Flashcards in KQ1 - What processes and factors give hot / semi-arid environments their distinctive characteristics? Deck (27)
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1
How much mean annual precipitation do arid environments get?
100-250 mm
2
How much mean annual precipitation do semi-arid environments get?
250-500 mm
3
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
4
What are the characteristics of semi-arid areas?
- High and more reliable rainfall
- Seasonal pattern of rainfall
- More continuous vegetation
- Arable farming is possible
5
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
6
Why does sinking are provide ideal air conditions?
- Dry air
- Cloudless and warm
- Persistant
7
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
8
Why does rain shadow / relief affect areas?
- High mountain ranges block the passage of rain-bearing winds which increases aridity on the sheltered side
9
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
10
What is mechanical weathering?
- occurs as a result of sudden temperature changes between hot days and freezing nights
11
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.
12
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.
13
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
14
Why is wind erosion so effective?
- Extreme pressure differences
- Little vegetation to slow wind
- A lot of fine debris from mechanical weathering
15
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
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How does wind transport?
• Suspension: fine sediment carried within the air
• Saltation: usually sand grains hopping along the surface
• Surface creep: rolling along the ground
17
Name the landforms formed by wind erosion?
- Yardang
- Rock pedestals
- Desert pavement
- Ventifact
18
What is a yardang?
• Steep-crested, linear ridges of rock orientated parallel to the prevailing wind
• Occur in groups
• Develop is easily eroded soft, but cohesive, sediments e.g. silt and clay and in more resistant rocks e.g. limestone and sandstone
• Length to width ratio = 4:1
• Can vary in height from a few cm’s to over 100m and can be several km’s in length
19
What is a rock pedestal?
• Zeugen= a type of pedestal rock with a resistant cap rock
• The top is case hardened by rising ground water that contains salts
• Case hardened: formation of a mineral coating on the surface of porous rock by evaporation of a mineral-bearing solution
• These salts crystallise (a form of weathering) in the upper part, so it becomes hardened to erosion and further weathering
• The bottom part/stem is thing due to the process of wind abrasion- saltating grains abrade the rock between 0-60cm. it is more effective here as it is not case hardened.
20
What is a ventidact?
- wind-polished stones which look like Brazil nuts and are a few centimetres in size
21
What is a landform formed by wind deposition?
Sand dune e.g. Namib Sand seas
• Description → a mound of sand built by Aeolian processes. A desert covered with dunes is known as a sand sea. There are many different types of dunes such as transverse dunes, linear dunes and star dunes. Dunes are found all over the world
• Explanation → they are formed depending on the direction of the wind. Sand is transported by suspension, saltation or by surface creep; sand deposited as dunes by the wind can be divided into 2 main types
22
Name the types of water erosion
• Flash floods: rare thunderstorms but ground is baked so runoff is rapid
• Exogenous rivers (a permanent river deriving its flow from beyond the desert margin): flow from wetter areas
• Historic water: at the end of the last ice ages, these climates were wetter
• Underground water: groundwater
• Fog may bring moisture to these arid areas
23
What are landforms formed by water erosion?
- Alluvial Fans
- Salt Pans / Playa Lake
- Canyon
- Wadis
24
What are alluvial fans?
• A fan shaped deposit made up of sediment overtime, often found at the base of mountain ranges where intermittent streams flow out e.g. at the end of a wadi or canyon
• Sediment drops out of stream flow as energy decreases due to widening of the channel
• Sediment builds up overtime in the channels at the base of the mountains and forces the stream to carve another channel
25
What are salt pans / playa lake?
• Formed where water pools evaporate
• Large and flat areas of land that were once lake beds
• Covered with salt and other minerals, often look white due to salt presence
26
What are wadis?
• Dry river bed with steep sides and a wide floor covered with channel deposits
• Form when flash floods/seasonal rain creates an ephemeral river (temporary river flowing seasonally/intermittently)
• High discharge with the availability of loose, dry sediment in the channel bed encourages the transport of large amounts of sediment
• Boulders are abraded and underlying bed rock is scoured
• Vary in size from a single channel a few metres long to dense networks many km’s in length
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