lo 8 stgis
TRANSCRIPT
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The work of water
Hot, arid environments
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Water is essential for the development of desert landforms. - mechanical and chemical weathering- important for erosion
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Sources of water in deserts
- Rainfall (may be low and irregular) & may cause occasional flash floods- Deflation may expose an oasis- Rivers that flow through deserts; can be classified as exotic (exogenous), endoergic and ephemeral
Gobi Desert - oasis
River Nile
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Exotic and Exogenous River
Exotic and Exogenous Rivers have their source in another, mist environment and then flows through a desert.
Ex: Nile in Egypt, being fed by the White Nile, which rises in the equatorial Lake Victoria
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Endoreic River
Endoreic Rivers drain into an inland lake of sea.
Ex: The River Jordan, which drains into the Dead Sea
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Ephemeral Rivers
Ephemeral Rivers flow seasonally or after storms. They show high discharges (Winderosion) and high sediment levels.
This is a result of following factors:- limited interception (lack of vegetation)- rain-splash erosion displacing fine particles, which in turn seal off the surface and make it impermeable
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Canyons
- Usually dry; if there is a river, it is usually exotic- Very deep gorges- Ex: Fish River Canyon in Namibia
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Wadis
- Dry gullies that have been eroded by flash floods
- Heavy rainstorms (100 – 250 mm) create rushing torrents on steep slopes
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Pediments
- Shallow slopes former at the base of a cliff, steep hill or mountain
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Plateau, Mesa and Butte
PlateauMesa – After heavy flash
floods water erodes the plateau and cuts off separate flat hills.
Buttle – worn down mesa
Monument Valley, Utah
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Wind Action in Deserts
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• Many of the world’s deserts are dominated by subtropical high-pressure systems
• Sediment is more likely to be moved if there is a lack of vegetation, and if it is dry, loose and small, which is the case in deserts
• Movement of sediment is induced by drag and lift forces, but is reduced by particle size and friction.
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Deflation: is the progressive removal removal of small material leaving behind larger materials. This forms a stony desert. Deflation may remove sand to form a deflation hollow Qattara Depression in Egypt
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Abrasion: is erosion carried out by wind-borne particles. They act like sandpaper, smoothing surfaces and exploiting weaker rocks.Examples of erosional features carved out by abrasion include yardangs, zeugens and ventifacts.
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Sand particles are moved by three processes:• Suspension- particles light enough to be carried
substantial distances• Saltation- a rolling particle gains sufficient velocity
for it to ‘jump’• Surface creep- larger grains that are dislodged by
saltating grains
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Weathering
Chemical and physical process, change the characteristics of earth’s surfaceAlso the preparation for Erosion
Human processes such as pollution, can significantly speed up chemical rain
(acid rain,..)
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Weathering Process
• Occurs when rocks are exposed to hydrosphere and atmosphere
• Breaks down rocks different types of sediment• Boulders• Cobbels• Pebbels• Sand• Silt• Clay
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Physical Weathering
• Rocks are broken down without changing chemical composition of the rock
• Different types of physical weathering
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Types of Weathering
• Frost action/ice wedging is the breakup of rock caused by the freezing and thawing (contracting and expansion) of water
• Abrasion is the physical wearing down of rocks as they rub or bounce against each other. This process is most common in windy areas, under glaciers, or in stream channels.
• Exfoliation is the peeling away of large sheets of loosened materials at the surface of a rock. Common in shale, slate, and mica.
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Chemical Weathering
• Rock broken down by chemical action resulting from changing the chemical composition of the rock
• Main factors of chemical weathering are: oxygen, rainwater, carbon dioxide, and acids
• produced by decaying plants and animals that leads to the formation of soil
• Few types of chemical weathering
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• Oxidation occurs when oxygen interacts chemically with minerals (iron and oxygen rusts)
• Hydration occurs when water interacts chemically with minerals (hornblende and feldspar unite with water they eventually form into clay)
• Carbonation when carbon dioxide interacts with minerals. (when CO2 dissolves in water carbon acid dissolves large masses of limestone, creating caves and caverns sink holes, karst topography, stalactites and stalagmites.
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Weathering rates
• Depends on…• Particle size/surface area exposed to the surface• Mineral Composition• Climate
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• Major product of weathering is soil• combination of particles of rocks, minerals, and
organic matter produced through weathering processes
• nutrients to support various forms of plant and animal life
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• Because of weathering and biological processes….
• soil horizons (layers) form.• Also vary from climate and ecosystems in
depth a composition
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Weathering in Deserts
• Through salt crystallization– 2 types of salt growth
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Type 1 (Exfoliation)
• In areas where temp. fluctuate between 26-28 degrees
• Sodium sulphate and sodium carbonate expands to 300%
• Created preassure on rock crack
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Type 2 (Exfoliation)
• Water evaporates (salt crystals are left behind)• Temp rises, pressure on the rock– Crack
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• Both of this factors are frequent in hot desert areas with low rainfall which allows salts to accumulate just below the surface
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Disintegration
• In hot deserts where a large range of diurnal temperatures occur.
• Rocks a bad temp. conductors which leads to tension within the rock which causes stress only in the outer layers (exfoliation)
• However moisture is essential for this to happen, often together with the pressure of the salts