KQ1 - What processes and factors give hot / semi-arid environments their distinctive characteristics? Flashcards
How much mean annual precipitation do arid environments get?
100-250 mm
How much mean annual precipitation do semi-arid environments get?
250-500 mm
What are the characteristics of arid areas?
- Infrequent and unreliable rainfall
- Some grasses, shrubs and trees grow
- Pastoral farming only possible where there are perennial streams / aquifers
What are the characteristics of semi-arid areas?
- High and more reliable rainfall
- Seasonal pattern of rainfall
- More continuous vegetation
- Arable farming is possible
What causes aridity?
- High pressure / latitude
- Hot arid and semi-arid areas are affected by subtropical high pressure cells found 30 degrees north and south of the equator
- Continentality
- Rain Shadow / relief
- Ocean currents / offshore winds
Why does sinking are provide ideal air conditions?
- Dry air
- Cloudless and warm
- Persistant
Why does continentality affect areas?
Areas in the centre of land masses (central Australia) are dry because they are far from rain-bearing winds which collect moisture from the sea
Why does rain shadow / relief affect areas?
- High mountain ranges block the passage of rain-bearing winds which increases aridity on the sheltered side
What is the distribution of arid areas?
- Hyper-arid mainly in Africa
- Deserts further north and south of the equator
- Concentrated on the west side of continents
- Hyper arid surrounded by arid surrounded by semi-arid
What is mechanical weathering?
- occurs as a result of sudden temperature changes between hot days and freezing nights
Name mechanical weathering processes
- Shattering: Rocks which have neither coarse grains nor blocky structure ma shatter into irregular fragments with sharp edges.
- Exfoliation: expansion and contraction causes the surface layer flakes off, as it is more exposed to temperature. The peeling of process is known as ‘onion-skin weathering’
- Granular disintegration: grain rocks e.g. granite break down into grains of sand. Granite breaks down quickly as it contains black and white crystals which heat up and cool down at different rates.
Name chemical weathering proccesses
- Hydrolysis: the breakdown of rock by acidic water to produce clay and soluble salts
- Oxidation: the breakdown of rock by oxygen and water, often giving iron-rich rocks a rusty coloured weathered surface
- Crystal growth: salt crystals grow between pores and joints when high temps draw saline groundwater to the surface and the water evaporates. They lead to granular and block disintegration.
What are the other forms of weathering?
Wetting and drying (rain also chills the rocks): repeated expansion on wetting and contraction on drying causes the rock to disintegrate
Why is wind erosion so effective?
- Extreme pressure differences
- Little vegetation to slow wind
- A lot of fine debris from mechanical weathering
What are the wind processes?
- Abrasion: when wind-blown sand abrades rock surfaces
* Attrition: the action of sand grains colliding with each other and in doing so becoming smaller