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
