Arid - KQ1 (processes and factors giving arid environment their distinctive characteristics) Flashcards
What are lines of latitude?
Imaginary lines around the Earth running parallel to the equator
- Measured in degrees N or S of the equator
What is altitude?
A measure of an area’s height above sea level
What is aridity?
When an area receives less than 250mm of rainfall a year
- Measured in aridity index = p (annual average precipitation) / PET (annual potential evapotranspiration)
What are the characteristics of hyper-arid environments?
- Aridity index = less than 0.3
- Mean annual precipitation = less than 100mm
- 7.5% of world’s surface
- Extremely dry = no rain for several years
- Very few shrubs
- Nomadic pastoralism only = move herds of sheep from place to place
- EXAMPLE = Sahara Desert, Morocco
What are the characteristics of arid environments?
- Aridity index = 0.03 - 0.2
- Mean annual precipitation = 100-250mm
- 12.1% of World’s surface
- Infrequent and unreliable rainfall
- Some grasses, shrubs and trees
- Pastoral farming possible where perennial streams (flow all year round)
What are the characteristics of semi-arid environments?
- Aridity index = 1.2-1.5
- Mean annual precipitation = 250-500mm
- 17.7% of world’s surface
- High and more reliable rainfall
- Usually a seasonal pattern of rainfall
- More continuous vegetation
- Arable farming possible
What causes aridity?
- High pressure/latitude
- Continentality
- Rain shadow/relief
- Ocean currents/offshore winds
How does high pressure/latitude cause aridity?
- Often subtropical high pressure cells (30N/S of equator)
- Intense solar insolation and converging air streams at ground level at equator = ascending air stream = Hadley Cell
- Air continues to rise until trop tropopause = cannot rise anymore = deflected polewards = cools and sinks
- Air in these cells = converges, sinks and subsidises, compresses, warms
- Subsidence prevents air from rising from ground surface = from cooling, condensing and forming cloud/rain
Why does sinking air provide ideal arid conditions?
- Dry air = lost most moisture as dropped in convectional rainstorms in equatorial regions
- Cloudless and warm = can hold more moisture than cold air = no clouds form
- Persistent = warm air can hold more moisture than cold air = lower air nearer surface = warm and dry
How does continentality cause aridity?
- The tendency for the middle of large continents to experience extremes of temperature and less rain
- Areas in centre of land masses (eg Simpson Desert in Central Australia) = dry because they are far from rain bearing winds which collect moisture from sea
- Moisture is precipitated near coast = little moisture for continent centre
How does a rain shadow/relief cause aridity?
- High mountain ranges block passage of rain bearing winds which increases aridity on leeward (sheltered side)
- Moist air is forced to rise on mountain’s leeward side = air expands, cools and condenses to produce rain and cloud
- Rains = any air on leeward side = already lost moisture = dryer and warmer on leeward
- EXAMPLE = winds from SE reaching Andes and Rockies = moisture won’t reach west-facing slopes such as the Atacama Desert
How are arid areas distributed?
- Hyper-arid mainly in Africa
- Deserts further N and S of equator
- Concentrated on west side of continents
- Hyper-arid surrounded by arid surrounded by semi-arid
- EXAMPLE = Arabian, Thar, Irania, Gobi, Sonoran, Great Bason, Sahara
What is mechanical weathering?
Occurs as a result of sudden temperature changes between hot days and freezing nights
- Granular disintegration
- Block separation
- Shattering
- Exfoliation
What is granular disintegration?
Mechanical weathering
- Grain rocks (eg granite) break down into grains of sand quickly as contains black and white crystals which heat up and cool down at different rates
What is block separation?
Mechanical weathering
- Well jointed sedimentary rocks (eg limestone) start to break down along their joints and bedding planes which are their main lines of weakness
What is shattering?
Mechanical weathering
- Rocks which have neither coarse grains nor blocky structure shatter into irregular fragments with sharp edges
- Basalt is black and metallic so absorbs heat and expands rapidly
What is exfoliation?
Mechanical weathering
- Expansion and contraction causes the surface layer to flake off = more exposed to temperature
- Peeling process = onion skin weathering
What is chemical weathering?
The erosion or disintegration of rocks, building materials, etc, caused by chemical reactions
- Hydrolysis
- Oxidation
- Crystal growth
What is hydrolysis?
Chemical weathering
- Breakdown of rocks by acidic water to produce clay and soluble salts
What is oxidation?
Chemical weathering
- Breakdown of rock by oxygen and water = often giving iron rich rocks a rusty coloured weathered surface
What us crystal growth?
Chemical weathering
- Salt crystals grow between pores and joints when high temperatures draw saline groundwater to the surface and the water evaporates
- Lead to granular and block disintegration
What is wetting and drying?
Repeated expansion on wetting and contraction on drying causes rock to disintegrate
- Type of weathering
What is hydration?
Minerals (eg anhydrate) absorb water and expand which causes stress and granular disintegration
- Water and anhydrite = gypsum – susceptible to other weathering processes such as carbonation
Why is wind erosion effective?
- Extreme pressure differences
- Little vegetation to slow wind/hold soil together
- Lots of fine debris from mechanical weathering
What are the wind processes?
- Deflation = entrainment and removal by wind of unconsolidated material (Deflation hollows = large enclosed depressions partly created by deflation)
- Abrasion = when wind-blown sand abrades rock surfaces- Attrition = the action of sand grains colliding with each other and in doing so becoming smaller
How does the wind transport?
- Suspension = fine sediment carried within the air
- Saltation =usually sand grains hopping along the surface
- Surface creep = rolling along the ground
What are landforms by wind erosion?
- Yardangs
- Rock pedestals
- Desert pavements
- Ventifact
What are yardangs?
Landforms by wind erosion
- Steep-crested linear ridges of rock orientated parallel to prevailing winds
- Occur in groups
- Can vary in height from few cms to over 100m and can be several kms in length
- EXAMPLE = Kharga yardang field in Egypt
What are rock pedestals?
- Top is case hardened by rising fround water that contains salts
- Case hardened = formation of mineral coating on surface of porous rock by evaporation of mineral-bearing solution
- Salts crystallise in upper part = harder to erode
- Bottom stem is thin due to process of wind abrasion = saltating grains abrade bottom of rock between 0-60cm = more effective as case not hardened
- EXAMPLE = Ameib Ranch, Namib Desert
What is a desert pavement?
Surface of stones resting on a finer material such as salt, silt or clay
- Form when wind/water removes finer material to leave behind larger stones
What are ventifacts?
Wind-polished stones which look like brazil nuts and are few cms in size
What are landforms by wind deposition?
Sand dunes
- Mound of sand built by Aeolian processes
- Types = transverse, linear and star dunes
- Formed depending on wind direction
- Sand is transported by suspension, saltation or surface creep = sand deposited as dunes by wind can be divided into two main types
- EXAMPLE = Namib Sand seas
What are types of water erosion?
- Flash floods = rare thunderstorms but ground is baked so runoff rapid
- Exogenous rivers (permanent river deriving its flow from beyond desert margin) = flow from wetter areas
- Historic water = at end of last ice ages = wetter climates
- Underground water = groundwater
- Fog may bring moisture to these arid areas
What are the landforms by water?
- Alluvial fans
- Playa lakes
- Canyons
- Wadis
What are alluvial fans?
Landform by water
- Fan shaped deposit made up of sediment overtime = often found at base of mountain ranges where intermittent streams flow out
- Sediment drops out of stream flow as energy decreases due to widening of channel
- Sediments build up overtime in channels at base of mountains and forces stream to carve another channel
- EXAMPLE = Death Valley, California
What are playas?
Landform by water
- Formed where rock pools evaporate
- Large and flat areas of land that were once lake beds
- Covered with salt and other minerals (often white due to salt presence)
- EXAMPLE = Bonneville Salt Flats, Utah
What are canyons?
Landform by water
- Gorge with a deep, narrow channel bounded by resistant rocks
- Result from vertical erosion by rivers = carry coarse loads at times of high flow = abrades river bed = deeper
- Valley floor often occupied by exogenous river
- EXAMPLE = Grand Canyon, USA
What are wadis?
Landform by water
- Dry river bed with steep sides and wide flood covered with channel deposits
- Form when flash floods/seasonal rain creates and ephemeral river (temporary river flowing seasonally/intermittently)
- High discharge with availability of loose, dry sediment in channel bed = encourages transport of large amounts of sediment
- Boulders are abraded and underlying bed rock is sourced
- Vary in size from a single channel a few metres long to dense networks many kms in length
- EXAMPLE = Wadi Rum, Jordan
