{"id":71,"date":"2014-06-21T03:34:23","date_gmt":"2014-06-21T03:34:23","guid":{"rendered":"https:\/\/courses.candelalearning.com\/env131fmusu14\/?post_type=chapter&p=71"},"modified":"2017-11-07T16:33:34","modified_gmt":"2017-11-07T16:33:34","slug":"glaciers","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/suny-earthscience\/chapter\/glaciers\/","title":{"raw":"Glaciers","rendered":"Glaciers"},"content":{"raw":"A\u00a0glacier<\/b>\u00a0(US<\/small>\u00a0\/\u02c8\u0261le\u026a\u0283\u0259r\/\u00a0or\u00a0UK<\/small>\u00a0\/\u02c8\u0261l\u00e6si\u0259\/) is a persistent body of dense ice that is constantly moving under its own weight; it forms where the accumulation of snow exceeds its ablation (melting and sublimation) over many years, often centuries. Glaciers slowly deform and flow due to stresses induced by their weight, creating crevasses, seracs, and other distinguishing features. They also abrade rock and debris from their substrate to create landforms such as cirques and moraines. Glaciers form only on land and are distinct from the much thinner sea ice and lake ice that form on the surface of bodies of water.\r\n\r\n\"\"<\/a>\r\n
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\r\n\r\nThe Baltoro Glacier in the Karakoram, Baltistan, Northern Pakistan. At 62 kilometres (39\u00a0mi) in length, it is one of the longest alpine glaciers on earth.\r\n\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n
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\r\n\r\nIce calving from the terminus of the Perito Moreno Glacier in western Patagonia, Argentina\r\n\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n
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\r\n\r\nThe Aletsch Glacier, the largest glacier of the Alps, in Switzerland\r\n\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n
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\r\n\r\nThe Quelccaya Ice Cap is the largest glaciated area in the tropics, in Peru\r\n\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\nOn Earth, 99% of glacial ice is contained within vast ice sheets in the polar regions, but glaciers may be found in mountain ranges on every continent except Australia, and on a few high-latitude oceanic islands. Between 35\u00b0N and 35\u00b0S, glaciers occur only in the Himalayas, Andes, Rocky Mountains, a few high mountains in East Africa, Mexico, New Guinea and on Zard Kuh in Iran.<\/span>[1]<\/a><\/sup>\r\n\r\nGlacial ice is the largest reservoir of freshwater on Earth.[2]<\/a><\/sup>\u00a0Many 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. Within high altitude and Antarctic environments, the seasonal temperature difference is often not sufficient to release meltwater.\r\n\r\nBecause glacial mass is affected by long-term climate 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.\r\n\r\n \r\n

Etymology and related terms<\/h2>\r\nThe word\u00a0glacier<\/i>\u00a0comes from French. It is derived from the Vulgar Latin\u00a0glacia<\/i>\u00a0and ultimately from Latin\u00a0glacies<\/i>\u00a0meaning \"ice\".[3]<\/a><\/sup>\u00a0The processes and features caused by glaciers and related to them are referred to as\u00a0glacial<\/b>. The process of glacier establishment, growth and flow is called\u00a0glaciation<\/b>. The corresponding area of study is called glaciology. Glaciers are important components of the global cryosphere.\r\n

Types<\/h2>\r\n
Main article: Glacier morphology<\/div>\r\n
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\r\n\r\nMouth of the Schlatenkees Glacier near Innergschl\u00f6\u00df, Austria\r\n\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\nGlaciers are categorized by their morphology, thermal characteristics, and behavior.\u00a0Alpine glaciers,<\/i>\u00a0also known as\u00a0mountain glaciers<\/i>or\u00a0cirque glaciers<\/i>, form on the crests and slopes of mountains. An alpine glacier that fills a valley is sometimes called a\u00a0valley glacier<\/i>. A large body of glacial ice astride a mountain, mountain range, or volcano is termed an ice cap or ice field.[4]<\/a><\/sup>\u00a0Ice caps have an area less than 50,000\u00a0km\u00b2 (20,000\u00a0mile\u00b2) by definition.\r\n\r\nGlacial bodies larger than 50,000\u00a0km\u00b2 are called ice sheets or continental glaciers.[5]<\/a><\/sup>\u00a0Several kilometers deep, they obscure the underlying topography. Only\u00a0nunataks<\/i>\u00a0protrude from their surfaces. The only extant ice sheets are the two that cover most of Antarctica and Greenland. They contain vast quantities of fresh water, enough that if both melted, global sea levels would rise by over 70 meters.[6]<\/a><\/sup>Portions of an ice sheet or cap that extend into water are called\u00a0ice shelves;<\/i>\u00a0they tend to be thin with limited slopes and reduced velocities.[7]<\/a><\/sup>\u00a0Narrow, fast-moving sections of an ice sheet are called\u00a0ice streams.<\/i>[8]<\/a><\/sup>[9]<\/a><\/sup>\u00a0In Antarctica, many ice streams drain into large ice shelves. Some drain directly into the sea, often with an ice tongue, like Mertz Glacier.\r\n
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\r\n\r\nSightseeing boat in front of a tidewater glacier, Kenai Fjords National Park, Alaska\r\n\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\nTidewater glaciers<\/i>\u00a0are glaciers that terminate in the sea, including most glaciers flowing from Greenland, Antarctica, Baffin 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\u00a0calve<\/i>, 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 the climate change than those of other glaciers.\r\n\r\nThermally, a\u00a0temperate glacier<\/i>\u00a0is at melting point throughout the year, from its surface to its base. The ice of a\u00a0polar glacier<\/i>\u00a0is always below freezing point from the surface to its base, although the surface snowpack may experience seasonal melting. A\u00a0sub-polar glacier<\/i>includes both temperate and polar ice, depending on 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 the temperature at its base alone. A\u00a0cold-based<\/i>\u00a0glacier is below freezing at the ice-ground interface, and is thus frozen to the underlying substrate. A\u00a0warm-based<\/i>\u00a0glacier is above or at freezing at the interface, and is able to slide at this contact.[10]<\/a><\/sup>\u00a0This 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.[11]<\/a><\/sup>\u00a0Glaciers which are partly cold-based and partly warm-based are known as\u00a0polythermal<\/i>.[10]<\/a><\/sup>\r\n

Formation<\/h2>\r\n
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\r\n\r\nGorner Glacier in Switzerland\r\n\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\nGlaciers form where the accumulation of snow and ice exceeds ablation. The area in which a glacier forms is called a cirque (corrie or cwm) - a typically armchair-shaped geological feature (such as a depression between mountains enclosed by ar\u00eates) - which collects and compresses through gravity the snow which falls into it. This snow collects and is compacted by the weight of the snow falling above it forming n\u00e9v\u00e9. Further crushing of the individual snowflakes and squeezing the air from the snow turns it into extremely dense 'glacial ice'. This glacial ice will fill the cirque until it 'overflows' through a geological weakness or vacancy, such as the gap between two mountains. When the mass of snow and ice is sufficiently thick, it begins to move due to a combination of surface slope, gravity and pressure. On steeper slopes, this can occur with as little as 15\u00a0m (50\u00a0ft) of snow-ice.\r\n\r\nIn 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 and denser firn. Over a period of years, layers of firn undergo further compaction and become glacial ice. Glacier ice is slightly less dense than ice formed from frozen water because it contains tiny trapped air bubbles.\r\n\r\nGlacial 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 is blue for the same reason. The blue of glacier ice is sometimes misattributed to Rayleigh scattering due to bubbles in the ice.[12]<\/a><\/sup>\r\n
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\r\n\r\nA glacier cave located on the Perito Moreno Glacier in Argentina.\r\n\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n

Structure<\/h2>\r\nA glacier originates at a location called its\u00a0glacier head<\/i>\u00a0and terminates at its\u00a0glacier foot<\/i>, or terminus.\r\n\r\nGlaciers are broken into zones based on surface snowpack and melt conditions.[13]<\/a><\/sup>\u00a0The\u00a0ablation zone<\/i>\u00a0is the region where there is a net loss in glacier mass. The\u00a0equilibrium line<\/i>\u00a0separates the ablation zone and the accumulation zone; it is the altitude where the amount of new snow gained by accumulation is equal to the amount of ice lost through ablation. The upper part of a glacier, where accumulation exceeds ablation, is called the\u00a0accumulation zone<\/i>. In general, the accumulation zone accounts for 60\u201370% 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.\r\n\r\nThe accumulation zone can be subdivided based on its melt conditions.\r\n
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  1. The\u00a0dry snow zone<\/i>\u00a0is a region where no melt occurs, even in the summer, and the snowpack remains dry.<\/li>\r\n \t
  2. The\u00a0percolation zone<\/i>\u00a0is 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 melting point.<\/li>\r\n \t
  3. Near the equilibrium line on some glaciers, a\u00a0superimposed ice zone<\/i>\u00a0develops. This zone is where meltwater refreezes as a cold layer in the glacier, forming a continuous mass of ice.<\/li>\r\n \t
  4. The\u00a0wet snow zone<\/i>\u00a0is the region where all of the snow deposited since the end of the previous summer has been raised to 0\u00a0\u00b0C.<\/li>\r\n<\/ol>\r\nThe \"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 snowcovered at the end of the melt season, and a terminus with vigorous flow.\r\n\r\nFollowing 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\u20131985, but since 1985 glacier retreat and mass loss has become larger and increasingly ubiquitous.[14]<\/a><\/sup>[15]<\/a><\/sup>[16]<\/a><\/sup>\r\n

    Motion<\/h2>\r\n
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    \r\n\r\nShear 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.\r\n\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n
    Main article: Ice sheet dynamics<\/div>\r\nGlaciers move, or flow, downhill due to gravity and the internal deformation of ice.[17]<\/a><\/sup>\u00a0Ice behaves like a brittle solid until its thickness exceeds about 50\u00a0m (160\u00a0ft). The pressure on ice deeper than 50\u00a0m causes plastic flow. At the molecular level, ice consists of stacked layers of molecules with relatively weak bonds between layers. When the stress on the layer above exceeds the inter-layer binding strength, it moves faster than the layer below.[18]<\/a><\/sup>\r\n\r\nGlaciers also move through basal sliding. In this process, a glacier slides over the terrain on which it sits, lubricated by the presence of liquid water. The water is created from ice that melts under high pressure from frictional heating. Basal sliding is dominant in temperate, or warm-based glaciers.\r\n

    Fracture zone and cracks<\/h3>\r\n
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    \r\n\r\nIce cracks in the Titlis Glacier\r\n\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\nThe top 50 metres (160\u00a0ft) of a glacier are rigid because they are under low pressure. This upper section is known as the\u00a0fracture zone<\/i>; it mostly moves as a single unit over the plastically flowing lower section. When a glacier moves through irregular terrain, cracks called crevasses develop in the fracture zone. Crevasses form due to differences in glacier velocity. If two rigid sections of a glacier move at different speeds and directions, shear forces cause them to break apart, opening a crevasse. Crevasses are seldom more than 150 feet (46\u00a0m) deep but in some cases can be 1,000 feet (300\u00a0m) or even deeper. Beneath this point, the plasticity of the ice is too great for cracks to form. Intersecting crevasses can create isolated peaks in the ice, called seracs.\r\n\r\nCrevasses 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 from the edge of the glacier, due to the reduction in speed caused by friction of the valley walls. Marginal crevasses are usually largely transverse to flow. Moving glacier ice can sometimes separate from stagnant ice above, forming a\u00a0bergschrund<\/i>. Bergschrunds resemble crevasses but are singular features at a glacier's margins.\r\n\r\nCrevasses make travel over glaciers hazardous, especially when they are hidden by fragile snow bridges.\r\n
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    \r\n\r\nCrossing a crevasse on the Easton Glacier, Mount Baker, in the North Cascades, United States\r\n\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\nBelow 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.[19]<\/a><\/sup>\r\n

    Speed<\/h3>\r\nThe 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 side walls, which slows the edges relative to the center.\r\n\r\nMean speeds vary greatly, but is typically around 1 meter per day.[20]<\/a><\/sup>\u00a0There 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\u201330\u00a0m per day, such as in Greenland's Jakobshavn Isbr\u00e6 (Greenlandic:\u00a0Sermeq Kujalleq<\/i>). Velocity increases with increasing slope, increasing thickness, increasing snowfall, increasing longitudinal confinement, increasing basal temperature, increasing meltwater production and reduced bed hardness.\r\n\r\nA few 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 far greater than normal speed.[21]<\/a><\/sup>\u00a0These surges may be caused by failure of the underlying bedrock, the pooling of meltwater at the base of the glacier[22]<\/a><\/sup>\u00a0\u2014 perhaps delivered from a supraglacial lake\u00a0\u2014 or the simple accumulation of mass beyond a critical \"tipping point\".[23]<\/a><\/sup>\r\n\r\nIn glaciated areas where the glacier moves faster than one km per year, glacial earthquakes occur. These are large scale tremblors that have seismic magnitudes as high as 6.1.[24]<\/a><\/sup>[25]<\/a><\/sup>\u00a0The number of glacial earthquakes in Greenland peaks every year in July, August and September and is increasing over time. 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. This increase in the numbers of glacial earthquakes in Greenland may be a response to global warming.[24]<\/a><\/sup>[25]<\/a><\/sup>\r\n

    Ogives<\/h3>\r\nOgives<\/i>\u00a0are 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.[26]<\/a><\/sup>\u00a0Sometimes ogives consist only of undulations or color bands and are described as wave ogives or band ogives.[27]<\/a><\/sup>\r\n

    Geography<\/h2>\r\n
    For more details on this topic, see List of glaciers, and Retreat of glaciers since 1850.<\/div>\r\n
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    \r\n\r\nBlack ice glacier near Aconcagua, Argentina\r\n\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\nGlaciers are present on every continent and approximately fifty countries, excluding those (Australia, South Africa) that have glaciers only on distant subantarctic island territories. Extensive glaciers are found in Antarctica, Chile, Canada, Alaska, Greenland and Iceland. Mountain glaciers are widespread, especially in the Andes, the Himalayas, the Rocky Mountains, the Caucasus, and the Alps. Mainland Australia currently contains no glaciers, although a small glacier on Mount Kosciuszko was present in the last glacial period.[28]<\/a><\/sup>\u00a0In New Guinea, small, rapidly diminishing, glaciers are located on its highest summit massif of Puncak Jaya.[29]<\/a><\/sup>\u00a0Africa has glaciers on Mount Kilimanjaro in Tanzania, on Mount Kenya and in the Rwenzori Mountains. Oceanic islands with glaciers occur on Iceland, Svalbard, New Zealand, Jan Mayen and the subantarctic islands of Marion, Heard, Grande Terre (Kerguelen) and Bouvet. During glacial periods of the Quaternary, Taiwan, Hawaii on Mauna Kea[30]<\/a><\/sup>\u00a0and Tenerife also had large alpine glaciers, while the Faroe and Crozet Islands[31]<\/a><\/sup>\u00a0were completely glaciated.\r\n\r\nThe 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\u00b0 to 27\u00b0 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 metres (21,330\u00a0ft). Between 19\u02daN and 19\u02daS, however, precipitation is higher and the mountains above 5,000 metres (16,400\u00a0ft) usually have permanent snow.\r\n\r\nEven 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,[32]<\/a><\/sup>and central and northern Alaska,[33]<\/a><\/sup>\u00a0though extraordinarily cold, had such light snowfall that glaciers could not form.[34]<\/a><\/sup>[35]<\/a><\/sup>\r\n\r\nIn addition to the dry, unglaciated polar regions, some mountains and volcanoes in Bolivia, Chile and Argentina are high (4,500 metres (14,800\u00a0ft) - 6,900\u00a0m (22,600\u00a0ft)) 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.\r\n

    Glacial geology<\/h2>\r\n
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    \r\n\r\nDiagram of glacial plucking and abrasion\r\n\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\n
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    \r\n\r\nGlacially plucked granitic bedrock near Mariehamn, \u00c5land Islands\r\n\r\n<\/div>\r\n<\/div>\r\n<\/div>\r\nGlaciers erode terrain through two principal processes:\u00a0abrasion<\/b>\u00a0and\u00a0plucking.<\/b>\r\n\r\nAs 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. 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.\r\n\r\nAbrasion occurs when the ice and its load of rock fragments slide over bedrock 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\u00a0mm in size. Abrasion leads to steeper valley walls and mountain slopes in alpine settings, which can cause avalanches and rock slides. These add even more material to the glacier.\r\n\r\nGlacial 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.\r\n\r\nThe rate of glacier erosion is variable. Six factors control erosion rate:\r\n