Arid Environments

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  • Created by: Rosa
  • Created on: 30-04-13 13:35

Processes and Factors influencing climate

Hot and semi-arid environments are characterised by having high temperatures , low precipitaion and sparse vegetation. An Arid environment has less than 250mm of annual rainfall, as a semi-arid environment has between 250-500mm of annual rainfall.(Aridity Index take into account evaporation and transpiration losses) 

Temperature- during the day under cloudless skies intense solar radiation heats the ground , which warms the air by conduction (transfer of heat loss or gain through direct contact) or convection (transfer of heat through conective air moments). At night under clear skies terrestial radiation (radiation emitted from earth) rapidly escapes and the ground cools , which then chills the air by conduction. The reason why the areas have high temperatures is because they are located at low latitudes , as here radiation is more concentrated and second the solar beam has a shorter distance to travel through the atmosphere and therefore is less diluted by reflection , scattering & absorption (however hotter than at equator as at equator there is nore cloud cover). Different arid areas experience different temperatures due to their latitude , height above sea level , distance from sea & albedo (% of solar radiation reflected, as opposed to absorbed , by the earths surface). Deserts located in middle of continents heat up and cool down faster than those by the seas so therefore greater temp. range. Sakt-encrusted , dried out lake surfaces reflecy more solar radiation than areas of dark rock (absorbs more). Offshore currents depress temperatures. - Sun is more overhead tropic deserts so os warmer. Precipitation varies and can be in the form of coastal fog , flash floods (triggered by intense heating of ground and convectional activity)

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Factors Causing Aridity

  • Latiude= affected by subtropical high pressure cells which are found about 30 north and south of equator. Air in the cells are subsiding & becomes compressed and drier. Subsidence also prevents air from rising from the ground surface , cooling , condensing and forming cloud and rain. Occasionally sub-tropical air allows up-draughts to rise forming thunder clouds and flash floods (developed over sea - however influence extend to land)
  • Offshore Winds= ARID AREAS LOCATED here prevailing winds blow from land to sea and carry little moisture
  • Continentality= areas in centre of land masses are dry because they are remote from rain-bearing winds which collect moisture from the sea
  • Relief= mountain ranges block passage of rain-bearing winds. Moist air is forced to rise on windward side of mountain which cools and condenses to produce cloud and rain. Leeward side is dry as most moisture has been lost, also the air when descending becomes compresses and becomes warmer and drier
  • Cold Offshore Currents = depress coastal desert air temperatures, cold air holds less moisture than warm air so rainfall on land is limited. Instead air above the cold current is chilled and produces advection fog (created when warm air comes into contact with cold surface)- fog carried inland and eventually dissipiates . Fog is frequent as it is trapped under an inversion layer(where air temp rises rather than falls with height), by sinking air in a tropical high pressure cell- when fog condenses it adds to plant growth and weathering processes
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Seasonal rain in semi-arid areas

Rainfall occurs when as at the equator intense heating causes air to rise , expand , cool and condense producing heavy rainfall. Area of high rainfall and winds is known as INTER-TROPICAL CONVERGANCE ZONE. At altidude the air spreads about , it then at 30 degrees N &S of equator it begins to sink and as it sinks other air compresses it making it warmer and drier , the sinking air creates a zone of subtropical high pressure , when the air reaches the ground moves towards the equator - HADLEY CELL

Polar front = about 50 -60 degrees S &N of equator where cool air from the poles meets warm air from sub-tropical high pressure cells producing rain

Winds - have an influence of landforms. In the centre of a sub-tropical high pressure cells or anticyclone is calm , but circulating around them are global winds. The winds shift seasonally with the migration of the sun , which means for part of the year an arid may be calm but at other times it is windy. Monsoon winds affect arid areas which are drawn towards continents in the summer. Mountains by obstructing wind flow create local winds on their leeward sides and can also funnel winds. 

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Climate and weathering

Insolation Weathering = high temps during the day heat up the surface layers of the rock which expand while at night they cool and contract- repeated cycles cause layers to peel and flake known as 'onion-skin weathering', heating and cooling of well jointed rock 'block disintergration' ('granular disintergration' dark minerals in granite) - repeated heating and cooling process called insolation weathering

Crystal Growth = weathering in porous sedimetary rocks such as sandstone. High temps draw saline groundwater to surface that then evaporates - salt crystals grow between the joints and lead to block and granular disintergration - causing flaking

Wetting & Drying = occurs when there are flash floods or seasonal rain encourages clay mineral in rocks to swell- repeated expansion & contraction causes rock disintergrate 

Frost Shattering= freezing and thawing of water between joints causes shattering-scree

Solution = water dissolves minerals such as salt which is then removed in soloution

Oxidation = oxygen dissolved in water reacts with minerals such as iron to create oxides and hydroxides- red staining on rocks- minerals that are oxidises increase volume which weakens rock

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Climate and weathering

ALVEOLES (small hollows 5-50cm , occuring in clusters with thin pairtrons) & TAFONI (hollows a few metres across developed along lines of weaknesses) , crystal growth , wetting and drying and hydration , wind scouring & case hardening (a hard layer of salt encrusted rock formed where salts have been brough to the surface by capillary action)

Pedestral Rock (isolated pillar of rock) formed by salt crystal rock  , high evaporation rates draws water to the surface capillary action , the salts from case-hardening protects the top of pedestral rock , while rocks below are weathered- also form where a harder cap rock protects and is less weathered than less resisitant rocks below. 

Pressure release - is the gradual removal of the surface layers of rock can cause those underneath to expand which produces lines of weaknesses where other weathering processes can take place . Pressure release can cause formation of inselbergs (upstanding masses of crystalline rock that project above the plains)

Uluru is an example of an inselberg on a central plain in Australia. 340m above ground, 2.5km long , and 0.5km wide and made out of sandstone . Flaking of rock surface due to insolation weathering and crystal growth. Rock layers etched with tafoni. Rock appears red as water has dissolved the iron. Pressure release cause for sheet fractures - 2km thick. Curved slopes formed at base of slope by chemical weathering e.g. hydrolysis when climate was warm and wetter. 

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Role of wind in shaping land forms

Deflation erosion - wind picks up and removes unconsolidated material creating deflation hollows (large , enclosed depressions) salt pans (flat areas encrusted with salt) desert pavements (surface of stones resting on a finer material such as silt/clay)

Corrasion erosion  - sand carried in wind abrades rock surfaces producing sculptured rocks and ventifacts ( wind-polished stones) 

Attrition erosion - sand grains collide with eachother and become smaller (amount of erosion depends on strength , duration of wind & struture of rocks)

Wind erosion creatures features such as yardangs , steep crested , linear ridges of rock parallel to the prevailing winds. They are found in groups and ridges are seperated by wind-scoured grooves. They develop in soft rocks which are easily eroded such as clay as well as resistant rock such as limestone. They have shape like upside down boat, end facing wind is highest and broadest, and vary in height. Abrasion smooth pits and grooves on the side of the yardang , while deflation helps form shape.  e.g Kharga Egypt

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Role of wind in shaping land forms

Wind Deposition- wind transports sand in different ways small particels are moved by suspension , sand-sized particles move in a series of hops on the ground by saltation (main way) , heavier particles rolled along the surface by surface creep. . 

Winds pick up sand from near rivers and lakes and seas and it is carried huge distances into deserts where it is deposited. 60% of worlds sandy arid environments are covered by sand seas (contain variety types of dunes).Dunes develop around obstacles such as rocks, or where a rough surfacre such as sand causes friction and deposition  (or where winds come from different directions). Shape and speed of movement of sand dune depends on by strength and direction of winds, volume & grain size of sand supply , shape of land and presence of vegetation. Sandy areas without dunes are called sand sheets dune type found here called 'zibars'. Sand dunes can be fixed anchor dunes formed around obstacles such as a rock or free tunes where no obstacles are present, further sub-divided into tranverse dunes where sand movement is normal to wind direction or linear dunes where sand movement is parallel to the wind

Namib is a desert which is compromised of gravel plains , isolated inselbergs and dune fields

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Water Erosion

Seasonal Rainfall is often heavy and therefore water runs across the surface rather than infiltrates. Little vegetation to intercept rainfall and high temps bake soil to crust

Wadis are a landform formed by water erosion are a dry river valley with steep sides and a wide floor covered with channel deposits. It forms when flash-floods creates an ephemeral river (temporary). High discharge and avalibilty of lose ,dry sediment in the channel bed encourages the transport of large amount of sediment. Amount of discharge downstream is reduced as storms are localised , some water evaporater or infiltrates river bed. As discharge declines stream loses its competence and BRAIDING (river split into smaller channels) occurs. They vary in size.

Canyons - a gorge with a deep , narrow channel bounded by resistant rocks. Valley occupied by exogenous river(a pernament river) , aridity limits weathering and erosion which preserves steepness of valley sides . Deep canyons form when area experiences tectonic uplift. Seasonal rainfall & flash-flooding causes the river discharge to vary and when low it cannot carry load so braiding occurs. USA has variety of 'canyon-land' type landforms , steep sided plateaus of rock called mesas and smalles 'buttes' are common

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Water Deposition

SALT PANS , PLAYAS , SABKHAS are feature of water deposition and are all flat and low-lying with little vegetation and parly occupied by saline lakes ( due to precipitation & surface run off fill the lake). When the lake dries out the clay floor cracks and curls up. Water evaporates leaving behin deposits of sodium chloride and calcium carbonate. Plants which are salt-torelant occupy the edges of the pan. 

Playas and Pans vary in size. When the lake dries out , salt weathering loosens the material on the floor of the pan which is then eroded by the wind. The material is then deposited as a crescentic , lunette dune on the downward side of the pan. Other pans form when animals over-graze around waterholes or develop in depressions between linear dunes. Term playa and pan are can be used for the same purpose. Sabahka refers to a type of salt pan fringing . 

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Plant Adpataions

  • Ephemerals - plants remain as seeds until rain falls and then they germinate, flower and seed within a matter of weeks- they are small with shallow roots
  • Succulents - e.g. cacti- cope with drought by storing water during in their fleshy leaves or stems during seasonal rainfall - reduce transpiration (loss of wtaer vapour to the atmosphere) by closing stomata during the day and open them at night when it is cooler. Many have thick or waxy leaves or cuticles which act as a waterproofing to reduce water loss. Many succulents have CAM which allows them to carry out photosynthesis when their stomata are closed during the day
  • Phreatophytes- obtain water from long roots which extend to ground water below the table e.g. mesquite bush which grows beside streams and salt pans
  • shrubs and trees have small leaves and spines to reduce transpiration losses - some plants lose their leaves in the dry season but their green stem continues to photosynthesise e.g. Palo Verde
  • Plants grow in salt pans and are called halophytes - and grow in saline soils
  • acacia tree have evolved spines and thorns to protect them from over grazing- sodum apple contain bitter poisonous latex which animals avoid - creotose bush give off toxic substances to deter other plants from growing nearby
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Animal Adaptations

  • Small & light in colour
  • highly mobile so can travel long distances to find water e.g. gazelles
  • some animals are nocturnal to avoid heat of the dayhe.g. golden mole- lizards shade under rocks
  • many store water in their tissues , often as fat and may concentrate urine to reduce water loss
  • desert foxes have large ears with veins close to the surface to increase heat loss
  • reptiles & insects have hard skins to reduce moisture loss
  • insects have long legs to keep their bodies away from sand
  • insects have mechansims which enable fog to condense on their bodies
  • camels store fat in their humps - large water capacity - thick lips and mouth so can eat thorny plants - long eyelashes protect eyes from sand storms - padded feet so can walk on hot sands
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  • The Sonoran desert is roughly the same size as the United Kingdom and is located over Southern California , Arizona & Mexico
  •  The Sonoran desert is so dry due to the lack of moisture in the air and this is because though the Pacific Ocean lies just to the west but the coastal mountain ranges effectively stop most moisture reaching inland. To the South- East lies the Gulf of Mexico any moisture from there must travel 1700km and cross the continental divide before it reaches the Sonoran desert. It is a difficult place for moisture to reach and therefore it there are little clouds as there is a lack of moisture. 
  •   A common drought enduring small tree growing on the upper slopes is called the Palo Verde , the plant loses it leaves in the dry season , while it greens bark enables it to carry out photosynthesis. Lower gentler slopes are covered with creosote bushes, which have small , dark resinous leaves to reduce transpiration. They have roots which reach down vertically and out horizontally in order to collect as much water as possible. The leaves have a type of varnish on them so that they can seal in moisture. The smell and taste of the plant puts off any animal from trying to eat it.  Plant spacing is controlled by water availability, that is creosote bushes grow further apart when supplies are scarce . Ephemerals such as the brown-eyed primrose commonly grown among the creosotes bushes.   Phreatophytes , such as the mesquite bush , grow beside streams and on the edges of salt pans. Halophytes such as inkweed, salt grass and pickleweed occupy saline soils on salt flats. Where water reaches the surface , such as along the San Adean fault , groves of Californian fan palm grow. Their very large leaves indicate water supply is plentiful and there is no need to conserve supplies. The Saguaro Cactus when it rains they collect water using their spines and a horizontal web of roots. The water is then stored in the stem to sustain the cactus, until the next rains come. The cactus can lose up to 82% of its water before it dies of dehydration. The ocotillo , looks dead but is awaiting for the next rain


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  •  The rain arrives in the desert in two distinct seasons , in the winter cold wet fronts from the Pacific are sometimes strong enough to bring rain to the desert despite the intervening mountains. In the late summer warm wet air from the gulf of Mexico sometimes makes it over the continental divide and is swept in over the desert often forming thunderstorms. 
  •  Soils in the desert are thin , relatively infertile and alkaline. Seasonal rains leach soluble salts down through the soil which are drawn up again under high evaporation in the dry season. Flash-flooding can compact soil creating impermeable surfaces. Salt flats in the Salton Trough are covered with thick crusts of sodium chloride, gypsum and calcium carbonate. 






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Disturbances - Due to water being pumped out and used in cities parts of Arizona are literally sinking into the ground , the dry rock compacts and the surface sinks – massive line sand holes in the soil . The soil does not stretch well so there are great tares in the earth.

  •  Many of the deserts rivers have dried up – water and many riverside habitats have disappeared and with them the wildlife-many animals on the verge of extinction
  •  Towns and people will have to move soon as there is little water- turn into ghost towns
  •  Many farms and towns and cities now in the desert such as Phoenix- use of lots of water
  • More wells have been built underground as more people move to the desert – water table had dropped by nearly 3m a year and is still dropping- pumping out more water be=is becoming more expensive and the water quality is poor and is not sustainable
  • In 20 years population of the state has doubled- Houses have water features – new houses – all need water - Use water from Colorado river
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  • Cities such as Phoenix have replaced deserts covered with cacati
  • taking water from rivers and groundwater absatraction to supply for a growing population has affected mesquite bushes + cotton wood trees
  • road construcion and gas and water pipes have displaced natural habitats- fenced highways restrict movement of animals - prevents pronghorn reacin water supplies
  • trampling and off road vehicles damaged soil and vegetation
  • intoduction of non-native species such as a tamarisk has displaced native cotton woodsBufle grass spread for Mexico to Arizona- occupies gaps in sparse vegetaion cover- increasing fuel load and fire risk- in 1994 engulfed 1150 acres of Saguaro National Park-20% saguaro cactus destroyed
  • Saguaro cacus sometimes illegally removed and used as a garden ornamental as well as desrt toroises
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  • generates forein currency earnings - benfits countries with few other resources
  • creates direct employment and indirect mployment -multiplier effect
  • encourage local people to learn new skills (Tunisia hotel traiing schools)
  • tourists spendings hlp to preserve or revialise tradionals crafts and rituals
  • develops infrastructure such as roads and water supply
  • preserves natural landscape e.g. Kuene National Park proects threatened black Rhinos


  • jobs are often low paid and low skilled - jobs are seasonal
  • tourist revenue transferred overseas , dont benefit host country-'all inclusice packages'
  • over reliance of tourism is risky as for the terrorist attack on red sea - Shalm-el Sheik
  • tourists who behave badly or dress inapproprialey cause offence to local people - shorts
  • local culure customs and rituals e.g. aboriginal dances are simplified for a tourist audience
  • discharge of untreaed sewage from hotels into sea or local water supplies causes illness
  • high demand of water in hotels - laundry , swimming pools e.g. golf courses Palm Springs
  • rubbish is an eye sore and endangeres wildlife - helicoper rides GC disturb wildlife & pollution- damage ancien sites
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Ancient egyptian tombs on the west bank of the Nile are under pressure from tourism.

  • Large no. of visitors visit the tombs each year and their raises the humidity in the tomb sofetning the plaster so it no longer sticks to the wall - people touching or brushing against paintings cause damage
  • Irigation of sugar cane is causing problems as water-table rises , saline water evaporates into the tombs and salt crystals grow - in 1990 one tombs ceiling collapsed - rising water-tables and visitor pressure has led to the closure of one of the finest tombs - Seti I
  • Efforts to reduce tourists impact included , dehumidifiers , placing glass screens in front of the paintings and closing tombs on a temporary basis to allow humidity levels to fall, numbers entering the tombs are controlled and popular tombs such as Tutankhamen have a higher entry fee
  • A new visitor centre opened in 2007 which should help spread visitor flow
  • Paths have been widened  between tombs to prevent overcrowding - and not to damage soils
  • Movin car parks and replacing tarmac roads with those of sand and gravel so they are less visually intrusive
  • Future option include - extending opening hours , limiting no. of tomba tourists can visit are possible options to reduce overcrowding
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Sustainable Tourism

  • involving local people in managing toursist facilities e.g. aboriginies at Uluru work as rangers
  • creating national parks to protect landforms , plants & desert animals 
  • controlling visitor numbers at popular sites e.g. Valley of the Kings 
  • establishing tourist codes e.g. entering Arches national park visitors reminded removal of rocks and artiefacts is prohibited and also not feed wild aniamls or drop litter
  • restoring historical sites using entrance fees
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Agricultural opputrunities and challenges

  • Hunting & gathering still practiced by some indeginous people but where water is scarce  main agriculture is nodasim (a wandering form of existence in search of good pasture and water for livestock, collection of fruits and roots)
  • rivers which flow through deserts e.g. Nile provide oppurtunity for irrigated agriculture e.g. tunisa grows olives , figs , oranges - date palm can produce dences , roofs , mats
  • On Niles bank , beans , cotton , rice etc. are all grown- transferring water to fields involves simple dykes to sprinkler systems - Aswan Dam built once seasonal rain and is regulated to ptrovide water for crops growing all year round 
  • Hydroponic used in Mexico and USA is hydroponics (grwoing of crops without soil)
  • Unreliable rainfall , strong winds , thin infertile soil pose serious challenges - drought and mismanagement causes desertification (conversion of marginal land to desert)- human factors include deforestation and overgrazing- demands of poverty and high pop.growth
  • another problem is salinisation - when high temps ecourage saline water to be drawn up through the soil and salt crystalises in the upper layers- salt crusts stop root growth , reduce infiltration and increase surface run off - also can cauae waterloggin if a lot in one area- can be managed by salt tolerant plants such as barley and rye
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Located in Indian and south-east Pakistan , densley populated by peopel and livestock and covers an area of 2000km2. Precipitation varies from 100 - 50mm, much falls in the monsoon season between July and September. It is composed of sand dunes and plains , low hills and saline lakes  , Thar vegetated by scrub and scattered trees, wildlife includes Indian gazelle. 

  • Summer monsoon water is collected for consumption as the groundwater is too deep and saline for human consumption 
  • Canals cross the area and supply irrigated water for cultivation of wheat and cotton 
  • Hot dry conditions , strong winds and human activities such as deforestation and overgrazing are causing soil erosion - many livestock (sheeps and goats) overgraze & remove palatable grasses leaving behind inedible plants
  • Loss o fplant cover has exposed soils to wind erosion - heavy trampling (waterholes) has compacted the soil , limiting seed development , damaging plant roots  and encouraging runoff
  • Pop. pressure causes trees and shrubs to be removed and used as building material , fuel and medicine. Deforestation has caused a declin in organic material which normally supplies nutrients and helps to hold soil particles together- once vegetation removed the sun bakes the soil until it cracks and blows away 
  • Loss of native tree Khejari deprives soil of valuable nitrate fertiliser - crop recover perios is shorter - rising pop.
  • In an attemp to stop desrtification and extend the area under irrigation , IGNP Canal was constructed to bring water from the Himilayas to the desert . Irrigated water used to grow crops such as sugar cane- however seepage from irrogation channels resulted in water logging and salinisation- a rise in malaria-indian gazelle decline
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Sustainable Agriculture

To reduce overgrazing in Thar they are being urged to limit herd size , allow grass to recover and improve quality of livlestock- use solar devices instead of dung for fuel and use the manure for fertilisers instead- plant trees such as prosopis cineria is providing timber wood and fuel wood as well as green leaves for camels,goats, sheep. Sand dunes are being stabilised with trees and grasses such as the acacia tortillis-limiting water and recycling water , digging ponds to collect water - lining canals with concrete to avoid seepage underlying issuer how to stop population growth

Niger sustainable agriculture- a poor , semi-arid area in West Africa within the Sahel. Soils cannot produce enough food to feed the population which is increasing by 2.9% per year- practices have tried to be introduced: 

  • mulching -incorporating vegetation in to the soil to increaae organic matter and to improve infiltration and reduces soil erosion - mulches also stimulate termite activity which breaks up soil- however millet stalks also used for building material, firewood , medicine so farmers use it sparingly - other mulches include andropogon gayanus
  • planning pits or 'za'- small holes dug with a *** which collect runoff water & rainwater- small amounts of manure and seed is added
  • manuring - mineral fertilsers would increase crop yield however it is very expensive 
  • intercropping - trees , native grasses and shrubs are planted in strips between millets to act as a windbreak 
  • bunds - low stone walls built along the contours of the fields to retain earth and hold back water
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Australia is the leading producers of gold,  uranium , titanium , maganese , nickles and opals. Mining provides jobs in deserts areas where there is little other oppurtunities for example Olympic Dam employed 3000 in central Australia. The company owning the mine has contributed $45 million to the South Australian Government which has benefitted the area. There have been improvments in local service and has had the multiplier effect- however local housing prices have therfore increased and people cannot afford to buy them

Aboroginal groups benefit directly if mining takes place on their landby recieving an annual rent. Tracks and roads which companies create allow aboriginal groups to reach remote areas for hunting - however due to rigths some indigenous groups oppose to the new mines

Mining companies face big challenges as moving the ore from centre of Australia to the coast  e.g. copper from the Olympic Dam has to be trucked 560km to the port at Adelaide. Modern mining methods use large quantaties of water e.g Olympic Dam relies on water from pipelines from the coast and groundwater resources. Aborignals fear that the water will become endanger rare springs e.g. Lake Eyre- important source of waer in central Australia and are of cultural signifcance for aborgignes 

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Opponents claim there are many environmental impacts , construction of roads, pipelines removes vegetation and soils. The flat relief of deserts mean mining operations are visible for long distances , an eyesore- e.g. Prominent Hill open-cast mining hole is nearly 1km wide and 50m deep- large visual impact. Opponents argue that liquid waste from treatment plants contaminate water and soils. Concern in Beverly which will use situ leaching method to extract the ore acids will be forced into the ground to dissolve and remove uranium- mining creates soil heaps which scar the landscape source of dust

Mining companies aware of their responsabilities carry out environmental assesments before mining. To reduce visual impacts height of waste dumps is limited and re-landscaping occurs when mining ceases. Mining companies recyle water from treatment ponds and spray weeds to control their spread

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Mr A Gibson

Excellent resource (colour coded for clarity) which covers a plethora of infomation about Arid Environments from causes and effects to plants and animal adaptations.

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