Category: Earth Science
As mountains rise up, natural forces attack, and wear them down. The wearing away of land is called erosion.
There are several forces of erosion. In mountain areas, the process called weathering breaks up rocks. For example, water often fills cracks in the rocks during the day. At night, the water freezes, and because ice occupies more space than water the ice exerts pressure on the sides of the crack. Repeated freezing and thawing splits these rocks apart.
In temperate regions, rivers play a major part in wearing away the land. However, in cold or high mountainous regions the main reason of erosion is ice.
Over 10 percent of the world’s water is frozen into ice. Large bodies of ice include the vast ice sheets of Antarctica and Greenland, smaller ice caps, and rivers of ice called glaciers.
Ice forms from compacted snow, called firn, which accumulates in high mountain basins called cirques.
Gradually, the air is squeezed out and the firn becomes clear blue ice. The ice spills over the lip of the cirque and flows down the mountain valleys under the force of gravity. The rate at which the ice moves varies. In Antarctica, the ice sheet about 3 feet per year. However, many mountain glaciers move three feet per day.
The end, or the snout , of a glacier is where the rate of melting, evaporation or the breaking away of icebergs balances the rate of ice formation.
Loose rocks are broken up by weather and fall on to the ice or become frozen within it. Rocks transported by ice are called moraine. Ground moraine is frozen into the base of the ice. As the ice moves, the ground moraine scrapes over the land, and carves out deep valleys. Tributary valleys are not deepened to the same extent and they are left above the main valley. Cirques are also deepened. Where two cirques are back to back, the ice erodes the wall between them until it becomes an arête, a knife-edged ridge. Within three or more cirques a pyramid peak or horn is formed.
Other land features are formed when ice drops into the moraine. Some piles up in the low hills called terminal moraine. Boulder clay, a mixture of rocks and fine rock powder, ground down by the moving ice is then molded into hummocks called drumlins. Erratics are large boulders dropped by the ice. Melting ice at the snout forms streams, which carry moraine away from the glacier. This waterborne moraine may pile up in winding ridges, called eskers or kames.
Such land features helped geologists to discover that large parts of the northern hemisphere were covered in ice during the last ice age which ended about 12 thousand years ago.
Geiranger Fjord, in Norway is a deep valley worn out by glaciers. These glaciers flowed in from the Scandinavian ice sheet during the last ice age. The depth and the deep sides of the valley testify to the power of these long vanished glaciers. When the ice melted, the sea level rose and flooded the valley to form a long and deep sea inlet. Fjord is the name for all glaciated sea inlets.