Landforms in Hot Arid Environments (DP IB Geography)
Revision Note
Hot Arid Wind Landforms
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:
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
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
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
Examiner Tip
You may have to describe the formation of a landform. Refer to shape, size, geology and field relationship (the position of the landform in relation to the landscape). Make sure you are able to draw a labelled diagram of the landform to support your description.
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
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
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
Hot Arid Water Landforms
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
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
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
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
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
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
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