How do glaciers form?
Glaciers form where more snow falls in winter than melts in summer. Over many years, the snow piles up, gets compressed into dense ice and begins to flow slowly downhill under its own weight.
Fresh snow is light and fluffy. As layers build up, the weight of new snow presses on older snow below, squeezing out air. The snow first turns into grainy, compacted snow called firn and, over decades or centuries, into solid glacial ice. When the ice becomes thick enough, usually around 50 meters, it starts to move.
Glaciers flow like very slow rivers of ice, often moving a few centimeters to a few meters a day. As they move, they grind and carve the land beneath, creating U-shaped valleys, fjords and lakes. They pick up rocks and leave behind piles of debris called moraines.
Shrinking ice
Glaciers store about 69 percent of the world's fresh water, most of it in the ice sheets of Antarctica and Greenland. Rising temperatures are causing most glaciers worldwide to shrink, contributing to sea level rise and threatening water supplies for people who rely on glacier meltwater.
Did You Know?
Glacier ice often looks blue because dense ice absorbs red light and scatters blue light, similar to deep water.