The work of water

Hot, arid environments

Water is essential for the development of desert landforms. - mechanical and chemical weathering- important for erosion

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

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

Endoreic River

Endoreic Rivers drain into an inland lake of sea.

Ex: The River Jordan, which drains into the Dead Sea

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

Canyons

- Usually dry; if there is a river, it is usually exotic- Very deep gorges- Ex: Fish River Canyon in Namibia

Wadis

- Dry gullies that have been eroded by flash floods

- Heavy rainstorms (100 – 250 mm) create rushing torrents on steep slopes

Pediments

- Shallow slopes former at the base of a cliff, steep hill or mountain

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

Wind Action in Deserts

• 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.

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

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.

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

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,..)

Weathering Process

• Occurs when rocks are exposed to hydrosphere and atmosphere

• Breaks down rocks different types of sediment• Boulders• Cobbels• Pebbels• Sand• Silt• Clay

Physical Weathering

• Rocks are broken down without changing chemical composition of the rock

• Different types of physical weathering

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.

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

• 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.

Weathering rates

• Depends on…• Particle size/surface area exposed to the surface• Mineral Composition• Climate

• 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

• Because of weathering and biological processes….

• soil horizons (layers) form.• Also vary from climate and ecosystems in

depth a composition

Weathering in Deserts

• Through salt crystallization– 2 types of salt growth

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

Type 2 (Exfoliation)

• Water evaporates (salt crystals are left behind)• Temp rises, pressure on the rock– Crack

• Both of this factors are frequent in hot desert areas with low rainfall which allows salts to accumulate just below the surface

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