The Role Of Wind In Systems And Processes And Arid Lanscape Development

Question 1

Using examples, explain the difference between the physical and chemical weathering processes in hot desert environments

Answer 1

Chemical weathering in deserts is the process leading to the decomposition or breaking down of rocks due to chemical reactions. This most often requires the presence of water, and/ or exposure to the air. Includes crystal growth, hydration, hydrolysis and oxidation.

Physical weathering in deserts involves extreme changes in temperatures. The high range causes a rapid breakdown due to the lack of vegetation or well developed soils. Includes thermal fracture (insolation weathering), granular and block disintegration, and exfoliation

Thermal fracture
When the large diurnal range (rapid heating and cooling) causes the rocks to expand and contract leading to fractures.

Exfoliation
‘Onion skin weathering’ - breaking, splitting or peeling of the outer rock layers.

Oxidation (Type of Chemical weathering)
When rocks containing iron are exposed to oxygen in the air or water. The iron reacts with the oxygen to form iron oxide which is weak, meaning the rock crumbles easily

Granular disintegration
Individual particles f rock contracting and expanding at different rates causing individual grains to break off

Block disintegration
The constant heating and cooling causes splitting along faults in the rock, causing pieces to break off

Question 2

Wind and aeolian processes are very common features of hot deserts environments and their margins because ?

Answer 2

1. The cloudless skies and high angle of incidence means air at the surface is heated and rises, so cooler air moves in to replaced it. It is this movement that creates winds.
2. Many desert regions are relatively barren with a free features to create friction to reduce wind speed or provide shelter. Therefore, winds can blow unimpeded for considerable distances.

Question 3

What are the two process of erosion by wind? And explain them?

Answer 3

1. Abrasion (also referred to as sandblasting)
   Where the arterial carried by the wind hits exposed rock surfaces and creates a range of erosional features.
2. Deflation
   Wind removes dry, unconsolidated sand, silt and clay particles from the surface and transports it away.

Question 4

What factors affect the rate of abrasion?

Answer 4

• the direction
• the frequency and velocity of the wind
• the lithology of the rocks
• size and nature of the particles carried in the wind

Question 5

What causes a desert pavement pavement?

Answer 5

When deflation occurs the wind only removes the finer material creating a surface covered in a concentration of coarse and fine pebbles known as desert pavements. Large stones left behind as lag deposit. Exposed to desert varnish

Question 6

What causes deflation hollows?

Answer 6

A depression caused by strong winds which removes loose materials from the surface. A underlaying basin structure within rocks in covered by a thick accumulation of sand. The strong winds remove some of the loose material to reveal part of the basin shape. The depth of deflation hollows in controlled by underlying groundwater. Can form playas in water table is high enough
- some of largest hollows are found in North Africa, where the extend over hundreds of quart kilometres.

Question 7

What are the factors determine the transportation by wind?

Answer 7

• the wind strength and direction
• amount of turbulence
• duration of the wind
• the relief and surface features
• the amount and nature of any vegetation

Question 8

What are the three main aeolian processes to transport material?

Answer 8

1. Suspension
   Transportation by wind where the smallest particles, generally less than 0.2mm, are held in the air.
   - With strong and persistent winds, clouds of dust can be carried significant altitudes and over hundreds or thousands of kilometres. Eg, when the ‘red rain’ of Saharan sand occasionally falling in the British isles.
2. Saltation
   A process where sand sized particles are transported by bouncing and hopping along the surface.
   - particles are usually transported at one half to one third of the speed of wind
3. Surface creep
   Where saltating particles return to the surface and hit larger particles that are too heavy to hop; they slowly creep (slide or roll) along the surface from a combination of the push of the saltating grain and the movement of wind
   - believe surface creep accounts for as much as 25% of all grain movement in hot deserts

Question 9

What is the role of deposition in hot desert environments?

Answer 9

1. Deposition occurs when the velocity of the wind decreases until it no longer transports the grains it is carrying.
2. Wind deposited materials can often give clues to the scale and direction of winds in the past.
3. For example, analysis if the huge wind-blown deposits of silt (loess) on the Loess Plateau in China, and similar but smaller accumulations in Europe and the Americas can help map the pattern of previous surface winds.
4. On a more local level, deposition may occur as winds meets an obstacle and is slowed on the downwind side. This can lead to the formation of dunes.

Question 10

What are Ventifacts?

Answer 10

Small scale features found at desert pavements. Individual exposed rocks with clearly eroded face aligned with prevailing winds lying on the desert surface.

Question 11

What are yardangs?

Answer 11

A stream lined parallel ridge of rock
Elongated ridges separated by deep grooves cut into the desert surface (VERTICALLY LAYERED ROCK). Weaker rocks are eroded by abrasion to form deep troughs. Hard rock is left standing (this is the yardang).

Question 12

What are Zeugens?

Answer 12

The collective name for features such as rock pillars, rock pedestals and rock mushrooms

Develop in HORIZONTALLY LAYERED ROCK. Most of the abrasion in concentrated within a metre of the desert surface which creates a narrow lower portion. This can create rock pedestals.

Question 13

What are Barchan dunes?

Answer 13

Crescent-shaped sand dunes. Also known as transverse dunes.
Form in areas of limited sand supply and unidirectional winds.
Dunes usually stat with deposition on leeward side of rock. As more sand it deposited it becomes shaped by wind.

Question 14

What are Seif dunes?

Answer 14

Knife-edged ridges of sand separated by wind scoured depressions
They develop under the influence of multiple winds. Is alternates which elongates the longitudinal dune. Over time more slip faces develop.
Can reach up to 200m high and 100km long.