Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Learning episodes-1 Location and characteristics of hot deserts and their margins (arid and semi-arid) – climate, soils and vegetation. Causes of aridity – atmospheric processes relating to pressure, winds, continentality, relief and cold ocean currents Arid geomorphological processes: mechanical weathering. The importance of wind – erosion: deflation and abrasion; transportation; suspension, saltation, surface creep, deposition. Landforms resulting from – wind action: defletion hollows, yardangs, zeugen and sand dunes – 1 Learning episodes-2 The importance of water – sources: exogenous, endoreic and ephemeral. The role of flooding Landforms resulting from water action: pediments, inselbergs, mesas and buttes, salt lakes, alluvial fans, wadis and badlands. Desertification: distribution of areas at risk, physical and human causes, impact on land, ecosystem and populations. Case study of desertification in the Sahel Managing hot desert environments and their margins – to consider and evaluate the strategies adopted with regard to land use and agriculture in areas such as the Sahel and contrast with the development of areas such as south western USA or southern Spain. Implications and potential for sustainability. 2 JUN10: Distinguish between arid and semi-arid areas. (2) Arid areas receive less than 250mm of rainfall per year (1), whilst semi-arid are those areas receiving between 250 and 500mm rainfall per year (1). The semi-arid areas are found on the fringes of arid areas – the deserts (1). Describe the distribution of arid and semi-arid areas shown in Figure 7. These areas mainly occur between 30 degrees north and south of the Equator. With the exception of an area in east Africa, they are not found at the Equator. They are on the western sides of continents and extend inland to varying degrees – this is most limited in South America and most extensive cross North Africa, across the Middle East into Asia. The semi-arid areas are generally on the edges of the arid ones. These areas are most extensive in North Africa, Asia and Australia. 3×1. JAN 13: Describe the location of areas of hot deserts (no map) (4) Hot deserts are located near to the lines of the Tropics of Cancer and Capricorn between approximately 20° and 30° north and south. They are usually found on the western side of continents – such as the Atacama in South America and Sonoran Desert in North America. Sometimes, they extend the width of the continent – as in the case of the Sahara in North Africa. There are no deserts present in the vicinity of the Equator. Hot deserts are present in all continents, with the exception of Europe and Antarctica. Inland if qualified. In lee of mountains. Allow up to two for naming locations. 4×1 Negative water balance Amount of rainfall not enough to define hot deserts. Need to link hot deserts to the water balance. Key characteristic 2: very high rate of potential evapotranspiration Negative water balance since the amount of precipitation received is much less than the potential evapotranspiration . Aridity Index 1.2 Characteristics of hot deserts and their margins (arid and semi-arid) – climate, soils and vegetation. (a) Climate Figure 3 Climate graph for In Salah, Algeria • Yearly Range is high (13 – 37 deg C°) • Diurnal range is high (daily) up to 30 deg C° • Level of rainfall < 250mm / year • Rainfall is sporadic • Level of humidity is low. JUN09 Describe the characteristics of the hot desert climate. (4) Temperatures are characterised by large annual ranges (1) with figures to support related to location (1). Need to say more than hot - needs an appropriate figure, e.g. approx 20 to 30 + degrees C (1). Diurnal range is also high (1). 15 - 30º C (1). Very high temperatures have been recorded in the shade - upper 50’s in Death Valley and Libya (1).Rainfall is low - usually defined as less than 250mm p.a.(1), but often much lower, e.g. Death Valley averages 40mm per year (1). It is also unreliable - there may be a number of years without any rainfall (1). Often very intense, and thunderstorms when it does fall (1). Low relative humidity. Winds tend to be local and seasonal (1) e.g. harmattan- a dry, dusty easterly or northeasterly wind on the West African coast, occurring from December to February-. (1). 4 x 1 Soils: key words • Organic: Derived from living things eg plants, animals. • Inorganic: Derived from non living things eg weathered rock. • Humus: The amorphous, ordinarily dark-colored, colloidal matter in soil; a complex of the fractions of organic matter of plant, animal, and microbial origin that are most resistant to decomposition. The stuff that plants need to grow. Main characteristics of a desert soil (aridisol): - yellow-red to grey brown in colour -very thin -low organic content (under 2%) -high pH value, 7.0-8.5 A profile is a vertical section cut through a soil from the surface to the underlying bedrock. It reveals a number of layers, or horizons, each with its own characteristics. Why? Soil is a mixture of organic and inorganic material. Deserts tend to have very poorly developed and thin soils, due to: - lack of moisture (hinders production of humus from dead vegetation) - high rates of evaporation - spare vegetation cover. (lack of organic matter) The climate is too dry and the vegetation too sparse for any significant chemical weathering of bedrock or the accumulation of organic material. In the few places where the water table is close to the surface, moisture can be drawn upwards by capillary action. Salts and mineral bases (magnesium, calcium and sodium) are also drawn upwards and deposited in the upper layers to give a slightly alkaline soil. What is vegetation like in a hot desert environment? Vegetation has to have a high tolerance to the moisture budget deficit, intense heat and, often salinity - Few species, sparsely distributed (sporadic cover) Little can grow due to the low level of precipitation, the high level of evaporation, high temperatures and strong sunshine. They are xerophytes -: They are adapted to withstand drought. Succulents are part of this group. They can store water in their tissues. Many succulents have fleshy stems and some have swollen leaves. Cacti absorb large amounts of water when it rains. Transpiration takes place from the stems, but it is reduced by the stomata closing during the day. -Most plants have small, spiky or waxy leaves to reduce transpiration. Some plants have silver coloured leaves which reflect sunlight and so plants stay cooler and minimise transpiration. -Roots are either very long to tap groundwater supplies- those of the acacia exceed 15 m- or spread out over wide areas near to the surface to take the maximum advantage of any rain or dew, like those of the creosote bush. - Seeds can lie dormant for months or several years until the next rainfall. Many plants are ephemerals . They have a short life cycle. They come to life only when it rains, they complete their life cycle by flowering and germinating their seeds in up to 3 weeks. An example is the Boerhavia found in the Sahara. -Others, like the saltbush, are halophytic- adapted to salty conditions – and can survive in salty depressions. Oasis vegetation Oasis, Sahel Oasis vegetation is found where there are sources of water in the desert. The classic oasis form where a layer of permeable rock has taken water from outside the desert. The layer then runs into an aquifer beneath the surface, carrying water between impermeable rocks. The aquifer comes to the surface because of erosion or folding (like the picture). Plant grow nearby. Most oases are now settled and cultivated. • There are two types of vegetation clearly shown – cover is sporadic. One type is a cactus (Saguaro) that stands above the other vegetation. It is mainly one large stem, with limited branches emerging from the main trunk. It is dark green in colour, but covered in white needles. The smaller shrubs (creosote bushes) have many branches which are quite thin. The leaves are plentiful, but small and light green in colour. • Allow up to 3 on either vegetation type, up to 2 for naming species shown. • 4×1 • Explain how vegetation in hot deserts is adapted to the climate. (7) Small leaves (and their waxy nature) to reduce evapotranspiration; the stomata on these plants close during the day, minimising water loss. The cactus is a succulent and water is stored in the fleshy stem and branches to be used during periods of dryness. Cacti have many long shallow roots allowing them to maximise intake of water during periods of rainfall. Other plants like the creosote bush have deeper roots and search for water downwards and where there is permanent groundwater. Some plants have silver coloured leaves that reflect sunlight and so plant stays cooler. There may be reference to plants that have a short growing season following rain to avoid the lack of water – these may be annuals that germinate, flower, seed and die quickly or perennials that have limited bursts of activity before becoming dormant again. • Marks here are for explanation only with reference to vegetation. 3- Causes of aridity. Atmospheric processes related to a) Atmospheric pressure and winds (global atmospheric circulation) b) Continentality c) Relief d) Cold ocean currents a) Global atmospheric circulation Annotate Figure of global atmospheric circulation to explain why most deserts are found around 30° North and South of the Equator Stable, clear skies occur here, with few clouds. It is dry because most water vapour was dropped as convection rainfall near the equator. This creates regions known as sub-tropical anticyclones. The air that has risen in the convection currents stops when it reaches a layer in the atmosphere called the tropopause. It cannot continue to rise, so is forced to flow Pole wards. As air flows away from its main heat source, the equator, it sinks. Cooling makes the air denser, and much of this denser air sinks back towards the earth and 30˚ north or south of the equator. (Descending limb of the Hadley Cell.) The ITCZ - Inter Tropical Convergence Zone - is an area of low atmospheric pressure that forms where the Northeast Trade Winds meet the Southeast Trade Winds near (actually just north of) the earth's equator. This causes thick clouds and torrential rain. (Rising limb of the Hadley Cell.) The hot air contains large amounts of water vapour, evaporated from the ground or transpired from vegetation. This heated air rises in the form of convection currents and then cools, creating an area of low pressure known as the ITCZ. The sun’s rays are most concentrated here – leading to the heating of the air lying above the ground surface. Booklet page 29 • Outline how the atmospheric circulation is responsible for the aridity in the Sahara. (4) • The Sahara Desert is found in an area of high pressure (1). • Here air that has risen near the Equator/further north in temperate low (1) • has cooled/become denser (1) • and so sinks to the surface (1). • It is dry and its descent makes rainfall improbable (1). • The North East Trades blow offshore from the continental interior (1), • these have blown across large extents of land so rainfall is improbable (1). Sinking / high pressure interchangeable. 4 x 1 Winds Winds are affected by the presence of the high pressure. Winds blow from areas of high pressure and so winds blow out from these areas of land – receiving no moisture- this contributes to the aridity. For example, the North East Trade winds blow across north Africa across the Sahara from about 30 degrees north. Explain how continentality is a cause of aridity. (3) JUN12 Continentality refers to areas within the interiors of land masses away from the influence of the sea. Central areas of continents more arid than coastal areas. Why? Moist wind from the sea move inland and the moisture held is dropped as precipitation. So when the wind reaches the centre of a large continent it’s carrying very little moisture, so very little rain falls. Relief Some very dry areas lie in the rain shadow of high mountains ranges. As the prevailing winds in the sub-tropics are the trade winds, blowing from the north-east in the northern hemisphere and the south-east in the southern hemisphere, then any barrier, such as the Andes, prevent moisture from reaching the western slope. Cold currents In some places cold ocean currents run along the coastline. Wind is cooled as it travels over the cold water and its ability to hold moisture is reduced. Moisture that’s stored in the atmosphere is released as precipitation over the ocean before reaching the land. So when the wind reaches the land there’s very little moisture left so very little rainfall falls. For example, the Namib Desert in Africa exists because of the Benguela Current, a cold ocean current that rubs up the west coast of Africa. Atacama DesertRain shadow and cold current Booklet page 28 Outline how the cold ocean current and the mountains are responsible for aridity in the Atacama desert. (4) The cold current (Humboldt/Peruvian) resulting from water being transferred from Antarctic (1). This causes air above the Pacific off the coast to cool and condensation to occur (1). As air drifts in over land, it is warmed and moisture is evaporated (1). Any winds crossing the cold water will cool, rainfall may occur over water (1); its capacity to hold moisture is limited and it too will warm up over the land (1). The Andes are the cause of a rainshadow effect (1). The air coming from the east has to rise over the mountains (1), will cool and condensation will occur (1). Thus rain will fall as the air ascends; as it descends the air will be drier and will be warmed, increasing its ability to hold moisture and so rain will not fall. 4 x 1 per basic point, 2 x (1+1) per developed point. Any combination. Maximum 3 on either mountains or cold current. • Explain the distribution of arid and semi-arid areas. (4) • Explain two or more causes of aridity. (15) Explain the distribution of arid and semi-arid areas. (4) There are a variety of reasons that might be considered – 1-the presence of high pressure as a result of the areas being on the falling limbs of the Hadley and Ferrel cells and the stability of the air, thus the warming air means precipitation will not occur; 2- Prevailing winds are out-blowing from high pressure belt and so as they are land based do not contain moisture; 3- The presence of cold currents off the western coasts results in winds coming across these precipitating over the sea; 4- Many of the areas are in rainshadow as a result of mountains elsewhere that have been crossed by prevailing winds earlier and 5-many are substantial distances from the sea – the influence of continentality. 4x1. Allow 1 mark for naming 2 factors. 3- Arid geomorphological processes: mechanical weathering. Define mechanical weathering. (2) Mechanical weathering involves the physical breakdown/disintegration of rock in situ. The pieces of rock are smaller, but they are the same as they were when attached to the main part of the rock – no chemical change or change in appearance has taken place. Direct link to climate. 2 x 1 Thermal expansion Exfoliation and granular disintegration Booklet page 20 4 (a) (ii) Study Figure 7 which shows the result of mechanical weathering in Death Valley, California. Describe evidence of mechanical weathering shown in Figure 7 and explain the processes involved. (4) D=Layers can be seen on the large boulder, partial layers appear to have peeled away. There is debris around the boulder, evidence of rock being weakened and pieces falling off. E= This is the result of temperature fluctuation on a daily basis. During the day, the surface layers expand in response to the high temperatures to a greater extent than those concealed beneath the surface. At night, cooling and contraction occurs at similarly different rates. This causes the outer layers to peel away from the lower layers – the process of exfoliation. Block separation This happens to rocks such as limestone, which are well jointed with bedding planes. The surface breaks down into block structure along the lines of weakness. The Importance of wind Erosion, transportation and deposition Transportation Movement of particles determined by several factors 1- Greatest where winds are strong (usually over 20km/hr). 2- Turbulent wind 3- come from a constant direction and blow steadily for a lengthy period of time 4- Nature of the regolith- the layer of unconsolidated rocky material covering bedrockmore likely to be moved if: - there is no vegetation to bind it together - particles are small enough to be transported. Mystery of Death Valley's Moving Stones Solved Booklet page 26 The largest material – between 0.25 and 2mm rolls along the surface in a process known as surface creep. Intermediate size of 0.25 – 0.15mm hops along the surface at heights of up to a metre – saltation whilst the finest material of under 0.15mm is carried within the atmosphere – up to 300m via suspension. 3×1. Allow 1 mark for identifying 2 processes. Erosion: Deflation and abrasion Deflation: the wind removes loose material and carries it away. As the fine material is transported, deflation leaves behind a rocky surface called a desert pavement. Desert pavement Deflation hollows Formed when the wind removes vast amounts of surface material. They form when there is some hollows. On cold nights the cold air sinks into these hollows and dew forms. The dew speeds up the weathering of the rock (chemical) and makes particles available for removal by wind. These processes can lead to depressions that are many metres deep. The Qattara Depression (Egypt) Abrasion Sandblasting action by materials as they are moved by saltation. It can pit the surface of rocks or it can wear then smooth. Since sand particles cannot be lifted very high, the zone of maximum erosion tends to be within 1.5 m of the earth’s surface. Abrasion produces a number of distinctive landforms such as rock pedestals, yardangs and zeugen. White Desert @ Egypt - Planet Earth (2006) Question Explain how methods of wind transportation affect the process of deflation and abrasion. (5) There is a clear link between suspension and deflation. Deflation occurs when the fine material is removed by the wind via suspension to leave behind a pebble strewn flat surface known as a desert pavement. Deflation hollows also result as material is picked up to create a basin shape. There is an equally clear link between saltation, suspension and abrasion. Material carried by the wind has a sandblasting effect on exposed rock surfaces, especially near the surface (1.5m). The material is carried and responsible for the abrasion process eroding areas of weakness – such as layers of soft rock to create landforms such as rock pedestals, yardangs and zeugen. Level 1 (1-3 marks) Describes the erosion processes. Shows some awareness of what wind transport does. Implicit link. Some use of appropriate terminology present at the higher end. Level 2 (4-5 marks) Explains clearly how transportation processes affect erosion processes – links explicit.Both erosion processes needed for top of level. Appropriate geographical terminology is used. 5- Landforms resulting from – wind action: defletion hollows, yardangs, zeugen and sand dunes – JUN13: Draw a labelled sketch to show the characteristics of yardangs. (4) Marks should be awarded for appropriate shape, reference to nature of rock layers and orientation, size – e.g. yardangs can be 100m high and continue for a number of km.4 x 1, including 1 mark for sketch. Dimensions – max 2. 1 mark for showing parallel ridges of appropriate shape linked to layers of hard rock. In addition, mark for labelling different rock layers/alignment, shape indicating widening near top, undercutting at base. Homework Draw a labelled sketch to show the characteristics of zeugens. (4) Describe the role of wind action in the formation of yardangs and zeugen. (7) JAN10: Describe the role of wind action in the formation of yardangs and zeugen. (7) Wind action causes yardangs – ridges of hard rock that stand above eroded lines of soft rock – etched out by prevailing wind, zeugen – ridges composed of a hard cap rock with underlying soft rock that is eroded more. Abrasion is the key process in forming both these landforms as material (sand) carried by the wind via saltation and suspension hits the masses of rock and wears it away – similar to sand blasting as is used to clean old buildings. This effect is most effective within 1.5 metres of the surface and causes rock pedestals/mushroom rocks to occur in a less extensive fashion than zeugen. The formation of such features is dependent on the geology of the area also. Wind deposition: sand dunes Form when sand grains carried in suspension are deposited as the wind slows down. May form around an obstacle as the wind is slowed down on the downward side of the barrier. As the wind velocity decreases, some of the sediment in suspension can no longer be held so it falls to create some form of deposit. The barchan is a small, crescent-shaped dune, about 30m high, which is moved by the wind. Booklet page 24 • The photograph shows sand dunes. They appear to occur in lines. One slope seems to be steeper than the other/asymmetrical. The middle part of the dune seems to be higher than the sides. They are crescent shaped/barchans/seif. • In the foreground, it is clear that the sand is not smooth, but is rippled. There is very little sparse / low lying vegetation present on the dunes. • Allow 1 mark for reference to Zone B and 1 mark for reference to vegetation. • 4x1 1- Explain the formation of the landscape shown… 2- 15 mark question Seifs • Seifs are longitudinal sand dunes. In places, they can stretch for several hundred metres. They form parallel to the prevailing winds. Barchans can develop into Seifs, 6- The importance of water – sources: exogenous, endoreic and ephemeral. The role of flooding Booklet page 22 • Rain is heavy and intense and so there is no time for infiltration. This gives rise to rapid surface runoff, meaning water reaches streams very quickly. Ground will be dry and baked, reducing the likelihood of infiltration further. After rain, evaporation rates high, reducing discharge quickly. Infiltration will begin to occur. • 2x1 Booklet page 18 • Question 1 Sources of water • The specification notes exogenous sources – those that originate outside the desert and continue through it without drying up, such as Colorado and Nile; endoreic – those that originate outside the desert but dry up or flow into inland lakes, such as Jordan and ephemeral – these are intermittent streams, present after rainfall. There may also be reference to aquifers and mist and fog, as well as direct precipitation. • 1 mark for list of two or more sources. • There must be reference to two sources for 3 marks. Landforms resulting from water action Booklet page 18 Booklet page 18 Impact of flooding on landforms Figure 5 suggests that water can have a significant effect and that it is present in substantial quantities at times. As such, the water has washed away a substantial depth of the material at the side of the road and has undercut it. 1 mark for description. This indicates a clear role for water in the desert in eroding and transporting material, and therefore, in shaping landforms – such as wadis, canyons. Material that has been transported will ultimately be deposited and such a substantial amount that has been moved will explain the subsequent formation of landforms such as alluvial fans. • Comment on the role of water in hot desert areas. (4) • Assess the role of water in the formation of desert landforms. (15) JAN12: Draw a labelled sketch to show characteristics of a wadi and explain its formation. (7) There should be a recognisable sketch of wadi likely showing a narrow, steep sided valley, perhaps with much material within it and no stream or the presence of an intermittent/ephemeral stream. A wider, more gently sloping valley in lower course is also acceptable. Explanation should refer to the significance of the intermittent stream and the power that it has to erode due to its flashy nature. The energy generated allows large items of material to be eroded and subsequently transported by traction and a huge amount of material to be carried in suspension. As the river cannot be sustained due to the limited duration of the rainfall, the capacity and competence of the stream reduces and it becomes choked with the sediment it is transporting. This is then deposited, strewn along the bed to await the next rainstorm. Pediment Inselberg Mesa and butte Wadi and alluvial fan Playa (Death Valley) Badlands https://plus.google.com/photos/+SpanishhighsC oUk/albums/5937574357394013201/5937575 358104020530?pid=5937575358104020530& oid=109502457382781256798 8- Desertification: distribution of areas at risk, physical and human causes, impact on land, ecosystem and populations. Booklet page 23 Areas at greatest risk from desertification are generally located adjacent to existing deserts. This is clearly the case adjacent to the Sahara – where there is the largest continuous area at risk to the south (the Sahel) – and Namibia, Western Australia and those in USA. Some adjacent areas appear not to be at risk – e.g. southern side of the Arabian Desert. The main areas at risk are around the Tropics, but desertification risk is clearly not confined to these areas in USA. • JAN10: To what extend do you agree that the human causes of desertification are more important than the physical causes? (15) Video: Desertification - A Visual Disaster (1.34m)-End: management Consequences: Video: Explaining the Sahel drought problem 9- Case study of desertification in the Sahel – (i) the struggle for survival to include the energy/ fuelwood crisis, water supply, the impact on food supply/farming and livelihoods and coping/ (ii) management strategies including external aid. Handout: JUN11: Describe the difficulties faced by people living in the Sahel and comment on the extent to which they result from desertification. (15) Management strategies including external aid. • Re-greening in Niger, a road trip with Dr Chris Reij Classwork JUN15: Life in the Sahel needs not be a struggle for survival; management strategies give hope for the future. To what extent do you agree with the statement? (15) -adverse conditions in the Sahel and issues such as removal of trees -impact of desertification; -various strategies aimed at improving life in the Sahel. Comment on question • Booklet page 21 Southern Spain Reasons for desertification and management JUN14: To what extent is development sustainable in areas such as South-western USA or Southern Spain? (7) JUN10: Assess the sustainability of different strategies used to manage land use, including agriculture in contrasting arid areas of the world. (15) Extra question on landforms • Booklet page 30