Arid Landscape development in contrasting settings Flashcards Preview

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Aeolian landforms (deflation) : Deflation hollows

- Wind removes dry sand, silt and clay. Where this material is 'scoped' out, a deflation hollow is left


Aeolian landforms (Deflation) : Dessert pavements

- As fine material is removed, coarse material and pebbles are left behind, forming what is known as a desert pavement


Aeolian landforms : Ventifacts

Fin-shaped rocks lying on the desert surface.

Shaped by wind-driven sand (abrasion) 

They have a smooth, flat windward side, and 
sharp narrow edges


Aeolian landforms : Yardangs

Formed when ridges of hard and soft rock run parallel to the direction of the prevailing wind. 

Wind erodes the softer rock (by abrasion) and leaves the ridges standing up as yardangs. 

Wind abrasion concentrates on weak strata; leaving harder material upstanding.


Aeolian landforms : Zeugen

- Collective term for rock pillars, rock pedestals and yardangs which have been undercut where less resistant rock underlies a layer of resistant rock

Zeugens are similar in appearance to yardang but formed on horizontal bedded structures rather than vertical beds. 

Often form where the surface has developed a duricrust (a brittle layer of mineral salts on the surface) 

If this surface gets broken, the softer rock beneath loses its protection so Aeolian erosion quickly strips away the underlying rock


Aeolian landforms :  dunes

- Dunes are mounds and ridges of blown sand

- To form they require a supply of sand, strong and frequent wind and an obstacle to trap the sediment

- Creep and saltation transport the sand up the windward slope, sand calculates on the peak and eventually a small avalanche will occur down the slip face to restore equilibrium

- In this way dunes advance in the direction of prevailing wind


Types of dune form

- Crescent dunes = wider than they are long with a concave slip face. There are two types : Barchans (with horns that face downwind) and transverse dunes

- Seif dunes = linear dunes, straight or slightly curved. More than 100km long and over 200m high with a slip face on alternative sides, cover large areas in parallel, knife edged ridges



Water landforms : Wadis

- steep sided, wide bottomed, gorge-like valleys formed by fluvial erosion

- Rarely filled with water and have a build up of sediment on the valley floor

- Either permanently dry or have ephemeral streams, which are fast-flowing and the result of intense storms


Water landforms : Alluvia fans + Bahadas

Deposition from wadis usually creates alluvial fans as sediment is washed out of the canyon and spreads out in a characteristic fan shape as large amounts of sediment can block channels resulting in a complex network of braiding

- Where many alluvial fans develop it is referred to as a bahada (bajada)


Water landforms : Pediments

- Gently sloping rock platforms found at the base of mountains in hot desert environments


Water landforms : Playas

- Where ephemeral streams flow into inland basins, salt lakes or playas form

- These are often temporary and where the water has evaporated, salts are left behind

- Sodium chloride, the most common, can give the appearance of a beach


Water landforms : Inselbergs

- Rounded, steep-sided hills that rise suddenly from a lowland plain

- They are generally composed of solid crystalline rocks such as granite and thought to be relics of previous geomorphological processes


Water landforms : Mesa


Mesas are plateau like features that are flat on top with steep edges

The land has probably been uplifted and there has been extensive erosion. There will be scree slopes.



Water landforms : Butes


A butte is a version of a mesa which may be smaller (erosion) or separate. It is a bit like a sea stack.


The relationship between process, time, landforms and landscapes :

1. Geographical context = geographical location is used to identify the worlds most well-known deserts

2. Time = timescale over which desert processes operate can vary. Some deserts have rapid and extreme temperature fluctuations over days, others have a slowed daily temperature change, others have been arid for years

3. Process = weathering processes are dependent on daily temperature ranges and the presence of moisture

4. Landforms = Desert processes combine to form a range of erosional and depositional landforms which can be aoleian or fluvial

5. Landscapes = desert landscapes vary widely from plat plants to fields of massive dunes to a variety of characteristic feature. Each landscape shows a unique interaction of process time and landform


Physical features of different desert landscapes

- Arabian = Almost entirely sandy, with some of the largest sand dune systems in the world

- Australia (Great sandy, Simpson and Great Victoria, Gibson and Sturt) = mostly sandy plains, mostly stony surfaces

- Chihuahuan (Arizona, New Mexico, Texas and North central Mexico) = High flat plateau with some stony surfaces and sandy soil, broken by mountain ranges and distinctive mesas

- Kalahari (Southwestern Africa) = Extension sand dunes interpreted with gravel plains

- Mojave (Arizona, California, Nevada) = a varied landscape including sandy soils, gravel pavements and salt flats

- Sahara (Northern Africa) = vast ranges of dunes among mountains and rocky areas, and gravel plains and salt flats

- That (India and Pakistan) = mostly sand dunes with areas of gravel and plains