Hot Desert Landforms (AQA A Level Geography)

Aeolian landforms of erosion

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

Ventifacts

• Ventifacts are faceted cobbles and pebbles that have been abraded and shaped by wind-blown sediment
• Formed in the direction of the prevailing winds
• The windward side is separated from the leeward side by sharp edges

Diagram showing the formation of a ventifact

Yardangs

• Yardangs look like an upturned boat
• They are elongated, streamlined ridges, that are less than 10m high and more than 100m long
• They are formed where vertical layers of resistant and less resistant rock are aligned to the direction of the prevailing wind
• The less resistant rock is eroded by abrasion, forming deep troughs and leaving behind vertical, yardangs of resistant rock
• 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

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 3-30m 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 2m of the desert floor, zeugens often have an eroded, narrower base

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

Pedestal rocks

• Pedestal rocks are also called 'mushroom rocks'
• Thought to be the final remains of a zeugen and are again primarily formed as a result of aeolian abrasion
• 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

Aeolian 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
• As the wind increases the wavelength and height, the crest height increases too
• Sand ripples form with a combination of saltation and surface creep
• Sand is removed from one crest to another, as sand is eroded from the windward side and deposited on the leeward side of the crest
• Asymmetrical ripples form when wind flows in a single direction
• Symmetrical ripples form where winds flow in two directions
• The main difference between a ripple and a dune is size, with dunes being taller than about 10 cm

Asymmetrical                                                                     Symmetrical

Asymmetrical ripples are non-uniform, whereas symmetrical ripples show distinct ridges and grooves

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
• Accumulations of sand build into mounds and ridges
• 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 is kept steep by wind eddies
• The bottom of the slip face is gentle and contains coarse-grained sand and may have further sand ripples
• A sand dune can move up to 30m a year and can be several meters high
  • The Duna Federico Kirbus, Argentina is the highest dune in the world, measuring 1234 meters in height (2845m 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 horns that curve towards the slip face, which are pushed forward by the wind

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, means 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

• A deep gorge, generally with a river running through it; due to the river helping to form the canyon in the first place
• Canyons are initially formed over millions of years, when water rushes through any kind of rock, but especially sandstone or limestone
• Exogenous rivers such as the Colorado (which rises in the Rocky Mountains, USA), form well beyond the desert margin and flow all year round 
• The Colorado river has cut down vertically to form the Grand Canyon
• The canyon in places is over 2000m (1 mile) deep, has steep sides due to its rock structure and lack of rainfall to degrade them
• 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 aeolian), 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
• An extreme example of a deep but narrow slot canyon would be one that is 1m or less at the top with a 30m or deeper drop
• A very 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 water 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, but not exclusively 
• 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 1.2km (3/4 mile) Siq slot canyon
  • In places, this canyon narrows to less than 0.5m across
  • Unlike most slot canyons, Siq was formed through tectonic uplift, splitting the mountain apart, but flash floods have subsequently smoothed the canyon walls

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

Wadis

• Channels which were once part of a perennial drainage system, now receive run-off from torrential storms
• Unhindered by vegetation, water erodes deep channel systems known as wadis or arroyos
• Wadis are found in lowland plains or as an incised valley/gully cut into a plateau
• Normally wadis are dry stream channels, but quickly fill after rain, and flash floods, which make these channels dangerous
• Wadis can vary in size and shape, and usually have steep sides, with a flat channel floor, showing channel braiding from water flow
• 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

• Flat topped, elevated, sedimentary rock landform
• Plateaus, also called high plains or tablelands, have a large, flat topped area with at least one steep sided slope, and rises sharply above the surrounded area
• Many processes form plateaus; including collisions of tectonic plates, upwelling of volcanic magma, extrusion of lava, and erosion by past fluvial and glacial action
• Plateaus can be considered a relict 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
• They have flat tops, steep eroded sides and scree slopes
• Wider than they are tall
• Mesas can occur in groups, but are more often stand 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 million of years to form; mesas are wide and buttes 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, erosional surface 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
• A pediment is typically covered by alluvium, derived from upland areas, and sheet-wash removes the sediment, creating 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

Formation of Uluru

• The sandstone inselbergs of Uluru and Kata Tjuta started to form about 550 million years ago, when water flowed from the Peterman Ranges bringing sand and rock to the plains in fans, one fan was mostly sand and the other rock
• Around 500 millions years ago, the area was covered in water, which compressed the mud and rock to form sandstone and conglomerate of basalt and granite, with sandstone cement
• Then around 400 million years ago, the sea retreated and tectonic shift folded, uplifted and tilted the region
• Over the last 300 million years, aeolian erosion has removed the softer rocks leaving behind the domed inselberg of Uluru and the boulder inselberg of Kata Tjuta behind
• These formations are much larger than they appear, as most of their mass is below the surface
• Uluru stands 348m above its plain, with a circumference of 9.4 km and a minimum depth of 2.5 km below ground
• Kata Tjuta (the Olgas) are a group of 36 monoliths (bolder inselbergs) with a total circumference of 22 km
• Both Uluru and Kata Tjuta are only the tips of a rock slab that continues underground for up to 6 km, with a distance of approximately 46 km

Uluru and Kata Tjuta are a slab of sandstone rock that stretches approximately 46km and up to 6km in depth

• Depositional fluvial landforms consist of:
  • Alluvial fans
  • Bajadas
  • Playas
  • Salt pans

Alluvial fans

• At the edges of mountain ranges, sediment is washed out through a wadi or canyon and deposited to form an alluvial fan
• Cones of sediment are deposited by rivers owing to an abrupt loss of energy as they leave their mountain channels and enter a plain
• Sediment is sorted in size from largest to smallest
• Found in desert areas with flash flooding
• Several fans may coalesce to form a bajada
• Alluvial fans can extend for several kilometres and reach a depth of 300m

Bajadas

• A bajada is the convergence, or blending, of many alluvial fans
• Bajadas are common in dry climates, such as the canyons of the American Southwest
• 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

• Salt pans are shallow containers or depressions in the ground in which salt water evaporates to leave a deposit of salt
• A salt pan is created when pools of seawater evaporate at a rate faster than it is replenished by rainfall
• 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, giving the surface its hard-white crust