GlaciationA glacier is a natural accumulation of land ice showing movement at some time. Many times during Earth's history, great ice sheets waxed and waned over the surface. What caused these periods of glaciation is still not fully understood and no single reason will probably be found. Causes of glaciationThe onset of a period or stage of glaciation is due to a change in
Earth temperature and circulation. It is generally accepted that a
global decrease of 4o to 5o C, especially during the summer,
and a substantial increase in the amount of snowfall in subarctic and
arctic regions is necessary for the onset of a glacial episode. Several
theories have been proposed for such a change in climate --
reductions in solar radiation due to meteorite collisions with the Earth, increased volcanism, the shifting location of continents, and the
uplift of vast mountain regions.
Milutin Milankovitch
Figure GS.* Snowball Earth
Anatomy of a GlacierWhatever the cause, the main reason glacial advances are initiated is
that winter accumulation exceeds the summer loss of snow
over a long period of time. Snow metamorphoses into glacial ice under
the increasing pressure of accumulated layers of snow. It first changes
to a granular form called firn, and ultimately into
ice. Glacial ice sometimes looks blue because it absorbs all colors
of the visible light spectrum except blue, which it transmits and hence
its blue appearance. Glacier ice may also appear white because some ice
is fractured with pockets of air that indiscriminately scatters the
visible light spectrum.
The mass balance of a glacier determines if it will advance across the surface or not. The mass balance is determined by the amount of gain and loss of ice from the glacier. The mass balance is positive when it accumulates more ice than it loses. A glacier has a negative mass balance if it loses more ice than it gains. Glaciers form in the zone of accumulation, the portion of the glacier over which accumulation exceeds ablation. Ablation is the loss of ice (or snow) from the glacier. Ablation includes sublimation, wind erosion, melting, and evaporation. The zone of accumulation for the large continental ice sheets resides at high latitudes. For mountain glaciers, the zone of accumulation is at a high altitude where temperatures are cold prevent complete summertime melt. The zone of ablation is where loss of ice mass is greater than accumulation. The boundary between these two zones is the firn or equilibrium line. If accumulation exceeds ablation the glacier will grow. If ablation exceeds accumulation, the glacier will retreat by melting in place. Listen to a glacier refreezing (Antarctica 2000). Glacier Movement
Once the ice reaches a thickness of about 20 meters (66 ft) it will
begin to move under the pressure of its weight. Glaciers move across the surface by
internal deformation and
basal slip. Under the weight of accumulating ice, the ice
is deformed and
begins to move by pseudo-plastic flow. Glaciers slip over the surface lubricated by meltwater
at their base. Generally speaking, flow velocity in a glacier is
greatest near the surface of the ice and decreases towards the bottom.
The surface moves faster than the base does due to internal deformation
and basal slipping. The actual forward movement of a particle of ice in
the glacier is about 1,000 feet per year. A typical glacier will move at
about 10 inches a day, though some move quite more rapidly like
Greenland's Jakobshavn glacier. [ A glacial surge occurs as an abrupt movement that can cover tens of meters per day. The exact cause is not well-known, but may result from water pressure building at the base which "floats" the glacier. In 1986 the Hubbard Glacier surged across the mouth of the Russell fjord in Alaska cutting it off from Yakut Bay. Glacier movement exceeded 112 feet per day, compared to a normal rate of 10 inches per day! Types of glaciers
Continental glaciers are vast ice sheets which originate in high latitudes. Here, cold temperatures allow snow to accumulate to great depths, metamorphosing into glacial ice. In the not so distant past, geologically speaking, the great ice sheets waxed and waned, penetrating into the midlatitudes as great lobes of ice. The continent of Antarctic and Greenland are the two major expanses of ice sheets on Earth today.
Alpine glaciers are those that form at high altitudes where the environment is conducive to glacier formation. Pushing outward from their zone of accumulation, alpine glaciers fill mountain valleys and sculpt the surface beneath. Upon retreat some of the most spectacular landscapes on Earth are revealed. Piedmont glaciers form by the merging of alpine glaciers at the base of mountains as they issue out of their valleys.
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