Water bubble burst on Mont Blanc

On the night of 12 July 1892, an erupted glacial lake claimed almost 200 lives. The masses of water caused a mudslide that overran the small resort town of Saint-Gervais while it slept.

A peaceful summer day comes to an end. Not suspecting anything bad, the inhabitants of Saint-Gervais go to bed on the evening of 11 July 1892. While they are sleeping, a catastrophe is brewing inside the Tête Rousse glacier at the foot of Mont Blanc. Over time, a dangerously large lake of meltwater has formed between the ice masses, encased in a shell of ice. In the night of 12 July, the water pressure inside the glacier becomes too great: the masses of water blast away the ice layer. An enormous tidal wave crashes into the valley, sweeping away everything in its path. The mudslide also overtakes the spa town of Saint-Gervais. 175 people die in the mass of water, mud and rock that flows towards Lake Geneva. Many of the dead are only recovered days later, others are never found.

Watercolour for the glacier

Global warming is making glaciers more and more dangerous: The more they melt, the greater the threat posed by erupting glacial lakes. Lakes inside the ice sheet are particularly dangerous because they often form unnoticed.

To detect such hidden lakes nevertheless, scientists use a simple trick: they colour the water at the upper reaches of a meltwater river. If the brightly coloured water flows out again below the glacier, the all-clear is given: the meltwater flows away again. If the coloured liquid remains, however, water is collecting somewhere inside the glacier. Then it is a matter of finding out where this water is in order to pump out the dangerous lake. The aim is to prevent a flood catastrophe like the one in 1892.

Danger from glaciers

Glaciers can pose a great danger to humans. Glacial lakes that suddenly burst their banks have already caused great damage. People have died in crevasses because they could not be rescued from them in time.

Crevasses are deep cracks in the ice. They occur when the glacier slides over a hilly ground or when the glacier ice flows at different speeds. Such crevasses are dangerous, especially for mountaineers and skiers, because sometimes they are not visible through an overlying snow cover.

When meltwater flows out of a glacier, large amounts of water can collect at its base: A glacial lake is formed. If too much meltwater and rainwater runs into the glacial lake in a short time, it overflows its banks and the water suddenly plunges into the valley.

Glacial lakes are particularly dangerous when they form under an ice sheet. Such lakes are called water pockets and are not visible from the outside. Therefore, they can grow bigger and bigger unnoticed. When the pressure of the enclosed water becomes too great, it blasts away the surrounding ice cover and the water erupts. Its flood sweeps away everything in its path on its way down into the valley.

Places that lie below glacial lakes are threatened by such outbursts. That is why experts there regularly check the extent of the glacial lakes to pump out the water in time and warn of an eruption.

What is a glacier?

Glaciers flow down from the mountains like white tongues. Others cover huge land masses as mighty sheets of ice. Glaciers consist mainly of ice and can be hundreds of metres thick and several kilometres long. Most of the fresh water on Earth is frozen into ice! But how do such ice masses come about in the first place?

Glacier ice forms where it is very cold all year round. Such low temperatures prevail far up in mountains, for example in the Alps. The snow that falls there does not even thaw completely in summer. The snow cover therefore becomes thicker and heavier. Under this load, the loose snowflakes are pressed over time first into grainy firn and then into dense ice.

Even in the areas around the North Pole or South Pole, more snow falls throughout the year than can thaw again. Glaciers then form, even in flat landscapes. The glaciers of the polar regions are thousands of metres thick. They have the shape of huge shields and are therefore called ice sheets.

Glaciers flow downhill very slowly under the weight of their own weight. Meltwater at their bottom makes it easier for them to glide over the ground. With their mass of ice, they also drag sand and rock fragments that have been blown off the ground by frost.

If a glacier eventually advances into warmer regions, its ice melts. The meltwater runs off in a trickle; if there is a large amount of water, a river forms. If the meltwater collects in a hollow, it forms a glacial lake.