Glaciers and ice sheets are made up of large masses of ice that flow like very slow rivers. They form when snow that has fallen over many years, compresses and transforms into ice. At present about 10% of the surface of the earth is ice-covered; most of this is found in the Arctic and Antarctic regions, and in high altitude mountain ranges. Glaciers erode and shape the land beneath them, scouring whole mountain sides and transporting large amounts of rock and soils.
Glaciers are very sensitive indicators of changes in climate; they grow and shrink due to changes in temperature or snowfall. In past, glaciers have covered much more of the earth’s surface, over 30% of the land and 30% of the oceans at the maximum of the last ice age. More recently glaciers have been shrinking and we are now concerned about sea-level rise, water supplies and, hazards such as avalanches and floods.
To find out more;
http://nsidc.org/cryosphere/glaciers
http://education.nationalgeographic.com/education/encyclopedia/glacier/?ar_a=1
Many of the world’s glaciers are clean ice glaciers where ice is visible at the surface and energy from the sun can directly melt the glacier.