Nature · Earth Science
Rivers of ice that reshape whole continents
A glacier is a huge, slow-moving mass of ice that forms on land where more snow falls each year than melts. Over centuries the snow compacts into dense ice that begins to flow under its own weight, grinding valleys, transporting rock and storing about three quarters of the world's fresh water.
Snow accumulates in a high 'zone of accumulation', is buried, and slowly recrystallises into firn and then solid glacial ice. Once thick enough, gravity makes the ice creep downhill toward a lower 'zone of ablation' where it melts or calves. The glacier survives as long as accumulation balances loss.
Snow gathers and turns to ice in the accumulation zone, flows downhill, then wastes away in the ablation zone — dumping rock as moraine.
Tongues of ice flowing down mountains
Born in high mountain hollows, these glaciers flow down existing valleys, widening and deepening them into the classic U-shape and leaving behind hanging valleys and moraines.
Continent-burying domes of ice
The largest glaciers of all, ice sheets smother whole continents. Only Antarctica and Greenland have them today, together storing the vast majority of Earth's fresh water.
Glaciers that calve into the sea
Where a glacier reaches the sea, chunks break away in thunderous calving events to form icebergs. These glaciers carve the deep fjords found in Norway, Alaska and Patagonia.
Small glaciers in mountain hollows
Nestled in high rocky hollows called cirques, these compact glaciers grind out steep-walled basins. When they melt, a small round tarn lake is often left behind.