Explainer: Ice sheets and glaciers

Here’s how and where these ice blankets form

A view of an Greenland ice sheet from 40,000 feet above.  Scientists are using planes and satellites to study how ice sheets and glaciers are changing around the world. 

Oceans Melting Greenland (OMG)/NASA

Ice covers nearly all of Antarctica’s 14 million square kilometers (5.4 million square miles) of land. In some places, that ice is nearly 4 kilometers (2.5 miles) thick. It formed as snow piled up over many thousands of years. Gradually, the snow compacted into ice. Scientists call this vast, frozen blanket an ice sheet. A second, smaller ice sheet covers much of Greenland.

An ice sheet may look stationary, but it is always moving.

The lower layers are under crushing weight — in some places more than 350 times the pressure of Earth’s atmosphere (or 5,000 pounds per square inch). This pressure squishes the hard ice like soft putty. The ice oozes, flowing from the interior of the continent out toward its edges. In some places, this flow is very slow — only a meter or so per year. But scientists have also found fast-flowing corridors where the ice moves hundreds of times that fast. These ice streams — better known as glaciers — can be 40 kilometers wide and hundreds of kilometers long.

Unlike ice sheets, glaciers are widespread on our planet. They are found in mountain ranges around the world, from the Andes in South America and the Himalayas in Asia to the Alps in Europe. Fed by snow that falls on the mountains, glaciers flow down U-shaped valleys. Some will go on to feed rivers that empty into lakes or the sea.

Most of Antarctica’s ice sheet is made up of slow-moving ice. This ice generally moves no more than a few meters, or tens of meters per year. But in some places there are corridors of ice that flow much more quickly. These rivers of ice, called ice streams, or glaciers, often flow hundreds of meters per year. In this map, the Antarctic Ice Sheet is color-coded according to how quickly the ice flows. The fast-flowing ice streams appear in blue or purple. Credit: Eric Rignot, NASA/JPL/UCI
Most of Antarctica’s ice sheet is made up of slow-moving ice. This ice generally moves no more than a few meters, or tens of meters per year. But in some places there are corridors of ice that flow much more quickly. These rivers of ice, called ice streams, or glaciers, often flow hundreds of meters per year. In this map, the Antarctic Ice Sheet is color-coded according to how quickly the ice flows. The fast-flowing ice streams appear in blue or purple. Eric Rignot, NASA/JPL/UCI

Douglas Fox is a freelance journalist who writes about life, earth and Antarctic sciences.

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