Arid: causes, landforms, weathering Flashcards
Causes of aridity
Global atmospheric circulation - dry descending air associated with sub tropical high pressure belt causes aridity 20-30 degrees north Distance from the sea - continentality limits the amount of water arrived across by winds Cold off shore currents - limit the amount of condensation in the overlying air Rainshadow effects - as air passed over mountsin
similarities between rainfall in semi arid and arid climates
Rainfall: low (overall); highly spatially variable BUT high evaporation rates
In arid areas rainfall tends to be below 250mm whereas in semi-arid the rainfall is higher, less than 500mm but usually seasonal and unreliable - movement of ITCZ e.g. Botswana.
Stable, subsiding air prevents convective up- draughts, which rarely reach sufficient heights for the development of cumulonimbus clouds.
In coastal areas (both), the formation of fog can instead provide moisture e.g. in Namibia fog provides 35-45 mm of precipitation per year.
what are the similarities between semi arid and arid temperatures
high.
Both experience cooler temperatures if on the coast (e.g. Peruvian current reduces temp. by 10 degrees), due to the presence of cool, upwelling currents.
Both experience ranges in temperature (diurnally, seasonally)
wind in both arid and semi arid climates?
high energy due to high temperatures and lack of vegetation (reducing friction with the air movement).
This gives rise to frequent dust-storms as the ground is both unconsolidated, superficial and dry.
key features of arid environments
high wind energy (unpair weathering process, erosion, landforms, soil or vegetation) diurnal or seasonal variation in precipitation and temperature climate change lack of water lacks sustainability (population pressure)
how does desert weathering differ from normal weathering?
it’s superficial only happening on the surface this makes it highly superficial
explain exfoliation (insolation) weathering
considerable diurnal range of temperatures in deserts of up to 30 degreees very hot daytime temperatures up to 50 degrees in hot arid climates causing the expansion of surface layers (only surface as rock is a poor conductor of heat) followed by cooling in the nighttime and contraction overtime this leads to fracturing of surface layers of rocks resulting in layers peeling away
how does granular disintegration occur due to exfoliation?
The differential expansion of the various minerals found in exposed igneous rocks leads to grandiose disintegration if exposed rock surfaces physical weathering processes result in the build up of piles of angular rocks (scree) at the base of steep slopes
explain salt weathering in deserts
salt crystal growth causes the disintegration of rocks when saline solutions deep into cracks and joints in the rocks and then evaporate leaving salt crystals behind these crystals expand in the suns heat exerting pressure on the rock most powerful and notable shocks are sodium sulphate and magnesium sulphate some salts expand by up to 300% as temperatures rise
explain freeze thaw weathering in deserts
high altitude deserts such as the Colorado Plateau, USA water enters cracks in rocks and in cold conditions freezes and expands this expansion caused he cracks to widen and can lead to disintegration especially after repeated freezing and thawing
how is lighter material transported in deserts?
in dust storms
how is heavier material pushed along the surface?
saltation traction rolling or creep
explain the global distribution of hot arid environments
dry descending limb of Hadley Cell leading to high pressure continentality - greatly heated land surfaces far from the sea with an absence of rain bearing winds e.g. GOBI Desert rain shadow effects, leeward side of maintain ranges e.g. ATACAMA (Andes mountain range) cold offshore currents - water vapour condenses offshore e.g. Peruvian current leads to temperatures of 16 degrees, where they should be 25 degrees - ATACAMA
what is a wadi
deep valley eroded in past wetter climates with the deposition of sand and gravel as alluvial fans
explain the development of a pediment
two theories: 1) periods of episodic rainfall lead to lateral erosion by streams and sheetfloods 2) parallels retreat of the mountain front due to weathering and erosion weathered material is carried from the mountain front across the pediment to be deposited
what is a pediment
gently sloping max 6/7 degrees rock slope either bare or with a thin cover into lf sediment which stretches away from the foot of a mountain range
how do alluvial fans develop
they form when a stream laden with sediment, changes gradient – initially flowing from uplands, down through narrow canyons and onto open plains.
As the stream reaches the plain, the channel laterally expands out losing velocity, capacity and competence.
As this happens, deposition occurs just beyond the canyon mouth, building up the lowland area near the mountain front, with coarse material deposited first.
This means that the slope is steepest at its apex and gradually diminishes, along with grain size, with increasing distance from the canyon mouth.
If multiple adjacent alluvial fans converge, they form a bajada.
60km alluvial fan that can be found in Xinjiang, China
explain the process of chemical weathering for arid and semi arid environments
limited by lack of rainfall but with occasional rainfall and dew this can occur hydration occurs when minerals absorb water resulting in an increase in volume
explain thermal fracture for arid and semi arid environments?
repeated temperature changes over a prolonged time period leading to the expansion and contraction of rock stresses caused may lead to the cracking of rock due to grandiose disintegration difference in thermal capacity of rock minerals is significant
describe the distribution of hot arid environments
latitudes 15 and 30 degrees north and south of the equator associated with the subtropical high pressure belt leeward side of migraine continental interiors costal areas adjacent to cold ocean currents
what is the most arid continent
australia
how is aridity measured
comparing average long term water supply to long term average water demand if demand is greater supply the area is arid
describe wind erosion in deserts
achieved by abrasion sand blasting effect of particles blown by strong winds against rock surfaces and effective up to one metre above the surface dry and fine sand particles moved by deflation
what is a wadi
deep valley eroded in past wetter climates with the deposition of sand and gravel as alluvial fans
explain the development of a pediment
two theories: 1) periods of episodic rainfall lead to lateral erosion by streams and sheet floods carrying sediment 2) parallels retreat of the mountain front due to weathering and erosion weathered material is carried from the mountain front across the pediment to be deposited
what is a pediment
gently sloping max 6/7 degrees rock slope either bare or with a thin cover into lf sediment which stretches away from the foot of a mountain range
how do alluvial fans develop
form where ephemeral rivers or streams emerge from a canyon onto a flat plain these streams are usually laden with sediment, the drop in gradient and ability to expand laterally on exiting the upland area leads to a loss of energy and deposition the coarse material is deposited first, at the mouth of the canyon, with the fine sediments deposited at the end they can be up to 20kn wide and 300m at the highest point; steepest at its apex and gradually decreasing in gradient with difference
explain the process of chemical weathering for arid and semi arid environments
limited by lack of rainfall but with occasional rainfall and dew this can occur hydration occurs when minerals absorb water resulting in an increase in volume
explain thermal fracture/ heating and cooling for arid and semi arid environments?
heating and cooling can lead to grandular disintegration in heterogenous rocks rocks heat up by day, expanding away from the centre of the rock but contract by night this weakens the outer layer of the rock creating a peeling effect; this happens at varying levels of thermal stress as different igneous rocks absorb heat differently expressing a different coefficient of expansion this is possible as rock is a very poor conductor of heat; at an air temperature of 40 degrees rock temperatures can be as high as 80
describe the distribution of hot arid environments
latitudes 15 and 30 degrees north and south of the equator associated with the subtropical high pressure belt leeward side of migraine continental interiors costal areas adjacent to cold ocean currents
describe wind erosion in deserts
achieved by abrasion sand blasting effect of particles blown by strong winds against rock surfaces and effective up to one metre above the surface dry and fine sand particles moved by deflation
how does a sand dune form and migrate
Formed by wind deposition as the wind hits the obstacle it loses energy and deposits the sand it is transporting.
The obstacle becomes a sand dune when it is covered in sand.
The dunes becomes a bigger obstacle, increasing sand deposition exponentially.
The dune then may migrate when wind direction and velocity are relatively constant.
It maintains its general form as the movement is episodic; the dune advances only when the wind is strong enough to move sand from the upwind to the downwind side
features of all sand dunes
gently sloping windward face steeply sloping leeward face windward: 10-20 degrees leeward: up to 32 degrees windward face is usually hard packed and smooth leeward face soft and unstable reach heights of up to 500 metres covering thousands of square kilometres make up only 20% of desert
what is a barchan dune
crescent shaped dune with horns or tips pointing down and away from the wind they arise where sand supply is limited where the ground is hard wind direction is fairly constant form around shrubs or larger rocks act as anchors to hold the main part of the dune in place tips migrate with the wind they are mobile features and can change form of wind direction changes
what is the difference between a parabolic and barchan dune
it’s tips point into the wind
how are parabolic dunes formed
need for obstruction eg plant or rock parabolic is anchored at its tips by the obstruction which acts to block wind while it’s main body migrated with the wind forming a depression between tips
how does a star dune form
where there is plentiful sand and many dominant winds come from directions resembles a star with arms pointing out in different directions the crests on the arms slope upward meeting to form a point in the middle of the dune similar to a pyramid
how does a transverse dune form?
where there is a great sand supply this dune is a ridge of sand that forms perpendicularly to the direction of the wind the leeward face is very steep group of transverse dunes resembles sand ripples on a large scale
how does a linear or seif dune form
where sand is abundant and cross winds converge often along seacoasts where the winds from the sea and winds from the land meet and push the sand into long lines these high parallel dunes reach over 200m in height and over 100 km in length create or summits of linear dunes are often straight or slightly waves
why is erosion highly effective in deserts
high pressure lack of vegetation aridity of desert much loose material can be moved by the wind
what is a yardang?
a wind sculpted streamlined ridge stretches over one km in length 30m in height e.g. Sahara Atacama Arabian deserts all have yardangs visible by satellite imagery
how do yardangs form
when strong winds, carrying sand in suspension blowing primarily in one direction remove all the sand in an area down to the bedrock via abrasion if the bedrock is slightly porous winds will erode the bedrock sand blasting hollows out of the soft part of the surface overtime wind erosion removes soft rock faster, leaving a sleek shaped ridge of upstanding resistant
what is a zeugen?
more rare and form isolated table like masses of resistant rock when weaker underlying rock has been eroded sculpted by the wind can be as high as 30m ultimately undercut and gradually worn away
how are zeugens formed
Jointed rocks are widened by heating and cooling or freeze-thaw weathering, and are then further enlarged by wind abrasion.
This is most prevelant at the first 3 ft of the ground, causing the less resistant bases of outcrops to erode, creating deep furrows under the less resistant cap-rock.
Strong, uni-lateral wind cuts down low-lying areas into parallel ridges which gradually erode into separate hills that creates a fleet of zeugens of roughly the same size.
what is a wadi
steep sided valley with a flat valley floor formed by intense streak flows following a storm generally in the past pluvial
