What is a glacier?

Glaciers are made up of fallen snow that, over many years, compresses into large, thickened ice masses. Glaciers form when snow remains in one location long enough to transform into ice. What makes glaciers unique is their ability to flow. 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.

An ice cap is a dome-shaped glacier mass flowing in all directions, such as the ice cap on Ellesmere Island in the Canadian Arctic. An ice sheet is a dome-shaped glacier mass exceeding 50,000 square kilometers. The world's ice sheets are confined to Greenland and Antarrctica.

Bird and Darwin Glaciers
This photo was taken during a flight over the Antarctic Ice Sheet. Bird Glacier is visible in the background, with Darwin Glacier in the foreground. —Credit: Courtesy Ted Scambos and Rob Bauer

Last updated: 16 March 2020