A glacier is a tongue of ice sliding slowly from a mountain due to pressure, slope and the pull of gravity. It is like a stream of ice on the back of the glacier. There are large snowfields. The continuity or life of the glacier depends upon the snowfields.
According to Devis when the climate gets very cold and the river freezes, instead of rivers of water, there are rivers of ice which are called glaciers. A glacier is formed under the influence of pressure and moisture when snowflakes (layers) change into a granular ice mass, known as the nave in French. When the ice becomes so thick that the lower layer becomes plastic. As it flows downhill, an active glacier comes into being.
Types of Glaciers:
Almost 70% of the Earth’s freshwater is stored in the Cryosphere, or realm of ice and snow, primarily in the form of glaciers. Glaciers form in regions that have low temperatures and sufficient snowfall, conditions found at both high elevations and high latitudes. In mountains, glacial ice can develop even in tropical and equatorial zones if the elevation is high enough to keep average annual temperatures below freezing. Glaciers can take a variety of forms, but they can generally be divided into two broad categories- alpine glaciers and ice sheets, which are also called continental glaciers.
Alpine Glaciers also called mountain glaciers, are found in high mountain ranges where snow accumulates and temperature are cold enough to maintain year-round snow cover. The specific forms of alpine glaciers depend on where they occur and the prevailing weather patterns. In high mountains, glaciers flow from high elevation collecting grounds down to lower elevations, where temperatures are warmer. Here the ice melts and evaporates. Glacial meltwater provides freshwater sources for large portions of the globe. Himalayan glaciers, for example, feed rivers throughout China, Southeast Asia, and India. The recent decline in many alpine glaciers due to global climate change, however, presents a serious threat to the supply of freshwater for these regions.
Alpine glaciers originate from a snowfield that accumulates in a bowl-shaped depression called a cirque. When alpine glaciers are contained within these basins, they are called cirque glaciers. However, most alpine glaciers flow out of these basins to become valley glaciers, which occupy sloping stream valleys between steep rock walls. When a valley glacier flows out onto a surrounding plain, it appears as a piedmont glacier. When a valley glacier terminates in seawater, as a tide-water glacier, blocks of ice break off to become icebergs.
Large masses of ice often occur at the very tops of mountain ranges. Ice caps are continuous masses that cover mountaintops with distinctive domes. Ice fields consist of interconnected valley glaciers with protruding rock ridges or summits called nunataks. In caps and ice fields feed individual glaciers that travel down the mountainside to lower elevations.
In arctic and polar regions, temperatures are low enough year-round for the snow to collect over broad areas, eventually forming a vast layer of glacial ice. Snow begins to accumulate on uplands, which are eventually buried under enormous volumes of ice. The layers of ice can reach a thickness of several thousand meters. The ice is thickest at the interior and thins toward the margins. The ice then spreads outward, over surrounding lowlands, and covers all landforms it encounters. We call this extensive type of ice mass an ice sheet, or a continental glacier. At some locations, ice sheets extend long tongues to reach the sea, known as outlet glaciers.
Ice sheets covered parts of northern North America and Eurasia many times during the most recent ice age, which began in the late-Cenozoic era. Today, the only remaining ice sheets are those of Antarctica and Greenland.
Erosional Work of Glacier:
Erosion by glacier takes place due to-
Plucking- It is also known as frost wedging or quarrying. During the summer, the surface parts of a glacier may partially melt. This melted water or rainwater steps down along the sides of the ice mass finding its way into the cracks and fractures in the rocks along with the edges and at the head of the glacier. Whenever temperature freezes this water, it breaks up the rock by frost action and with the movement of the glacier they are frozen in suspension in the ice.
Rasping- This term is used to describe the scraping the glacial action. The front edge of the glacier functions as bulldozer pushing and scraping the ground in front of the ice.
Avalanching- It is a process of mass wasting. Along the margin of the valley glacier, the valley sides are scraped and blocks are broken off which become frozen into the ice and are carried away. This leads to under cutting of the sides of the valley make ground for sliding and debris avalanching. This brings great quantity of rock waste. But this avalanching is absent in case of continental glacier.