Copyright © 2007 Dorling Kindersley
About one-tenth of Earth’s dry land and one-eighth of its oceans are covered with ice. This ice is made of snow that collects and becomes compacted (pressed down). Most ice occurs in thick ICE SHEETS that cap the land in the polar regions. In the past, during long cold eras called Ice Ages, ice covered much more of the Earth’s surface than it does today. Scientists estimate that there have been over 15 Ice Ages in the last 2 million years.
Ninety per cent of the world’s ice is found in Antarctica. The ice cap here is 4,200 m (13,000 ft) deep in places. Over thousands of years, a thick ice sheet builds up over land when more snow falls during the winter months than melts each summer. The enormous weight of the ice pushes much of this vast, high landmass down below sea level.
Icebergs are not formed from salty sea ice, but from land ice that calves (breaks off) from ice sheets or glaciers on the coast. Only 12 per cent of the iceberg’s mass appears above the sea surface. The rest is hidden below. A fringe of sea ice also edges the Antarctic landmass, expanding in winter and melting in summer.
Glaciers are slow-moving rivers of ice that begin high on mountains. Fallen snow pressed down by new snow forms a dense ice called firn. When enough ice builds up, gravity and the glacier’s own weight set it sliding downhill at a rate of 1–2 m (3–6 1/2 ft) per day.
Moving ice is a powerful erosive force. As glaciers slip downhill they carve deep, U-shaped valleys, sharp peaks, and steep ridges. The gouging power of the ice is increased by rocks and boulders carried along at the front, sides, and beneath the glacier. When the glacier reaches the warmer lowlands, it melts.