Glaciers
All About Glaciers
A glacier is a large, perennial accumulation of crystalline ice, snow, rock, sediment, and often liquid water that originates on land and moves down slope under the influence of its own weight and gravity. Glaciers form on land, and they are made up of fallen snow that gets compressed into ice over many centuries. They move slowly downward from the pull of gravity. Out of all of the freshwater we have on Earth, only 1% comes from glaciers. Glaciers take up about 10% of the entire Earth. Glaciers can range in age from a couple hundred to thousands of years old. Most glaciers today are remnants of the massive ice sheets that covered Earth during the Ice Age.
Glaciers often form in places where:
mean annual temperatures are close to the freezing point
winter precipitation produces significant accumulations of snow
temperatures throughout the rest of the year do not result in the complete loss of the previous winter’s snow accumulation
Glaciers are keystones of Life on Earth. As giant freshwater reservoirs, they support the planet’s life systems and influence our day-to-day lives, even for communities who live far away from them. However, glaciers are disappearing.
The disappearance of glaciers makes visible the invisible. It makes tangible the current climate change that can be hard to perceive in other ecosystems. The recent evolution of glaciers found in World Heritage sites paints a true picture of their decline in a warming planet.
How Glaciers Form
Snowfall on a glacier is the first step in the formation of glacier ice.
As snow builds up, snowflakes are packed into grains.
The weight of the overlying snow causes the grains below to become coarser and larger. (Fresh snow is about 90 percent air.)
Melted snow quickly refreezes forming ice. How the snow changes and how much time it takes to develop into glacier ice depends on the temperature.
Glaciation Periods
Glaciers begin to form when snow remains in the same area year round, where enough snow accumulates to transform into ice. Each year, new layers of snow bury and compress the previous layers. This compression forces the snow to recrystallize, initially forming grains similar to the size and shape of sugar grains.
Gradually, the grains grow larger and the air pockets between the grains get smaller, causing the snow to slowly compact and increase in density. After about a year, the 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 the size of an adult fist.