Up-close view of a rough patch of ice shelf
Antarctica's Ice Sheet
The continent of Antarctica is covered by a thick sheet of ice, averaging over 2 kilometers deep. This frozen mass holds some ninety percent of the fresh water in the world, although much of the continent is paradoxically very dry. Precipitation levels measured at single-digit centimeters per year are common, making the icy expanse in fact one of the "driest" places on Earth.
An ice sheet consists of a large amount of ice that stays in one place and does not appreciably melt or otherwise disappear - at least on the brief timescale of civilization. Antarctica's ice sheet is divided in two different sections: the Eastern- and Western- Antarctic Ice Sheet. The Eastern Antarctic Ice Sheet is grounded on a continental shelf above sea level, just like most glacial fields in the Northern Hemisphere. However, the Western Antarctic Ice Sheet rests on the sea floor, stuck in place under the massive weight of ice.
The basic principle of ice sheet formation is as follows. Ice sheets form over thousands to millions of years when the amount of snow falling during cold periods exceeds the amount of water escaping during summer. Over time, layer builds upon layer, re-freezing into glacial ice — like the formation of an ice dam on the eves of a roof. In some places, water melts and escapes directly as runoff, but in most of Antarctica the temperature is consistently below freezing, so ice is primarily lost at the margins where the shelf runs into the sea.