10 Arid and semi-arid environments
the strata are vertical. These ridges can be as high as 100 m and stretch for many kilometres.
Sand dunes
Winds deposit the sand they carry as dunes. There are many types of dune. Their shape and size depend on the supply of sand, direction of wind, nature of the ground surface, and presence of vegetation. Only about 25–33% of the world’s deserts are covered by dunes and in North America only 1–2% of the deserts are ergs (sandy). Large ergs are found in the Sahara and Arabia.
Typical mistake Many students forget that most deserts are stony and that sand dunes only occur in a relatively small number of deserts.
Types of dune
Nebkhas are small dunes formed behind trees or shrubs, whereas lunette dunes are formed in the lee of depressions. Barchan dunes are crescent-shaped and are found in areas where sand is limited but there is a constant wind supply. They have a gentle windward slope and a steep leeward slope up to 33°. Parabolic dunes have the opposite shape to barchans – they are crescent shaped but point downwind. They occur in areas of limited vegetation or soil moisture. Linear dunes or seifs occur as ridges 200–500 m apart. They may extend for tens, if not hundreds, of kilometres. It is believed that some regularity of turbulence is responsible for their formation. Where the winds come from many directions, star dunes may be formed, with limbs extending from a central peak. Star dunes can be up to 150 m high and 2 km wide.
Now test yourself 6 Distinguish between barchan dunes and parabolic dunes. 7 Define the terms exogeneous, endoreic and ephemeral with respect to desert rivers. 8 Explain how (a) alluvuial fans and (b) pediments are formed.
Answers on p.220
Tested
10.3 Soils and vegetation Nutrient flow Deserts have low rates of biomass productivity. On average net primary productivity is 90 g/m2/yr. This is due to the limited amount of organic matter caused by extremes of heat and lack of moisture. Productivity can generally be positively correlated with water availability. Owing to the low and irregular rainfall, inputs to the nutrient cycle (dissolved in rain and as a result of chemical weathering) are low (Figure 10.5). Most of the nutrients are stored in the soil, and there are very limited stores in the biomass and litter. This is due to the limited amount of biomass and litter in the desert environment. In some deserts nutrient deficiency (especially nitrogen and/or phosphorus) may become critical. The rapid growth of annuals after a rain event rapidly depletes the store of available nutrients, while their return in decomposition is relatively slow. Despite the extreme short-term variability of the desert environment, the desert ecosystem is considered, in the long-term, to be both stable and resilient. This is due to the adaptations of desert organisms to survive water stress – in some cases for years.
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Cambridge International AS and A Level Geography Revision Guide
Revised
