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
what is an inselberg
prominent steep sided hill composed of strong solid rock that is resistant isolated in desert plain eg ayers rock in centrral australia rounded due to weathering
what are the dunes formed by multi directional winds
star dunes seif dunes
what are the three types of rivers
exogenous endorheic ephemeral
what is an endorheic river
rivers that flow into in land lakes eg river jordan flowing into the dead sea
what is exogenous river
rivers which were formed in wet areas outside the desert but flow across it eg river nile in egypt formed from heavy highland rain
what is an ephemeral river
rivers that only flow after rain when rain falls in arid areas it often comes as heavy short lived storms flash floods
why are there high diurnal ranges in arid environments
insolation levels are high due to clear skies night time terrestrial radiation gives rise to dew and massive drops in temperature
how are wadis formed
A deep, steep-sided canyon like feature that contain debris material, formed by erosional processes, as can be seen in North-East Egypt.
High discharge, combined with steep channel gradients, means that the ephemeral rivers high energy for fluvial erosion.
This means sediment and coarse material can erode the banks of the river, suspended or rolled along the riverbed.
In areas of weak, easily eroded clays, they cut into unconsolidated material, forming highly dissected badland topography.
When the flash flood ends, the capacity and competence of the stream reduces and it becomes choked with sediment it is transporting. This is then deposited, strewn along the bed until the next pluvial.
how does continetaljty cause aridity
winds are drier, having lost their water content in areas nearer the sea and so increases likelihood of areas being dry and therefore deserts.
e.g. north east trades over north Africa and the Sahara
how regularly do sand dunes occur in deserts
only a small number 25-33% sahara and arabian desert - middle east and north africa
where do pediments form
at the base of s cliff or steep hill
what is a salt pan
site of former or occasional lakes
explain the rainshadow effect
winds rise to cross mountain they are cooled and condense on the eastern side once the peak is reach the winds descend and are warmed dry due to compression eg snowy mountains of australia desert
where do the highest desert temperature occur
upper 50s in death valley and libiya
explain how atmospheric pressure causes aridity and clear skies
high pressure sinking limb of hadley cell air rises to 15,000m then cools and becomes denser, sinking at 10-30 degrees north and south of the equator the air is compressed and heated as a result air can’t rise in an anticyclone as the sinking air puts a “lid” on the atmosphere clear skies and no rain
describe the appearance of a barchan dune
Barchan dunes, when viewed from above, resemble crescent moons, with curved outward backs and steep, curved inward faces. The curved tips of the crescent point downwind and partially enclose a single slip-face. A barchan dune may grow as tall as a several-story building. Barchan dunes are the most common, occuring widely in deserts around world.
what conditions do barchan dunes occur in
Barchans usually form where there is a limited supply of sand, relatively flat ground, and a fairly constant flow of wind from one direction.
what conditions do transverse dunes occur in
They form when there is an abundant supply of sand and relatively weak winds.
how do linear dunes form
A linear, or longitudinal, dune is one that forms where sand is abundant and strong cross winds converge from at least two directions, pushing the sand into long lines or ridges.
what dune needs wind from the same quadrant
longitudinal eg SW AND Nw
example of an alluvial fan
60km alluvial fan in the Taklamakan Desert in Xinjiang
explain the process of hydration
clay minerals absorb water and explains shake expands by 1600% minerals incorporate water into their molecular structure which causes swelling and chemical change anhydrate to gympsum new mexico, white sands deserts
example of seasonal rainfall in arid environments?
Gaborone in Botswana has 85mm of rain in January and only 5mm of rain in July
how does sinking air lead to no clouds and no rain
air can’t rise in an anticyclone as the sinking air puts a lid on the atmosphere
how do cold off shore currents lead to aridity
warm, moist air that encounters the cold currents is cooled to a low temperature, it condenses - falling as rain
the air holds little moisture (30% of original) when it is re-heated over land, where it may provide fog or mist, but rarely rain. (Namib and Atacama).
how does continentality cause aridity
Most water in atmosphere is evaporated from the sea, condensing and falling as precipitation
Land closer to the sea receives much of this moisture.
As air moves inland, it gets depleted of moisture and precipitation drops:
currents have already lost the moisture they once carried.
how does rain shadow effects cause aridity
rain falls on the windward side of the mountain range leaving the leeward area dry e.g. Atacama desert
how does human activity contribute to aridity
when an area becomes desertification through overgrazing of animals, there is a huge reduction in biomass fewer plants mean that there is less evaporation and evapotranspiration so there is less water available to produce rainfall widespread desertification over an area is enough to reduce rainfall below the 500mm
what is a playa and how does it form
a lake developed in an internal drainage basin from intermittent streams from the occasional rainfall lake forms is rapidly evaporated and it’s site occupied by a saline crust
what is a butte
a flat topped feature separated from a plateau by stream erosion in faulted lines of weakness
when is the subtropical anticyclone overhead in the southern hemisphere
January- April e.g. Gaborone, Botswana
when is the subtropical anticyclone overhead in the northern hemisphere
July-October e.g. Death Valley, California
why are soils light grey in colour
accumulation of calcium carbonate in soil
this happens due to leaching
what is meant by biomass productivity
Biomass productivity is a measure of the amount of biomass produced per square metre a year.
In hot arid areas this is very low i.e. about 0.003 kilos per square metre.
present day erosional landforms
wadis, mesas and buttes, pediments, inselbergs,
depositional present day landforms
dunes, playas, alluvial fans and bahadas
formation of an inselburg
Volcanic processes may give rise to a body of rock resistant to erosion, inside a body of softer rock such as limestone, which is more susceptible to erosion.
When the less resistant rock is eroded away to form a plain, the more resistant rock is left behind as an isolated mountain.
The strength of the uneroded rock is often attributed to the tightness of its jointing
explain how wadis are formed by both past and present processes
Wadis are no doubt relic features carved out by earlier rivers
but present day flash floods can be intensive so may contribute to sheet wash erosion
describe the process of wind erosion in deserts
Wind: Predominantly through ‘sand blasting’ and most effective within the first metre above ground.
Above that, particles are essentially dust (ineffective at erosion).
Only takes place when the wind is loaded with loose materials, such as sand grains.
Wind throws the particles of sand against rock face, abrading it via sand blasting.
Wind smooths and polishes homogenous rock, but heterogeneous rocks (weakened by joints or faults) will become etched, grooved and fluted e.g. forming zeugens and yardangs.
describe the process of wind transportation
Fine particles can be transported by suspension
most sand is transported by saltation
traction is limited due to the fine/fragile nature of desert material
describe the process of wind deposition in deserts
Deposition occurs where wind carrying particles are forced to rise over some obstruction such as rocks with accumulation on the lee side.
Sand may also be trapped by vegetation and accumulate.
evidence of past pluvials
Geomorphological evidence suggesting water levels were whigher such as dry valley systems (Ahaggar Mountains, Algeria), wadis (Egypt) and Lake Chad (was previously 120m deeper)
Signs of former human occupation of the deserts by people who fished lakes and rivers that no longer exist. For example, the Garamantes civilization of the Ahaggar Mountains, who left behind cave paintings in Algeria, dating as early as 6000BC.
Great quantities of fossil groundwater that exist hundreds of metres beneath the surface of limestone and sandstone areas could not accumulate in present conditions in hot deserts where evaporation greatly exceeds rainfall.
Evidence of sand dunes in areas which are now rainy (sand dunes can’t form in over 150mm). For example, in Savannah Africa, where rainfall is up to 1000m in the present day, there is evidence of ancient degraded dunes.
Evidence of fossil soils of a more humid type, including laterite horizons. This is a soil type rich in iron and aluminium, commonly considered to have formed in hot and wet tropical areas, and can be found in western Egypt
why is pressure release most common in semi arid areas
they tend to be more populated, giving rise to a higher number of quarries which remove material, reducing pressure on rocks, allowing them to crack along horizontal planes
why is carbonation not so common in hot arid areas
the high temperatures prevent water from dissolving large amounts of CO2
explain the process of carbonation
Carbonation involves rocks being dissolved by carbonic acid from the atmosphere or soil; particularly common in limestone, which turns to olivine in solution.
explain the process of oxidation
involves the reaction of iron with oxygen – rocks containing iron-bearing materials change in the presence of moisture and oxygen.
what are the products of hydrolysis
a chemical reaction with water, involving the action of H and OH ions; e.g. feldspar (found in igneous rocks), reacts to form kaolinite clay.
Can result in black, thick ‘desert varnish’ seen in the Nevada desert.
describe the process of water erosion in arid environments
When rain falls in arid environments, water runs down slopes without vegetation, picking up and moving unconsolidated sediment.
Dry channels change to flooded streams and transport material until velocity decreases and deposition occurs.
Seepage and evaporation cause the streams to dry up and material is deposited. Leads to the formation of alluvial fans.
explain how rainshadow effects cause aridity
Moisture-laden air encounters a mountain and is forced up. The ascending air is cooled and condenses on the windward side of the range.
Once over the mountain, the air descends warming as it does so, so increasing its evaporative power.
difference between yardang and zeugen
y: vertical bedding planes of weakness
z: horizontal bedding planes of weakness
what initates yardangs and zeugen formation
dew and temperature changes initate joint opening
what is the difference between a mesa and a butte
mesa is wider
what is the pressure of the subtropica high
1026mb
how do semi arid and arid areas differ from arid areas for rainfall
Rainfall: Semi-arid areas experience greater rainfall, seasonally. This happens as the subtropical anticyclone moves away from the area, allowing depressions to bring rain e.g. Botswana.
Some hot arid areas get 0mm of rainfall, such as Central and Southern Africa.
Semi-arid areas are more variable – years of drought may be followed by storms with hundreds of mm of rainfall (giving rise to drought tolerant plants and coarse grass). For example, in Tunisia 319mm of rainfall fell in 3 days in 1969, compared to an annual average of 275mm. This is in contrast to Death Valley in SW USA, with most rainfall events producing just 3-4mm of rainfall.
how do semi arid and arid areas differ in temperature
In arid areas tends to be mid-upper 20s; seasonal with a range of about 10 C. Diurnal range can also be large with night time cooling sometimes to freezing e.g. Death Valley up to 40 degrees.
Lack of cloud cover, lack of surface vegetation to retain heat: radiation escapes.
The range is generally less in semi-arid areas.
how does convectional rain occur
when the land becomes very hot, it heats the air above it; this air expands and rises; as it rises cooling and condensation take place – more common in summer
why are temperatures generally high in arid areas
the sun is high in the sky so the quality and quantity of insolation recieved is great
large amount of incoming (short wave) solar radiation
40% of the sun’s radiation is recieved in cloudless sky vs clouds 23%
the less cloud cover there is the more radiation reaches the earths surface
explain the large diurnal temperature range in hot arid areas
large amount of shortwave incoming solar radiation (up to 40%) - the less cloud cover the more radiation reaches the earths surface
low vegetation cover means reflected solar radiation is minimal
long wave radiation is lost on a large scale as clouds do not reflect the radiation back to the surface - energy loss maximised
explain the process of pressure release
the unloading of overlying rocks causes underlying rocks to expand and fracure parallel to the surface
this leads to the formation of cracks and joints at right angles to the unloading surface - lines of weakness in the rock
biological weathering in arid areas
Animal activity e.g. in Zimbabwe herds are concentrated near water holes, accentuating initial depressions
Tectonic activity – especially uplift
Plant roots (phreatophytes) break up already weak soil
most famous cold off shore current
Peruvian current flows along the western coast of South America affecting the Atacama desert
explain the nature of semi arid climates
They are generally characterised by the high temperatures of arid areas, although there can be a seasonality to both temperatures and to the rainfall which is typically higher than arid areas at around 250–500 mm per annum.
Rainfall is unreliable as many of these areas are on the periphery of influence from ITCZ movement.
The dryness is mostly the result of the influence of the descending air of the sub-tropical high pressure systems, rain shadow effects, proximity to continental interiors and in some cases to cold offshore currents.
Climatic conditions are more variable for semi-arid than hot arid areas.
explain the development of a butte
The more resistant rock forms the vertical core with some almost detached pillars. The sloping slopes are characterised by horizontal bands of more resistant rock which is covered in detritus.
The slope angles are very even and give rise to a prominent knick between it and the surrounding gentle sloping pediment or broad alluvial plain.
The butte represents a residual feature that has been produced from the lateral erosion of streams cutting into the interfluves.
Eventually the interfluves coalesce to leave isolated features. Such extensive fluvial activity took place in previous pluvial periods.
explain why semi-arid areas get seasonal rainfall (summer and winter)
areas towards the equatorial edge of the subtropical high pressure belt will get rain in summer (high pressure belts move north)
because they will come under the influence of the equatorial low pressure belt
air rises, causing rain
e.g. Botswanna summer = January-April 85mm of rain av.
areas towards the polar edge of the sub tropical high pressure belt get rain in winter
because they come under the influence of the mid latitude low pressure belt in this season
e.g. Southern spain winter = July-October 75mm of rain av.
why is the diurnal temperature range less in costal areas
the sea acts to cool the temperature during the day
so the diurnal range is much less than inland
costal Peru diuranl range is 11 degrees - half of inland
how does seasonality differ from the equator
the further away a desrt is
the greater temperature difference between summer and winter
deserts near the equator do not have clear seasons
why is the seasonal temperature range less in costal deserts
the sea is colder than the land in summer and lowers the temperature of costal places nearby
the sea is warmer than the ladn in winter, raising the temperature of costal areas
what are deflation hollows
hollows carved out of weaker parts of the rock by wind
Zimbabwe animal trampling
the Quattara Depression in Egypt is 128m below sea level
how do salt lakes/playas form
tectonically: the collapse of rocks down fault lines/folding
by solution of the underlying limestone rock
by wind erosion of soft rock - deflation
