Geologists class glacier ice as a rock, and it flows
To a geologist, glacier ice is a metamorphic rock made almost entirely of one mineral, sitting just below its melting point. It creeps downhill under its own weight, stores about 69 percent of the world's fresh water, and outside the poles no country has more of it than Pakistan, with over 7,000 known glaciers.
A glacier forms wherever more snow piles up than melts or evaporates over many years. The snow collects, often in an armchair-shaped hollow called a cirque, refreezes into granular névé, is crushed into denser firn, and eventually becomes glacial ice with most of its air squeezed out. Once thick enough, sometimes only about 15 metres on a steep slope, it starts to move, bending in some places and cracking into crevasses in others. The upper accumulation zone, usually 60 to 70 percent of the surface, gains mass; the lower ablation zone loses it.
The famous blue is often explained wrongly. It is sometimes blamed on light scattering from bubbles, but the main cause is that water molecules absorb a little red light, the same reason deep water looks blue. Because glacier ice has so few bubbles, which would otherwise make it look white, the blue shows through.
About 99 percent of glacial ice lies in the two surviving ice sheets on Antarctica and Greenland. Antarctica's ice covers roughly 98 percent of the continent, averaging 2,100 metres thick, and if both sheets melted, sea levels would climb more than 70 metres. Glaciers also exist on every continent except mainland Australia; between 35 degrees north and south they cling only to high ranges such as the Himalayas and Andes and a few peaks in East Africa, Mexico, New Guinea and Iran. Glaciers that end in the sea calve icebergs, and their long cycles of advance and retreat respond less to climate than others do.
Glaciers are among the most sensitive gauges of climate. They have shrunk substantially since the Little Ice Age ended around 1850. A slight cooling let many alpine glaciers advance between 1950 and 1985, but since 1985 retreat has grown larger and more widespread. For many communities, summer meltwater from glaciers is a crucial water supply when other sources run dry.
Source: Glacier
