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How do we build a glacier? We start with a snowflake. Snow, over time, is compressed into firnand then into glacier ice. Snow falls in cold regions, such as mountain tops or in polar regions. In glaciology, snow refers to material that has not changed since it fell 1.
Snow is very light and fluffy, and has a very low density. If the snow is wetter, it will have an increased density. Snowflakes have a hexagonal structure, and fallen snow has a ificant amount of air in it. Firn is usually defined as snow that is at least one year old and has therefore survived one melt season, without being transformed to glacier ice. Firn is transformed gradually to glacier ice as density increases with depth, as older snow is buried by newer snow falling on top of it.
Year after year, successive accumulation layers are built up. In the accumulation zone of a glacier, density therefore increases with depth; the rate depends on the local climate and rate of accumulation 1. Firn transforms to glacier ice at a density of kg m Firn becomes ice at a depth of about 13 m 1. At sites like this with rapid snow accumulation, the depth of a firn layer, and the load on it, increases rapidly with depth.
However, in cold, dry East Antarctica, firn becomes ice at a depth of 64 m at Byrd and 95 m at Vostok. Low temperatures slow the transformation. In addition, slow accumulation gives slow burial, and a small load each year; the increase in density takes much longer. Typically, the transformation of firn to ice takes years, and a depth of 50 — 80 m 1.
Air is isolated in separate bubbles. This occurs at a density of kg m The air space between particles is reduced, bonds form between them, and the particles grow larger. This is a process known as sintering. Increasing pressure compresses the bubbles, placing the enclosed air under pressure and increasing the density of the ice 2. Fresh snowflakes, which have a complex shape, have a large surface area. Over time and under pressure, the surface area is reduced, the surface is smoothed, and the total surface area is reduced.
Fresh, complex snowflakes are transformed into rounded particles. The transformation of firn to ice is much faster where there is melting and refreezing 2. Meltwater can percolate downwards, infilling porespaces, and the displaced air escapes upwards. This will produce high density ice much more rapidly than in colder regions without melting. The density of pure glacier ice is usually taken as kg m Below 4 km of ice, such as at the centre of the East Antarctic Ice Sheet, the pressure would increase the density to kg m Bubbles are common in glacier ice.
Bubbles can contain liquid water or atmospheric gases, making them very useful for ice core research. The air in the bubble largely reflects the atmospheric concentrations when the ice formed 1. With greater depth in polar ice sheets, bubbles shrink as the overlying ice increases. The gas pressure within the bubbles therefore increases, and at certain depths, the gas attains a dissociation pressure.
The bubbles begin to disappear as the gas molecules form clathrate hydrates 1. This process takes thousands of years. Glacier ice contains various impurities in tiny amounts. By most scales, glacier ice is a very pure solid-earth material, because the processes leading to snowfall on a glacier — evaporation, condensation, precipitation — act as a natural distillation system 1.
However, glaciers can contain impurities. The dirtiest glaciers are mountain glaciers, where debris can fall directly onto the ice surface. On ice sheets and glaciers, dust and other debris may blow onto the ice surface. Debris on the ice surface can affect the absorption of energy at the ice surfaceand lead to increased or decreased melting. Glaciers are composed of sedimentary layers in their accumulation zones, formed of annual layers of snowfall.
These layers are initially parallel to the glacier surface.
This is the primary stratification in structural glaciology. In temperate and subpolar settings, the annual sedimentary layers consist of alternating thick layers of bubble-rich ice, which originated as winter snow, and thin layers of clear ice, which are the refrozen meltwater from the summer melt season.
Debris horizons may form, when summer melting concentrates debris such as rockfall and wind-blown dust on the ice surface. In cold polar regions, annual layering forms instead by seasonal variation of snow metamorphism and wind deposition 1. Glacier ice is blue because the longer visible wavelengths are absorbed.
The more energetic, blue, wavelengths are scattered back 2. The effect is greatest with deep, basal ice, which is bubble free and has large crystals.
The blue colour tends therefore to be most intense in the calls of calved icebergs or fresh fractures. Rough, weathered ice and fresh snow will appear white because preferential absorption does not occur. Cuffey, K. The Physics of Glaciers, 4th edition. Academic Press, Benn, D. Hodder Education, Related Articles What is the global volume of land ice and how is it changing? Save my name,and website in this browser for the next time I comment.
This site uses Akismet to reduce spam. Learn how your comment data is processed. Snow How do we build a glacier? Snow flakes by Wilson Bentley. Bentley was a bachelor farmer whose hobby was photographing snow flakes. From Wikimedia Commons. A scientist collecting snow and ice samples from the wall of a snow pit. Fresh snow can be seen at the surface and en:glacier ice at the bottom of the pit wall.
The snow layers are composed of progressively denser en:firn. Formation of glacier ice. Luis Maria Benitez, Wikimedia Commons. Glacier ice with many bubbles exposed on the ice shelf. It is melting and thinning rapidly. Close up of white bubble-rich ice.
Note the sharp junction between the coarse-clear ice and bubble-rich ice. Iceberg laden with debris from a glacier, Antarctic Peninsula. Glacier ice exposed in an ice-cored moraine. Note the foliation with coarse clear ice and white bubble-rich ice. This iceberg is formed from basal glacier ice.
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