How Do Glaciers Form?
A glacier is a great river of ice that moves slowly down a mountain. Glaciers form in high mountain valleys, where summers are so cold that most of the winter snow never melts, but piles higher, year after year.
The ice grows thicker and begins to creep down the slope. The movement of a glacier is so slow that it is measured in inches a day rather than in miles an hour. When a glacier reaches the sea, great chunks of ice, called icebergs, break off and drift out to sea.
Glaciers begin to form when snow remains in the same area year-round, where enough snow accumulates to transform into ice. The great weight of snow presses the snowflakes together and turns them into solid ice.
Each year, new layers of snow bury and compress the previous layers. This compression forces the snow to re-crystallize, forming grains similar in size and shape to grains of sugar.
Gradually the grains grow larger and the air pockets between the grains get smaller, causing the snow to slowly compact and increase in density. It takes about two winters, then snow turns into firn—an intermediate state between snow and glacier ice.
At this point, it is about two-thirds as dense as water. Over time, larger ice crystals become so compressed that any air pockets between them are very tiny. In very old glacier ice, crystals can reach several inches in length. For most glaciers, this process takes more than a hundred years.