The Mer de Glace (Sea of Ice) is a glacier located on the northern slopes of the Mont Blanc massif, in the Alps. At 7 kilometres long and 200 metres deep, it is the longest glacier in France.
The Mer de Glace, like all glaciers, is constantly renewed under the effect of two phenomena: accumulation, notably due to snowfall and ablation, essentially due to melting. The Mer de Glace flows permanently under the effect of its own weight, crusting crevasses, seracs or pockets of water to form, depending on the type of ground.
The glacier’s speed, although not perceptible to the naked eye, is considerable. From more than 120 meters a year in its upper part, the Mer de Glace moves about 90 meters per year in the region of Montenvers, which is about one centimeter per hour.