Glacier

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Glacier of the Geikie Plateau in Greenland. Geikie Plateau Glacier.JPG
Glacier of the Geikie Plateau in Greenland.
The Taschachferner in the Otztal Alps in Austria. The mountain to the left is the Wildspitze (3.768 m), second highest in Austria. Wildspitze seen from Hinterer Brunnkogel, with visible ascent track of ski mountaineer.jpg
The Taschachferner in the Ötztal Alps in Austria. The mountain to the left is the Wildspitze (3.768 m), second highest in Austria.
With 7,253 known glaciers, Pakistan contains more glacial ice than any other country on earth outside the polar regions. At 62 kilometres (39 mi) in length, the pictured Baltoro Glacier is one of the world's longest alpine glaciers. Baltoro glacier from air.jpg
With 7,253 known glaciers, Pakistan contains more glacial ice than any other country on earth outside the polar regions. At 62 kilometres (39 mi) in length, the pictured Baltoro Glacier is one of the world's longest alpine glaciers.

A glacier ( US: /ˈɡlʃər/ ; UK: /ˈɡlæsiər,ˈɡlsiər/ ) is a persistent body of dense ice that is constantly moving under its own weight. A glacier forms where the accumulation of snow exceeds its ablation over many years, often centuries. It acquires distinguishing features, such as crevasses and seracs, as it slowly flows and deforms under stresses induced by its weight. As it moves, it abrades rock and debris from its substrate to create landforms such as cirques, moraines, or fjords. Although a glacier may flow into a body of water, it forms only on land and is distinct from the much thinner sea ice and lake ice that form on the surface of bodies of water.

Contents

On Earth, 99% of glacial ice is contained within vast ice sheets (also known as "continental glaciers") in the polar regions, but glaciers may be found in mountain ranges on every continent other than the Australian mainland, including Oceania's high-latitude oceanic island countries such as New Zealand. Between latitudes 35°N and 35°S, glaciers occur only in the Himalayas, Andes, and a few high mountains in East Africa, Mexico, New Guinea and on Zard-Kuh in Iran. [2] With more than 7,000 known glaciers, Pakistan has more glacial ice than any other country outside the polar regions. [3] [1] Glaciers cover about 10% of Earth's land surface. Continental glaciers cover nearly 13 million km2 (5 million sq mi) or about 98% of Antarctica's 13.2 million km2 (5.1 million sq mi), with an average thickness of ice 2,100 m (7,000 ft). Greenland and Patagonia also have huge expanses of continental glaciers. [4] The volume of glaciers, not including the ice sheets of Antarctica and Greenland, has been estimated at 170,000 km3. [5]

Glacial ice is the largest reservoir of fresh water on Earth, holding with ice sheets about 69 percent of the world's freshwater. [6] [7] Many glaciers from temperate, alpine and seasonal polar climates store water as ice during the colder seasons and release it later in the form of meltwater as warmer summer temperatures cause the glacier to melt, creating a water source that is especially important for plants, animals and human uses when other sources may be scant. However, within high-altitude and Antarctic environments, the seasonal temperature difference is often not sufficient to release meltwater.

Since glacial mass is affected by long-term climatic changes, e.g., precipitation, mean temperature, and cloud cover, glacial mass changes are considered among the most sensitive indicators of climate change and are a major source of variations in sea level.

A large piece of compressed ice, or a glacier, appears blue, as large quantities of water appear blue, because water molecules absorb other colors more efficiently than blue. The other reason for the blue color of glaciers is the lack of air bubbles. Air bubbles, which give a white color to ice, are squeezed out by pressure increasing the created ice's density.

The word glacier is a loanword from French and goes back, via Franco-Provençal, to the Vulgar Latin glaciārium, derived from the Late Latin glacia, and ultimately Latin glaciēs, meaning "ice". [8] The processes and features caused by or related to glaciers are referred to as glacial. The process of glacier establishment, growth and flow is called glaciation. The corresponding area of study is called glaciology. Glaciers are important components of the global cryosphere.

Types

Classification by size, shape and behavior

The Quelccaya Ice Cap in Peru is the second-largest glaciated area in the tropics Quelccaya Glacier.jpg
The Quelccaya Ice Cap in Peru is the second-largest glaciated area in the tropics

Glaciers are categorized by their morphology, thermal characteristics, and behavior. Alpine glaciers form on the crests and slopes of mountains. A glacier that fills a valley is called a valley glacier, or alternatively, an alpine glacier or mountain glacier. [9] A large body of glacial ice astride a mountain, mountain range, or volcano is termed an ice cap or ice field . [10] Ice caps have an area less than 50,000 km2 (19,000 sq mi) by definition.

Glacial bodies larger than 50,000 km2 (19,000 sq mi) are called ice sheets or continental glaciers. [11] Several kilometers deep, they obscure the underlying topography. Only nunataks protrude from their surfaces. The only extant ice sheets are the two that cover most of Antarctica and Greenland. [12] They contain vast quantities of freshwater, enough that if both melted, global sea levels would rise by over 70 m (230 ft). [13] Portions of an ice sheet or cap that extend into water are called ice shelves; they tend to be thin with limited slopes and reduced velocities. [14] Narrow, fast-moving sections of an ice sheet are called ice streams . [15] [16] In Antarctica, many ice streams drain into large ice shelves. Some drain directly into the sea, often with an ice tongue, like Mertz Glacier.

Tidewater glaciers are glaciers that terminate in the sea, including most glaciers flowing from Greenland, Antarctica, Baffin, Devon, and Ellesmere Islands in Canada, Southeast Alaska, and the Northern and Southern Patagonian Ice Fields. 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 impact as the iceberg strikes the water. Tidewater glaciers undergo centuries-long cycles of advance and retreat that are much less affected by climate change than other glaciers. [17]

Classification by thermal state

Webber Glacier on Grant Land is an advancing polar glacier Ellesmere Island 06.jpg
Webber Glacier on Grant Land is an advancing polar glacier

Thermally, a temperate glacier is at a melting point throughout the year, from its surface to its base. The ice of a polar glacier is always below the freezing threshold from the surface to its base, although the surface snowpack may experience seasonal melting. A subpolar glacier includes both temperate and polar ice, depending on the depth beneath the surface and position along the length of the glacier. In a similar way, the thermal regime of a glacier is often described by its basal temperature. A cold-based glacier is below freezing at the ice-ground interface and is thus frozen to the underlying substrate. A warm-based glacier is above or at freezing at the interface and is able to slide at this contact. [18] This contrast is thought to a large extent to govern the ability of a glacier to effectively erode its bed, as sliding ice promotes plucking at rock from the surface below. [19] Glaciers which are partly cold-based and partly warm-based are known as polythermal. [18]

Formation

A glacier cave located on the Perito Moreno Glacier in Argentina 153 - Glacier Perito Moreno - Grotte glaciaire - Janvier 2010.jpg
A glacier cave located on the Perito Moreno Glacier in Argentina

Glaciers form where the accumulation of snow and ice exceeds ablation. A glacier usually originates from a cirque landform (alternatively known as a corrie or as a cwm) – a typically armchair-shaped geological feature (such as a depression between mountains enclosed by arêtes) – which collects and compresses through gravity the snow that falls into it. This snow accumulates and the weight of the snow falling above compacts it, forming névé (granular snow). Further crushing of the individual snowflakes and squeezing the air from the snow turns it into "glacial ice". This glacial ice will fill the cirque until it "overflows" through a geological weakness or vacancy, such as a gap between two mountains. When the mass of snow and ice reaches sufficient thickness, it begins to move by a combination of surface slope, gravity, and pressure. On steeper slopes, this can occur with as little as 15 m (49 ft) of snow-ice.

In temperate glaciers, snow repeatedly freezes and thaws, changing into granular ice called firn. Under the pressure of the layers of ice and snow above it, this granular ice fuses into denser firn. Over a period of years, layers of firn undergo further compaction and become glacial ice. [20] Glacier ice is slightly more dense than ice formed from frozen water because glacier ice contains fewer trapped air bubbles.

Glacial ice has a distinctive blue tint because it absorbs some red light due to an overtone of the infrared OH stretching mode of the water molecule. (Liquid water appears blue for the same reason. The blue of glacier ice is sometimes misattributed to Rayleigh scattering of bubbles in the ice.) [21]

Structure

The overhanging icefront of the advancing Webber Glacier with waterfalls (Borup Fiord area, Northern Ellesmere Island) on July 20, 1978. Debris rich layers have been sheared and folded into the basal cold glacier ice. The glacier front is 6 km broad and up to 40 m high Ellesmere Island 05.jpg
The overhanging icefront of the advancing Webber Glacier with waterfalls (Borup Fiord area, Northern Ellesmere Island) on July 20, 1978. Debris rich layers have been sheared and folded into the basal cold glacier ice. The glacier front is 6 km broad and up to 40 m high

A glacier originates at a location called its glacier head and terminates at its glacier foot, snout, or terminus.

Glaciers are broken into zones based on surface snowpack and melt conditions. [22] The ablation zone is the region where there is a net loss in glacier mass. The upper part of a glacier, where accumulation exceeds ablation, is called the accumulation zone. The equilibrium line separates the ablation zone and the accumulation zone; it is the contour where the amount of new snow gained by accumulation is equal to the amount of ice lost through ablation. In general, the accumulation zone accounts for 60–70% of the glacier's surface area, more if the glacier calves icebergs. Ice in the accumulation zone is deep enough to exert a downward force that erodes underlying rock. After a glacier melts, it often leaves behind a bowl- or amphitheater-shaped depression that ranges in size from large basins like the Great Lakes to smaller mountain depressions known as cirques.

The accumulation zone can be subdivided based on its melt conditions.

  1. The dry snow zone is a region where no melt occurs, even in the summer, and the snowpack remains dry.
  2. 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 the melting point.
  3. 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.
  4. 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 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 is snow-covered at the end of the melt season, and they have a terminus with a vigorous flow.

Following the Little Ice Age's end around 1850, glaciers around the Earth have retreated substantially. A slight cooling led to the advance of many alpine glaciers between 1950 and 1985, but since 1985 glacier retreat and mass loss has become larger and increasingly ubiquitous. [23] [24] [25]

Motion

The stress-strain relationship of plastic flow (teal section): a small increase in stress creates an exponentially greater increase in strain, which equates to deformation speed. Stress-strain1.svg
The stress–strain relationship of plastic flow (teal section): a small increase in stress creates an exponentially greater increase in strain, which equates to deformation speed.

Glaciers move downhill by the force of gravity and the internal deformation of ice. [26] At the molecular level, ice consists of stacked layers of molecules with relatively weak bonds between layers. When the amount of strain (deformation) is proportional to the stress being applied, ice will act as an elastic solid. Ice needs to be at least 30 m (98 ft) thick to even start flowing, but once its thickness exceeds about 50 m (160 ft) (160 ft), stress on the layer above will exceeds the inter-layer binding strength, and then it'll move faster than the layer below. [27] This means that small amounts of stress can result in a large amount of strain, causing the deformation to become a plastic flow rather than elastic. Then, the glacier will begin to deform under its own weight and flow across the landscape. According to the Glen–Nye flow law, the relationship between stress and strain, and thus the rate of internal flow, can be modeled as follows: [28] [26]

where:

= shear strain (flow) rate
= stress
= a constant between 2–4 (typically 3 for most glaciers) that increases with lower temperature
= a temperature-dependent constant
Differential erosion enhances relief, as clear in this incredibly steep-sided Norwegian fjord. Geirangerfjord (6-2007).jpg
Differential erosion enhances relief, as clear in this incredibly steep-sided Norwegian fjord.

The lowest velocities are near the base of the glacier and along valley sides where friction acts against flow, causing the most deformation. Velocity increases inward toward the center line and upward, as the amount of deformation decreases. The highest flow velocities are found at the surface, representing the sum of the velocities of all the layers below. [28] [26]

Because ice can flow faster where it is thicker, the rate of glacier-induced erosion is directly proportional to the thickness of overlying ice. Consequently, pre-glacial low hollows will be deepened and pre-existing topography will be amplified by glacial action, while nunataks, which protrude above ice sheets, barely erode at all – erosion has been estimated as 5 m per 1.2 million years. [29] This explains, for example, the deep profile of fjords, which can reach a kilometer in depth as ice is topographically steered into them. The extension of fjords inland increases the rate of ice sheet thinning since they are the principal conduits for draining ice sheets. It also makes the ice sheets more sensitive to changes in climate and the ocean. [29]

Although evidence in favor of glacial flow was known by the early 19th century, other theories of glacial motion were advanced, such as the idea that meltwater, refreezing inside glaciers, caused the glacier to dilate and extend its length. As it became clear that glaciers behaved to some degree as if the ice were a viscous fluid, it was argued that "regelation", or the melting and refreezing of ice at a temperature lowered by the pressure on the ice inside the glacier, was what allowed the ice to deform and flow. James Forbes came up with the essentially correct explanation in the 1840s, although it was several decades before it was fully accepted. [30]

Fracture zone and cracks

Ice cracks in the Titlis Glacier TitlisIceCracksDeep.jpg
Ice cracks in the Titlis Glacier

The top 50 m (160 ft) of a glacier are rigid because they are under low pressure. This upper section is known as the fracture zone and moves mostly as a single unit over the plastic-flowing lower section. When a glacier moves through irregular terrain, cracks called crevasses develop in the fracture zone. Crevasses form because of differences in glacier velocity. If two rigid sections of a glacier move at different speeds or directions, shear forces cause them to break apart, opening a crevasse. Crevasses are seldom more than 46 m (150 ft) deep but, in some cases, can be at least 300 m (1,000 ft) deep. Beneath this point, the plasticity of the ice prevents the formation of cracks. Intersecting crevasses can create isolated peaks in the ice, called seracs.

Shear or herring-bone crevasses on Emmons Glacier (Mount Rainier); such crevasses often form near the edge of a glacier where interactions with underlying or marginal rock impede flow. In this case, the impediment appears to be some distance from the near margin of the glacier. Chevron Crevasses 00.JPG
Shear or herring-bone crevasses on Emmons Glacier (Mount Rainier); such crevasses often form near the edge of a glacier where interactions with underlying or marginal rock impede flow. In this case, the impediment appears to be some distance from the near margin of the glacier.

Crevasses can form in several different ways. Transverse crevasses are transverse to flow and form where steeper slopes cause a glacier to accelerate. Longitudinal crevasses form semi-parallel to flow where a glacier expands laterally. Marginal crevasses form near the edge of the glacier, caused by the reduction in speed caused by friction of the valley walls. Marginal crevasses are largely transverse to flow. Moving glacier ice can sometimes separate from the stagnant ice above, forming a bergschrund. Bergschrunds resemble crevasses but are singular features at a glacier's margins. Crevasses make travel over glaciers hazardous, especially when they are hidden by fragile snow bridges.

Below the equilibrium line, glacial meltwater is concentrated in stream channels. Meltwater can pool in proglacial lakes on top of a glacier or descend into the depths of a glacier via moulins. Streams within or beneath a glacier flow in englacial or sub-glacial tunnels. These tunnels sometimes reemerge at the glacier's surface. [31]

Subglacial processes

Erosion rates of subglacial sediment caused by the motion of different glaciers across the world Davies 2018 glacier sediment erosion rates.png
Erosion rates of subglacial sediment caused by the motion of different glaciers across the world

Most of the important processes controlling glacial motion occur in the ice-bed contact—even though it is only a few meters thick. [33] The bed's temperature, roughness and softness define basal shear stress, which in turn defines whether movement of the glacier will be accommodated by motion in the sediments, or if it'll be able to slide. A soft bed, with high porosity and low pore fluid pressure, allows the glacier to move by sediment sliding: the base of the glacier may even remain frozen to the bed, where the underlying sediment slips underneath it like a tube of toothpaste. A hard bed cannot deform in this way; therefore the only way for hard-based glaciers to move is by basal sliding, where meltwater forms between the ice and the bed itself. [34] Whether a bed is hard or soft depends on the porosity and pore pressure; higher porosity decreases the sediment strength (thus increases the shear stress τB). [33]

Porosity may vary through a range of methods.

Bed softness may vary in space or time, and changes dramatically from glacier to glacier. An important factor is the underlying geology; glacial speeds tend to differ more when they change bedrock than when the gradient changes. [34] Further, bed roughness can also act to slow glacial motion. The roughness of the bed is a measure of how many boulders and obstacles protrude into the overlying ice. Ice flows around these obstacles by melting under the high pressure on their stoss side; the resultant meltwater is then forced into the cavity arising in their lee side, where it re-freezes. [33]

As well as affecting the sediment stress, fluid pressure (pw) can affect the friction between the glacier and the bed. High fluid pressure provides a buoyancy force upwards on the glacier, reducing the friction at its base. The fluid pressure is compared to the ice overburden pressure, pi, given by ρgh. Under fast-flowing ice streams, these two pressures will be approximately equal, with an effective pressure (pi – pw) of 30 kPa; i.e. all of the weight of the ice is supported by the underlying water, and the glacier is afloat. [33]

Basal melting and sliding

A cross-section through a glacier. The base of the glacier is more transparent as a result of melting. Glacier cross-section.jpg
A cross-section through a glacier. The base of the glacier is more transparent as a result of melting.

Glaciers may also move by basal sliding, where the base of the glacier is lubricated by the presence of liquid water, reducing basal shear stress and allowing the glacier to slide over the terrain on which it sits. Meltwater may be produced by pressure-induced melting, friction or geothermal heat. The more variable the amount of melting at surface of the glacier, the faster the ice will flow. Basal sliding is dominant in temperate or warm-based glaciers. [35]

τD = ρgh sin α
where τD is the driving stress, and α the ice surface slope in radians. [33]
τB is the basal shear stress, a function of bed temperature and softness. [33]
τF, the shear stress, is the lower of τB and τD. It controls the rate of plastic flow.

The presence of basal meltwater depends on both bed temperature and other factors. For instance, the melting point of water decreases under pressure, meaning that water melts at a lower temperature under thicker glaciers. [33] This acts as a "double whammy", because thicker glaciers have a lower heat conductance, meaning that the basal temperature is also likely to be higher. [34] Bed temperature tends to vary in a cyclic fashion. A cool bed has a high strength, reducing the speed of the glacier. This increases the rate of accumulation, since newly fallen snow is not transported away. Consequently, the glacier thickens, with three consequences: firstly, the bed is better insulated, allowing greater retention of geothermal heat. [33]

Secondly, the increased pressure can facilitate melting. Most importantly, τD is increased. These factors will combine to accelerate the glacier. As friction increases with the square of velocity, faster motion will greatly increase frictional heating, with ensuing melting – which causes a positive feedback, increasing ice speed to a faster flow rate still: west Antarctic glaciers are known to reach velocities of up to a kilometre per year. [33] Eventually, the ice will be surging fast enough that it begins to thin, as accumulation cannot keep up with the transport. This thinning will increase the conductive heat loss, slowing the glacier and causing freezing. This freezing will slow the glacier further, often until it is stationary, whence the cycle can begin again. [34]

Location and diagram of Lake Vostok, a prominent subglacial lake beneath the East Antarctic Ice Sheet. Lake Vostok drill 2011.jpg
Location and diagram of Lake Vostok, a prominent subglacial lake beneath the East Antarctic Ice Sheet.

The flow of water under the glacial surface can have a large effect on the motion of the glacier itself. Subglacial lakes contain significant amounts of water, which can move fast: cubic kilometres can be transported between lakes over the course of a couple of years. [36] This motion is thought to occur in two main modes: pipe flow involves liquid water moving through pipe-like conduits, like a sub-glacial river; sheet flow involves motion of water in a thin layer. A switch between the two flow conditions may be associated with surging behaviour. Indeed, the loss of sub-glacial water supply has been linked with the shut-down of ice movement in the Kamb ice stream. [36] The subglacial motion of water is expressed in the surface topography of ice sheets, which slump down into vacated subglacial lakes. [36]

Speed

The formation of supraglacial lakes at Baltoro Glacier in April 2018 (top) had substantially accelerated its melting and motion in the following summer months (bottom) Wendleder 2024 Baltoro supraglacial acceleration.jpg
The formation of supraglacial lakes at Baltoro Glacier in April 2018 (top) had substantially accelerated its melting and motion in the following summer months (bottom)

The speed of glacial displacement is partly determined by friction. Friction makes the ice at the bottom of the glacier move more slowly than ice at the top. In alpine glaciers, friction is also generated at the valley's sidewalls, which slows the edges relative to the center.

Mean glacial speed varies greatly but is typically around 1 m (3 ft) per day. [38] There may be no motion in stagnant areas; for example, in parts of Alaska, trees can establish themselves on surface sediment deposits. In other cases, glaciers can move as fast as 20–30 m (70–100 ft) per day, such as in Greenland's Jakobshavn Isbræ. Glacial speed is affected by factors such as slope, ice thickness, snowfall, longitudinal confinement, basal temperature, meltwater production, and bed hardness.

A few glaciers have periods of very rapid advancement called surges. These glaciers exhibit normal movement until suddenly they accelerate, then return to their previous movement state. [39] These surges may be caused by the failure of the underlying bedrock, the pooling of meltwater at the base of the glacier [40]  — perhaps delivered from a supraglacial lake  — or the simple accumulation of mass beyond a critical "tipping point". [41] Temporary rates up to 90 m (300 ft) per day have occurred when increased temperature or overlying pressure caused bottom ice to melt and water to accumulate beneath a glacier.

In glaciated areas where the glacier moves faster than one km per year, glacial earthquakes occur. These are large scale earthquakes that have seismic magnitudes as high as 6.1. [42] [43] The number of glacial earthquakes in Greenland peaks every year in July, August, and September and increased rapidly in the 1990s and 2000s. In a study using data from January 1993 through October 2005, more events were detected every year since 2002, and twice as many events were recorded in 2005 as there were in any other year. [43]

Ogives

Forbes bands on the Mer de Glace glacier in France Forbes Bands on Mer de Glace in France.jpg
Forbes bands on the Mer de Glace glacier in France

Ogives or Forbes bands [44] are alternating wave crests and valleys that appear as dark and light bands of ice on glacier surfaces. They are linked to seasonal motion of glaciers; the width of one dark and one light band generally equals the annual movement of the glacier. Ogives are formed when ice from an icefall is severely broken up, increasing ablation surface area during summer. This creates a swale and space for snow accumulation in the winter, which in turn creates a ridge. [45] Sometimes ogives consist only of undulations or color bands and are described as wave ogives or band ogives. [46]

Geography

Fox Glacier in New Zealand finishes near a rainforest Fox-Gletscher1.jpg
Fox Glacier in New Zealand finishes near a rainforest

Glaciers are present on every continent and in approximately fifty countries, excluding those (Australia, South Africa) that have glaciers only on distant subantarctic island territories. Extensive glaciers are found in Antarctica, Argentina, Chile, Canada, Alaska, Greenland and Iceland. Mountain glaciers are widespread, especially in the Andes, the Himalayas, the Rocky Mountains, the Caucasus, Scandinavian mountains, and the Alps. Snezhnika glacier in Pirin Mountain, Bulgaria with a latitude of 41°46′09″ N is the southernmost glacial mass in Europe. [47] Mainland Australia currently contains no glaciers, although a small glacier on Mount Kosciuszko was present in the last glacial period. [48] In New Guinea, small, rapidly diminishing, glaciers are located on Puncak Jaya. [49] Africa has glaciers on Mount Kilimanjaro in Tanzania, on Mount Kenya, and in the Rwenzori Mountains. Oceanic islands with glaciers include Iceland, several of the islands off the coast of Norway including Svalbard and Jan Mayen to the far north, New Zealand and the subantarctic islands of Marion, Heard, Grande Terre (Kerguelen) and Bouvet. During glacial periods of the Quaternary, Taiwan, Hawaii on Mauna Kea [50] and Tenerife also had large alpine glaciers, while the Faroe and Crozet Islands [51] were completely glaciated.

The permanent snow cover necessary for glacier formation is affected by factors such as the degree of slope on the land, amount of snowfall and the winds. Glaciers can be found in all latitudes except from 20° to 27° north and south of the equator where the presence of the descending limb of the Hadley circulation lowers precipitation so much that with high insolation snow lines reach above 6,500 m (21,330 ft). Between 19˚N and 19˚S, however, precipitation is higher, and the mountains above 5,000 m (16,400 ft) usually have permanent snow.

Black ice glacier near Aconcagua, Argentina Black-Glacier.jpg
Black ice glacier near Aconcagua, Argentina

Even at high latitudes, glacier formation is not inevitable. Areas of the Arctic, such as Banks Island, and the McMurdo Dry Valleys in Antarctica are considered polar deserts where glaciers cannot form because they receive little snowfall despite the bitter cold. Cold air, unlike warm air, is unable to transport much water vapor. Even during glacial periods of the Quaternary, Manchuria, lowland Siberia, [52] and central and northern Alaska, [53] though extraordinarily cold, had such light snowfall that glaciers could not form. [54] [55]

In addition to the dry, unglaciated polar regions, some mountains and volcanoes in Bolivia, Chile and Argentina are high (4,500 to 6,900 m or 14,800 to 22,600 ft) and cold, but the relative lack of precipitation prevents snow from accumulating into glaciers. This is because these peaks are located near or in the hyperarid Atacama Desert.

Glacial geology

Diagram of glacial plucking and abrasion Arranque glaciar-en.svg
Diagram of glacial plucking and abrasion

Glaciers erode terrain through two principal processes: plucking and abrasion. [56]

As glaciers flow over bedrock, they soften and lift blocks of rock into the ice. This process, called plucking, is caused by subglacial water that penetrates fractures in the bedrock and subsequently freezes and expands. [57] This expansion causes the ice to act as a lever that loosens the rock by lifting it. Thus, sediments of all sizes become part of the glacier's load. If a retreating glacier gains enough debris, it may become a rock glacier, like the Timpanogos Glacier in Utah.

Abrasion occurs when the ice and its load of rock fragments slide over bedrock [57] and function as sandpaper, smoothing and polishing the bedrock below. The pulverized rock this process produces is called rock flour and is made up of rock grains between 0.002 and 0.00625 mm in size. Abrasion leads to steeper valley walls and mountain slopes in alpine settings, which can cause avalanches and rock slides, which add even more material to the glacier. Glacial abrasion is commonly characterized by glacial striations. Glaciers produce these when they contain large boulders that carve long scratches in the bedrock. By mapping the direction of the striations, researchers can determine the direction of the glacier's movement. Similar to striations are chatter marks, lines of crescent-shape depressions in the rock underlying a glacier. They are formed by abrasion when boulders in the glacier are repeatedly caught and released as they are dragged along the bedrock.

Glacially plucked granitic bedrock near Mariehamn, Aland PluckedGraniteAlandIslands.JPG
Glacially plucked granitic bedrock near Mariehamn, Åland

The rate of glacier erosion varies. Six factors control erosion rate:

When the bedrock has frequent fractures on the surface, glacial erosion rates tend to increase as plucking is the main erosive force on the surface; when the bedrock has wide gaps between sporadic fractures, however, abrasion tends to be the dominant erosive form and glacial erosion rates become slow. [58] Glaciers in lower latitudes tend to be much more erosive than glaciers in higher latitudes, because they have more meltwater reaching the glacial base and facilitate sediment production and transport under the same moving speed and amount of ice. [59]

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

Larger pieces of rock that are encrusted in till or deposited on the surface are called "glacial erratics". They range in size from pebbles to boulders, but as they are often moved great distances, they may be drastically different from the material upon which they are found. Patterns of glacial erratics hint at past glacial motions.

Moraines

Glacial moraines above Lake Louise, Alberta, Canada MorainesLakeLouise.JPG
Glacial moraines above Lake Louise, Alberta, Canada

Glacial moraines are formed by the deposition of material from a glacier and are exposed after the glacier has retreated. They usually appear as linear mounds of till, a non-sorted mixture of rock, gravel, and boulders within a matrix of 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. Medial moraines are formed when two different glaciers merge and the lateral moraines of each coalesce to form a moraine in the middle of the combined glacier. Less apparent are ground moraines, also called glacial drift, which often blankets the surface underneath the glacier downslope from the equilibrium line. The term moraine is of French origin. It was coined by peasants to describe alluvial embankments and rims found near the margins of glaciers in the French Alps. In modern geology, the term is used more broadly and is applied to a series of formations, all of which are composed of till. Moraines can also create moraine-dammed lakes.

Drumlins

Drumlins around Horicon Marsh, Wisconsin, in an area with one of the highest concentration of drumlins in the world. The curved path of the Laurentide Ice Sheet is evident in the orientation of the various mounds. Drumlins around Horicon Marsh in Wisconsin.jpg
Drumlins around Horicon Marsh, Wisconsin, in an area with one of the highest concentration of drumlins in the world. The curved path of the Laurentide Ice Sheet is evident in the orientation of the various mounds.

Drumlins are asymmetrical, canoe-shaped hills made mainly of till. Their heights vary from 15 to 50 meters, and they can reach a kilometer in length. The steepest side of the hill faces the direction from which the ice advanced (stoss), while a longer slope is left in the ice's direction of movement (lee). Drumlins are found in groups called drumlin fields or drumlin camps. One of these fields is found east of Rochester, New York; it is estimated to contain about 10,000 drumlins. Although the process that forms drumlins is not fully understood, their shape implies 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.

Glacial valleys, cirques, arêtes, and pyramidal peaks

Features of a glacial landscape Glacial landscape.svg
Features of a glacial landscape

Before glaciation, mountain valleys have a characteristic "V" shape, produced by eroding water. During glaciation, these valleys are often widened, deepened and smoothed to form a U-shaped glacial valley or glacial trough, as it is sometimes called. [60] The erosion that creates glacial valleys truncates any spurs of rock or earth that may have earlier extended across the valley, creating broadly triangular-shaped cliffs called truncated spurs. Within glacial valleys, depressions created by plucking and abrasion can be filled by lakes, called paternoster lakes. If a glacial valley runs into a large body of water, it forms a fjord.

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

At the start of a classic valley glacier is a bowl-shaped cirque, which have escarped walls on three sides but is open on the side that descends into the valley. Cirques are where ice begins to accumulate in a glacier. Two glacial cirques may form back to back and erode their backwalls until only a narrow ridge, called an arête is left. This structure may result in a mountain pass. If multiple cirques encircle a single mountain, they create pointed pyramidal peaks; particularly steep examples are called horns.

Roches moutonnées

Passage of glacial ice over an area of bedrock may cause the rock to be sculpted into a knoll called a roche moutonnée, [61] or "sheepback" rock. Roches moutonnées may be elongated, rounded and asymmetrical in shape. They range in length from less than a meter to several hundred meters long. [62] Roches moutonnées have a gentle slope on their up-glacier sides and a steep to vertical face on their down-glacier sides. The glacier abrades the smooth slope on the upstream side as it flows along, but tears rock fragments loose and carries them away from the downstream side via plucking.

Alluvial stratification

As the water that rises from the ablation zone moves away from the glacier, it carries fine eroded sediments with it. As the speed of the water decreases, so does its capacity to carry objects in suspension. The water thus gradually deposits the sediment as it runs, creating an alluvial plain. When this phenomenon occurs in a valley, it is called a valley train. When the deposition is in an estuary, the sediments are known as bay mud. Outwash plains and valley trains are usually accompanied by basins known as "kettles". These are small lakes formed when large ice blocks that are trapped in alluvium melt and produce water-filled depressions. Kettle diameters range from 5 m to 13 km, with depths of up to 45 meters. Most are circular in shape because the blocks of ice that formed them were rounded as they melted. [63]

Glacial deposits

Landscape produced by a receding glacier Receding glacier-en.svg
Landscape produced by a receding glacier

When a glacier's size shrinks below a critical point, its flow stops and it becomes stationary. Meanwhile, meltwater within and beneath the ice leaves stratified alluvial deposits. These deposits, in the forms of columns, terraces and clusters, remain after the glacier melts and are known as "glacial deposits". Glacial deposits that take the shape of hills or mounds are called kames . Some kames form when meltwater deposits sediments through openings in the interior of the ice. Others are produced by fans or deltas created by meltwater. When the glacial ice occupies a valley, it can form terraces or kames along the sides of the valley. Long, sinuous glacial deposits are called eskers . Eskers are composed of sand and gravel that was deposited by meltwater streams that flowed through ice tunnels within or beneath a glacier. They remain after the ice melts, with heights exceeding 100 meters and lengths of as long as 100 km.

Loess deposits

Very fine glacial sediments or rock flour [64] is often picked up by wind blowing over the bare surface and may be deposited great distances from the original fluvial deposition site. These eolian loess deposits may be very deep, even hundreds of meters, as in areas of China and the Midwestern United States. Katabatic winds can be important in this process.

Climate change

Glacer Retreat Photo.png
South Cascade Glacier in Washington photographs from 1928 to 2003 showing the recent rapid glacier retreating
2015-2100 Impacts of global warming on glaciers and sea level rise.svg
Based on current national pledges, global average temperature increase is projected to cause loss of ~half of Earth's glaciers by 2100 and raise sea level by ~115 mm [65] (not counting rise from melting ice sheets).

Glaciers, which can be hundreds of thousands of years old, are used to track climate change over long periods of time. [66] Researchers melt or crush samples from glacier ice cores whose progressively deep layers represent respectively earlier times in Earth's climate history. [66] The researchers apply various instruments to the content of bubbles trapped in the cores' layers in order to track changes in the atmosphere's composition. [66] Temperatures are deduced from differing relative concentrations of respective gases, confirming that for at least the last million years, global temperatures have been linked to carbon dioxide concentrations. [66]

Human activities in the industrial era have increased the concentration of carbon dioxide and other heat-trapping greenhouse gases in the air, causing current global warming. [67] Human influence is the principal driver of changes to the cryosphere of which glaciers are a part. [67]

Ice lagoon Jokulsarlon at the foot of the Vatnajokull Glacier, Iceland, 2023 Vatnajokull glacier.jpg
Ice lagoon Jökulsárlón at the foot of the Vatnajökull Glacier, Iceland, 2023

Global warming creates positive feedback loops with glaciers. [68] For example, in ice–albedo feedback, rising temperatures increase glacier melt, exposing more of earth's land and sea surface (which is darker than glacier ice), allowing sunlight to warm the surface rather than being reflected back into space. [68] Reference glaciers tracked by the World Glacier Monitoring Service have lost ice every year since 1988. [69] A joint study by the University of Graz, the French National Centre for Scientific Research, the Université de Fribourg and the University of Lausanne has shown that the flow velocity of glaciers in the Alps accelerates and slows down to a similar extent at the same time, despite large distances. This clearly shows that their speed is controlled by the climate change. [70] Another indicator for glacier loss is the Glacier Loss Day.

Water runoff from melting glaciers causes global sea level to rise, a phenomenon the IPCC terms a "slow onset" event. [71] Impacts at least partially attributable to sea level rise include encroachment on coastal settlements and infrastructure, increase in populations inhabiting 100-year coastal flood zones, existential threats to small islands and low-lying coasts, declines in coastal fishery resources, losses of coastal ecosystems and ecosystem services, groundwater salinization, increased risks to coastal food and water security, and compounding damage from tropical cyclones, flooding, storm surge, and land subsidence. [71]

Isostatic rebound

Isostatic pressure by a glacier on the Earth's crust Glacier weight effects LMB.png
Isostatic pressure by a glacier on the Earth's crust

Large masses, such as ice sheets or glaciers, can depress the crust of the Earth into the mantle. [72] The depression usually totals a third of the ice sheet or glacier's thickness. After the ice sheet or glacier melts, the mantle begins to flow back to its original position, pushing the crust back up. This post-glacial rebound, which proceeds very slowly after the melting of the ice sheet or glacier, is currently occurring in measurable amounts in Scandinavia and the Great Lakes region of North America.

A geomorphological feature created by the same process on a smaller scale is known as dilation-faulting. It occurs where previously compressed rock is allowed to return to its original shape more rapidly than can be maintained without faulting. This leads to an effect similar to what would be seen if the rock were hit by a large hammer. Dilation faulting can be observed in recently de-glaciated parts of Iceland and Cumbria.

On other planets

Protonilus Mensae, Ismenius Lacus quadrangle, Mars Wide view of glacier showing image field.JPG
Protonilus Mensae, Ismenius Lacus quadrangle, Mars

The polar ice caps of Mars show geologic evidence of glacial deposits. The south polar cap is especially comparable to glaciers on Earth. [73] Topographical features and computer models indicate the existence of more glaciers in Mars' past. [74] At mid-latitudes, between 35° and 65° north or south, Martian glaciers are affected by the thin Martian atmosphere. Because of the low atmospheric pressure, ablation near the surface is solely caused by sublimation, not melting. As on Earth, many glaciers are covered with a layer of rocks which insulates the ice. A radar instrument on board the Mars Reconnaissance Orbiter found ice under a thin layer of rocks in formations called lobate debris aprons (LDAs). [75] [76] [77]

In 2015, as New Horizons flew by the Pluto-Charon system, the spacecraft discovered a massive basin covered in a layer of nitrogen ice on Pluto. A large portion of the basin's surface is divided into irregular polygonal features separated by narrow troughs, interpreted as convection cells fuelled by internal heat from Pluto's interior. [78] [79] Glacial flows were also observed near Sputnik Planitia's margins, appearing to flow both into and out of the basin. [80]

See also

Related Research Articles

<span class="mw-page-title-main">Moraine</span> Glacially formed accumulation of debris

A moraine is any accumulation of unconsolidated debris, sometimes referred to as glacial till, that occurs in both currently and formerly glaciated regions, and that has been previously carried along by a glacier or ice sheet. It may consist of partly rounded particles ranging in size from boulders down to gravel and sand, in a groundmass of finely-divided clayey material sometimes called glacial flour. Lateral moraines are those formed at the side of the ice flow, and terminal moraines were formed at the foot, marking the maximum advance of the glacier. Other types of moraine include ground moraines and medial moraines.

<span class="mw-page-title-main">Till</span> Unsorted glacial sediment

Till or glacial till is unsorted glacial sediment.

<span class="mw-page-title-main">Glaciology</span> Scientific study of ice and natural phenomena involving ice

Glaciology is the scientific study of glaciers, or, more generally, ice and natural phenomena that involve ice.

<span class="mw-page-title-main">Crevasse</span> A deep crack, or fracture, in an ice sheet or glacier

A crevasse is a deep crack that forms in a glacier or ice sheet. Crevasses form as a result of the movement and resulting stress associated with the shear stress generated when two semi-rigid pieces above a plastic substrate have different rates of movement. The resulting intensity of the shear stress causes a breakage along the faces.

<span class="mw-page-title-main">Jökulhlaup</span> Type of glacial outburst flood

A jökulhlaup is a type of glacial outburst flood. It is an Icelandic term that has been adopted in glaciological terminology in many languages. It originally referred to the well-known subglacial outburst floods from Vatnajökull, Iceland, which are triggered by geothermal heating and occasionally by a volcanic subglacial eruption, but it is now used to describe any large and abrupt release of water from a subglacial or proglacial lake/reservoir.

<span class="mw-page-title-main">Outwash plain</span> Plain formed from glacier sediment transported by meltwater

An outwash plain, also called a sandur, sandr or sandar, is a plain formed of glaciofluvial deposits due to meltwater outwash at the terminus of a glacier. As it flows, the glacier grinds the underlying rock surface and carries the debris along. The meltwater at the snout of the glacier deposits its load of sediment over the outwash plain, with larger boulders being deposited near the terminal moraine, and smaller particles travelling further before being deposited. Sandurs are common in Iceland where geothermal activity accelerates the melting of ice flows and the deposition of sediment by meltwater.

<span class="mw-page-title-main">Glacial motion</span> Geological phenomenon

Glacial motion is the motion of glaciers, which can be likened to rivers of ice. It has played an important role in sculpting many landscapes. Most lakes in the world occupy basins scoured out by glaciers. Glacial motion can be fast or slow, but is typically around 25 centimetres per day (9.8 in/d).

<span class="mw-page-title-main">Glacial landform</span> Landform created by the action of glaciers

Glacial landforms are landforms created by the action of glaciers. Most of today's glacial landforms were created by the movement of large ice sheets during the Quaternary glaciations. Some areas, like Fennoscandia and the southern Andes, have extensive occurrences of glacial landforms; other areas, such as the Sahara, display rare and very old fossil glacial landforms.

<span class="mw-page-title-main">Terminal moraine</span> Type of moraine that forms at the terminal of a glacier

A terminal moraine, also called an end moraine, is a type of moraine that forms at the terminal (edge) of a glacier, marking its maximum advance. At this point, debris that has accumulated by plucking and abrasion, has been pushed by the front edge of the ice, is driven no further and instead is deposited in an unsorted pile of sediment. Because the glacier acts very much like a conveyor belt, the longer it stays in one place, the greater the amount of material that will be deposited. The moraine is left as the marking point of the terminal extent of the ice.

<span class="mw-page-title-main">Lyngen Alps</span> Mountain range in Norway

The Lyngen Alps are a mountain range in northeastern Troms county in Norway, east of the city of Tromsø. The mountain range runs through the municipalities of Lyngen, Balsfjord, and Storfjord. The mountains follow the western shore of the Lyngen fjord in a north-south direction. The length of the range is at least 90 kilometres (56 mi) and the width is 15–20 kilometres (9.3–12.4 mi). The mountains dominate the Lyngen Peninsula, which is bordered by the Lyngen fjord to the east, and the Ullsfjorden to the west.

<span class="mw-page-title-main">Outwash fan</span> Type of sediment deposition by a melting glacier

An outwash fan is a fan-shaped body of sediments deposited by braided streams from a melting glacier. Sediment locked within the ice of the glacier gets transported by the streams of meltwater, and deposits on the outwash plain, at the terminus of the glacier. The outwash, the sediment transported and deposited by the meltwater and that makes up the fan, is usually poorly sorted due to the short distance traveled before being deposited.

<span class="mw-page-title-main">Tunnel valley</span> Glacial-formed geographic feature

A tunnel valley is a U-shaped valley originally cut under the glacial ice near the margin of continental ice sheets such as that now covering Antarctica and formerly covering portions of all continents during past glacial ages. They can be as large as 100 km (62 mi), 4 km (2.5 mi) wide, and 400 m (1,300 ft) deep.

<span class="mw-page-title-main">Meltwater</span> Water released by the melting of snow or ice

Meltwater is water released by the melting of snow or ice, including glacial ice, tabular icebergs and ice shelves over oceans. Meltwater is often found during early spring when snow packs and frozen rivers melt with rising temperatures, and in the ablation zone of glaciers where the rate of snow cover is reducing. Meltwater can be produced during volcanic eruptions, in a similar way in which the more dangerous lahars form. It can also be produced by the heat generated by the flow itself.

A subaqueous fan is a fan-shaped deposit formed beneath water, that is commonly related to glaciers and crater lakes.

Fluvioglacial landforms or glaciofluvial landforms are those that result from the associated erosion and deposition of sediments caused by glacial meltwater. Glaciers contain suspended sediment loads, much of which is initially picked up from the underlying landmass. Landforms are shaped by glacial erosion through processes such as glacial quarrying, abrasion, and meltwater. Glacial meltwater contributes to the erosion of bedrock through both mechanical and chemical processes. Fluvio-glacial processes can occur on the surface and within the glacier. The deposits that happen within the glacier are revealed after the entire glacier melts or partially retreats. Fluvio-glacial landforms and erosional surfaces include: outwash plains, kames, kame terraces, kettle holes, eskers, varves, and proglacial lakes.

<span class="mw-page-title-main">Supraglacial lake</span> Pond of liquid water on the top of a glacier

A supraglacial lake is any pond of liquid water on the top of a glacier. Although these pools are ephemeral, they may reach kilometers in diameter and be several meters deep. They may last for months or even decades at a time, but can empty in the course of hours.

<span class="mw-page-title-main">Overdeepening</span> Characteristic of basins and valleys eroded by glaciers

Overdeepening is a characteristic of basins and valleys eroded by glaciers. An overdeepened valley profile is often eroded to depths which are hundreds of metres below the lowest continuous surface line along a valley or watercourse. This phenomenon is observed under modern day glaciers, in salt-water fjords and fresh-water lakes remaining after glaciers melt, as well as in tunnel valleys which are partially or totally filled with sediment. When the channel produced by a glacier is filled with debris, the subsurface geomorphic structure is found to be erosionally cut into bedrock and subsequently filled by sediments. These overdeepened cuts into bedrock structures can reach a depth of several hundred metres below the valley floor.

Subglacial streams are conduits of glacial meltwater that flow at the base of glaciers and ice caps. Meltwater from the glacial surface travels downward throughout the glacier, forming an englacial drainage system consisting of a network of passages that eventually reach the bedrock below, where they form subglacial streams. Subglacial streams form a system of tunnels and interlinked cavities and conduits, with water flowing under extreme pressures from the ice above; as a result, flow direction is determined by the pressure gradient from the ice and the topography of the bed rather than gravity. Subglacial streams form a dynamic system that is responsive to changing conditions, and the system can change significantly in response to seasonal variation in meltwater and temperature. Water from subglacial streams is routed towards the glacial terminus, where it exits the glacier. Discharge from subglacial streams can have a significant impact on local, and in some cases global, environmental and geological conditions. Sediments, nutrients, and organic matter contained in the meltwater can all influence downstream and marine conditions. Climate change may have a significant impact on subglacial stream systems, increasing the volume of meltwater entering subglacial drainage systems and influencing their hydrology.

The glacial series refers to a particular sequence of landforms in Central Europe that were formed during the Pleistocene glaciation beneath the ice sheets, along their margins and on their forelands during each glacial advance.

<span class="mw-page-title-main">Glacial stream</span> Body of liquid water that flows down a channel formed by a glacier

A glacier stream is a channelized area that is formed by a glacier in which liquid water accumulates and flows. Glacial streams are also commonly referred to as "glacier stream" or/and "glacial meltwater stream". The movement of the water is influenced and directed by gravity and the melting of ice. The melting of ice forms different types of glacial streams such as supraglacial, englacial, subglacial and proglacial streams. Water enters supraglacial streams that sit at the top of the glacier via filtering through snow in the accumulation zone and forming slush pools at the FIRN zone. The water accumulates on top of the glacier in supraglacial lakes and into supraglacial stream channels. The meltwater then flows through various different streams either entering inside the glacier into englacial channels or under the glacier into subglacial channels. Finally, the water leaves the glacier through proglacial streams or lakes. Proglacial streams do not only act as the terminus point but can also receive meltwater. Glacial streams can play a significant role in energy exchange and in the transport of meltwater and sediment.

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Bibliography

  • Huggett, Richard John (2011). Fundamentals Of Geomorphology. Routledge Fundamentals of Physical Geography Series (3rd ed.). Routledge. ISBN   978-0-203-86008-3.

General references

  • Hambrey, Michael; Alean, Jürg (2004). Glaciers (2nd ed.). Cambridge University Press. ISBN   978-0-521-82808-6. OCLC   54371738. A less-technical treatment of all aspects, with photographs and firsthand accounts of glaciologists' experiences. All images of this book can be found online (see Weblinks: Glaciers-online)
  • Benn, Douglas I.; Evans, David J.A. (1999). Glaciers and Glaciation. Arnold. ISBN   978-0-470-23651-2. OCLC   38329570.
  • Bennett, M.R.; Glasser, N.F. (1996). Glacial Geology: Ice Sheets and Landforms. John Wiley & Sons. ISBN   978-0-471-96344-8. OCLC   33359888.
  • Hambrey, Michael (1994). Glacial Environments. University of British Columbia Press, UCL Press. ISBN   978-0-7748-0510-0. OCLC   30512475. An undergraduate-level textbook.
  • Knight, Peter G. (1999). Glaciers . Cheltenham: Nelson Thornes. ISBN   978-0-7487-4000-0. OCLC   42656957. A textbook for undergraduates avoiding mathematical complexities
  • Walley, Robert (1992). Introduction to Physical Geography. Wm. C. Brown Publishers. A textbook devoted to explaining the geography of our planet.
  • Paterson, W.S.B. (1994). Physics of Glaciers (3rd ed.). Pergamon Press. ISBN   978-0-08-013972-2. OCLC   26188. A comprehensive reference on the physical principles underlying formation and behavior.

Further reading