937 and inscribed as World Heritage in 1981, gathers 47 major glaciers and Lago Argentino, the country's largest lake at 1,466 km².

Almost all of those glaciers are retreating, some spectacularly. The Upsala, which ends in the same lake a few kilometres to the north, has retreated about 9 kilometres since 1986 according to imagery from the European Copernicus programme; the arm that joined it to its neighbour Bertacchi separated a few years ago. That is the backdrop against which Perito Moreno is read: in an ice field where retreat is the rule, a front that barely moved during the twentieth century stands out on its own.

That is where the misunderstanding comes from. If all of them retreat and this one does not, the easiest conclusion is that the climate here is different or that the glacier is «winning». Neither is true. Perito Moreno shares the same climate, the same snow and the same temperatures as its neighbours. What it does not share is the shape of the valley where it ends.

A glacier is a balance. Up high, in the accumulation zone, snow compacts and becomes ice; lower down, in the ablation zone, ice melts and, if it ends in a lake, calves. When what comes in equals what goes out, the front stays where it is. At Perito Moreno that balance has three supports that most of its neighbours do not have at the same time.

The first is snow. In a 2007 study, Stuefer, Rott and Skvarca measured a net accumulation close to 5.5 metres of water equivalent a year in the upper part, with an equilibrium line at about 1,170 metres and 70 % of the glacier's surface above it. Moist air masses from the Pacific unload over the cordillera and the glacier receives an amount of snow that few places in the world can match. That gives it a lot of ice to push downhill.

The second is the peninsula. The same study attributes the stability to a «particular geometry»: the Magallanes Peninsula blocks the glacier's central flow and forces the ice to split into two fronts. A glacier that runs into land cannot advance freely, and a front resting on rock, rather than floating in deep water, calves more slowly. As early as 1997, Skvarca and Naruse had described a self-regulating mechanism: when the front advances into deeper water, calving increases and slows it down; when it retreats into shallower water, calving decreases.

The third was hidden under the water and under the ice, and was only confirmed recently. In March 2022 a German-Argentine team flew over the glacier with a radar slung from a helicopter to measure the ice thickness, and mapped the lake bed in front of the glacier. The result, published in 2025 by Moritz Koch and colleagues in Communications Earth & Environment, showed a large rock ridge beneath the front. The glacier was literally resting on it. As long as the ice was thick enough to sit on that ridge, the front could not retreat far: the rock held it.

Perito Moreno was not defying the climate. It had a lot of snow, a peninsula that slowed it and a rock ridge that held it. It was a matter of shape, not of temperature.

With the geometry understood, the rupture stops being a mysterious spectacle and becomes logical. When the glacier advances far enough, its front touches the Magallanes Peninsula and closes the passage between the two halves of the lake. Brazo Rico, to the south, becomes a reservoir: rivers keep flowing in, but the water can no longer flow out to the Canal de los Témpanos. So it rises. In 2004, according to the study by Chinni and Warren, the level reached 10.5 metres above the main lake; in 1988 the difference was about 19 metres, and in earlier episodes differences of up to 30 metres have been documented.

The impounded water looks for a way out. First it seeps beneath the ice; the seepage widens into a tunnel; the tunnel grows into an arch through which Brazo Rico drains, sometimes over days; and in the end the arch, with nothing beneath it, collapses. That is «the rupture»: not the glacier splitting apart, but the collapse of an ice bridge the glacier itself had built by advancing. In 2004 the tunnel began to drain on 12 March and the roof fell on the 14th, two days later, with between 3 and 4 km³ of water released into the main lake.

The history of the ruptures is therefore the history of the advances. The first documented one was in 1917. Between 1917 and 1988 there were 16 closures, some lasting months and others up to three years. Then nothing between 1988 and 2003: fifteen years in which the glacier did not reach the peninsula. It touched it again in 2004, 2006, 2008, 2012, twice in 2013, 2016, in March 2018, with a night-time collapse that nobody saw, and at the end of 2019. In other words: a rupture means the glacier had advanced far enough to close off the lake. It is a sign of push, not of weakness.

This is where the walkway sentence becomes outdated. The study by Koch and colleagues compared the glacier's surface height and velocity between 2000 and 2024. Between 2000 and 2019 the front thinned by about 0.34 metres a year and retreated less than 100 metres over the whole period: the stability the guidebooks talk about, measured. Between 2019 and 2024 the thinning rose to 5.5 metres a year, more than sixteen times faster, and in some stretches the front has retreated more than 800 metres. The ice, moreover, is flowing faster.

The explanation connects back to the rock ridge. Thinner ice weighs less and the lake water pushes it upward more. According to the authors, the glacier «is detaching from its anchor point»: as it loses contact with the ridge, the water beneath the front gets deeper, calving increases and the retreat speeds up. If the front comes off that ridge entirely, the authors suggest it could retreat several kilometres into deeper water, a scenario they themselves describe as probably irreversible.

What this means for the visitor is simple to say and hard to accept: Perito Moreno was never an exception to the changing climate, but a glacier with shock absorbers. Abundant snow, the peninsula and the rock ridge delayed the response; they did not cancel it. When the accumulated thinning was enough, the shock absorbers stopped being enough. The enormous calving events of recent years, which some guides narrate as spectacle, are also this: the front coming off its support.

First: «the glacier is growing». No. For a century the front held a similar position, with advances and retreats of tens or hundreds of metres. Holding is not growing, and the glacier was in any case losing ice every day through the calving you see yourself. What happened is that it replaced it.

Second: «it is proof that the climate isn't changing». Also no. A single glacier with an unusual geometry does not refute what the other forty-six in the park are doing, and even less now that it has joined them. It was an explainable delay, and the explanation has been measured with radar and satellite.

Third: «the rupture is the glacier destroying itself». The opposite: for a rupture to happen the glacier must have advanced far enough to touch the peninsula. It is a phenomenon of push. That no rupture has been confirmed since the end of 2019 is, in fact, consistent with a retreating front. And fourth: «a big calving event means retreat». Not necessarily. Calving is what every glacier that ends in water does, every day. Retreat is measured over years and with instruments, not by the roar of one afternoon.

None of this takes the emotion out of the visit; it moves it elsewhere. Instead of waiting for a calving event with the camera ready, you can read the landscape for what it is: a balance in plain view. These are the things worth looking for from the walkway or from the boat.

The transformation this article proposes is modest and concrete. Before reading it, Perito Moreno was «the glacier that doesn't retreat», a fact to admire and repeat. Afterwards, it is a glacier with a lot of snow, a peninsula that slows it and a rock ridge that held it for a century and that it is now losing. The rupture stops being a spectacle to wait for and becomes the trace of an advance; and the calving stops being the glacier «breaking» and becomes what a glacier does every day. With that in mind, the walkway is not a theatre box: it is a place from which a balance can be read.

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