- Written by Nadia O’Carroll
Glaciers are moving masses of ice that can only develop where frozen precipitation such as snow, sleet and wind drift snow can accumulate in winter, then survive the summer without melting, evaporating or splitting away.
A glacier forms when accumulated snow remains unmelted for a season, then becomes compressed into a granular form of ice called firn. These sugar like crystals continue to grow until they eventually form glacial ice crystals, which may be several centimetres in length. As the layer of snow and ice accumulates and becomes denser through crystallisation, the sheer weight combined with the force of gravity eventually creates enough pressure to produce a flow outwards or downwards.
The specific geographic and climatic conditions for glaciers mainly occur near the poles (polar glaciers) but can also exist at high altitudes (alpine glaciers) and these different locations produce several different types of glaciers.
Continental glaciers or ice sheets are enormous masses of glacial ice and snow that occur in Antarctica and Greenland. The extensive Antarctic ice sheet is over 4200 metres thick in places, because the Antarctic is a cold desert with low precipitation these huge glaciers are very slow growing and its estimated age is 40 million years. When the ice sheets extend over the ocean and float they are known as ice shelves, the edge may split away to form icebergs.
Alpine glaciers develop from mountain icefields; they may flow across mountains or down into valleys like tongues of ice. If they reach the sea they are described as tidewater glaciers.
Glaciers produce distinctive landscapes due to their powerful erosive force. The base and sides of a glacier collect rocks as it moves along, the weight and abrasion carve out distinctive U shaped valleys that feature truncated spurs, hanging valleys, bowl shaped depressions (cirques), mountain lakes (tarns) and debris deposits such as moraines and drumlins. Landform features such as fiords and horn mountains such as the Matterhorn are products of glacier erosion.
All glaciers are sensitive to changes in climatic conditions because they are constantly balancing between accumulation / advance and ablation (melting and evaporation) / retreat. Observations show that in the twentieth century the vast majority of glaciers have retreated rapidly.
Glaciers are a major part of the planet’s cryosphere - they cover 10% of land surface, contain 75% of the world’s freshwater and if glaciers and all land ice melted, it is estimated that sea levels would rise by up to 70 metres. Consequently massive changes to glaciers have the potential to cause major environmental impacts.