Arid: causes, landforms, weathering Flashcards

Question

explain the development of a pediment

Answer 1

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

Answer 2

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

Answer 3

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

Answer 4

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

Answer 5

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

Answer 6

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

Answer 7

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

Answer 8

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

Answer 9

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

Answer 10

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

Answer 11

it’s tips point into the wind

Answer 12

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

Answer 13

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

Answer 14

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

Answer 15

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

Answer 16

high pressure lack of vegetation aridity of desert much loose material can be moved by the wind

Answer 17

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

Answer 18

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

Answer 19

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

Answer 20

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.

Answer 21

steep sided valley with a flat valley floor formed by intense streak flows following a storm generally in the past pluvial

Answer 22

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

Answer 23

star dunes seif dunes

Answer 24

exogenous endorheic ephemeral

Answer 25

rivers that flow into in land lakes eg river jordan flowing into the dead sea

Answer 26

rivers which were formed in wet areas outside the desert but flow across it eg river nile in egypt formed from heavy highland rain

Answer 27

rivers that only flow after rain when rain falls in arid areas it often comes as heavy short lived storms flash floods

Answer 28

insolation levels are high due to clear skies night time terrestrial radiation gives rise to dew and massive drops in temperature

Answer 29

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.

Answer 30

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

Answer 31

only a small number 25-33% sahara and arabian desert - middle east and north africa

Answer 32

at the base of s cliff or steep hill

Answer 33

site of former or occasional lakes

Answer 34

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

Answer 35

upper 50s in death valley and libiya

Answer 36

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

Answer 37

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.

Answer 38

Barchans usually form where there is a limited supply of sand, relatively flat ground, and a fairly constant flow of wind from one direction.

Answer 39

They form when there is an abundant supply of sand and relatively weak winds.

Answer 40

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.

Answer 41

longitudinal eg SW AND Nw

Answer 42

60km alluvial fan in the Taklamakan Desert in Xinjiang

Answer 43

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

Answer 44

Gaborone in Botswana has 85mm of rain in January and only 5mm of rain in July

Answer 45

air can’t rise in an anticyclone as the sinking air puts a lid on the atmosphere

Answer 46

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).

Answer 47

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.

Answer 48

rain falls on the windward side of the mountain range leaving the leeward area dry e.g. Atacama desert

Answer 49

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

Answer 50

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

Answer 51

a flat topped feature separated from a plateau by stream erosion in faulted lines of weakness

Answer 52

January- April e.g. Gaborone, Botswana

Answer 53

July-October e.g. Death Valley, California

Answer 54

accumulation of calcium carbonate in soil this happens due to leaching

Answer 55

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.

Answer 56

wadis, mesas and buttes, pediments, inselbergs,

Answer 57

dunes, playas, alluvial fans and bahadas

Answer 58

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

Answer 59

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

Answer 60

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.

Answer 61

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

Answer 62

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.

Answer 63

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

Answer 64

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

Answer 65

the high temperatures prevent water from dissolving large amounts of CO2

Answer 66

Carbonation involves rocks being dissolved by carbonic acid from the atmosphere or soil; particularly common in limestone, which turns to olivine in solution.

Answer 67

involves the reaction of iron with oxygen – rocks containing iron-bearing materials change in the presence of moisture and oxygen.

Answer 68

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.

Answer 69

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.

Answer 70

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.

Answer 71

y: vertical bedding planes of weakness z: horizontal bedding planes of weakness

Answer 72

dew and temperature changes initate joint opening

Answer 73

mesa is wider

Answer 74

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.

Answer 75

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.

Answer 76

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

Answer 77

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

Answer 78

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

Answer 79

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

Answer 80

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

Answer 81

Peruvian current flows along the western coast of South America affecting the Atacama desert

Answer 82

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.

Answer 83

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.

Answer 84

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.

Answer 85

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

Answer 86

the further away a desrt is the greater temperature difference between summer and winter deserts near the equator do not have clear seasons

Answer 87

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

Answer 88

hollows carved out of weaker parts of the rock by wind Zimbabwe animal trampling the Quattara Depression in Egypt is 128m below sea level

Answer 89

tectonically: the collapse of rocks down fault lines/folding by solution of the underlying limestone rock by wind erosion of soft rock - deflation

Arid: causes, landforms, weathering Flashcards

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