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120-425: A jökulhlaup ( Icelandic pronunciation: [ˈjœːkʏl̥ˌl̥œyp] pronunciation ) (literally "glacial run") 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

240-424: 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

360-525: A recessional moraine . Tunnel valleys from successive glaciations may crosscut one another. Tunnel valleys frequently run along roughly parallel courses. They originate in and run through regions which include clear evidence of glacial erosion through abrasion and may exhibit striations and roche moutonnée . Depositional forms such as terminal moraines and outwash fans are found at their terminal end. In Michigan tunnel valley channels have been observed to diverge slightly with an average spacing between

480-428: A channel and sustaining the discharge. Hence, combining this data and analysis with Icelandic jökulhlaup observations, there is experimental evidence that some form of the jökulhlaup hypothesis with features of the steady state model is correct. Subglacial meltwater flow is common to all theories; hence a key to understanding channel formation is an understanding of subglacial meltwater flow. Meltwater may be produced on

600-420: A function of the glacial recession. The filled configuration is significant because filled tunnel valleys become excellent reservoirs for either water (aquifer) or for oil. This results since relatively coarse-grained sandstones are located on the valley floors and valley margins and valley floor because the coarser-grained sediments settle out more easily and accumulate preferentially in the flowing water common to

720-458: A general mapping of the glacier thickness when the tunnel valleys were formed, particularly if the original surface relief under the glacier was limited. The rapid, high-volume discharge is highly erosive, as evidenced by the debris found in tunnels and at the mouth of tunnels, which tends to be coarse rocks and boulders. This erosive environment is consistent with creation of tunnels over 400 m deep and 2.5 km wide, as have been observed in

840-511: 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

960-595: 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. 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

1080-411: 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. Most of the important processes controlling glacial motion occur in the ice-bed contact—even though it is only a few meters thick. The bed's temperature, roughness and softness define basal shear stress, which in turn defines whether movement of

1200-408: A kilometer per year. 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. The flow of water under the glacial surface can have

1320-404: A large effect on the motion of the glacier itself. Subglacial lakes contain significant amounts of water, which can move fast: cubic kilometers can be transported between lakes over the course of a couple of years. 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

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1440-460: A lower heat conductance, meaning that the basal temperature is also likely to be higher. 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. Secondly,

1560-403: A period of less than a year. As the flow subsided, the weight of ice closed the tunnel and sealed the lake again. The water flow was modeled satisfactorily with channeling in ice and in sediment. The analytic model shows that over some regions, the ice-bedrock geometry included sections which would have frozen, blocking off flow, unless erosion of the sedimentary substrate was the means of creating

1680-491: A period of weeks with the largest flow near the end, or it climbs much faster during the course of some hours. These patterns are suggested to reflect channel melting, and sheet flow under the front, respectively. Similar processes on a very large scale occurred during the deglaciation of North America and Europe after the last ice age (e.g., Lake Agassiz and the English Channel ), and presumably at earlier times, although

1800-420: A result of the hydraulic transmissivity of the subsoil under the glacier. If the rate of production exceeds the rate of loss through the aquifer, then water will collect in surface or subglacial ponds or lakes. The signatures of supraglacial and basal water flow differ with the passage zone. Supraglacial flow is similar to stream flow in all surface environments—water flows from higher areas to lower areas under

1920-594: A result, the ice-flow patterns and the debris accumulation are different in interlobate zones. Specifically, tunnel valleys and eskers indicate water flow toward the interlobate zones, which are elevated as the result of debris carried and deposited there. Glacially formed tunnel valleys have been identified on every continent. Tunnel valleys associated with the Late Ordovician glaciation have been observed in north African countries, including Libya . These large-scale channel-fill sandstone bodies (tunnel valleys) are

2040-503: A striking sedimentological feature of the glacially related deposits on the old North Gondwanaland margin. They range from 10–200 m (33–656 ft) in depth, and 500–3,000 m (1,600–9,800 ft) wide. The tunnel valleys are incised into the bedrock and can be traced for 2–30 km (1.2–18.6 mi) in length. In one example, in Mauritania , in the western Sahara , Late Ordovician siliciclastic glacial features and deposits on

2160-486: A subglacial lake. The hydraulic head of the water collected in a basal lake will increase as water drains through the ice until the pressure grows high enough to either develop a path through the ice or to float the ice above it. Sources of water and water drainage routes through and below temperate and sub-polar glaciers are reasonably well understood and provide a basis for understanding tunnel valleys. For these glaciers, supraglacial water ponds or moves in rivers across

2280-785: A surface area of 351 km (136 sq mi). Northern Idaho and Montana show evidence of tunnel valley formation under the Purcell lobe and the Flathead Lobe of the Cordilleran Ice Sheet. Tunnel valleys in southeast Alberta form an interconnected, anabranching network comprising Sage Creek, the Lost River and the Milk River and generally drain southeast. Tunnel valleys have been observed in Minnesota , Wisconsin and Michigan at

2400-484: A thin layer. A switch between the two flow conditions may be associated with surging behavior. Indeed, the loss of sub-glacial water supply has been linked with the shut-down of ice movement in the Kamb ice stream. The subglacial motion of water is expressed in the surface topography of ice sheets, which slump down into vacated subglacial lakes. The speed of glacial displacement is partly determined by friction . Friction makes

2520-410: 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. 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

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2640-807: A tunnel valley may go uphill: water can flow uphill if it is under pressure in an enclosed pipe: for example in Doggerland (submerged land which is now part of the bed of the North Sea ) are some infilled tunnel valleys that flowed from north to south across the hollow of the Outer Silver Pit . They vary in channel depth and width; Danish examples run from 0.5–4 km (0.31–2.49 mi) wide and from 50–350 m (160–1,150 ft) deep. They vary in depth along their course, exhibiting overdeepening ; overdeepened sections cut into bedrock and typically are significantly deeper than either upstream or downstream sections of

2760-438: 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 . Since jökulhlaups emerge from hydrostatically sealed lakes with floating levels far above the threshold, their peak discharge can be much larger than that of a marginal or extra-marginal lake burst. The hydrograph of a jökulhlaup from Vatnajökull typically either climbs over

2880-456: Is above or at freezing at the interface and is able to slide at this contact. 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. Glaciers which are partly cold-based and partly warm-based are known as polythermal . Glaciers form where the accumulation of snow and ice exceeds ablation . A glacier usually originates from

3000-407: 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. These surges may be caused by the failure of the underlying bedrock,

3120-661: Is an indicator of the presence of oil in these areas. Tunnel valleys represent a substantial fraction of all meltwater drainage from glaciers. Meltwater drainage influences the flow of glacial ice, which is important in understanding of the duration of glacial–interglacial periods, and aids in identifying glacial cyclicity, a problem that is important to palaeoenvironmental investigations. Tunnel valleys are typically eroded into bedrock and filled with glacial debris of varying sizes. This configuration makes them excellent at capturing and storing water. Hence they serve an important role as aquifers across much of Northern Europe, Canada and

3240-411: Is because these peaks are located near or in the hyperarid Atacama Desert . Glaciers erode terrain through two principal processes: plucking and abrasion . 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. This expansion causes

3360-406: Is by basal sliding, where meltwater forms between the ice and the bed itself. 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 τ B ). 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

3480-434: Is called glaciology . Glaciers are important components of the global cryosphere . 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 . A large body of glacial ice astride a mountain, mountain range, or volcano

3600-416: 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

3720-490: Is evidence of ice erosion such as linear striations in the bedrock, these are observed only in the widest valleys, and are believed to have played a secondary role. The subglacial layout of valley tunnels is predominantly oriented parallel to glacial ice flow lines – essentially they stretch from areas of thicker sheet ice toward areas of thinner sheet ice. They can exhibit reverse gradients, which result when pressurized meltwater flows over obstacles such as ridges or hills along

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3840-628: Is higher, and the mountains above 5,000 m (16,400 ft) usually have permanent snow. 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

3960-575: Is located in the southern Southern Patagonian Ice Field , terminating in Lake Argentino . It divides Lake Argentino into the Los Témpanos channel, and the Rico branch, blocking the channel and forming an ice dam. Lake Argentino periodically breaks through in outburst floods with drainage initially through a tunnel with subsequent roof collapse to form an open channel. There have been five known ice ages in

4080-402: Is lower in areas of thinner ice; hence subglacial water tends to converge on the interlobate joint. The separate lobes move at different speeds, generating friction at the ice boundary; the heat released melts ice to release additional water. The surface of the interlobate area is crevassed, allowing surface meltwater, which runs down the ice surface to the lower area, to penetrate into the ice. As

4200-401: Is present, the water is initially released in a broad-front jökulhlaup which can have a flow front that is tens of kilometres wide, spreading out in a thin front. As the flow continues, it tends to erode the underlying materials and the overlying ice, creating a tunnel valley channel even as the reduced pressure allows most of the glacial ice to settle back to the underlying surface, sealing off

4320-515: Is still much debate among geologists as to where these events occurred, they likely took place when the ice sheet receded from the Adirondack Mountains and the St. Lawrence Lowlands . On 7 February 2021, part of Nanda Devi Glacier broke away in the 2021 Uttarakhand glacier burst , triggering outburst flood sweeping away a power plant. More than 150 people were feared dead. Around 9500 BC

4440-897: Is termed an ice cap or ice field . Ice caps have an area less than 50,000 km (19,000 sq mi) by definition. Glacial bodies larger than 50,000 km (19,000 sq mi) are called ice sheets or continental glaciers . 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. They contain vast quantities of freshwater, enough that if both melted, global sea levels would rise by over 70 m (230 ft). 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. Narrow, fast-moving sections of an ice sheet are called ice streams . In Antarctica, many ice streams drain into large ice shelves . Some drain directly into

4560-413: 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

4680-402: Is the underlying geology; glacial speeds tend to differ more when they change bedrock than when the gradient changes. 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

4800-552: Is then forced into the cavity arising in their lee side , where it re-freezes. As well as affecting the sediment stress, fluid pressure (p w ) 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, p i , given by ρgh. Under fast-flowing ice streams, these two pressures will be approximately equal, with an effective pressure (p i – p w ) of 30 kPa; i.e. all of

4920-952: 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. Mainland Australia currently contains no glaciers, although a small glacier on Mount Kosciuszko was present in the last glacial period . In New Guinea, small, rapidly diminishing, glaciers are located on Puncak Jaya . Africa has glaciers on Mount Kilimanjaro in Tanzania, on Mount Kenya , and in

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5040-761: The Baltic Ice Lake was tapped on water as the ice front retreated north of mount Billingen . Glacial A glacier ( US : / ˈ ɡ l eɪ ʃ ər / ; UK : / ˈ ɡ l æ s i ər , ˈ ɡ l eɪ s i ər / ) is a persistent body of dense ice that is constantly moving downhill 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

5160-703: The British Columbian Coast Mountains , resulting in a jökulhlaup. The flood surge of from 100 to 300 m/second flowed 11 km through Farrow Creek to terminate in Chilko Lake , causing significant erosion. The ice dam has not reformed. Similar British Columbian jökulhlaups are summarized in the table below. As the Laurentide Ice Sheet receded from its maximum extent from around 21,000 to 13,000 years ago, two significant meltwater rerouting events occurred in eastern North America . Though there

5280-636: The Faroe and Crozet Islands 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

5400-750: The Himalayas , Andes , and a few high mountains in East Africa, Mexico, New Guinea and on Zard-Kuh in Iran. With more than 7,000 known glaciers, Pakistan has more glacial ice than any other country outside the polar regions. Glaciers cover about 10% of Earth's land surface. Continental glaciers cover nearly 13 million km (5 million sq mi) or about 98% of Antarctica 's 13.2 million km (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. The volume of glaciers, not including

5520-688: The Late Paleozoic Pilbara ice sheet . Tunnel valleys and related glacial impacts have been identified in Russia, Belarus, Ukraine, Poland, Germany, Northern France, the Netherlands, Belgium, Great Britain, Finland, Sweden, Denmark and Norway. They have been studied in detail in Denmark, north Germany and north Poland where the thick ice sheet of the Weichsel and earlier Glaciations , having flowed down from

5640-649: The Niagara Escarpment flowed through tunnel valleys beneath the ice expanded to form a west-to-east passage between the main Laurentide Ice Sheet and a mass of ice in the Lake Ontario basin. Cedar Creek Canyon is a tunnel valley located in Allen County, Indiana . It is a very straight, narrow gorge about 50 to 100 ft (15 to 30 m) deep that contains part of the lower segment of Cedar Creek ,

5760-551: The Quaternary , Manchuria , lowland Siberia , and central and northern Alaska , though extraordinarily cold, had such light snowfall that glaciers could not form. 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

5880-860: The Ruppiner See (a lake in Ostprignitz-Ruppin , Brandenburg ), the Werbellinsee , and the Schwielochsee , all in Germany. Okanagan Lake is a large, deep ribbon lake in the Okanagan Valley of British Columbia which formed in a tunnel valley from the Okanogan lobe of the Cordilleran Ice Sheet . The lake is 135 km (84 mi) long, between 4 and 5 km (2.5 and 3.1 mi) wide, and has

6000-521: 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 and Tenerife also had large alpine glaciers, while

6120-448: 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. Ogives or Forbes bands 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

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6240-566: The Antarctic. Piotrowski has developed a detailed analytic model of the process, which predicts a cycle as follows: A subglacial lake in Iceland was inadvertently triggered by a borehole drilled through the overlying ice. The authors suggested that hydrofracturing crevasses and flooding of moulins by precipitation events may be natural triggers of jökulhlaups. Whilst jökulhlaups were originally associated with Vatnajökull, they have been reported in

6360-483: The Antarctic. Piotrowski's model predicts a cycle as follows: Tunnel valleys have similar characteristics, irrespective of whether they are formed on land or in a submerged environment. This is because they are formed by high pressure water under a thick ice sheet – in a submerged environment they still have sufficient pressure to erode tunnel valleys into configurations comparable to those generated on land. Tunnel valleys may remain open, partially filled or filled, as

6480-484: The North Gondwana continental shelf include incised channels identified as tunnel valleys. The filled tunnel valley are several kilometers long and several hundred meters wide. Reconstructions conclude that these structures were located in glacier ice-margin regions; the cross-sections of the valleys are comparable to those confirmed to have formed glacially, the valleys end in outwash fans similar to tunnel valleys, and

6600-624: The North Sea, the Atlantic and in waters near Antarctica. Tunnel valleys appear in the technical literature under several terms, including tunnel channels, subglacial valleys, iceways , snake coils and linear incisions. Tunnel valleys play a role in identifying oil-rich areas in Arabia and North Africa. The Upper Ordovician –Lower Silurian materials there contain a roughly 20 m (66 ft) thick, carbon-rich layer of black shale. Approximately 30% of

6720-466: The Nye channel formation which has been observed in sediments, a weakness of the steady state theory is that it requires that tunnel valleys be excavated in unconsolidated sediment, in which meltwater is initially forced through an initially narrow subglacial conduit. With progressive sediment erosion by the meltwater, ice deforms under its own weight into the cavity to creating an ever-larger tunnel valley. However

6840-610: The United States. Examples include Oak Ridges Moraine Aquifer , Spokane Valley-Rathdrum Prairie Aquifer, Mahomet Aquifer , the Saginaw Lobe Aquifer, and the Corning Aquifer. Tunnel valleys have been observed as open valleys and as partially or totally buried valleys. If buried they may be partially or totally filled with glacial outwash or other debris. The valleys may be incised in bedrock, sand, silt, or clay. A part of

6960-412: The advance of many alpine glaciers between 1950 and 1985, but since 1985 glacier retreat and mass loss has become larger and increasingly ubiquitous. Glaciers move downhill by the force of gravity and the internal deformation of ice. 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

7080-520: 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

7200-430: 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. 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. This acts as a "double whammy", because thicker glaciers have

7320-585: The amounts of ice accumulated through precipitation and lost through ablation . The increased gradient increases the shear stress on a glacier until it begins to flow. The flow velocity and deformation are also affected by the slope of the ice, the ice thickness and temperature. Punkari identified that continental ice sheets typically flow in fan-shaped lobes, which converge from separate sources and move at differing speeds. Lobes are separated by interlobate zones, which have thinner ice coverage. Water collects in this interlobate area. The hydraulic head (pressure)

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7440-713: 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. Sometimes ogives consist only of undulations or color bands and are described as wave ogives or band ogives. 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, Pakistan, Alaska, Greenland and Iceland. Mountain glaciers are widespread, especially in

7560-530: The annual production of water from one typical catchment of 642,000,000 m (2.27 × 10  cu ft) would normally drain through its associated tunnel valley in less than 48 hours. The debris found in tunnels and at the mouth of tunnels tends to be coarse rocks and boulders – this is indicative of high flow velocities and an extremely erosive environment. This erosive environment is consistent with creation of tunnels over 400 m (1,300 ft) deep and 2.5 km (1.6 mi) wide, as have been observed in

7680-432: 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. Glaciers in lower latitudes tend to be much more erosive than glaciers in higher latitudes, because they have more meltwater reaching

7800-445: 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. The rate of glacier erosion varies. Six factors control erosion rate: When

7920-425: The broad front release and channelizing the flow. The direction of the channel is defined primarily by the overlying ice thickness and second by the gradient of the underlying earth, and may be observed to "run uphill" as the pressure of the ice forces the water to areas of lower ice coverage until it emerges at a glacial face. Hence the configuration of the various tunnel valleys formed by a specific glaciation provides

8040-685: The channels of 6 km (3.7 mi) and a standard deviation of 2.7 km (1.7 mi). Tunnel valley channels often start or stop abruptly. They have convex-up longitudinal profiles. They are often occupied by elongated lakes of underfit streams. They frequently show signs of subsequent depositions such as eskers. Evidence suggests that erosion in a tunnel valley is primarily the result of water flow. They erode by meltwater, which it has been argued, episodically drains in repeated jökulhlaups from subglacial lakes and reservoirs; examples of such motion have been observed in Antarctica . Although there

8160-559: 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". 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

8280-467: 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. Although evidence in favor of glacial flow was known by the early 19th century, other theories of glacial motion were advanced, such as

8400-483: 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: where: The lowest velocities are near the base of the glacier and along valley sides where friction acts against flow, causing

8520-638: The eastern Scotian Shelf off Nova Scotia . They originate from the Laurentian Channel south of the Cabot Strait . Additionally, seismic profiles show deeply buried post-Miocene channels, some of which lie 1,100 m (3,600 ft) below modern sea level, cutting across the eastern part of the outer Laurentian Channel which have also tentatively been determined to be tunnel valleys. Seismic profiles have also mapped large tunnel valleys on Banquereau Bank and Sable Island Bank . The Perito Moreno Glacier

8640-418: The essentially correct explanation in the 1840s, although it was several decades before it was fully accepted. 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

8760-475: 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

8880-405: The geological record is not well preserved. Subglacial meltwater may be produced on the glacier surface (supraglacially), below the glacier (basally) or in both locations. Ablation (surface melting) tends to result in surface pooling. Basal melting results from geothermal heat flux out of the earth, which varies with location, as well as from friction heating which results from the ice moving over

9000-952: The glacial base and facilitate sediment production and transport under the same moving speed and amount of ice. 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: Tunnel valley Tunnel valleys were formed by subglacial erosion by water and served as subglacial drainage pathways carrying large volumes of meltwater. Their cross-sections often exhibit steep-sided flanks similar to fjord walls. They presently appear as dry valleys, lakes, seabed depressions, and as areas filled with sediment. If they are filled with sediment, their lower layers are filled primarily with glacial, glaciofluvial or glaciolacustrine sediment, supplemented by upper layers of temperate infill. They can be found in areas formerly covered by glacial ice sheets including Africa, Asia, North America, Europe, Australia and offshore in

9120-444: The glacial ice (called Rothlisberger channels), eventually flowing out at the ice margin. On the simplest level, the tunnel valley can be considered a larger-scale version of these phenomena. Tunnel valleys or tunnel channels are produced by meltwater flows beneath glacial ice. Tunnel valleys are often buried or partially buried by sediment accumulation during periods of ice advance and retreat. Although attractive since it scales up

9240-406: The glacier bed. Tunnel valleys can be formed under extremely thick glacial ice – examples have been observed on the bottom of Lake Superior and in the oceans offshore in Antarctica. The course of a tunnel valley typically runs from thickest glacial ice to the glacier margin; as a result the glacial ice pressurizes the water such that it runs uphill toward its end. Although there is agreement on

9360-427: The glacier surface (supraglacially), below the glacier (basally) or both. Meltwater may flow either supraglacially or basally as well; the signatures of supraglacial and basal water flow differ with the passage zone. Supraglacial flow is similar to stream flow in all surface environments – water flows from higher areas to lower areas under the influence of gravity. Basal flow exhibits significant differences. In basal flow

9480-453: 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

9600-428: 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

9720-504: 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. The health of

9840-614: 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. 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

9960-420: The ice sheets of Antarctica and Greenland, has been estimated at 170,000 km . Glacial ice is the largest reservoir of fresh water on Earth, holding with ice sheets about 69 percent of the world's freshwater. 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

10080-544: 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 and function as sandpaper, smoothing and polishing the bedrock below. The pulverized rock this process produces

10200-488: The ice, called seracs . 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

10320-483: The ice, while water from basal melting collects under the glacier; either source can form a subglacial lake. The hydraulic head of the water collected in a basal lake will increase as water drains through the ice until the pressure grows high enough either to force a path through the ice or to float the ice above it. If meltwater accumulates, the discharges are episodic under continental ice sheets as well as under Alpine glaciers. The discharge results when water collects,

10440-411: 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

10560-418: The increased pressure can facilitate melting. Most importantly, τ D 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

10680-653: The infill is post-glacial typical of that observed for tunnel valleys. In southern Africa a Permo-Carboniferous tunnel valley system has been identified in northern Cape Province, South Africa. The active formation of tunnel valleys is observed in the present period beneath the Antarctic ice. During the late Ordovician , eastern Gondwana was covered with ice sheets. As a consequence, Jordan and Saudi Arabia exhibit regionally-extensive filled tunnel valley structures. Open-pit gold mines near Kalgoorlie , Western Australia, expose an extensive network of glacially-eroded valleys filled with tillite and shale cut below

10800-418: The influence of gravity . Basal flow under the glacier exhibits significant differences. In basal flow the water, either produced by melting at the base or drawn downward from the surface by gravity, collects at the base of the glacier in ponds and lakes in a pocket overlain by hundreds of metres of ice. If there is no surface drainage path, water from surface melting will flow downward and collect in crevices in

10920-423: 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.) 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. The ablation zone

11040-596: The largest tributary of the St. Joseph River . In the Laurentian Channel offshore eastern Canada, numerous tunnel valleys have been identified originating from the submerged valley of the St. Lawrence River , which is also of glacial origin. Seismic reflection profiles of the fill of tunnel valleys suggest that they are of various ages, with the youngest dating from shortly after the Late Glacial Maximum . They result from erosion by sub-glacial water crossing

11160-495: The largest cross-sectional area in the center of the course and terminate over a relatively short distance in elevated outwash fans at the ice-margin. Tunnel valleys are found to cross the regional gradient – as a result they may be crosscut by modern stream networks. In one example, tributaries of the Kalamazoo River cut at nearly right angles across buried tunnel channel filled with ice and debris. They frequently terminate at

11280-618: The literature over a broad range of locations including the present day Antarctic, and there is evidence that they also occurred in the Laurentian ice sheet and the Scandinavian ice sheet during the Last Glacial Maximum . Work in Iceland has categorised jökulhlaups by origin and size. The categories of origin are: In July 1994, an ice-dammed surface lake drained via a subglacial tunnel through Goddard Glacier  [ sv ] , in

11400-638: The margins of the Laurentide Ice Sheet . Examples of bedrock tunnel valleys in Minnesota include River Warren Falls and several valleys which lie deep beneath till deposited by the glaciers which created them, but can be traced in many places by the Chain of Lakes in Minneapolis and lakes and dry valleys in St. Paul . The Kawartha lakes of Ontario formed in the Late Wisconsinan glacial period. Ice melt from

11520-679: 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. 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. This explains, for example,

11640-445: 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

11760-504: The mountains of Scandinavia , began to rise up the north-European slope, driven by the altitude of the glacial ice accumulation over Scandinavia . Their alignment indicates the direction of ice flow at the time of their formation. They are found extensively in the United Kingdom with several examples reported from Cheshire for example. They are also to be found under the North Sea. Examples of lakes formed in tunnel valleys include

11880-432: The overlying ice is lifted, and the water moves outward in a pressurized layer or a growing under-ice lake. Areas where the ice is most easily lifted (i.e. areas with thinner overlying ice sheets) are lifted first. Hence the water may move up the terrain underlying the glacier if it moves toward areas of lower overlying ice. As water collects, additional ice is lifted until a release path is created. If no preexisting channel

12000-721: The pooling of meltwater at the base of the glacier  — perhaps delivered from a supraglacial lake  — or the simple accumulation of mass beyond a critical "tipping point". 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. The number of glacial earthquakes in Greenland peaks every year in July, August, and September and increased rapidly in

12120-400: The pressure of the ice forces the water to areas of lower ice coverage until it emerges at a glacial face. Hence the configuration of the various tunnel valleys formed by a specific glaciation provide a general mapping of the glacier thickness when the tunnel valleys were formed, particularly if the original surface relief under the glacier was limited. Analyses by Piotrowski demonstrate that

12240-410: 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. 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

12360-479: The role of meltwater in creation of tunnel valleys, several theories are still under consideration for the role of that meltwater: Periodic outbursts of subglacial water have been observed moving subglacial water between subglacial lakes beneath the East Antarctic Ice Sheet. Satellite data recorded a subglacial discharge totaling two km (0.48 cu mi) traveling ~260 km (160 mi) over

12480-467: The same tunnel valley. They have steep sides which are frequently asymmetric . Tunnel valleys frequently include relatively straight individual segments parallel to and independent of one another. Tunnel valley courses may be periodically interrupted; the interruption may include a stretch of elevated esker , indicating the channel ran through ice for a distance. The below-grade sections typically run 5–30 km (3.1–18.6 mi) in length; in some cases

12600-558: 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

12720-413: The sections form a larger pattern of an interrupted channel composed of strings of depressions which can extend from 70–100 km (43–62 mi). The upstream portion – that section furthest into the glacier – consists of a branching system forming a network, similar to the anastomostic branching patterns of the upper reaches of a river (as contrasted with dendritic patterns). They typically exhibit

12840-409: 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

12960-417: The steady state theory appears not to account for erosion into bedrock, which has been extensively observed. There is evidence that meltwater discharges are episodic. This can result because as water continues to collect, more ice is lifted, and the water moves outward in a growing under-ice lake. Areas where the ice is most easily lifted (i.e., areas with thinner overlying ice sheets) are lifted first. Hence

13080-423: 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. This means that small amounts of stress can result in a large amount of strain, causing

13200-438: 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

13320-411: The surface below it. In 1997 analyses concluded that, based on basal meltwater production rates, the annual production of subglacial water from one typical northwestern Germany catchment was 642 × 10 m during the last Weichselian glaciation . Meltwater may flow either above the glacier (supraglacially), below the glacier (subglacially/basally) or as groundwater in an aquifer below the glacier as

13440-453: The surface of the glacier until it drops down a vertical crevice (a moulin ) in the glacier. There it joins subglacial water created by geothermal heat; some portion of the water drains into aquifers below the glacier. Excess subglacial water that cannot drain through sediment or impermeable bedrock as groundwater, moves either through channels eroded into the bed of sediment below the glacier (called Nye channels ) or through channels upward into

13560-456: The tidal environment will show undertow dominated fans. The transitional environment is characterized by both mixed marine and fresh water life in a delta environment. In an essentially dry environment, the glacial flow carries sediment which accumulates much as it would in any stream bed. Ice flow within glaciers results from an increase in the surface slope of the glacier, which result from geographic features combined with an imbalance between

13680-455: The tunnel valley fill stages. The subglacial tunnel valley networks originally formed near the ice margin. Tunnel valleys are likely to fill with sediment as the result of meltwater release during glacial recession. Tunnel valleys fill in two main ways. In the first instance, debris carried by flow settles out and accumulates in the tunnel valley. Subsequently, once the ice has retreated sufficiently, marine deposits may be laid down, depending on

13800-408: The underlying materials and the overlying ice, creating a channel even as the reduced pressure allows most of the glacial ice to settle back to the underlying surface, sealing off the broad front release and channelizing the flow. The direction of the channel is defined primarily by the overlying ice thickness and secondarily by the gradient of the underlying earth, and may be observed to “run uphill” as

13920-496: The water depth at the ice front. The tunnel valley sedimentary record is controlled by meltwater release flow rates and sediment burdens during glacial recession. The sediment found in the tunnel valley provides insight into whether it was laid down in a tidal environment, a transitional environment, or an essentially dry environment with good drainage. In the glaciomarine environment, glacially-related deposits are interbedded with to those similar to those on non-glaciated tidal areas;

14040-418: The water may move up the terrain underlying the glacier if it moves toward areas of lower overlying ice. As water collects, additional ice is lifted until a release path is created. If no preexisting channel is present, the water is initially released in a broad-front jökulhlaup which can have a flow front that is tens of kilometers wide, spreading out in a thin front. As the flow continues, it tends to erode

14160-409: The water, either produced by melting at the base or drawn downward from the surface by gravity, collects at the base of the glacier in ponds and lakes in a pocket overlain by hundreds of meters of ice. If there is no surface drainage path, water from surface melting will flow downward and collect in crevices in the ice, while water from basal melting will collect under the glacier; either source will form

14280-417: The weight of the ice is supported by the underlying water, and the glacier is afloat. 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

14400-473: The world's oil is found in these shale deposits. Although the origin of these deposits is still under study, it has been established that the shale routinely overlies glacial and glacio-marine sediment deposited ~445 million years before the present by the Hirnantian glaciation . The shale has been linked to glacial meltwater nutrient enrichment of the shallow marine environment. Hence the presence of tunnel valleys

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