Glaciers

Glaciers are rivers of slow-moving ice. They form in mountain regions when it is too cold for snow to melt. They flow down through valleys, creeping lower until they melt in the warm air lower down.

Glaciers form when new snow falls on old. The new snow compacts the old snow into denser snow called firn.

In firn snow, all the air is squeezed out so it looks like ice. As more snow falls, firn gets more compacted and becomes glacier ice flowing slowly downhill.

Nowadays glaciers form only in high mountains and towards the North and South Poles. In the Ice Ages glaciers were widespread and left glaciated landscapes in many places that are now free of ice.

As glaciers move downhill, they bend and stretch, opening up deep cracks called crevasses. Sometimes these occur where the glacier passes over a ridge. The biggest crevasse is often called the bergschrund. It forms when the ice pulls away from the back wall of the hollow where the glacier starts.

Where the underside of a glacier is warmish (about 0°C), it moves by gliding on a film of water, made as pressure melts the glacier's base. This is basal slip.

Where the underside of a glacier is coldish (below 0°C), it moves as if layers are slipping over each other like a pack of cards. This is internal deformation.

Cirque glaciers are small glaciers that flow from hollows high up. Alpine valley glaciers form when several cirque glaciers merge. Pedmont glaciers form where valley glaciers join as they emerge from the mountains.