Extreme environments

Notes on IB SL Geography unit 7: Extreme environments

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  • Created by: Tassawar
  • Created on: 14-05-13 20:48

Definitions

Arid: having less than 250mm of precipitation per year.

Active layer: The highly mobile, often saturated surface layer of permafrost that melts in summer and freezes in winter. Its depth can vary from a few centimetres to 5 metres.

Albedo: The reflectivity of a surface. Snow-covered landscapes are highly reflective and have a high albedo, whereas dark surfaces (e.g. vegetation, tarmac) have a low albedo. 

Carrying capacity: The maximum number of individuals that a given environment can support with the resources available and without detrimental effects.

Glacial: characterised by the presence of ice masses, either at high altitude or at the poles. 

Periglacial: Snow and ice cover on the fringe of glaciated areas ("peri" = on the edge of), usually associated with permafrost or ground that remains frozen for at least 2 years. These regions include high mountain and tundra areas of northern Europe and North America.

Semi-arid: Having 250-500 mm of percipitation per year. 

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Distribution of extreme environments

Cold environments. Polar environments are located towards the North/ South pole where levels of insolation are low. In the Northern hemisphere, there is a linear belt of periglacial environment - not found in southern hemisphere. Periglacial environments usually found 60-65 degrees North/ South. Cold environments are also found in high mountains - areas of high ground in Asia e.g. Himalayas or in America - Rockies/ Andies. 

Hot environments: Desert/ semi-desert areas cover up to 1/3 of the Earth's surface. They are located around the tropics and associated with permanent high-pressure systems which limit rain formation. Four factors which encourage desert formation: 

High pressure conditions: e.g. Sahara/ Australian deserts. Due to the Hadley cell model above the equator/ tropics due to sun exposure which is just convection currents causing hot air to rise and cold air to fall, resulting in high pressure/ low precipitation. Continentality: distance from the sea will result in low precipitation e.g. Zambia as the air carrying moisture from the sea have a longer distance to travel and lose all their moisture along the way. Rain-shadow effect: mountains experience drought on the leeward side as the wind travels up and releases all precipitation on the windward side. Proximity to cold upwelling currents: limits the amount of moisture held in the air e.g. off the west coast of South America which limits precipitation. 

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Glacial environments

Glacier: a large body of ice and compacted snow moving slowly down a valley. 

Moraine: a line of loose rocks, weathered from the valley sides and carried by the glacial ice. 

Ablation: losses from the glacier system e.g. melting, evaporation, wind erosion, avalance, sublimation.

Accumulation: gains in the glacier system e.g. snowing.

The amount of and rate of erosion depends on local geology, velocity of the glacier and weight/ thickness of ice. Plucking: mostly occurs at base/ sides. Most effective on rocks weakened by freeze-thaw weathering - as the ice moves, meltwater seeps into joints in rocks, refreezes and rips out the rock to move with the glacier. Abrasion: the debris carried by glacier scrapes/ scratches the rock, leaving striations and grooves. 

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Glacial landforms

Corries: armchair-shaped hollows that have been scooped out by erosion at the head of glacier. They are found on north/ east facing slopes generally. A preglacial hollow is enlarged by nivation. Ice accumulates in the hollow. After reaching the critical weight, the ice moves out in a rotational manner, eroding the floor. Meltwater trickles down the crevasse formed, expanding the corrie via freeze-thaw. Example: Cirque de Gavarnie in Central Pyrenees, France. 

Aretes: formed by two back-to-back corries. Pyramidal peaks: formed by three or more corries. Hanging valleys: valleys that hang in the U-shaped valley that feed into the glacier (formed by tributary glaciers). 

Roche moutonnée: a bare mound of rock that varies in size from a few metres to hundreds high. These arre smoothed on their upvalley side by abrasion, but roughened on down-valley by plucking. They can be several kilometres long. There are examples in Glacier Bay, Alaska. 

Drift: glacial and fluvioglacial deposits left after the ice has melted.   Till: angular/ unsorted glacial deposits that include erratics, drumlins and moraines.     Erratic: a large boulder foreign to the local geology.    Drumlin: a small egg-shaped hill on the floor of a glacial trough. 

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Periglacial environments

Periglacial: relating to or found in a region that borders on a glacier. 

Permafrost: impermeable, permanently frozen ground. 

Freeze-thaw action: the action of water as it freezes and thaws in cracks in rock, causing the rock to shatter. 

Periglacial environments are characterised by permafrost, usually hundreds of metres deep and freeze-thaw action which continuously alters the ground surface. In the summer, the ice near the surface of the soil melts, leaving a marshy, boggy pit which is difficult to travel on. 

Landforms: Permafrost - 20% of the world's surface is covered in permafrost. 3 types of permafrost - continuous, discontinuous and sporadic, associated with mean annual temperatures of -5 to -50; -1.5 to -5 and 0 to 0.5. Above the permafrost is the active layer which seasonally thaws and freezes, and is associated with mass movement. The depth of the active layer depends on the amount of heat recieved. 

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Periglacial areas

Solifluction: "flowing soil". In winter, water freezes in the soil, causing expansion of the soil and segregation of individual soil particles. In spring, the ice melts and water flows downhill. It cannot infiltrate the soil due to impermeable permafrost. As the soil moves, it carries soil particles and deposits them downslope. Solifluction occurs only when temperatures are above zero and free liquid water is available in the active layer. Happens more when surface sediment is saturated. 

Frost creep: when the surface expands (freezing), rock trapped in the surface is pushed normal to the surface. Then, when it melts, it comes down vertically - like long shore drift as wel as down slope due to gravity. 

Rockfalls: occur when rock fragments break away from a cliff face due to freeze-thaw. 

Patterned ground: is a term describing stone-circles and polygons and stripes that are found in soils subjected to intense frost action e.g. on the slopes of Kerio crater, Iceland. On steeper slopes, stripes are more predominant. They are formed due to ice-sorting, differential frost heave, solifluction and the effect of vegetation. 

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Pingos and Thermokarst

Pingo: isolated, conical hill up to 90m high and 800m wide, which only develops in periglacial areas. They form due to movement/ freezing of water underpressure. 2 types of pingos: open-system and closed-system. The water comes from a distant, elevated source, open-system pingos form. If the supply is local, close-system pingos form due to expansion of permafrost - there are ~1,500 pingos found in Canada. They form due to ground-water moving upward from artesian, hydraulic pressure. In both systems, water is forced up due to hydrostatic pressure and intialises the formation of the ice core as the water is pushed up and subsequently freezes. 

Thermokarst: a landscape of hummocks and wet hollows resulting from subsidence caused by the melting of permafrost. This may be because of broad climatic changes or local environmental changes. 

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Hot/ arid environments

The distinctive topography is moulded by water. Due to dryness, wind erosion is also very potent. 

Water erosion: the water comes from the water table (exposed by deflation) and rivers. Rain comes in short, torrential downpours - high runoff. Flash floods rush down steep mountain slopes, gullies/ canyons, carrying a large amount of rock fragment. At the bottom of the desert, the stream spreads out. It then spreads out still more as it washes down to the lowest part of the basin where it may form a playa lake or a salt pan. 

Exotic/ Exogenous rivers: their source are wetter, different environments and then flow through a desert e.g. Nile in Egypt is exotic river, fed from the White Nile. 

Ephemeral rivers: flow seasonally or after storms and often have high discharges and high sediment levels. Even on gentle, 2 degree slopes, discharge can be made. This is due to: impermeable surfaces, limited interception, rainsplash erosion. 

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Weathering in deserts

Salt crystallisation: decomposition of rock by solutions of salt. When temperatues fluctuate, salts expand 300%, creating pressure in rock joints. When the water evaporates, salt is left behind - then the salt crystals expand rather than salt solutions. 

Disintegration: found in hot desert areas where there is a large diurnal temperature range. Temperatures exceed 40 degrees at day and fall below freezing at night. Rocks heat up by day and contract by night. As rock is a poor conductor, stresses occur on the outside layers only, causing exfoliation. 

Weathering produces regolith, an unconsolidated layer of sediment above the rock which can be transported by wind for erosion. 

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Wind erosion in deserts

World's largest deserts are dominated by subtropical, high-pressure weather systems and Kabatan winds. Areas are affected by trade winds, while local winds are important too. Wind action is important in areas where winds are strong/ turbulent/ come from a constant direction for a long period of time. 

Near the surface, wind speed is reduced by friction - but rougher ground = more turbulent. Sediment is moved if there is lack of vegetation. Sediment is moved by suspension, saltation and surface creep. 

Deflation: is the progressive removal of small material, leaving behind larger materials. This forms a stony reg. Deflation may remove sand to form deflation hollows e.g. Qattara Depression, Egypt.

Abrasion: erosion by wind/ water borne particles acting like sandpaper, smoothing surfaces and exploiting weak rocks. 

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