Landforms in Hot Arid Environments (SL IB Geography)

• Each desert landscape is unique due to past and present interactions of rocks and processes operating on them
• Wind and water action are the most important processes, although weathering, mass movement and vegetation also play a role

Wind landforms of erosion

• The wind is responsible for the formation of a number of distinct landforms, including:
  • Deflation hollows and desert pavements
  • Ventifacts
  • Yardangs and zeugens
  • Rock pedestals

Ventifacts

• Ventifacts are faceted cobbles and pebbles that have undergone erosion and shaping by wind-blown sediment 
• Formed in the direction of the prevailing winds
• There are sharp edges separating the leeward side from the windward side

Diagram showing the formation of a ventifact

Yardangs

• Yardangs look like an upturned boat
• They are elongated, streamlined ridges, that are less than 10 m high and more than 100 m long
• They are formed where vertical layers of resistant and less resistant rock are aligned to the direction of the prevailing wind
• Abrasion causes the less resistant rock to erode, leaving behind vertical yardangs of resistant rock and creating deep troughs.
• People are not 100% sure about their formation yet, but due to their alignment with the prevailing winds and the abrasion from sand erosion at their bases, this suggests that wind plays a part in their formation

Yardangs and zeugens form in the same way, but the layers of resistant and less resistant rock lie differently

Zeugens

• Zeugens form in the same way as yardangs, but the layers of resistant and less resistant rock lie horizontally 
• The resulting ridges can be anything from 3 to 30 metres high
• Joints in the resistant rock widen through weathering
• Abrasion deepens the furrows down into the less resistant rock beneath
• Undercutting of the furrows may also occur to give them a pedestal-like shape with a flat cap rock which protects the underlying, less resistant rock
• As the primary process is abrasion, which is concentrated within 2 m of the desert floor, zeugens often have an eroded, narrower base

Pedestal rocks

• Pedestal rocks are also called 'mushroom rocks'
• Thought to be the final remains of a zeugen, they are again primarily formed as a result of aeolian abrasion
• It can also be found in areas where isolated rock peaks are exposed to the surface
• Made of alternating, horizontal bands of sedimentary rock
• Winds carrying fine sand particles act as an abrasive and start cutting and polishing the exposed rock 
• Abrasion works up to a maximum height of 2m
• The softer, least resistant rock is eroded faster than the case hardened upper cap
• Effectively creating a mushroom-like structure 
• Continued erosion leads to the eventual collapse of the pedestal

Image showing the formation of a pedestal 'mushroom' rock

Wind landforms of deposition

• Wind-deposited material occurs as:
  • Sand sheets
  • Ripples
  • Dunes and sand seas

Sand sheets

• These are flat areas of sand with sand grains that are too large to saltate (bounce)
• 45% of all depositional surfaces are of this type, e.g. Selima in South Egypt

Sand ripples

• These are small landforms formed by the wind
• They consist of crests and troughs and develop at right angles to the wind
• The main difference between a ripple and a dune is size, with dunes being taller than about 10 cm

Sand dunes

• The wind eventually blows sand into a network of troughs, crests and ripples that are perpendicular to the wind direction
• They are the consequence of saltation
• They become a dune when the crest is about 30 cm high and the slip-face's angle of repose is 34°
• Dunes grow as sand particles move up the gentle, windward slope by the processes of saltation and surface creep
• The sand particles continually fall over the crest of the dune, onto the steeper, slightly concaved, leeward slope/slip face
• The top of the slip face is steep because it is made of fine-grained sand and it is kept steep by wind eddies
• The bottom of the slip face is gentle, contains coarse-grained sand and may have further sand ripples
• A sand dune can move up to 30 metres a year and can be several metres high
  • The Duna Federico Kirbus, Argentina is the highest dune in the world, measuring 1234 metres in height (2845 metres above sea level)
  • The Big Dipper, Merthyr Mawr, Wales, is home to the tallest dune in the UK and the second-largest in Europe, measuring around 61m

Image showing how a sand dune is formed, dependent on wind and loose sand.

Types of sand dunes

• There are many types of sand dunes but the two most common are: 
  • Barchan dunes
  • Seif or longitudinal dunes

Barchan dunes

• These are the typical crescent-shaped dunes
• Found in isolation in deserts where there is a limited supply of sand but a very dominant wind direction
• Barchan dunes form at right angles to the prevailing wind in one direction
• They have wind-pushed horns that curve in the direction of the slip face

Diagram showing the characteristic features and formation of barchan dunes

Seif or longitudinal dunes

• These are elongated, linear sand dunes
• Most often found in extensive areas of sand known as sand seas
• They can stretch for several hundred metres 
• Formed from two dominant prevailing winds in two different directions
• One blows in one direction for part of the year
• The second blows from the other direction for the remainder of the year
• Seif dunes form parallel to the wind direction and may develop from barchan dunes

Diagram showing the formation of seif dunes

• Despite low rainfall, temporary streams and rivers are powerful land-forming agents
• High-intensity rainfall and lack of plant cover mean that runoff is rapid, creating erosional landforms such as canyons
• Streams and rivers carry high sediment loads, creating depositional landforms such as alluvial fans
• Some of these landforms are still being actively formed (e.g. wadis), whilst others are being gradually removed through weathering, mass movement and erosion (e.g. mesas)

Canyons

• Canyons are deep gorges that usually have a river running through them
• Canyons are formed over millions of years when water rushes through any kind of rock, but especially sandstone and limestone (the Colorado river has cut down vertically to form the Grand Canyon)
• Formed through:  D U D E 
  
  • D: deposition, deposition of sediment from rivers builds up layers of sedimentary rock
  • U: uplift, the newly formed rock layers undergo uplift, where they rise up and form large plateaus
  • D: downcutting, hydraulic action deepens the channel of a stream or valley by removing material from the stream's bed or the valley's floor
  • E: erosion (not wind), erosion wears away at the sides of the plateaus and forms steeper gradients
• Slot canyons are narrow types of canyons that are deep rather than wide
• A special combination of flash flood water and rock is needed for a slot canyon to form, which makes them rare 
• They form when a crack is covered by a flash flood waters pooling in a natural wash/wadi/gully 
  
  • The water seeps into the crack, bringing with it rocks, sediment, and other debris that erode small areas from the inside edges of the crack 
  • Rain, flood, repeat
  • Slot canyons are usually formed in sandstone
• Found anywhere in the world, but are concentrated in the southwestern U.S. and Australia
  • Petra in Jordan, made famous in the Indiana Jones film ‘Last Crusade’, is entered via the 1.2 km (3/4 mile) Siq slot canyon
  • In some places, this canyon narrows to less than 0.5 m across
  • Unlike most slot canyons, Siq was formed through tectonic uplift, splitting the mountain apart, but flash floods have subsequently smoothed the canyon walls

Formation of a canyon

(Slot) canyons have taken a long time to form and are still being formed 

Wadis

• Wadis are channels that were once part of a constant drainage system but now receive run-off from torrential storms
• Water wears away deep trench systems called wadis or arroyos 
• Wadis can be found in lowland areas or as a valley or gully that is cut into a plateau
• Normally, wadis are dry stream channels, but quickly fill after rain and flash floods, which make these channels dangerous
• Wadis come in a range of sizes and shapes, but they usually have steep sides and a flat channel floor
• Wadis have thick sediment layers, loose debris from flash floods and may contain vegetation 

Wadis can vary in shape and size but have similar characteristics 

Plateau

• Plateaus, also called high plains or tablelands, are flat-topped, elevated, sedimentary rock landform that are wider than they are tall
• Many processes form plateaus, including tectonics, lava outflows, and erosion by past river and glacial action
• Plateaus can be considered a relict feature (old feature)
  • The Colorado Plateau or the Colorado Plateau Province is the largest plateau in America
  • It covers an area of 336,700 km², covering northern Arizona, western Colorado, north-western New Mexico, along with southern and eastern Utah

Mesas and buttes

• Formed in flat-lying sedimentary layers, with flat tops, steep eroded sides, and scree slopes, they are wider than taller
• Mesas can occur in groups but are more often isolated
• Mesas are remnant parts of a larger sedimentary table-top plateau
• Buttes (pronounced 'beaut') are remnant mesas that have undergone further weathering and erosion and are taller than they are wide 

Plateaus take millions of years to form; mesas are wide and buttes are taller than they are wide

Pediments

• A pediment forms at the foot of a steep slope or cliff of a receding mountain range by running water
• They have gentle, eroded slopes, or plains, with a low relief of between 2° and 7° 
• They mark the angle of change between the cliff face and the pediment plain
• Alluvium, which comes from upland areas, typically covers a pediment, and sheet-washing removes the sediment to leave a smooth rock surface

Much of the alluvial material on a pediment is in transit, moving during episodic storm events or blown by wind

Inselbergs

• Inselbergs rise abruptly out of desert landscapes and are known as 'island mountains'
• Inselbergs are usually formed from a granite intrusion (pluton), but can also be large sandstone deposits 
• Water (and wind) attack the original surface, leaving a round-topped inselbergs (through exhumation/uncovering) 
• The exposed inselberg has a deep-seated ‘decay’ origin where subsurface weathering has begun rounding the edges
• Or the rock may have extensive subsurface chemical weathering that has broken the inselberg into boulders 
• There are two major forms:
  • Domed inselbergs (bornhardts), e.g. Uluru, Australia
  • Boulder inselbergs (kopjes, rubbins), e.g. Matopos, Zimbabwe

The formation of inselbergs begin underground through chemical weathering, which changes to erosion as the rock is exposed

Depositional water landforms 

• Hot, arid water landforms consist of:
  • Alluvial fans
  • Bajadas
  • Playas
  • Salt pans

Alluvial fans

• In areas with flash flooding, rocks are washed out through a wadi, or canyon and land in an alluvial fan at the edges of mountain ranges
• Due to a sudden loss of energy, rivers leave their mountain channels and join a plain, leaving behind cones of sediment
• Sediment is sorted in size from largest to smallest
• A bajada can be made when several fans come together
• Alluvial fans can extend for several kilometres and reach a depth of 300m

Bajadas

• A bajada is the blending of many alluvial fans and are common in dry climates
• Bajadas can be narrow, from the flow of two or three streams of water, or they can be wide, where dozens of alluvial fans converge
• Bajadas and large alluvial fans are sources of groundwater

Playas 

• Playa lakes form in low areas of deserts following intense precipitation
• They are shallow, often saline, and short-lived, lasting from a few hours to several months
• When the water of a playa lake evaporates, the dry lake bed is referred to as a playa or salt pan
• Playas contain mud-cracks and salt deposits; some are thick enough to mine

Salt pans

• A salt pan is a small bowl or depression in the ground where salt water evaporates and the salt is left behind
• Seawater pools turn into salt pans when they dry up faster than they can be filled up by rain
• As the water evaporates, it leaves behind the minerals precipitated from the salt ions dissolved in the water
• Salt is typically the most abundant of these minerals, accumulating over many thousands of years and giving the surface its hard-white crust