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Desert environments

Desert environments and their margins

Objective 1 – know and understand the location of hot deserts and their margins (arid and semi- arid) – climate, soil and vegetation.

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Exam tip

As in all parts of the specification the knowledge of this section can be tested in short, low scoring questions (2 – 4 marks). Command words could include ‘define’, ‘outline’, ‘describe’.
• This section can also be used to test skills such as map description and photo interpretation
• Since deserts can be found on a global scale, you must learn how to describe a global distribution. Simple lists of desert names will not score highly but do name the deserts as examples of a type of location. Distributions should be described in terms of latitude, continentality etc. Look for overall patterns as well as anomalies.

What you need to know:

• Location:

– Hot deserts are found approximately between 25⁰ to 30⁰ N and S of the equator. There are deserts found on the western sides of continents as well as in continental interiors.
– Semi-arid areas are found on the margins of the hot deserts, both north and south e.g. The Sahel is found to the south of the Sahara desert and the southern Mediterranean region is found to the north.

• Climate:

– Temperatures (with annual and diurnal ranges);
– Precipitation amounts (e.g. arid areas are less than 250mm) and type. The annual pattern of precipitation, especially for semi-arid areas.

• Vegetation in hot deserts can be:

– Xerophytes reduce water loss, sometimes storing water. e.g. cactus
Phreatophytes get water from deep underground. e.g. the mesquite tree
– Ephemerals lie dormant until rainfall and then grow quickly and reproduce. E.g. the desert lily
– Halophytes are able to survive in the saltier parts of the deserts. E.g. the saltbush

• Soils in these areas are low in nutrients and organic material. They include:

– Aridisols in hot deserts
– Solonetz soils in semi-arid regions.

Questions

1. Outline the global distribution of hot deserts [4 marks]

2. Describe the vegetation shown in the image. [3 marks]

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3. Describe the main characteristics of desert soils. [4 marks]

Objective 2 – Be able to explain the causes of aridity – atmospheric processes relating to pressure, continentality, relief and cold ocean currents.

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Exam Tips

• Examination questions are likely to be structured, with the mark scheme demanding more than one cause of aridity.

What you need to know:

Pressure: Deserts occur where there is high atmospheric pressure. High pressure means that air is descending. As air descends it compresses and warms up. This increases its ability to hold water vapour and so any water droplets (clouds) are evaporated, giving clear skies. The hot deserts occur at the sub-tropical high pressure zone, between 20⁰ N and S of the equator.

• Winds: Winds that blow from continental interiors are dry. E.g. the NE trade winds blow from the interior of N Africa causing the Sahara desert to extend southwards.

• Continentality: The further away from the oceans, the less water there is in the atmosphere. Again, this can be seen in the Sahara desert.

• Relief: Mountain ranges create a rain-shadow on their down-wind side. This is particularly true of the Atacama desert where the SE winds blow over the Andes and create a rain-shadow to their west.

Cold ocean currents: Air blowing over a cold ocean current, e.g. the Benguela current off Namibia, cools down and causes offshore rain and fogs. Once the air moves over land it is warmed up and any water evaporates.

Questions

4. For one named hot desert, outline the causes of its aridity. [7 marks]

5. Using examples describe and explain the causes of hot desert climates. [15 marks]

Objective 3 – Arid geomorphological processes: mechanical weathering.

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Exam Tips

• Remember, just because this is a short section in the specification, it is just as important as the much longer sections and has the same chance of being asked

What you need to know:

There are 5 main mechanical weathering processes that take place in hot deserts. They are the result of diurnal temperature changes. These are:

• Granular disintegration: Rock types with large crystals e.g. granite break down grain by grain. Dark minerals absorb more heat than lighter coloured minerals. They absorb more heat and expand more the lighter minerals. This causes the rock to disintegrate.

• Block separation: This occurs in well-jointed sedimentary rocks. Joints and bedding planes are lines of weakness in rocks that can become opened up. This can result in angular blocks breaking off cliff faces.

• Shattering: This can occur in some rocks that have a uniform structure. The whole rock can expand and contract until it shatters into sharp-edged fragments.

• Exfoliation: Commonly known as ‘onion-skin weathering, this is caused by the differential expansion and contraction of surface layers of rock. The outermost layer of rock heats up the most, expands more than the interior and so creates a weakness parallel to the surface. This causes this outer layer to peel off.

Frost shattering takes place in mountainous areas within hot deserts. Water, mainly from dew, can be alternately frozen and melted because the high diurnal range takes night time temperatures below 0⁰C. On freezing the water expands and can open up cracks in the rock. These will eventually shatter.

Questions:

6. Explain the processes of mechanical weathering that take place in hot deserts. [6 marks]

7. Explain the role of mechanical weathering in the formation of a named desert landform.[7 marks]

Objective 4 – The effect of wind erosion: deflation and abrasion; transportation.

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Exam Tips

• Questions could be asked about individual aspects of the effect of wind i.e. erosion, transport or deposition.
• Alternatively, questions could be asked about the role of wind in the formation of a named landform.
• This could be the subject of an essay based on the general effects of wind in the desert.suspension; saltation, surface creep; deposition.

What you need to know:

• Erosion:

– Deflation: A process where loose surface material is picked up by the wind and removed from an area. This can expose the underlying rock and create large deflation hollows.
– Abrasion: This occurs when the wind picks up and carries loose sand particles which are then blown against rock surfaces, carving and polishing those surfaces.

• Transportation:

– Suspension: Small particles of sand and dust are picked up by strong winds and can be carried large distances. It is this process which gives sand storms.
– Saltation: A process where coarser sand grains move in a bouncing fashion across the desert surface. Grains are picked up in a gust of wind and then dropped again. The falling sand grain then dislodges more grains. This takes place just above the surface. The abrasion resulting from this moving sand undercuts rocks.
– Surface creep: This is where coarse sand grains are rolled along the ground.

Question

8. Outline how wind moves material of different sizes in the desert. [5 marks]

Objective 5 – Understand the effect of water – sources: exogenous, endoreic and ephemeral. The role of flooding.

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Exam Tips

• Questions could be asked directly on the sources of water in deserts.
• Alternatively, structured questions could be asked about the role of water in the formation of landforms mentioned in section 5.

What you need to know:

• Sources (particularly with regard to rivers):

– Exogenous: Literally meaning that the source of a river is outside the desert. Most exogenous rivers are perennial. Some exogenous rivers flow right through a desert e.g. the River Nile, while others are:
– Endoreic: These are rivers that start outside the desert but flow into an inland sea or lake e.g. The River Jordan flows into the Dead Sea.
– Ephemeral: These are rivers that are short-lived. They tend to flow either seasonally or after intense rainfall.

• Other sources:

– Aquifers: These are rocks below the surface that contain groundwater. This can sometimes be found close to the surface and form small pools, or be accessed by shallow wells. Deep groundwater can be accessed by deeper wells. E.g. the Great Artesian Basin of Australia.

The role of flooding: Flooding is rare in deserts. When it does occur it can result in large amounts of water flowing in what are usually dry river valleys. The lack of vegetation and the weathered surface means that large amounts of rock are removed in a very short time. This material is then deposited when the water dries up. Flooding also causes shallow aquifers to be recharged.

Question:

9. Using an example, describe one source of exogenous water in a hot desert environment. [3 marks]

Objective 6 – be able to understand and explain landforms resulting from wind action – yardangs, zeugen and sand dunes – water action: pediments, inselbergs, mesas and buttes, salt lakes, alluvial fans, wadis and bad lands.

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Exam Tips

• Questions are likely to be of the ‘Describe and explain’ type of structured question. These could be linked to processes outlined in sections 3, 4, and 5.
• Questions could involve the use of photograph interpretation.
• This section could form the basis of an essay question.

What you need to know:

• Landforms resulting from wind action:

• Yardangs: rock ridges that lie parallel to the prevailing wind. The strata are steeply dipping and the edge of the landform is defined by changes in rock type. They are formed by abrasion
• Zeugen: These too are rock ridges that lie parallel to the prevailing wind that are formed by abrasion. The strata are horizontal and the ridges often have a cap of resistant rock. A particular form of this is a mushroom rock.
• Sand dunes: These are formed by the shaping of loose sand by the wind. Their shape is dependent upon their relationship to the wind. They include:
– Barchans: these crescent-shaped dunes form across the direction of the prevailing wind.
– Linear ridges and seif dunes: these dunes form parallel to the direction of the prevailing wind. They may be tens of metres high and hundreds of kilometres long. The difference between them is that the seif dunes are not straight forward ridges, but have serrated crests and sides.
– Star dunes: These form where there is no dominant wind direction.

• Landforms resulting from water action:

• Pediments: Gently sloping rock surfaces found at the base of desert mountain ranges. They are low angled and may be completely bare surfaces or be scattered with boulders. Their origin is uncertain, but water, either sheet flow or meandering ephemeral streams seem to have worked together with weathering to denude the surface.
• Inselbergs: These are isolated, rounded hills. Again, their origin is uncertain, but they probably formed during times of higher rainfall.
• Mesas and buttes: These are a form of inselberg that are made from horizontal layers of rock. They have been cut out and isolated by fluvial erosion. Mesas are table-like plateaux with steep sides, often with a resistant cap rock. Buttes are pillar-like land forms that are the result of the erosion of mesas. Typically they have scree around their base.
• Salt lakes: These are found in basins isolated between mountains. They are formed where endoreic rivers dry up. When the river water dries, dissolved salts in the water are deposited. These salts are commonly rock salt (halite) but can have more complex salts depending upon the nature of the geology the source rivers flowed over. They are usually very flat and devoid of vegetation.
• Alluvial fans: These form where ephemeral rivers flow out of mountains onto a much lower angled surface. As the river emerges from the mountains, it spreads out. The increased friction and resultant loss in velocity cause deposition to occur.
• Wadis: This is a dry river valley in the desert that has been cut by ephemeral streams. They vary in size, but all have flat floors and steep, deeply incised valley sides.
• Badlands: These are found more often in semi-arid landcape. They are found where the rock type is impermeable (e.g. mudstone). The intense rainstorms of this area wash over the surface, eroding rapidly. They are characterised by deeply incised gullies, wadis and alluvial fans.

Questions:

10. Describe the role of wind action in the formation of sand dunes. [7 marks]

11. Assess the importance of wind on the shaping of desert landforms. [15 marks]

12. Describe and explain the importance of water in the formation of desert landforms. [15 marks]

13. Outline the main features of badlands topography. [4 marks]

Objective 7 – Be able to describe and explain desertification: distribution of areas at risk, physical and human causes, impact on land, ecosystems and populations.

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Exam Tips

• The first part of this section could be tested by either a straightforward question about distribution or as part of a map description. If the latter, then look at section 1 to see how to describe global distributions.
• This is a complex area of the specification and is likely to be asked as part of an essay question

What you need to know:

• You have to be aware of what is meant by desertification. The United Nations Convention to Combat Desertification defines it as ‘land degradation in arid, semi-arid and dry sub-humid areas resulting from various factors including climatic variations and human factors.’ This specification is restricted to the arid and semi-arid areas.
• The areas affected are found:

• Along the north coast of Africa, north of the Sahara desert.
• Along the southern fringes of the Sahara desert in the Sahel region
• In southern Africa and Madagascar
• In the southwest of the USA and northern Mexico
• Around the fringes of the Simpson desert in Australia
• In western Argentina.

• Physical causes of desertification:

• The main cause is climate change. Rainfall has decreased in many of these areas and it occurs less often. This is combined with rising temperatures so that the rainfall is less effective. Less rainfall has caused aquifers to dry up.

• Human causes of desertification:

• These include population growth and the resultant increased demand for fuel wood, intensification of agriculture and over-grazing.
• The growth of cash crop farming has pushed subsistence farmers to marginal land that cannot support agriculture.
• Unsuitable irrigation practices.

• Impacts:

• On land: There is a loss in agricultural productivity.
• On ecosystems: The loss of woodland has led to soil exposure and eventual soil erosion.
• On populations: The loss of agricultural land leads to famine and possible starvation. This results in migration to other, slightly better off areas. This can exacerbate the problems there.

Questions:

14. Describe the global distribution of areas at risk from desertification. [4 marks]

15. ‘Desertification is the result of both human and physical causes.’ Discuss this statement and evaluate the contribution made by both groups of causes. [15 marks]

Objective 8 – Provide a detailed case study of desertification in the Sahel – the struggle for survival to include the energy/ fuel wood crisis, water supply, the impact on food supply / farming and livelihoods and coping / management

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Exam Tips

• Experts do not like generic case studies. A good case study is when the Expert is able to read it and see that what has been written is specifically for the named location, rather than a generalised Sahel area.
• You cannot possibly learn details about the whole of the Sahel region. Have one case study prepared in great detail.
• Case studies for this topic are best done at a national scale. Then it is possible to get detail e.g. of deforestation and water supply. The impacts can be given at a smaller scale within the named country.
• Consider each factor with respect to its impact on the struggle for survival.strategies including external aid.

What you need to know:

For your chosen case study you must:

• Have details about demand for fuel wood and the resultant deforestation. The specific impacts of deforestation on your case study area.
• Have details about water supply. The specific impacts of poor water supply on your case study area.
• Changes in the food supply (i.e. the way farming has had to adapt to the changing conditions).
• Any coping strategies that have been adopted. This can be on a national scale (e.g. reforestation) or on a local scale (e.g. the introduction of drip irrigation). These coping strategies must include the role of at least one outside agency (a charity or the United Nations) in helping people in their struggle to survive.

Question:

16. Using a case study, describe the struggle to survive in the Sahel. Assess the sustainability of different strategies used in this struggle. [15 marks]

Objective 9 – Describe and explain managing hot dessert environments and their margins – to consider and evaluate the strategies adopted with regard to land use and agriculture in areas such as the south-western USA or southern Spain. Implications and potential for sustainability.

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Exam Tips

• There is no reason why you cannot use the same case study area in the Sahel that you learned for section 8.
• You only need to have one example from a contrasting area of the world. The bigger the contrast, the easier it is to compare management strategies.
• The specification says ‘land use and agriculture’. This means that answers must have agriculture and at least one other type of land use to get into the higher mark ranges.

What you need to know:

• The management of land in the Sahel (from section 8)
• The management of land in the contrasting region.
• The contrasts should be linked to levels of economic development.
• Be able to evaluate the sustainability of the two management systems.

Question:

17. Assess the potential for sustainability of different land use management strategies in contrasting semi-arid areas of the world. [15 marks]

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