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 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. After about two winters, 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 several inches in length. For most glaciers, this process takes more than a hundred years. Glacial ice often appears blue when it has become very dense. When glacier ice becomes extremely dense, the ice absorbs a small amount of red light, leaving a bluish tint in the reflected light, which is what we see. When glacier ice is white, that usually means that there are many tiny air bubbles still in the ice.
What makes glaciers unique is their ability to move. Due to sheer mass, glaciers flow like very slow rivers. Some glaciers are as small as football fields, while others grow to be dozens or even hundreds of kilometers long. Presently, glaciers occupy about 10 percent of the world’s total land area, with most located in polar regions like Antarctica, Greenland, and the Canadian Arctic. Glaciers can be thought of as remnants from the last Ice Age, when ice covered nearly 32 percent of the land, and 30 percent of the oceans. Most glaciers lie within mountain ranges that show evidence of a much greater extent during the ice ages of the past two million years, and more recent indications of retreat in the past few centuries.
Most of the world’s glaciers are found near the poles, but glaciers exist on all of the world’s continents. Glaciers require very specific climatic conditions. Most are found in regions of high snowfall in winter and cool temperatures in summer. These conditions ensure that the snow that accumulates in the winter is not lost during the summer. Such conditions typically prevail in polar and high alpine regions.The amount of precipitation, whether in the form of snowfall, freezing rain, avalanches, wind-drifted snow, is important to glacier survival. For instance, in very dry parts of Antarctica, low temperatures are ideal for glacier growth, but the small amount of net annual precipitation causes the glaciers to grow very slowly, or even to disappear due evaporation of the ice.
Glaciers melt, and ablation result from increasing temperature, evaporation, and wind scouring. Ablation is a natural and seasonal part of glacier life. As long as snow accumulation equals or is greater than melt and ablation, a glacier will remain in balance or even grow. Once winter snowfall decreases, or summer melt increases, the glacier will begin to retreat.