A glacier is a mass of ice that forms usually in mountainous areas where more snow accumulates in winter than melts during the summer. As the snow is being compacted into ice, the area formed is called an ice field. When the ice begins to move downhill under the force of gravity and its own weight, it is called a glacier.
Motion and change define a glacier's life. Glacial ice advances, then retreats. Glaciers grow and shrink in response to changing climate. Typically, glacier movement and shape shifting occur over long periods of time (hundreds to thousands of years). But not all glaciers move slowly. For example, surging glaciers experience dramatic increases in flow rate, sometimes travelling as much as ten to one hundred times faster than the normal rate of movement.
Glaciers can be divided in two distinct classes:
Valley or mountain glaciers
These are also known as Alpine glaciers and flow like tongues of ice through the mountain valleys. These glaciers are formed by descending river and later on are also fed by direct snowfall, avalanches and the drift of snow blown by the wind from higher levels.
Their shape and form undergoes transformation according to the configuration of the valley. Mountain glaciers are ordinarily not very extensive, their length ranging from two to three miles. But certain mountain glaciers are so vast that they go on flowing for a distance of 50-60 miles and appear like huge rivers of ice. The thickness of the ice also varies from ten feet to tens of thousand feet.
In Green Land and Antarctica all the precipitation is in the form of snow. The snow that falls from year to year goes on being accumulated because very little of it is wasted either by melting or evaporation. The result is that these regions are covered by an extensive ice mass, a gigantic ice dome that hides beneath it all the surface irregularities irrespective of their height or depth. This is known as a continental glacier.
Some interesting facts
* Presently, 10 per cent of the land area is covered by glaciers.
* Glaciers store about 75 per cent of the world's freshwater.
* Glacierized areas cover over 15,000,000sq km.
* Antarctic ice is over 4,200m thick in some areas.
* In the US, glaciers cover over 75,000sq km, with most of the glaciers located in Alaska.
* During the last Ice Age, glaciers covered 32 per cent of the total land area.
* If all land ice melted, sea level would rise approximately 70m worldwide.
* Glacier ice crystals can grow to be as large as baseballs.
* The land underneath parts of the West Antarctic Ice Sheet may be up to 2.5km below sea level, due to the weight of the ice.
* North America's longest glacier is the Bering Glacier in Alaska, measuring 204km long.
* The Malaspina Glacier in Alaska is the world's largest Piedmont glacier, covering over 8,000sq km and measuring over 193km across at its widest point.
* Glacial ice often appears blue because ice absorbs all other colours and reflects blue.
* The Kutiah Glacier in Pakistan holds the record for the fastest glacial surge. In 1953, it raced more than 12km in three months, averaging about 112m per day.
* In Washington state alone, glaciers provide 470 billion gallons of water each summer.
* Antarctic ice shelves may have icebergs that are over 80km long.
* Almost 90 per cent of an iceberg is below water - only about 10 per cent shows above water.
* The Antarctic ice sheet has been in existence for at least 40 million years.
* From the 17th century to the late 19th century, the world experienced a "Little Ice Age," when temperatures were consistently cool enough for significant glacier advances.
Work of glaciers
By their movement, glaciers mark change and this is one of the reasons for the study of glaciers. By monitoring glaciers over time and around the world, researchers construct valuable records of glacial activity and their response to climate variation.
By comparing contemporary observations with historical and environmental records, such as agricultural records, pre-historic temperature or climate profiles, glaciologists acquire and provide an enhanced understanding of global processes and change.
Nowadays there are three schools of thought regarding the work done by glaciers.
One school of thought regards glaciers as protectors of the crust and this may be called the Protectionist school. The chief protagonist of this view was Heim who enunciated this theory in 1885.
As against this is the Erosionist school founded by Hess in 1904. Hess greatly exaggerated the erosional power of glaciers and regarded them as powerful agents of erosion and excavation like the running water of rivers.
The third school which represents the most modern thought on this point believes that, ice so long as it is stable, does the work of protection but when it begins to move, it does erode the surface it moves on.
Therefore, from the study of glaciated regions, moving ice has been found to carry the following functions:
1) Erosion in mountainous areas.
2) Depositing of rock material in lowlands so that the previous topography is not only obscured but also new land areas are built.
3) Interference with drainage.
Cycle of glaciation
A study of the lithosphere shows that in many places more than one till sheet is found. Besides, two types of moraine are also found. The old moraines are very much weathered while the young ones are still fresh. It shows that the old moraines were exposed to weathering for long, before the new drift was laid over.
Thirdly, in between the different sheets of till and drift, peat deposits and plant remains are found. All these evidences go to show that they were separated by interglacial periods when the earth's crust was ice free. These came one after the other in a cyclic order and the whole is known as the cycle of glaciation.
Thus, it is seen that the influence of glaciers on man's life has been very comprehensive through changes in drainage and soil on the one hand and through extensive deposits of sand, gravel and clay along with the provision of underground water supply under glacial drift on the other.