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A glacier is a large, long-lasting river of ice that is formed on land and moves in response to gravity. A glacier is formed by multi-year ice accretion in mountainous or sloping terrain. 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 can be found on every continent except Australia.

Geologic features associated with glaciers include end, lateral, and medial moraines that form from glacially transported rocks and debris; U-shaped valleys and cirques (cwms) at their heads, and the glacier fringe, which is the area where the glacier has recently melted.

Types of glaciers

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. Most of the concepts in this article apply equally to alpine glaciers and continental glaciers.

Tidewater glaciers are glaciers that flow into 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 km long.

Formation of glaciers

The snow from which glaciers form is subject to repeated freezing and thawing, permitting the formation of a form of granular ice called névé. Under the pressure of the layers of ice and snow above it, this granular ice fuses into firn. Over a period of years, layers of firn undergo further compaction and become 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 dangerous. 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. In the summer, the melted ice from the glacier alone may be enough to create a stream and while the glacier may be a barren waste of dense ice, fertile land is often nearby.

The place where the glacier thins to nothing is called the ice front.

The upper part of a glacier that receives most of the snowfall is called the zone of accumulation. As a rule of thumb the accumulation-zone accounts for c. 60-70% of the glacier surface area. The rest is called the ablation-zone, where melting is larger than the snow turned into ice. The altitude where the two zones meet, i.e. where the ablation- and accumulation zone meet we call the Equilibrium-line altitude. This is sometimes referred to as the snow-line. At this altitude the accumulation equals ablation. Hence, the glacier is in balance at this zone. The snowfall creates a sufficient depth of ice to exert a downward force sufficient to cause deep erosion of rock in this area. This often leaves a bowl or amphitheater-shaped depression called a cirque. On the opposite end of the glacier, at its foot or terminal, 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.

The worldwide shrinking of 70% of glaciers is among the evidence for global warming. Approximately 30% of glaciers are advancing.

In accordance with the ablation-a and accumulation zones the "health" of glaciers are defined by the amount of solid precipitation (accumulation) and melting (ablation). There exists several non-linear relationships that defined the relation between accumulation and ablation.

Even in very cold climates, there may be unglaciated areas, which receive too little precipitation to form permanent ice. This was the case in most of Siberia, central and northern Alaska and all of Manchuria during glacial periods of the Quaternary. 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.

Classification of glaciers

Glaciers are classified according to size and their relationship with local topography. The smallest ones in mountain valleys are referred to as alpine or valley glaciers. On the other hand, some larger ice layers can cover an entire mountain, mountain chain or even a volcano; this type is known as an ice cap. Ice caps feed outlet glaciers, tongues of ice that extend into valleys belows far from the margins of those larger ice masses. In general, outlet glaciers are valley glaciers, which are formed by the movement of ice from a polar ice cap, or an ice cap from mountainous regions, to the sea.

The largest glaciers are continental ice sheets, enormous masses of ice that are not affected by the landscape and extend over the entire surface, except on the 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 were to melt, it would cause sea levels to rise some six meters all around the world, while if the Antarctic ice sheet melted, sea levels would rise up to 65 meters.

Other glaciers of smaller size include plateau glaciers. They resemble ice sheets, but on a smaller scale. They cover some plateaus and high-altitude areas. This type of glacier appears in many places, especially in Iceland and some of the large islands in the Arctic Ocean.

Piedmont glaciers occupy broad lowlands at the base of steep mountains, and form when one or more alpine glaciers surge from the confining walls of mountain valleys. The size of piedmont glaciers varies greatly: among the largest is the Malaspina glacier, which extends along the length of the southern coast of Alaska. It covers more than 5,000 km² of the coastal plain at the foot of the Saint Elias range. The highest alpine glacier in the world is the Siachen Glacier which interestingly is also a conflict zone between India and Pakistan.

Glacial motion

Ice behaves like an easily breaking solid until its thickness exceeds 50 meters (about 164 ft). Below that depth the increased pressure causes ice to behave like a plastic material, which allows it to flow. The glacial ice is made up of layers of molecules stacked on top of each other. The bonds between layers are weaker than the those within the layer. When stress exceeds the inter-layer binding strength the layers start to slide.

Another type of movement is basal gliding. This is produced when the whole glacier moves over the terrain on which it sits. In this process, thawed ice helps movement by lubricating the process. Melting can come from the decrease of the melting point as the pressure increases towards the base of the glacier. Two other sources of water from melting can be friction between ice and rock, which raises the temperature, and geothermal heat from the Earth's interior.

The movement of glaciers is not uniform since they are subject to friction and gravity. Friction causes the lower section of the glacier to move more slowly than the top.

In contrast to the lower section, the top 50 meters of the glacier is not subject to friction and is more rigid. This section is known as the fracture zone.

Ice on the fracture zone moves on top of the lower section. When the glacier moves through irregular terrain cracks form in the fracture zone. These cracks can be up to 50 meters deep, at which point they meet the plastic flow which seals them.

Speed of glacial movement

The speed of glacial displacement is partly determined by friction. Friction makes bottom ice move slower than the upper portion. In alpine glaciers, this also applies for the friction generated at valleys side walls, which slows the edges relative to the center. This has been confirmed by experiments in the 19th century, in which aligned stakes in alpine glaciers were used to analyze its movement, which proved that central regions had moved greater distances.

Mean speeds vary; some have speeds so slow that trees can establish themselves among the deposited scourings. In other cases they can move as fast as many meters per day, as is the case of Byrd glacier, an overflowing glacier in the Antarctica which moves 750-800 meters per year (some 2 meters (6 ft) per day), according to studies using satellites.

Many glaciers have periods of very rapid advancement called surges. These glaciers exhibit normal movement until suddenly they accelerate, then return to their previous state. During these surges, the glacier may reach velocities up to 1000 times greater than normal.

Moraines

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, also called glacial drift, which often blankets the surface underneath much of the glacier downslope from the line of equilibrium. Glacial meltwaters contain rock flour, an extremely fine powder ground from the underlying rock by the glacier's movement. Other features formed by glacial deposition include long snake-like ridges formed by streambeds under glaciers, known as eskers, and distinctive streamlined hills known as drumlins.

So-called "stoss and lee erosional features" are formed by glaciers and show the direction of its movement. Long linear rock scratches (that follow the glaciers's 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 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.

The most common name for glacial sediment is moraine. The term is of French origin, and it was coined by peasants to describe alluvial embankments and rims found near the margins of glaciers in the French Alps. Currently, the term is used more broadly, and is applied to a series of formations, all of which are composed of till.

Drumlins

Drumlins are asymmetrical hills with aerodynamic profiles made mainly of till. Their heights vary from 15 to 50 meters and can reach 1 km of length. The tilted side of the hill looks toward the direction from which the ice advanced, while the longer slope follows the ice's direction of movement.

Drumlins are not found isolated, on the contrary, they are found in groups called drumlin fields or drumlin camps. An example of these fields is found east of Rochester, New York, and it is estimated that it contains about 10,000 drumlins.

Although the process that forms drumlins is not absolutely understood, it can be inferred from their shape that they are products of the plastic deformation zone of ancient glaciers. It is believed that many drumlins were formed when glaciers advanced over and altered the deposits of earlier glaciers. heeeeyyy.

Glacial erosion

Rocks and sediments are added to glaciers through various processes. Glaciers erode the terrain principally through two methods: abrasion and plucking.

As the glacier flows over the bedrock's fractured surface, it softens and lifts blocks of rock that are brought into the ice. This process is known as plucking, and it is produced when fusion water penetrates the gaps and the diaclase separates them from the bedrock and glacier's bottom and freezes. When the water expands, it acts as a lever that loosens the rock by lifting it. This way, sediments of all sizes become part of the glacier's load.

Abrasion occurs when the ice and the load of rock fragments slide over the bedrock and function as sandpaper that smoothens and polishes the surface situated below. This pulverized rock is called rock flour. This flour is formed by rock grains of a size between 0.002 to 0.00625 mm. Sometimes the amount of rock flour produced is so high that currents of fusion waters acquire a grayish color.

Another of the visible characteristics of glacial erosion are glacial estriations. These are produced when the bottom's ice contains large chunks of rock that mark trenches in the bedrock. By mapping the direction of the flutes the direction of the glaciar's movement can be determined.

The velocity of a glaciar's erosion is variable. The diferential erosion undertaken by the ice is controlled by four important factors:

• Velocity of glacial movement
• Thickness of the ice
• Shape, abundance and hardness of rock fragments contained in the ice at the bottom of the glacier
• Relative ease of erosion of the surface under the glacier.

Material which becomes incorporated in a glacier may be carried as far as the zone of ablation before being deposited. Glacial deposits are of two distinct types:

• glacial till: Material directly deposited from glacial ice. Mixture of undifferentiated material ranging from clay to boulders, the usual composition of a moraine.
• Fluvial and outwash: Sediments deposited by water. Stratified through various processes, such as boulders being separated from finer particles.

The larger pieces of rock which are encrusted in till or deposited on the surface are called 'glacial erratics. They may range in size from pebbles to boulders, but as they may be moved great distances they may be of drastically different type than the material upon which they are found. Patterns of glacial erratics provide clues of past glacial motions.

Glacial valleys

Before glaciation, mountain valleys have a characteristic "V" shape, produced by vertical water erosion. However, during glaciation, these valleys widen and deepen, which creates a "U"-shaped glacial valley. Besides the deepening and widening of the valley, the glacier also smooths the valley due to erosion. This way, it eliminates the spurs of earth that extend across the valley. As a result of this interaction, triangular cliffs called truncated spurs are formed.

Many glaciers deepen their valleys more than their smaller tributaries. Therefore, when the glaciers stop receding, the valleys of the tributary glaciers remain above the main glacier's depression, and these are called hanging valleys.

In parts of the soil that were affected by abrasion and plucking, the depressions left can be filled by Pater Noster lakes, from the Latin for "Our Father", referring to a station of the rosary.

At the head of a glacier is a very important structure called the corrie, which has a bowl shape with escarped walls on three sides, but open on the side that descends into the valley. In the corrie an accumulation of ice is formed. These begin as irregularities on the side of the mountain which are later augmented in size by the coining of the ice. After the glacier melts, these corries are usually occupied by small mountain lakes called tarns.

At times when there are two glaciers separated by a diving ridge. This, located between the corries, is eroded to create a gorge. This structure is called a pass.

Glaciers are also responsible for the creation of fjords (deep coves or inlets) and escarpments that are found at high latitudes. With depths that can exceed 1,000 metres caused by the postglacial elevation of sea level and therefore, as it changed the glaciers changed their level of erosion.

Arêtes and horns

Besides the features that glaciers create in mountainous terrain, it is also common to find sinuous crests with sharp edges called arêtes, and pointed pyramidal peaks called horns.

Both features may have the same process behind their formation: the enlargement of cirques from glacial plucking and the action of the ice. Horns are formed by cirques that encircle a single mountain.

Arêtes emerge in a similar manner; the only difference is that the cirques are not located in a circle, but rather on opposite sides along a divide. Arêtes can also be produced by the collision of two parallel glaciers. In this case, the glacial tongues cut the divides down to size through erosion, and polish the adjacent valleys.

Sheepback rock

Some rock formations in the path of a glacier are sculpted into small hills with a shape known as roche moutonnée or sheepback. An elongated, rounded, asymmetrical, bedrock knob produced can be produced by glacier erosion. It has a gentle slope on its up-glacier side and a steep to vertical face on the down-glacier side. The glacier abrades the smooth slope which it flows along, while rock is torn loose from the downstream side and carried away in ice. Rock on this side is fractured by combinations of forces due to water, ice in rock cracks, and structural stresses.

Alluvial stratification

The water that rises from the zone of ablation moves away from the glacier and carries with is fine eroded sediments. As the water's speed decreases, so does its capacity to carry objects in suspension. The water then gradually deposits the sediment as it runs, creating an alluvial plain. When this phenomenon occurs in a valley, it is called a valley train.

Alluvial plains and valley trains are usually accompanied by basins known as kettles. Glaciar depressions are also produced in till deposits. These depressions are formed when large ice blocks get stuck in the glacial alluvium and after melting they leave holes in the sediment.

Generally, the diameter of these depressions does not exceed 2 km, except in Minnesota, where some depressions reach up to 50 km in diameter, with depths varying between 10 and 50 meters.

Deposits in contact with ice

When a glacier reduces in size to a critical point, its flow stops and the ice gets stuck. Meanwhile, fusion waters flow that flow over, inside, and below the ice leave stratified alluvial deposits. Because of this, as the ice melts, it leaves stratified deposits in the form of columns, terraces and clusters. These type of deposits are known as deposits in contact with ice.

When those deposits take the form of columns of tipped sides or mounds, which are called kames. Some kames form when fusion water deposits sediments through openings in the interior of the ice. In other cases, they are just the result of fans or deltas towards the exterior of the ice produced by fusion water.

When the glacial ice ocuppies a valley it can form terraces of kame along the sides of the valley.

A third type of deposit in contact with the ice is characterized by long, narrow sinuous crests composed fundamentally of sand and gravel. some of these crests have heights exceeding 100 meters and their lengths surpass 100 km. These are eskers, crests deposited by the fusion water rivers that flow over, inside and beneath a stuck glacial ice mass.

A quadruple division of the Quaternary glacial period has been established for North America and Europe. These divisions are based principally on the study of glacial deposits. In North America, each of these four stages was named for the state in which the deposits of these stages were well exposed. In order of appearance, the are: Nebraskan, Kansan, Illinoian, and Wisconsian. This classification was refined thanks to the detailed study of the sediments of the ocean floor. Because the sediments of the ocean floor, in contrast to that of the Earth's surface, are unaffected by statigraphical discontinuities, they are useful to determinine the climatic cycles of the planet.

In this matter, geologists have come to identify over twenty divisions, each of them lasting approximately 100,000 years. All these cycles fall within what is known as the Quaternary glacial period.

During its peak, the ice left its mark over almost 30% of Earth's surface, covering approximately 10 million km in North America, 5 million km in Europe and 4 million km² in Siberia. The glacial ice in the Northern hemisphere was double the amount of that found in the Southern hemisphere. This is because in the South Pole the ice cannot advance beyond the Antarctic landmass. It is now believed that the glacial period began between two and three million years ago, in what is known as the Pleistocene era.

The reason behind this rise of the crust is an isostatic adjustment, a theory that sustains that when a mass, such as a glacier, bends Earth's crust, which sinks because of the pressure; after the glacier melts, the crust begins to rise to its original position, that is to say, at its level of equilibrium.

Causes of ice ages

In spite of the knowledge acquired during the last few years, little is known about the causes of glaciations.

Generalized glaciations have been rare in the history of Earth. However, the Ice Age of the Pleistocene was not the only glaciative event, since tillite deposits have been identified. Tillite is a sedimentary rock formed when glacial till is lithified.

These deposits found in strata of differing age present similar characteristics as fragments of fluted rock, and some are superposed over bedrock surfaces of channeled and polished rock or associated with sandstone and conglomerates that have features of alluvial plain deposits.

Two Precambrian glacial episodes have been identified, the first approximately 2 billion years ago, and the second about 600 milion years. Also, in rocks of the late Paleozoic (of 250 million years of age) it was found a well-documented register of a previous glacial period.

Although there are several scientific hypotheses about the determining factors of glaciations, the two most important ideas are plate tectonics and variations in Earth's orbit.

Plate tectonics

Because glaciers can form only on dry land, plate tectonics suggest that the evidence of previous glaciations is currently present in tropical latitudes due to the drift of tectonic plates from tropical latitudes to circumpolar regions. Evidence of glacial structures in South America, Africa, Australia, and India support this idea, because it is known that they experienced a glacial period near the end of the Paleozoic Era, some 250 million years ago.

The idea that the evidence of middle-latitude glaciations is closely related to the displacement of tectonic plates and was confirmed by the absence of glacial traces in the same period for the higher latitudes of North America and Eurasia, which indicates, as obvious, that their locations were very different than today.

Climatic changes are also related to the positions of the continents, which has made them vary in conjunction with the displacement of plates. That also affected ocean current patterns which caused changes in heat transmission and humidity. Since continents drift very slowly (about 2 cm per year), similar changes occur in periods of millions of years.

A study of marine sediment that contained certain climatically sensitive microorganisms until about half a million years ago were compared with studies of the geometry of Earth's orbit, and the result was forceful: climatic changes are closely related to periods of obliquity, precession, and eccentricity of the Earth's orbit.

In general, with the collected data it can be affirmed that plate tectonics is only applicable to very long periods of time, while Milankovitch's proposal, backed up by the work of others, adjusts to the periodic alterations of glacial periods of the Pleistocene. It must be taken into account that these proposals are subject to criticism, and it is still not known with certainty if there are other factors involved.

See also

• Perito Moreno Glacier
• Glacial motion
• List of glaciers
• Icefall
• Ice sheet
• Quaternary period

References

• This article draws heavily on the corresponding article in the Spanish-language Misplaced Pages, which was accessed in the version of July 24, 2005.
• Michael Hambrey and Jürg Alean, Glaciers, 2nd ed. (Cambridge University Press, 2004, ISBN 0-521-82808-2) An excellent less-technical treatment of all aspects, with superb photographs and firsthand accounts of glaciologists' experiences.
• Douglas I. Benn and David J. A. Evans, Glaciers and Glaciation (Arnold, 1999)
• M. R. Bennett and N. F. Glasser, Glacial Geology: Ice Sheets and Landforms (John Wiley & Sons, 1996)
• Michael Hambrey, Glacial Environments (University of British Columbia Press, UCL Press, 1994) An undergraduate-level textbook.
• Robert Walley, Introduction to Physical Geography (Wm. C. Brown Publishers, 1992) A textbook devoted to explaining the geography of our planet.
• W. S. B. Paterson, Physics of Glaciers, 3rd ed. (Pergamon Press, 1994) A comprehensive reference on the physical principles underlying formation and behavior.

External links

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• National Snow and Ice Data Center - Glaciers
• USGS Glacier Studies Project
• Glaciers and Glacial Hazards - USGS
• 2003-08-15 Scientists Rewrite Laws Of Glacial Erosion
• Fjords National Park, Alaska
• NOVA scienceNOW - A 7 minute video of the NOVA broadcast that aired on PBS, July 26, 2005. Hosted by Robert Krulwich, the video is about the world's fastest glacier and why it is moving way too fast.

• Glaciology
• Forms of water
• Glaciers