When ice lets go

Steep glaciers can break off. Warming changes where and how.

Hanging glaciers cling to steep faces. Many of them are cold, frozen to the rock beneath. As the Alps warm, meltwater can reach the bed and loosen that grip, and whole sections can fail at once, as on the Altels in 1895.

Glacier tongues that end on steep ground calve blocks of ice, like the Allalin above Mattmark. And a glacier can be pushed beyond its limits from outside: in 2025 rockfall debris loaded the Birch Glacier until it collapsed onto Blatten.

Radar and GPS now watch dozens of unstable glaciers in Switzerland. When a glacier accelerates towards failure, its speed often follows a predictable curve, which is how the 2017 collapse on the Weissmies was forecast to the day.

Retreating ice leaves lakes behind. Some are held back by ice or loose moraine.

Since 1850 about 1,200 lakes have formed in the areas Swiss glaciers left behind; 987 of them still existed in 2016. Most are small and harmless. Some are dammed by ice or by unstable moraine, and can drain suddenly.

An outburst can release millions of cubic metres of water in hours, and on steep ground it can pick up debris and become a debris flow. The Giétro flood of 1818 is the historical worst case; Grindelwald in 2008 and Lenk in 2018 are recent ones.

Engineering helps: tunnels and channels keep lakes low at Grindelwald and on the Plaine Morte. But new lakes keep forming, 180 of them in the decade to 2016 alone.

Glaciers and permafrost hold mountains together. Take them away and slopes move.

Glacier ice supports the valley walls around it. When the glacier thins, the slopes lose that support and can start to slide. This is called debuttressing, and the Moosfluh above the Aletsch Glacier is the textbook Swiss example.

Higher up, permafrost, ground that stays frozen all year, acts as a cement in steep rock faces. As it thaws, rockfalls become more frequent, often in places that were considered safe.

When a rock avalanche falls onto ice, it can take the ice and meltwater with it and turn into a fast debris flow, as at Piz Cengalo in 2017.