A glacier is a perennial mass of ice which moves over land. A glacier forms in locations where the mass accumulation of snow and ice exceeds ablation over many years. The word glacier comes from French via the Vulgar Latin glacia, and ultimately from Latin glacies meaning ice. The corresponding area of study is called glaciology.
Glacier ice is the largest reservoir of fresh water on Earth, and is second only to oceans as the largest reservoir of total water. Glaciers cover vast areas of the polar regions and are found in mountain ranges of every continent except Australia. In the tropics glaciers are restricted to the highest mountains. The processes and landforms caused by glaciers and related to them are referred to as glacial. The process of glacier growth and establishment is called glaciation. Glaciers are indicators of climate and are important to world water resources and sea level variation. They are an important component of the more encompassing cryosphere.
Types of glaciers
external image 180px-Glacier_mouth.jpg
external image 180px-Glacier_mouth.jpg
Main article: Glacier morphologyMouth of the Schlatenkees Glacier near Innergschlöß, Austria.
Glaciers are categorized in many ways including by their morphology, thermal characteristics, or their behavior. Two common types of glaciers are Alpine glaciers, which originate in mountains, and Continental ice sheets, which cover larger areas.
Alpine glaciers form on mountain slopes and are also known as mountain, niche, or cirque glaciers. An alpine glacier that fills a valley is referred to as a valley glacier. Larger glaciers that cover an entire mountain, mountain chain, or volcano are known as an ice cap or ice field, such as the Juneau Icefield. Ice caps feed outlet glaciers, tongues of ice that extend into valleys below, far from the margins of the larger ice masses.
Ice sheets are the largest glaciers. These enormous masses of ice are not visibly affected by the landscape as they cover the entire surface beneath them, with possible exception near the glacier margins where they are thinnest. Antarctica and Greenland are the only places where continental ice sheets currently exist. These regions contain vast quantities of fresh water. The volume of ice is so large that if the Greenland ice sheet melted, it would cause sea levels to rise six meters (20 ft) all around the world. If the Antarctic ice sheet melted, sea levels would rise up to 65 meters (210 ft). Ice shelves are areas of floating ice, commonly located at the margin of an ice sheet. As a result they are thinner and have limited slopes and reduced velocities.[4] Ice streams are fast moving sections of an ice sheet. They can be several hundred kilometers long. Ice streams have narrow margins and either side ice flow is usually an order of magnitude less. In Antarctica, many ice streams drain into large ice shelves. However, some drain directly into the sea, often with an ice tongue, like Mertz Glacier. In Greenland and Antarctica, ice streams ending at the sea are often referred to as tidewater glaciers or outlet glaciers, such as Jakobshavn Isbræ (Kalaallisut: Sermeq Kujalleq).
Tidewater glaciers are glaciers that terminate in the sea. As the ice reaches the sea pieces break off, or
calve, forming icebergs. Most tidewater glaciers calve above sea level, which often results in a tremendous splash as the iceberg strikes the water. If the water is deep, glaciers can calve underwater, causing the iceberg to suddenly explode up out of the water. The Hubbard Glacier is the longest tidewater glacier in Alaska and has a calving face over ten kilometers long. Yakutat Bay and Glacier Bay are both popular with cruise ship passengers because of the huge glaciers descending hundreds of feet to the water. This glacier type undergoes centuries-long cycles of advance and retreat that are much less affected by the climate changes currently causing the retreat of most other glaciers. Most tidewater glaciers are outlet glaciers of ice caps and ice fields.
In terms of thermal characteristics, a
temperate glacier is at melting point throughout the year, from its surface to its base. The ice of a polar glacier is always below freezing point from the surface to its base, although the surface snowpack may experience seasonal melting. A sub-polar glacier has both temperate and polar ice, depending on the depth beneath the surface and position along the length of the glacier.


external image 180px-ByrdGlacier_HiLoContrast.jpg
external image 180px-ByrdGlacier_HiLoContrast.jpg
Low and high contrast images of the Byrd Glacier. The low-contrast version is similar to the level of detail the naked eye would see — smooth and almost featureless. The bottom image uses enhanced contrast to highlight flow lines on the ice sheet and bottom crevasses. Formation of glacial ice
Glaciers form where the accumulation of snow and ice exceeds ablation. As the snow and ice thicken, they reach a point where they begin to move, due to a combination of the surface slope and the pressure of the overlying snow and ice. On steeper slopes this can occur with as little as 50 feet of snow-ice. The snow which forms temperate glaciers is subject to repeated freezing and thawing, which changes it into a form of granular ice called firn. Under the pressure of the layers of ice and snow above it, this granular ice fuses into denser and denser firn. Over a period of years, layers of firn undergo further compaction and become glacial ice. Glacier ice has a slightly reduced density from ice formed from the direct freezing of water. The air between snowflakes becomes trapped and creates air bubbles between the ice crystals.
The distinctive blue tint of glacial ice is often wrongly attributed to Rayleigh scattering due to bubbles in the ice. The blue color is actually created for the same reason that water is blue, that is, its slight absorption of red light due to an overtone of the infrared OH stretching mode of the water molecule.


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external image 180px-Glacier.swiss.500pix.jpg
The Upper Grindelwald Glacier and the Schreckhorn, in the Swiss Alps, showing accumulation and ablation zones
The location where a glacier originates is referred to as the glacier head. A glacier terminates at the glacier foot, or terminus. Glaciers are broken into zones based on surface snowpack and melt conditions. The ablation zone is the region where there is a net loss in glacier mass. The equilibrium line separates the ablation zone and the accumulation zone. At this altitude, the amount of new snow gained by accumulation is equal to the amount of ice lost through ablation. The accumulation zone is the region where snowpack or superimposed ice accumulation persists. A further zonation of the accumulation zone distinguishes the melt conditions that exist. The dry snow zone is a region where no melt occurs, even in the summer, and the snowpack remains dry. The percolation zone is an area with some surface melt, causing meltwater to percolate into the snowpack. This zone is often marked by refrozen ice lenses, glands, and layers. The snowpack also never reaches melting point. Near the equilibrium line on some glaciers, a superimposed ice zone develops. This zone is where meltwater refreezes as a cold layer in the glacier, forming a continuous mass of ice. The wet snow zone is the region where all of the snow deposited since the end of the previous summer has been raised to 0°C. The upper part of a glacier that receives most of the snowfall is called the accumulation zone. In general, the glacier accumulation zone accounts for 60-70% of the glacier's surface area, more if the glacier calves icebergs. The depth of ice in the accumulation zone exerts a downward force sufficient to cause deep erosion of the rock in this area. After the glacier is gone, this often leaves a bowl or amphitheater-shaped isostatic depression ranging from large lake basins such as the Great Lakes or Finger Lakes to smaller mountain basins, known as cirques.
The "health" of a glacier is usually assessed by determining the glacier mass balance or observing terminus behavior. Healthy glaciers have large accumulation zones, more than 60% of their area snowcovered at the end of the melt season, and a terminus with vigorous flow.
Following the Little Ice Age, around 1850, the glaciers of the Earth have retreated substantially through the 1940s (see Retreat of glaciers since 1850). A slight cooling led to the advance of many alpine glaciers from 1950-