Y2) TERM 1- DESERTS Flashcards
Describe typical hot desert characteristics
Typical hot desert characteristics would include dry (less than 250mm precipitation per yea), large diurnal temperature range, little biomass, and hot! (20-30oC)
How does atmospheric circulation cause aridity?
Atmospheric circulation causes aridity as high pressure (dry and cloudless) is created at the tropics due to the falling limb of the Hadley and Ferrel cells (driven by convection at the equator)
How does continentality cause aridity?
Continentality causes aridity since moisture becomes less available with distance from a large body of water – the further inland you go, the drier it gets
How does relief/ orography cause aridity?
Relief/ orography causes aridity by forcing warm and moist air to rise, cool and condense, which forms clouds (which rains!) – the leeward side therefore receives no rainfall, and is likely to be arid
How do cold ocean currents cause aridity?
Cold ocean currents cause aridity by limiting the amount of evaporation that can occur (as air is cooled, thus unable to contain much moisture) – air may be foggy, but not really “wet”!
What is the aridity index?
The Aridity index is a measure of dryness, as a ratio on precipitation vs potential evapotranspiration; the lower the value, the drier a place is.
What is the aridity index?
The Aridity index is a measure of dryness, as a ratio on precipitation vs potential evapotranspiration; the lower the value, the drier a place is.
Describe the “Soil Moisture Budget”
The “Soil Moisture Budget” shows how soil moisture varies throughout a year, depending on the relationship between precipitation (input) and evapotranspiration (output)
What is “positive feedback”? (Can you think of an example in a desert setting?)
“Positive feedback” is a self-perpetuating/ self-reinforcing cycle, that spiral further away from equilibrium. E.g., a lack of vegetation = reduced evapotranspiration = reduced rainfall = reduced vegetation = reduced evapotranspiration….
What is “negative feedback”? (Can you think of an example in a desert setting?)
“Negative feedback” is where a “check” occurs (an event that stops everything spiralling out of control/ further from equilibrium) Example: weathering = build up of sediment at slope bases (called “scree”) = protection of the slope = reduced weathering
describe desert soils
Desert soils – thin, infertile, alkaline (pH 8.5), often saline, minimal leaching, soil development is slow (hundreds of years), lack of vegetation (organic matter), capillary action occurs where evaporation exceeds precipitation, which draws salts upwards towards the surface from bedrock,. E.g. aridosols (which tend to be coarse in structure, free draining and, if hot enough a hard crust can form on the surface
Describe the soil moisture budget
The “Soil Moisture Budget” shows how soil moisture varies throughout a year, depending on the relationship between precipitation (input) and evapotranspiration (output)
Summerise typical desert soil characteristics
Desert soils – thin, infertile, alkaline (pH 8.5), often saline, minimal leaching, soil development is slow (hundreds of years), lack of vegetation (organic matter), capillary action occurs where evaporation exceeds precipitation, which draws salts upwards towards the surface from bedrock,. E.g. aridosols (which tend to be coarse in structure, free draining and, if hot enough a hard crust can form on the surface
Describe typical desert vegetation
Typical desert vegetation is small, occurs in patches, huddles together in each others shade, may have long/wide roots, will be succulent, corrugated, hydronasty
What are the inputs into a desert system?
Desert inputs include: water, heat and sediment
Summarise typical desert soil characteristics
Desert soils – thin, infertile, alkaline (pH 8.5), often saline, minimal leaching, soil development is slow (hundreds of years), lack of vegetation (organic matter), capillary action occurs where evaporation exceeds precipitation, which draws salts upwards towards the surface from bedrock,. E.g. aridosols (which tend to be coarse in structure, free draining and, if hot enough a hard crust can form on the surface
Describe typical desert vegetation
Typical desert vegetation is small, occurs in patches, huddles together in each other’s shade, may have long/ wide roots, be succulent, corrugated, hydronasty…
What is weathering?
Weathering is the process of breaking down a rock in situ (i.e. no movement); water and heat are very important (but deserts have very little water!)
List some examples of weathering…
Weathering types:
- Mechanical: Thermal fracture // Salt weathering // Frost shattering
- Chemical: Oxidation // Hydration // Carbonation
- Biological: Roots // Lichen
Abrasion…deflation… what’s the difference?
Abrasion is the “sand-blasting” effect of particles colliding with surfaces up to ~2m in height, gradually wearing it away; deflation is the ground surface gradually lowering as finer material is removed
Saltation…suspension…traction… what’s the difference?
Saltation is where small particles “bounce” along, suspension is where smaller particles are “carried” along, and traction/ surface creep is where the heaviest material is dragged/ rolled along
What are the 3 main types of river found in a desert?
The 3 main types of river found in a desert are exogenous, endoreic, ephemeral
What is episodic rainfall?
sodic rainfall = occasional/ sporadic rainfall
Water in the desert
Water in the desert – rare, but powerful
Why is the Sahara Desert a desert?
The Sahara Desert is a desert due to atmospheric circulation (high pressure, falling limbs of Hadley and Ferrel cells), cold ocean current (Canary Current), possible relief in some places (Highlands), and central areas due to continentality
What is desertification?
Desertification is “Land degradation in arid & semi-arid areas resulting from various factors including climatic variations and human activities”
Why does climate change naturally?
Natural climate change: Milankovitch cycles // Plate tectonics (alter ocean currents and global heat distribution) // Volcanic eruptions (alter atmospheric composition) // Variations in sunspot activity/ solar flares // Changes in ice coverage, albedo and positive feedback mechanisms
Why does climate change due to humans?
Human causes of climate change: Aka anthropogenic causes (enhanced greenhouse effect – fossil fuels/ deforestation/ concrete/ agriculture…)
What impact are current climate trends likely to have on deserts?
Current climate trends are likely to lead to deserts becoming hotter and drier
what desert landforms are Fluvial
Plateau
Mesa
Butte
Inselbergs
Pediments
Playas (aka salt pan)
Wadis
Canyon
Alluvial fans
Bahadas (bajadas)
what desert landforms are Aeolian
Deflation hollow
Desert pavement
Yardang
Zeugen
Ventifact
Barchan dune
Seif dune
Plateau
Large elevated flat areas
Colorado plateau - 2,000m asl
Mesa
Fluvial
Erosion
Remnants of erosion of a mesa/ section of a plateau ,Wider than its taller
100,000+ years to form
Butte
Fluvial
Erosion
Continues erosion of a mesa/ section of plateau , taller than it is wider
100,000 + years to form
Inselbergs
Fluvial
Erosion
Differential erosion usually by water- hard rock remains , (lines of hard rock)
100,000 + years to form
Pediments
Fluvial
Erosion
Sheet flooding at the mountain foot, abrasion, traction , saltation
100,000+ years
Playas (aka salt pan)
Fluvial
Erosion
Deflation may become a temporary lake in times of rainfall, unless the water table is nearby. often forms salty crusts around edges. Plants need to be halophytic
100,000+ years to form
Death Valley , California
Wadis
Fluvial
Erosion
Abrasion by ephemeral streams
100+ years
Canyon
Fluvial
Erosion
Continued erosion of a wadi, or a landscape formed by exogenous rivers over millions of years
1,000+ years
Alluvial fans
Fluvial
Depositional
Drop In energy due to friction with valley floor, hence sediment dropped as water spreads out in all directions . heaviest sediments dropped first, lightest further away
10+ years
Bahadas (bajadas)
Fluvial
Depositional
Same process as alluvial fans - the merger of Alluvial fans
100+ years ago
Deflation hollow
Aeolian
Erosion
Deflation removes sediment, leaving behind a large, shallow area, which may form an oasis if shallow enough/ close enough to the water table
100,000+ years
Desert pavement
Aeolian
Erosion
Deflation removes the finest material, leaving behind gravel, rock, pebbles that are too heavy to transport
10,000 + years
Yardang
Aeolian
Erosion
Differential erosion/ abrasion
10,000+ years
Zeugen
Aeolian
Erosion
Abrasion
10,000+ years
Ventifact
Aeolian
Erosion
Abrasion ‘mushrooms rocks’
10+ years
Barchan dune
Aeolian
Deposition
Drop in energy = deposition formed in the direction of the prevailing wind . Migrate 30km/yr
5+ years
Seif dune
Aeolian
Depositional
Drop in energy = Depositional. Initially formed in the direction of the prevailing wind (Barchan) then modified by secondary winds. Migrate - 10m/yr
5+ years
