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), Gobi Desert - oasis
endoergic and ephemeral
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
Plateau
Mesa – 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 surface
Also 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