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GLACIERS

Specialty Definition: GLACIERS

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Specialty Definition: Glacier

(From Wikipedia, the free Encyclopedia)

A glacier is a large, long-lasting flow of ice that is formed on land and moves in response to gravity. The glacier fringe is the area where the glacier has recently melted. There are two main types of glaciers: alpine glaciers, which are found in mountain terrains, and continental glaciers, which are associated with ice ages and can cover large areas of continents. Glacier ice is the largest reservoir of fresh water on Earth, and second only to the oceans as the largest reservoir of total water. Glaciers are found on every continent except Australia. This article will concentrate on Alpine glaciers, but most of the concepts also apply to continental glaciers except the scale is quite different.

Austria's longest glacier, the Pasterze, winds its 8 km (5 mile) route at the foot of Austria's highest mountain, the Grossglockner.
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A number of geologic features are associated with glaciers, including moraines that form from glacially transported rocks and debris at the terminus of the glacier; distinctive U-shaped valleyss ; and cirques (cwms) at their heads.

The upper part of a glacier that receives most of the snowfall is called the zone of accumulation. The snowfall here is sufficiently heavy to exert a downward force and cause deep erosion of rock in this area, often leaving a bowl or amphitheater-shaped depression called a cirque. On the opposite end of the glacier, at its foot or terminal end, is the zone of deposition (also called the zone of wastage or the zone of ablation) where upward and lateral forces predominate and deposition of sediment occurs. Between these two zones is the line of equilibrium where the downward erosive forces of the zone of accumulation and the upward deposition forces of the zone of deposition cancel. Erosive lateral forces are not canceled; therefore, glaciers turn v-shaped river-carved valleys into u-shaped glacial valleys.

Glacial moraines are formed from the deposition of material from a glacier and are exposed after the glacier has retreated. These features usually appear as linear mounds of till, which is a poorly-sorted mixture of rock, gravel and boulders that are within a matrix of a fine powdery material. Terminal or end moraines are formed at the foot or terminal end of a glacier, lateral moraines are formed on the sides of the glacier, and medial moraines are formed down the center. Less obvious is the ground moraine which often blankets the surface underneath much of the glacier downslope from the line of equilibrium. Other features formed by glacial deposition include distinctive streamlined hills known as drumlins, and long snake-like ridges formed by streambeds under glaciers, known as eskers. Glacial meltwaters contain rock flour, an extremely fine powder ground from the underlying rock by the glacier's movement.

The Upper Grindelwald Glacier and the Schreckhorn, at Grindelwald, Switzerland.
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So-called "stoss and lee erosional features" are formed by glaciers and show the direction of its movement. Long linear features that follow the direction of movement are called glacial striations and divots in the rock are called chatter marks. These two features are both left on the surfaces of stationary rock that were once under a glacier and were formed when loose rocks and boulders in the ice were transported over the rock surface. Transport of fine-grained material within a glacier can smooth or polish the surface of rocks, leading to what geologists call glacial polish. Glacial erratics are rounded boulders that were left by a melting glacier and are often seen perched precariously on exposed rock faces after glacial retreat.

Some glaciers are quite small, while others may be tens of kilometers in length. Today, they are found on very high mountains in equatorial and mid-latitude regions and progressively lower as one approaches the poles. Greenland and Antarctica are heavily glaciated, to the point of being almost entirely covered by ice. Glaciation of this extent is called continental glaciation.

The downstream end of continental glaciers often flows into the sea. As the ice reaches the ocean, it breaks off, forming icebergs. Glaciers are really rivers of ice that move slowly downhill. Even in very cold climates, there may be unglaciated areas, which receive too little precipitation to form permanent ice.

The snow from which glaciers form is subject to repeated freezing and thawing, permitting the formation of a form of granular ice called nevé. Under the pressure of the layers of ice and snow above it, this granular ice fuses into solid glacial ice. Glacial ice contains minute air bubbles as a result, giving it a distinctive blue tint due to Rayleigh scattering. The lower layers of glacial ice flow and deform plastically under this pressure, allowing the glacier as a whole to move slowly like a viscous fluid. Glaciers do not need a slope to flow, being driven by the continuing accumulation of new snow at their source. The upper layers of glaciers are more brittle, and often form deep cracks known as crevasses as they flex. These crevasses make travel over glaciers a risky proposition. Glacial meltwaters flow throughout and underneath glaciers, carving channels in the ice similar to caves in rock and also helping to lubricate the glacier's movement.

During ice ages, continental glaciers may be as much as 1500 meters thick. A more extreme instance of glacial growth may have occurred during the Snowball Earth period. In the past several centuries the Earth's glaciers have generally been retreating, often dramatically.

See also: West Antarctica

External links

• 2003-08-15 Scientists Rewrite Laws Of Glacial Erosion

  Source: adapted by the editor from Wikipedia, the free encyclopedia under a copyleft GNU Free Documentation License (GFDL) from the article "Glacier."

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Ice age

(From Wikipedia, the free Encyclopedia)

An ice age is a period of long-term downturn in the temperature of Earth's climate, resulting in an expansion of the polar ice caps and mountain glaciers ("glaciation"). Glaciologically, ice age is often used to mean a period of ice sheets in the northern and southern hemispheres; by this definition we are still in an ice age. More colloquially, and of the last 4 Myr, ice age is used to refer to colder periods with extensive ice sheets over the North American and European continents: in this sense, the last ice age ended about 10,000 years ago. This article will use the term 'glacial periods' for colder periods during ice ages and 'interglacial' for the warmer periods.

Major periods of glaciation

There have been four major periods of glaciation in the Earth's past. The first, and possibly most severe, occurred from 800 to 600 million years ago (the late Proterozoic Age). It has been suggested that the end of this cold period was responsible for the subsequent Cambrian Explosion, though this theory is recent and controversial. A minor series of glaciations occurred from 460 to 430 million years ago. There was extensive glaciations from 350 million years before present to 250 million. The present Pleistocene ice age has seen more or less extensive glaciation on 40,000 and 100,000 year cycles. The last glacial period ended about 10,000 years ago.

Interglacials

In between periods of glaciation, there are multi-million year periods of more temperate climate, but also within these abovementioned periods (or at least within the last one), temperate and severe periods occur. The colder periods are called 'glacial periods', the warmer periods 'interglacials'.

We are in an interglacial period now, the last retreat ending about 10,000 years ago. There appears to be a folk wisdon that "the typical interglacial period lasts ~12,000 years" but this is hard to substantiate from the evidence of ice core records. Nonetheless, this led to some fear of a new glacial period starting soon, a global cooling concern. Many now believe that anthropogenic forcing from increased "greenhouse gases" would outweigh any Milankovitch (orbital) forcing; and more recent consideration of the orbital forcing suggests that even in the absence of human perturbation the present interglacial would last at least 50 kyr.

Causes of ice ages

The cause of ice ages remain controversial, but the general consensus is that it is a combination of up to three different factors: atmospheric composition (particularly the fraction of CO₂ and methane), changes in the Earth's orbit around the Sun known as Milankovitch cycles (and possibly the Sun's orbit around the galaxy), and the arrangement of the continents.

The present ice ages are the most studied and best understood, particularly the last 400 kyr, since this is the period covered by ice cores that record atmospheric composition and proxies for temperature and ice volume. Within this period, the match of ice age frequencies to orbital forcing periods is so good that orbital forcing is the generally accepted explanation, even though the strength of the forcing appears to be too small. Feedback from CO₂ may explain this mismatch.

The first of these three factors is probably responsible for much of the change, especially for the first ice age. The "Snowball Earth" hypothesis maintains that the severe freezing in the late Proterozoic was both caused and ended by changes in CO₂ levels in the atmosphere. However, the other two factors do matter.

For reasons that are unclear, an abundance of land in the arctic and antarctic circles appears to be a necessity for an ice age. The Earth's orbit does not have a great effect on the long term causation of ice ages, but does seem to dictate the pattern of multiple freezings and thawings that take place within the current ice age. The complex pattern of changes in Earth orbit and the change of albedo may influence the occurrence of glacial and interglacial phases - this was first explained by the theory of Milutin Milankovic which is supported by the recently discovered details about the last ice age.

While Milankovic forcing predicts that cyclic changes in the Earth's orbital parameters can be expressed in the glaciation record, additional explanations are necessary to explain which cycles are observed to be most important in the timing of glacial periods. In particular, during the last 800 thousand years the dominant glacial oscillation has been 100 thousand, which corresponds to changes in Earth's eccentricity and inclination, and yet is by far the weakest of the three frequencies predicted by Milankovic. During the period 3.0 - 0.8 million years ago the dominant pattern of glaciation corresponded to the 41 thousand year period of changes in Earth's obliquity. The reasons for prefering one frequency to another are poorly understood and an active area of current research, but the answer probably relates to some form of resonance in the Earth's climate system.

Recent glacial and interglacial phases

The last glacial and interglacial phases of the Pleistocene are named, from most recent to most distant, as follows (names before the '/' are North America, names after it Northern European, dates in thousand years BCE - note that in Eastern Europe and the Alps yet other names are used):

• Wisconsinan/Weichsel (or Vistula) (glacial period, 70-15)
• Sangamon/Eem (interglacial, 130-70)
• Illinoian/Saale (glacial, 180-130)
• Yarmouth/Holstein (interglacial, 230-180)
• Kansan/Elster (glacial, 300-230)
• Aftonian/Cromer (interglacial, 330-300)
• Nebraskan/Gunz (glacial, 470-330)
• -/Waalian (interglacial, 540-470)
• -/Donau II (glacial, 550-540)
• -/Tiglian (interglacial, 585-550)
• -/Donau I (glacial, 600-585)

The Wisconsinan glaciation has had a considerable effect on the landscape of the Northern Hemisphere. The Great Lakes were carved out of the Canadian Shield by ice, Long Island was formed from glacial till, and the watersheds of Canada were so severely disrupted that they are still sorting themselves out -- the plethora of lakes in northern Canada can be almost entirely attributed to the action of the ice. Before the theory of ice ages, such catastrophic changes were usually attributed to flood.

The end of the last glaciation also corresponds quite closely to the development of permanent human settlements and agriculture, and it is possible that there is a connection between the two events.

Origin of ice age theory

The theory of ice ages was first published by Louis Agassiz in his book Étude sur les glaciers of 1840 and was based on observations made in Switzerland.

See also Little Ice Age.

Ice Age is also the title of an animated movie; see Ice Age (movie).

Source: adapted by the editor from Wikipedia, the free encyclopedia under a copyleft GNU Free Documentation License (GFDL) from the article "Ice age."

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Commercial Usage: GLACIERS

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Image Slideshow: GLACIERS

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Photo Album: GLACIERS

Thumbnail	Description & Credit	Thumbnail	Description & Credit
	Along Greenland's western coast, a small field of glaciers surrounds Baffin Bay. Credit: NASA.		Alaskan Glaciers. Credit: NASA.
	Extensive wetlands lie near the town of Yellowknife, near the Great Slave Lake in Northwest Territories, Canada. The shallow lakes seen in this image have formed in grooves in the landscape that were carved by glaciers during the last Ice Age. Credit: NASA.
Source: pictures compiled by the editor from various references; see picture credits.

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Digital Photo Gallery: GLACIERS
 

"Lake Tekapo" by Wendy Cain Commentary: "Glaciers melt and the water is turquoise, it was quite windy that day but its normally calm and tranquil."

Source: photographs selected by the editor, with permission from the photographers.

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Non-Fiction Usage: GLACIERS

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Source: compiled by the editor from several corpora; see credits.

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