Arid Landscape development in contrasting settings

Question 1

Aeolian landforms (deflation) : Deflation hollows

Answer 1

• Wind removes dry sand, silt and clay. Where this material is ‘scoped’ out, a deflation hollow is left

Question 2

Aeolian landforms (Deflation) : Dessert pavements

Answer 2

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

Question 3

Aeolian landforms : Ventifacts

Answer 3

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

Question 4

Aeolian landforms : Yardangs

Answer 4

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.

Question 5

Aeolian landforms : Zeugen

Answer 5

• 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

Question 6

Aeolian landforms : dunes

Answer 6

• 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

Question 7

Types of dune form

Answer 7

• 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

Question 8

Water landforms : Wadis

Answer 8

• 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

Question 9

Water landforms : Alluvia fans + Bahadas

Answer 9

• 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)

Question 10

Water landforms : Pediments

Answer 10

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

Question 11

Water landforms : Playas

Answer 11

• 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

Question 12

Water landforms : Inselbergs

Answer 12

• 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

Question 13

Water landforms : Mesa

Answer 13

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.

Question 14

Water landforms : Butes

Answer 14

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

Question 15

The relationship between process, time, landforms and landscapes :

Answer 15

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

Question 16

Physical features of different desert landscapes

Answer 16

• 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