Glaciers form over hundreds of thousands of years atop land in cold climates where snow and ice remain year-round. As snow layers accumulate, their weight forces underlying snowflakes closer together, thereby reducing the air spaces between them. The individual flakes crystallize and combine with other crystals around them to become glacial ice.

The largest glaciers, called continental ice sheets, blanket more than 50,000 square kilometers of land in ice and snow thousands of meters deep. Today, continental ice sheets are found only in Greenland and Antarctica. Divided into two unequal parts by a mountain range, the 14-million-square-kilometer Antarctic Ice Sheet alone contains about 70 percent of Earth's fresh water supply. The larger, eastern part rests entirely above sea level and covers Greater Antarctica. Most of the smaller, western part rests on rock at or below sea level and covers nearly the entire island archipelago collectively named Lesser Antarctica.

As more and more snow collects on an ice sheet and turns to ice, it forms a slight dome. The weight of the ice at the top of the dome becomes so great that gravity causes the underlying ice to slowly deform and spread down slope. The rate of movement varies from less than a meter per year near the dome to several hundred meters per year near the edges. Eventually, the ice flows into the sea, forming floating platforms called ice shelves.

To better understand both the nature and speed of ice sheet movement, glaciologists look for evidence within the ice sheets themselves. One particularly instructive type of movement, shearing, occurs when adjacent planes slide past one another at different speeds. Evidence of shearing tells scientists that although the ice sheet's entire mass is in constant motion down slope, its upper part moves faster than its lower part. This happens because the base of the ice sheet is actually frozen to the bedrock.

Studies show that in Antarctica, both the top and bottom of the West Sheet are moving much faster in places than the East Sheet. This is significant because, if the entire West Sheet were to slide into the sea, it would raise the global sea level by 6 meters.

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