Arid Landscapes

Question 1

How are dune formed?

Answer 1

The formation of dunes involve the dual action of wind transportation and deposition. When saltating wind grains encounter small patches of sand, their kinetic energy dissipate as they accumulate. A slip face forms when the height of accumulations increases above 30cm and this constant flow of new material would build up as it moves over the crest to the brink; then it avalanches, falls and cascades down as the slip face continually adjusts. The dune migrates in a downward direction in which sands transporting winds are blowing.

Question 2

How are rills and gullies formed?

Answer 2

Rills form as a result of water flowing over bare soil, particularly downhill. With sparse vegetation, water can flow unhindered due to less interception, and fall on the surface directly as rain resulting in rain wash. As water flows over the land as overland flow, it picks up pieces of soil away and this creates a rill. Gullies eventually form when rills get large enough.

Question 3

How are yardangs formed?

Answer 3

• yardangs often form in alternate vertical bands of hard and soft rocks
• the weaker rocks are less resistant and erode more quickly via abrasion
• the harder rocks are more resistant and left as ridges
• as time passes, the ridges become more pronounced and can be up to 15m high
• they occur parcel to the prevailing wind direction as this is the direction in which most erosion occur

Question 4

What are the conditions for loess to form?

Answer 4

• a source of silt (e.g. by past glacial grinding of rocks, or by wind abrasion and weathering processes such as salt weathering and chemical weathering)
• wind to transport the silt
• a suitable site for deposition and accumulation: dust must be deposited on rough surfaces (eg vegetated surfaces) as deposits on a dry and smooth surface are vulnerable to wind erosion and transport

Question 5

How are loess formed?

Answer 5

• winds blow across the desert such as Gobi desert in Asia, it picks up and carries fine particles such as sand crystals made of quartz or mica
• on the edge of the desert, the moisture in the air causes the particles and dust to settle on the ground
• the presence of vegetation such as grass traps the dust and hold it to the ground
• more dust slowly accumulates to form loess