A glacier is a large, perennial mass of ice that flows under its own weight. Glaciers are found on every continent except Australia and Antarctica. They occur in nearly all mountain ranges, and at high elevations in the Arctic and Antarctic. The largest glacier in the world is the Aletsch Glacier in Switzerland, which is about 23 kilometers (14 miles) long.
Glaciers form when snow accumulates over time and turns to ice. The weight of the accumulating snow compresses the lower layers of snow into ice. glaciers can take many thousands of years to form. As a result, they are often very thick—some are more than 3 kilometers (2 miles) deep!
During periods of warm weather, glaciers can melt and retreat. When temperatures drop, glaciers can advance again. This cyclical behavior is known as glacial oscillation. Glacial oscillation is thought to be driven by changes in Earth’s orbit around the sun (known as Milankovitch cycles), which affect the amount of sunlight that reaches our planet’s surface.