Why Does the Perito Moreno Glacier Keep Growing While Others Retreat?

Perito Moreno glacier in Argentina’s Los Glaciares National Park held steady — and even advanced — for most of the twentieth century because exceptional snowfall from the Southern Patagonian Ice Field replenished its ice as fast as the front calved into Lago Argentino. That same advance causes it to periodically dam the lake’s Brazo Rico arm and release roughly a quarter of a cubic mile of water in just a few days when the ice arch finally gives way.

What Makes Perito Moreno Unusual Among Glaciers?

Almost every glacier on Earth has been in retreat for decades. Perito Moreno was a striking exception. Nineteen miles long, with an ice face rising 240 feet above the lake surface — roughly the height of a 24-story building — it sits inside a shallow basin where it locks against a rocky peninsula called Península Magallanes. That geographic anchor, combined with some of the world’s heaviest snowfall feeding down from the Southern Patagonian Ice Field, kept the glacier in a rare equilibrium: new ice piling on at the top matched what was constantly calving off the front.

The Southern Patagonian Ice Field is the third-largest freshwater reserve on Earth, after Antarctica and Greenland. Perito Moreno draws directly from that reserve, which is why its snowfall budget was so different from glaciers in drier or warmer mountain ranges. Since around 2020, however, even this balance has begun to break down, and the glacier has shown signs of net retreat for the first time.

How Does the Glacier Dam Lago Argentino?

Because Perito Moreno advances rather than pulling back, its front periodically pushes all the way across a narrow channel and presses against the far shore, sealing off the Brazo Rico arm of Lago Argentino from the main lake. Once the seal is complete, water trapped in Brazo Rico has nowhere to go. It simply rises.

In 2004 — one of the most documented rupture events — the water level in Brazo Rico climbed 34 feet above the rest of the lake. Picture an ice wall holding back a small inland sea, with millions of tons of water pressing against it from every direction. The dam does not hold forever.

What Happens When the Ice Arch Collapses?

The failure begins subtly. Rising water eventually seeps beneath the glacier’s base. Buoyancy slowly lifts the ice off its rocky support, opening a small channel on the Brazo Rico side. Water rushes in, and the tunnel grows rapidly: flowing water melts the walls, the ceiling loses support, and massive blocks begin breaking free from above. Once the process starts, it accelerates quickly.

In 2004, approximately a quarter of a cubic mile of water drained through that collapsing tunnel in just three and a half days. The water that drained in less than four days had taken about four months to accumulate behind the ice. When the arch finally falls, the ice face sheds blocks the size of apartment buildings into the channel below, generating waves large enough to be seen clearly from the public viewing galleries onshore.

How Often Has This Happened?

The rupture is not a once-in-a-lifetime event. It has occurred more than twenty times since the first recorded instance in 1917. Notable ruptures were documented in 1917, 1988, 2004, 2016, and 2018. The cycle — advance, seal, flood, collapse, repeat — is one of the most dramatic recurring geological events visible to the public anywhere on the planet.

The interval between ruptures varies considerably. Some cycles last only a few years; others stretch for decades. The timing depends on how quickly the glacier re-advances after calving and how fast precipitation refills Brazo Rico behind the new ice dam.

Why Is This Glacier Scientifically Important?

Perito Moreno sits within Los Glaciares National Park, a UNESCO World Heritage Site, and it serves as a living laboratory for glaciologists studying ice dynamics, mass balance, and subglacial hydrology. Its rare stability — now fading — made it a benchmark for understanding what keeps a glacier in equilibrium. The periodic dam-and-release cycle offers researchers a controlled, repeating experiment in glacial outburst floods that would be impossible to engineer artificially. As climate change accelerates the retreat of the Southern Patagonian Ice Field as a whole, Perito Moreno’s shifting behavior is being watched closely as an indicator of larger systemic change.