Lava Lake

Lava lakes are large volumes of molten lava, usually basaltic, contained in a volcanic vent, crater, or broad depression. The term is used to describe both lava lakes that are wholly or partly molten and those that are solidified. Lava lakes can form in three ways:

1. From one or more vents in a crater that erupts enough lava to partially fill the crater
2. When lava pours into a crater or broad depression and partially fills the crater
3. Atop a new vent that erupts lava continuously for a period of several weeks or more and slowly builds a crater higher and higher above the surrounding ground.

As of 2010 there were five volcanoes with persistent lava lakes in the world:

• Erta Ale, Ethiopia
• Mount Erebus, Antarctica
• Kīlauea, Hawaii
• Nyiragongo, Democratic Republic of the Congo
• Marum, Ambrym, Vanuatu

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Crater Lake

A crater lake is a lake that forms in a volcanic crater or caldera, such as a maar, or in an impact crater caused by a meteorite. Sometimes lakes which form inside calderas are called caldera lakes, but often this distinction is not made. Crater lakes covering active (fumarolic) volcanic vents are sometimes known as volcanic lakes, and the water within them is often acidic, saturated with volcanic gases, and cloudy with a strong greenish color. Lakes located in dormant or extinct volcanoes tend to have fresh water, and the water clarity in such lakes can be exceptional due to the lack of inflowing streams and sediment.

Crater lakes form as incoming precipitation fills the depression. The lake deepens until an equilibrium is reached between the rate of water coming in and the rate of water loss due to evaporation, subsurface drainage, and possibly also surface outflow if the lake fills the crater up to the lowest point on its rim. Surface outflow can erode the deposits damming the lake, lowering its level. If the dam erodes rapidly, this can produce a breakout flood.

A well-known crater lake, which bears the same name as the geological feature, is Crater Lake in Oregon, USA. It is located in the caldera of Mount Mazama, hence the name "Crater Lake" is somewhat of a misnomer. It is the deepest lake in the United States with a depth of 594 m (1,949 ft). Crater Lake is fed solely by falling rain and snow, with no inflow or outflow at the surface, and hence is one of the clearest lakes in the world.

The highest volcano in the world, 6,893 metres (22,615 ft) Ojos del Salado, has a permanent crater lake about 100 metres (300 ft) in diameter at an elevation of 6,390 m (20,960 ft) on its eastern side. This is most likely the highest lake of any kind in the world.

Due to their unstable environment, some crater lakes exist only intermittently. Caldera lakes in contrast can be quite large and long-lasting; for instance, Lake Toba formed after its eruption around 70,000 years ago and has an area of over 1,000 square kilometres.

While many crater lakes are picturesque, they can also be deadly. Gas discharges from Lake Nyos suffocated 1,800 people in 1986, and crater lakes such as Mount Ruapehu's often contribute to destructive lahars.

Lakes can also fill impact craters, but these are not usually referred to as crater lakes except in a few isolated cases. Example of such impact crater lakes include Manicouagan in Canada, Lake Bosumtwi in Ghana and Siljan in Sweden.

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Rift Lake

A rift lake is a lake formed as a result of subsidence related to movement on faults within a rift zone, an area of extensional tectonics in the continental crust. They are often found within rift valleys and may be very deep. Rift lakes may be bounded by large steep cliffs along the fault margins.

Examples of Rift Lake

• Lake Khuvsgul in northern Mongolia
• Rift Valley lakes, eastern Africa
• Lake Baikal in Siberia
• Lake Vostok in Antarctica may have formed in a rift setting
• Lake Balaton in Hungary
• Salton Sea, southern California
• The Orcadian Lakes - rift lakes formed during the Middle Devonian in northern Scotland
• Lake Lockatong - a rift lake of Triassic age formed in the Newark Basin.

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Formation of Oxbow Lake

An oxbow lake is formed when a river creates a meander, due to the river's eroding the bank through hydraulic action and abrasion/corrosion. After a long period of time, the meander becomes very curved, and eventually the neck of the meander will touch the opposite side and the river will cut through the neck, cutting off the meander to form the oxbow lake.

When a river reaches a low-lying plain, often in its final course to the sea or a lake, it meanders widely. In the vicinity of a river bend, deposition occurs on the convex bank (the bank with the smaller radius). In contrast, both lateral erosion and undercutting occur on the cut bank or concave bank (the bank with the greater radius.) Continuous deposition on the convex bank and erosion of the concave bank of a meandering river cause the formation of a very pronounced meander with two concave banks getting closer. The narrow neck of land between the two neighboring concave banks is finally cut through, either by lateral erosion of the two concave banks or by the strong currents of a flood. When this happens, a new straighter river channel is created and an abandoned meander loop, called a cut-off, is formed. When deposition finally seals off the cut-off from the river channel, an oxbow lake is formed. This process can occur over a time scale from a few years to several decades and may sometimes become essentially static.

Gathering of erosion products near the concave bank and transporting them to the convex bank is the work of the secondary flow across the floor of the river in the vicinity of a river bend. The process of deposition of silt, sand and gravel on the convex bank is clearly illustrated in point bars.

River flood plains containing rivers with a highly sinuous platform will be populated by longer oxbow lakes than those with low sinuosity. This is because rivers with high sinuosity will have larger meanders and greater opportunity for longer lakes to form. Rivers with lower sinuosity are characterized by fewer cutoffs and shorter oxbow lakes due to the shorter distance of their meanders.

The effect of the secondary flow can be demonstrated using a circular bowl. Partly fill the bowl with water and sprinkle dense particles such as sand or rice into the bowl. Set the water into circular motion with one hand or a spoon. The dense particles will quickly be swept into a neat pile in the center of the bowl. This is the mechanism that leads to the formation of point bars and contributes to the formation of oxbow lakes. The primary flow of water in the bowl is circular and the streamlines are concentric with the side of the bowl. However, the secondary flow of the boundary layer across the floor of the bowl is inward toward the center. The primary flow might be expected to fling the dense particles to the perimeter of the bowl, but instead the secondary flow sweeps the particles toward the center.

The curved path of a river around a bend causes the surface of the water to be slightly higher on the outside of the river bend than on the inside. As a result, at any elevation within the river the water pressure is slightly greater near the outside of the river bend than on the inside. There is a pressure gradient toward the convex bank which provides the centripetal force necessary for each parcel of water to follow its curved path. The boundary layer flowing along the floor of the river is not moving fast enough to balance the pressure gradient laterally across the river. It responds to this pressure gradient and its velocity is partly downstream and partly across the river toward the convex bank. As it flows along the floor of the river, it sweeps loose material toward the convex bank. This flow of the boundary layer is significantly different from the speed and direction of the primary flow of the river, and is part of the river's secondary flow.

When a fluid follows a curved path, such as around a circular bowl, around a bend in a river or in a tropical cyclone, the flow is described as vortex flow: the fastest speed occurs where the radius is smallest, and the slowest speed occurs where the radius is greatest. The higher fluid pressure and slower speed where the radius is greater, and the lower pressure and faster speed where the radius is smaller, are all consistent with Bernoulli's principle.

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Oxbow Lake

An oxbow lake is a U-shaped body of water formed when a wide meander from the main stem of a river is cut off to create a lake. This landform is called an oxbow lake for the distinctive curved shape, named after part of a yoke for oxen. In Australia, an oxbow lake is called a billabong, derived from an indigenous language. The word "oxbow" can also refer to a U-shaped bend in a river or stream, whether or not it is cut off from the main stream.

The Reelfoot Lake in west Tennessee is an oxbow lake formed when the Mississippi River changed course following the New Madrid Earthquake of 1811–1812. There are many oxbow lakes alongside the Mississippi River and its tributaries. The largest oxbow lake in North America, Lake Chicot (located near Lake Village, Arkansas), was originally part of the Mississippi River.

The Oxbow (Connecticut River), a 2.5-mile (4.0 km) bend in the Connecticut River, is disconnected at one end.

The town of Horseshoe Lake, Arkansas is named after the horseshoe-shaped oxbow lake on which it is located.

Cuckmere Haven in Sussex, England contains a widely meandering river with many oxbow lakes, often referred to in physical geography textbooks.

Kanwar Lake Bird Sanctuary, India contains rare and endangered migratory birds and is one of Asia's largest oxbow lakes.

Carter Lake, Iowa was created after severe flooding in 1877 led to the river shifting approximately 1.25 mi to the southeast.

Oxbow lakes may be formed when a river channel is straightened artificially to improve navigation or for flood alleviation. This occurred notably on the upper Rhine in Germany in the nineteenth century.

An example of an entirely artificial waterway with oxbows is the Oxford Canal in England. When originally constructed, it had a very meandering course, following the contours of the land, but the northern part of the canal was straightened out between 1829 and 1834. The work reduced its length from 91 to 77 and a half miles (approximately) and left a number of oxbow-shaped sections isolated from the new course.

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Meromictic Lake, Characteristics and List

A meromictic lake has layers of water that do not intermix. In ordinary, "holomictic" lakes, at least once each year there is a physical mixing of the surface and the deep waters. This mixing can be driven by wind, which creates waves and turbulence at the lake's surface, but wind is only effective at times of the year when the lake's deep waters are not much colder or warmer than its surface waters.

The term "meromictic" was coined by the Austrian Ingo Findenegg in 1935, apparently based on the older word "holomictic". The concepts and terminology used in describing meromictic lakes were essentially complete following some additions by G. Evelyn Hutchinson in 1937.

Characteristics of Meromictic Lake

Most lakes are holomictic; that is, at least once a year, physical mixing occurs between the surface and the deep waters. In monomictic lakes the mixing occurs once a year; in dimictic lakes the mixing occurs twice a year (typically spring and autumn), and in polymictic lakes the mixing occurs several times a year. In meromictic lakes, the layers of the lake water remain unmixed for years, decades, or centuries.

Among the consequences of this stable layering (or stratification) of lake waters is that the deeper layer (the "monimolimnion") receives little oxygen from the atmosphere. The monimolimnion becomes depleted of oxygen. While the surface layer (the "mixolimnion") may have 10 mg/l or more dissolved oxygen in summer, the monimolimnion in a meromictic lake has less than 1 mg/l. Very few organisms can live in this oxygen-poor environment. One exception is purple sulfur bacteria. These bacteria, which are commonly found at the top of the monimolimnion in meromictic lakes, use sulfur compounds for photosynthesis; sulfur compounds are one of the products of sediment decomposition in "anoxic" (oxygen poor) environments.

This type of lake may form for a number of reasons:

• the basin is unusually deep and steep-sided compared to the lake's surface area
• the lower layer of the lake is highly saline and denser than the higher levels of water

The layers of sediment at the bottom of a meromictic lake remain relatively undisturbed because there is very little physical mixing and few living organisms to stir them up, and very little oxygen or chemical decomposition. For this reason corings of the sediment at the bottom of meromictic lakes are important research tools in tracing climate history at the lake.

When the layers do mix for whatever reason the consequences can be devastating for organisms that normally live in the mixolimnion. This layer is of a much smaller volume than the monimolimnion and therefore when they mix the oxygen concentration in mixolimnion will decrease dramatically. This may result in the death of many organisms such as fish that require oxygen.

Occasionally carbon dioxide (CO₂) or other dissolved gasses can build up relatively undisturbed in the lower layers of a meromictic lake. When the stratification is disturbed, as could happen due to an earthquake, a limnic eruption may result. In 1986, a notable event of this type took place at Lake Nyos in Cameroon, causing nearly 1,800 deaths.

While it is mainly lakes that are meromictic, the world’s largest meromictic basin is the Black Sea. Here the deep waters below 50 metres (150 feet) do not mix with the upper layers that receive oxygen from the atmosphere. As a result, over 90% of the deeper Black Sea volume is anoxic water. The Caspian Sea is anoxic below 100 metres (300 feet). The Baltic Sea is persistently stratified with large hypoxic sediment areas below its halocline.

List of meromictic lakes

There are meromictic lakes all over the world. The distribution appears to be clustered, but this may be due to incomplete investigations. Depending on the exact definition of "meromictic", the ratio between meromictic and holomictic lakes are between 1:1000 and 1:3000.

Meromictic Lake in Africa

• Lake Nyos and Lake Monoun in Cameroon
• Lake Kivu in Rwanda
• Lake Tanganyika in Burundi, The DRC, Tanzania and Zambia

Meromictic Lake in Antarctica

• Lake Vanda in Ross Dependency
• Organic Lake in Vestfold Hills

Meromictic Lake in Asia

• Pantai Keracut (Keracut Beach) Lake, Penang National Park, northwest Penang island, Malaysia
• Jellyfish Lake (Ongeim'l Tketau), on Eil Malk in Palau

Meromictic Lake in Australia

• Lake Fidler, in Tasmania's Wilderness World Heritage Area, Australia.

Meromictic Lake in Europe

• Kärntner Seen (Alpine lakes in the Austrian province of Carinthia; studied by Ingo Findenegg in the 1930s).
• Lake Vähä-Pitkusta in Finland.
• Salsvatnet, Kilevann, Tronstadvatn, Birkelandsvatn, Rørholtfjorden, Botnvatn, Rørhopvatn and Strandvatn lakes in Norway.
• Lake Cadagno is a "crenogenic" meromictic lake in Switzerland, and the location of the Alpine Biology Center (Centro Biologia Alpina).
• Lac Pavin and Lac du Bourget in France
• The Black Sea is also considered to be meromictic.

Meromictic Lake in North America

• Ballston Lake, 30 km NNW of Albany, New York
• Crawford Lake near Milton, Ontario
• Fayetteville Green Lake and Round Lake, in Green Lakes State Park near Syracuse, New York
• Glacier Lake, in Clark Reservation State Park near Syracuse, New York
• Great Salt Lake near Salt Lake City, Utah
• Irondequoit Bay near Rochester, New York is also considered meromictic; use of road salt has been cited as the main reason for its change
• Lower Mystic Lake in Arlington and Medford, Massachusetts
• McGinnis Lake in Petroglyphs Provincial Park, Ontario
• Mahoney Lake in the Okanagan Valley, British Columbia
• Pink Lake in Gatineau Park, Quebec
• Redoubt Lake near Sitka, Alaska; one of North America's largest meromictic lakes.
• Soap Lake in Washington
• Sunfish Lake near Waterloo, Ontario
• Devil's Bathtub near Rochester, New York in Mendon Ponds Park
• Blackcat Lake near Dorset, Ontario in Frost Centre
• Chapel Lake, in Pictured Rocks National Lakeshore, near Munising, Michigan

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Endorheic Lake

Endorheic lakes are bodies of water that do not flow into the sea. Most of the water falling on earth finds its way to the oceans through a network of rivers, lakes and wetlands. However, there is a class of water bodies that are located in closed or endorheic watersheds where the topography prevents their drainage to the oceans. These endorheic watersheds (containing water in rivers or lakes that form a balance of surface inflows, evaporation and seepage) are often called terminal lakes or sink lakes.

Endorheic lakes are usually in the interior of a body mass, far from an ocean. Their watersheds are often confined by natural geologic land formations such as a mountain range, cutting off water access to the ocean. The inland water flows into dry watersheds where the water evaporates, leaving a high concentration of minerals and other inflow erosion products. Over time this input of erosion products can cause the endorheic lake to become relatively saline (a "salt lake"). Since the main outflow pathways of these lakes are chiefly through evaporation and seepage, endorheic lakes are usually more sensitive to environmental pollutants inputs than water bodies that have access to oceans.

An endorheic basin is a closed drainage basin that retains water and allows no outflow to other bodies of water such as rivers or oceans. Normally, water that has accrued in a drainage basin eventually flows out through rivers or streams on Earth's surface or by underground diffusion through permeable rock, ultimately ending up in the oceans. However, in an endorheic basin, rain (or other precipitation) that falls within it does not flow out but may only leave the drainage system by evaporation and seepage. The bottom of such a basin is typically occupied by a salt lake or salt pan. Endorheic basins are also called internal drainage systems.

Endorheic regions, in contrast to exorheic regions which flow to the ocean in geologically defined patterns, are closed hydrologic systems. Their surface waters drain to inland terminal locations where the water evaporates or seeps into the ground, having no access to discharge into the sea. Endorheic water bodies include some of the largest lakes in the world, such as the Aral Sea and the Caspian Sea, the world’s largest saline body of water cut off from the ocean.

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Artificial Lake - Reservoir

A reservoir or artificial lake is used to store water. Reservoirs may be created in river valleys by the construction of a dam or may be built by excavation in the ground or by conventional construction techniques such as brickwork or cast concrete.

The term reservoir may also be used to describe underground reservoirs such as an oil or water well.

Types of Artificial Lake or Reservoir

Valley dammed reservoir

A dam constructed in a valley relies on the natural topography to provide most of the basin of the reservoir. Dams are typically located at a narrow part of a valley downstream of a natural basin. The valley sides act as natural walls with the dam located at the narrowest practical point to provide strength and the lowest practical cost of construction. In many reservoir construction projects people have to be moved and re-housed, historical artifacts moved or rare environments relocated. Examples include the temples of Abu Simbel ( which were moved before the construction of the Aswan Dam to create Lake Nasser from the Nile in Egypt ) and the re-location of the village of Capel Celyn during the construction of Llyn Celyn.

Construction of a reservoir in a valley will usually necessitate the diversion of the river during part of the build often through a temporary tunnel or by-pass channel.

In hilly regions reservoirs are often constructed by enlarging existing lakes. Sometimes in such reservoirs the new top water level exceeds the watershed height on one or more of the feeder streams such as at Llyn Clywedog in Mid Wales. In such cases additional side dams are required to contain the reservoir.

Where the topography is poorly suited to a single large reservoir, a number of smaller reservoirs may be constructed in a chain such as in the River Taff valley where the three reservoirs Llwyn-on Reservoir, Cantref Reservoir and Beacons Reservoir form a chain up the valley.

Bank-side reservoir

Where water is taken from a river of variable quality or quantity, bank-side reservoirs may be constructed to store the water pumped or siphoned from the river. Such reservoirs are usually built partly by excavation and partly by the construction of a complete encircling bund or embankment which may exceed 6 km in circumference. Both the floor of the reservoir and the bund must have an impermeable lining or core, often made of puddled clay. The water stored in such reservoirs may have a residence time of several months during which time normal biological processes are able to substantially reduce many contaminants and almost eliminate any turbidity. The use of bank-side reservoirs also allows a water abstraction to be closed down for extended period at times when the river is unacceptably polluted or when flow conditions are very low due to drought. The London water supply system is one example of the use of bank-side storage for all the water taken from the River Thames and River Lee with many large reservoirs such as Queen Mary Reservoir visible along the approach to London Heathrow Airport.

Service reservoir

Service reservoirs store fully treated potable water close to the point of distribution. Many service reservoirs are constructed as water towers, often as elevated structures on concrete pillars where the landscape is relatively flat. Other service reservoirs are entirely underground, especially in more hilly or mountainous country. In the United Kingdom, Thames Water has many underground reservoirs built in the 1800s by the Victorians, most of which are lined with brick. Honor Oak Reservoir, which was completed in 1909, is believed to one of the largest of this type in Europe. The roof is supported on large brick pillars and arches and the outside surface is grassed over.

Service reservoirs perform several functions including ensuring sufficient head of water in the water distribution system and providing hydraulic capacitance in the system to even out peak demand from consumers enabling the treatment plant to run at optimum efficiency. Large service reservoirs can also be managed to so that energy costs in pumping are reduced by concentrating refilling activity at times of day when power costs are low.

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Glacial Lake

A glacial lake is a lake with origins in a melted glacier. Near the end of the last ice age, roughly 10,000 years ago, glaciers began to retreat. A retreating glacier often left behind large deposits of ice in hollows between drumlins or hills. As the ice age ended, these melted to create lakes. This is apparent in the Lake District in Northwestern England where post-glacial sediments are normally between 4 and 6 metres deep. These lakes are often surrounded by drumlins, along with other evidence of the glacier such as moraines, eskers and erosional features such as striations and chatter marks.

The scouring action of the glaciers pulverizes minerals in the rock over which the glacier passes. These pulverized minerals become sediment at the bottom of the lake, and some of the rock flour becomes suspended in the water column. These suspended minerals support a large population of algae, making the water appear green.

These lakes are clearly visible in aerial photos of landforms in:

• Canada
• the northern US
• Russia
• the flat areas of Argentina
• Iceland
• southern New Zealand
• Tibet
• Scotland
• Norway
• Sweden
• Tasmania, Australia

and other regions that were glaciated during the last ice age. The coastlines near these areas are typically very irregular, reflecting the same geological process.

By contrast, other areas have fewer lakes that often appear attached to rivers. Their coastlines are smoother. These areas were carved more by water erosion.

As seen in the English Lake District, the layers of the sediments at the bottom of the lakes can then tell you the rate of erosion by taking into account the rate of erosion of the glacier and its subsequent placement of the sediment. The elemental make up of the sediments are not associated with the lakes the themselves, but by the migration of the elements within the soil, such as iron and manganese.

The spreading of these elements, within the lake bed, are contributed to the condition of the drainage basin and the chemical composition of the water.

Sediment deposition can also be influenced by animal activity; including the distribution of biophile elements, which are elements that are found in organic organisms, such as phosphorus and sulfur.

The less halogen and boron found in the sediments accompanies a change in erosional activity. The rate of deposition reflects the amount of halogen and boron in the deposited sediments.

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Subglacial Lake

A subglacial lake is a lake under a glacier, typically an ice cap or ice sheet. There are many such lakes, with Lake Vostok in Antarctica being by far the largest known at present.

Characteristics of Subglacial Lake

The water below the ice remains liquid since geothermal heating balances the heat loss at the ice surface. The pressure causes the melting point of water to be below 0°C. The ceiling of the subglacial lake will be at the level where the pressure melting point of water intersects the temperature gradient. In Lake Vostok the ice over the lake is thus much thicker than the ice sheet around it.

The water in the lake can have a floating level much above the level of the ground threshold. In fact, theoretically a sub-glacial lake can even exist on the top of a hill, provided that the ice over it is so much thinner that it creates the required hydrostatic seal.

The floating level can be thought of as the water level in a hole drilled through the ice into the lake. It is equivalent to the level at which a piece of the ice over it would float if it were a normal ice shelf. The ceiling can therefore be conceived as an ice shelf that is grounded along its entire perimeter, which explains why it has been called a captured ice shelf. As it moves over the lake, it enters the lake at the floating line, and it leaves the lake at the grounding line.

For the lake to exist there must be a hydrostatic seal along the entire perimeter, if the floating level is higher than the threshold. A hydrostatic seal is created when the ice is so much higher around the lake that the equipotential surface dips down into impermeable ground. Water from underneath this ice rim is then pressed back into the lake by the hydrostatic seal. The ice surface is ten times more important than the bed surface in creating the hydrostatic seal. This means that a 1 m rise in the ice surface at the ice rim is as efficient as a 10 m rise in the bed level below it. In Lake Vostok the ice rim has been estimated to a mere 7 m, while the floating level is about 3 km above the lake ceiling.

If the hydrostatic seal is penetrated when the floating level is high, the water will start flowing out in a jökulhlaup. Due to melting of the channel the discharge increases exponentially, unless other processes allow the discharge to increase even faster. Due to the high head that can be achieved in subglacial lakes, jökulhlaups may reach very high rates of discharge.

Antarctica

A map of 124 subglacial lakes across Antarctica was published in 2009, most of them newly discovered using lasers on NASA's ICESat satellite.

While interior lakes tended to be static, many coastal lakes changed significantly. Some are connected by channels under the ice hundreds of kilometres long.

Water flowing under glaciers can act as a lubricant, causing land ice to accelerate into the sea and add to rising sea levels.

Extraterrestrial

There is also evidence that there are subglacial lakes on Jupiter's moon Europa. Not all lakes with perennial ice cover can be called sub-glacial, though, since there are also those that are covered by regular lake ice. A criterion for glacial ice is that it is flowing. Ice needs to be approximately thirty metres thick to start flowing, so frozen-over lakes are unlikely ever to transform themselves into subglacial lakes.

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Periglacial Lake

A periglacial lake is one formed where the natural drainage of the topography is obstructed by an ice sheet, ice cap or glacier. Periglacial lakes are not typical of areas under the modern Periglacial definition, since most of them formed temporarily during the last deglaciation and are not necessarily associated to landforms created by the freezing of water (glaciers not accounted). A common ecological community in periglacial areas is the tundra.

Extent of periglaciation

Some earth scientists have likened the extent of periglaciation to that of permafrost. Under this definition large areas in Siberia, Canada and Alaska and smaller areas in Fennoscandia, Tibet, Iceland, Greenland, Antarctica and the Andes are periglaciated. Not all scientists agree with this definition as many areas without permafrost show significant action of processes related to the freezing of water.

Periglaciation is the corresponding noun. It means 'periglacial conditions', that is principally, an area of permafrost - intense freezing, perhaps with freeze/thaw of the surface. That is to say, the surface layer melts briefly in summer. Periglaciation occurs near mountain glaciers. At lower levels it forms a zone of cold around continental glaciers in areas of high latitudes, covering perhaps 20% of the earth’s land surface.

Periglacial conditions in the Pleistocene created landscapes and geological conditions moulded by frost action; the repeated freezing and thawing of material over many years. Around a third of the Earth's land surface can be considered as having been subject to periglacial conditions at some time.

Factors affecting location

• Latitude – temperatures tend to be higher towards the equator. Periglacial environments tend to be found in higher latitudes. Since there is more land at these latitudes in the north, most of this effect is seen in the northern hemisphere. However, in lower latitudes, the direct effect of the sun's radiation is greater so the freeze-thaw effect is seen but permafrost is much less widespread.
• Altitude – Air temperature drops by approximately 1 °C for every 100 m rise above sea level. This means that on mountain ranges, modern periglacial conditions are found nearer the Equator than they are lower down.
• Ocean Currents – Cold surface currents from polar regions, reduce mean average temperatures in places where they exert their effect so that ice caps and periglacial conditions will show nearer to the Equator as in Labrador for example. Conversely, warm surface currents from tropical seas increases mean temperatures. The cold conditions are then found only in more northerly places. This is apparent in western North America which is affected by the North Pacific current. In the same way but more markedly, the Gulf Stream affects Western Europe.
• Continentality – Away from the moderating influence of the ocean, seasonal temperature variation is more extreme and freeze-thaw goes deeper. In the centres of Canada and Siberia, the permafrost typical of periglaciation goes deeper and extends further towards the Equator. Similarly, solifluction associated with freeze-thaw extends into somewhat lower latitudes than on western coasts.

Landforms associated with periglacial environments

Periglacial environments shows a wide range of different processes, some of which may occur on other environments. There is no set of processes that are present in all periglacial areas but rather different combinations in each place.

Periglaciation results in a variety of ground conditions but especially those involving irregular, mixed deposits created by ice wedges, solifluction, gelifluction, frost creep and rockfalls. Periglacial environment are from a geomorphological point view relatively stable, and difficult to alter because all the work done on it has make it insensitive, by having for example low slope angles.

Coombe and head deposits Coombe deposits are chalk deposits found below chalk escarpments in Southern England. Head deposits are more common below outcrops of granite on Dartmoor.

Patterned Ground is stones which form circles, polygons and stripes. Local topography affects which of these are expressed. A process called frost heaving is responsible for these features.

Solifluction lobes are formed when waterlogged soil slips down a slope due to gravity forming U shaped lobes.

Blockfields or Felsenmeer are areas covered by large angular blocks, traditionally believed to have been created by freeze-thaw action. A good example of a blockfield can be found in the Snowdonia National Park, Wales. Blockfields are common in the unglaciated parts of the Appalachian Mountains in the northeastern United States, such as at the River of Rocks or Hickory Run Boulder Field, Lehigh County, Pennsylvania.

Other landforms include:

• Palsa
• Pingo
• Rockglacier
• Thermokarst
• Loess

River activity

Most areas under periglaciation have relatively low precipitation (if not the areas would likely be glaciated) and low evapotranspiration. which makes average river discharge rates low. Many rivers flowing into the Arctic sea of northern Canada and Siberia have despite this a very strong erosive capacity due to the fact that thaw occurs first in the upper part of the drainage basin leading to large areas being flooded further down (north) because of obstructing river ice. When these dams melt or break large amounts of water are released with destructive and erosive power.

Chemical and physical weathering

Despite ordinary beliefs there is no predominance of physical weathering over chemical weathering in periglacial areas, however the action of physical weathering is more relatively more important if compared to weathering activity in warmer areas.

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