Physical Deserts

Front 1

Where are desert margins found?

Back 1

30• north and south of the equator, in the tropics of Cancer and Capricorn.
Some can be found in the interior of continents (Gobi)
Some next to mountain ranges (Atacama)

Front 2

What causes aridity?

Back 2

1. atmospheric circulation (Hadley Cell)
2. cold ocean currents
3. continental
4. rainshadow effect

Front 3

1. Hadley Cell

Back 3

air moves in circular patterns between and around the equator called Hadley cells.
The warm, moist air rises at the equator, where sun rays are direct, which then cools, condenses and falls as rain leaving the air cool and dry.
This dry air moves both north and south, and descends to be warmed up. it then moves back to the equator.

Front 4

2. cold ocean currents

Back 4

wind is cooled as it runs over the cold water, so its ability to hold moisture is reduced.

the clouds condense and fall as rain over the sea, leaving cold, dry clouds to reach land, which then becomes warm and dry.

Front 5

3. continentality

Back 5

air over the sea absorbs water to then condense as clouds and move to land, where precipitation occurs. this then leaves dry air, which moves across the land, remaining dry. the further in land, the dryer the air becomes.

Front 6

4. rainshadow effect

Back 6

-air picks up moisture from warm ocean
-air gets warmed up, rises up the mountain and the -cools and condenses, precipitating.
-due to high atmospheric pressure, this cool, dry air sinks and warms & dries up, causing the rainshadow

Front 7

barchan dunes

Back 7

formation: one direction of wind deposits sand from suspension over an obstacle. There is a gentle slope in the direction of the wind with horns forming either side of the dune building up. In the middle, there is a steep slope.

Front 8

star dune

Back 8

dunes formed from several different different directions of wind.

Front 9

seif dunes

Back 9

starts as a barchan, then another direction of wind comes and elongates one of the horns.

Front 10

zeugan

Back 10

bands of hard and soft rock on top of each other. can be 100ft high

Front 11

yardangs

Back 11

streamlined hills carved from abrasion and deflation. bands of hard and soft rock next to each other get eroded by abrasion. 3 or 4 times longer than wide. can be several inches to miles/100 metres high.

Front 12

Weathering in deserts

Back 12

Biological and Chemical not as significant because of lack of water.

Mechanical/Physical is the dominant:
-insolation weathering (onion peeling/exfoliation)
-freeze thaw (quite rare)
-salt crystallisation (when it rains, salt in the ground extracted but because evaporation is so fast, the salts get left on the ground, called DURICRUST)

Front 13

Wind proccess

Back 13

ABRASION: the wind picks up sand particles and sand-blasts the lower part of a rock in the way, creating a mushroom shape.

DEFLATION: wind blows the light sand off the surface, leaving behind a pebbly surface. over time this eddying wind deepens the ground to cause a deflation hollow.

Front 14

Wind transportation and deposition

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-surface creep
-saltation
-suspension

differences between water and wind transport:
1. particles more round (attrition much more strong)
2. wind sorts out much more effectively

Front 15

Water processes

Back 15

Exogenous - source: mountain.

Front 16

Landforms from WIND

Back 16

-yardang - bands of hard and soft rock (sandstone). wind erodes the rock. steep side facing wind, curves down to another steep side (lions).

-zeugen - (THINK OF A MUSHROOM) rock has horizontal layers of sedimentary soft and hard rock, usually sandstone.
it gets eroded by the wind through abrasion (sandblasting) at the bottom, so the base is very thin.

DUNES: -barchan: dominant wind direction. crescent shaped dunes with two horns. they can migrate. gentle sloping up in direction of wind, steep slope against the wind.
-seif dune: there is a barchan, but the wind changes direction causing one of the 'horns' to elongate. then the other wind equals out, blows, causing a hook on the longer horn to form.

Front 17

Landforms from water (relic features)

Back 17

mesa - an isolated flat-topped hill with steep sides, resistant cap rock on top

butte - an isolated hill with steep sides and a flat top (similar to but narrower than a mesa)

inselberg - rounded, develop in rocks such as granite

pediment - gentle slopes of rock covered by sediment (e.g. alluvial fan) at the foot of a mountain range/feature. could be formed by rivers at foot of mountain.

playa - when rivers carve through pediments, sometimes there can be a depression causing a salt lake/playa, due to the rapid evaporation

badland - a landscape, usually in a semi-arid area carved up by rivers. they develop where rocks are weak and impermeable or poor land management.

Front 18

vegetation

Back 18

-ephemerals - dormant seeds that germinate when water arrives

-phreatophytes - have long roots to reach down in the soil

-xerophytes - withstand dry conditions, spiky leaves and silky hairs to prevent water loss, waxy cuticle and tough bark to prevent waterloss.

halophytes - adapted to salty conditions they can grow on the edges of salt pans, e.g. salt bush

Front 19

weathering

Back 19

thermal expansion (onion peeling) - during the day, rock heats up and expands, contracts at night, repeats till rock loosens and breaks up

salt weathering - rain water (containing a bit of salt) or ground water gets into cracks of a rock. water evaporates, leaving salt crystals, which expand when the water evaporates

frost shattering/freeze thaw weathering - water gets into a crack, freezes over night, expands, melts in the day, contracts, till the rock breaks off

wetting and drying - some desert rocks are clay, when clay gets wet, it expands, causing the rock to break due to pressure.

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rivers in deserts

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exogenous: starts outside, goes through, ends outside

endoreic: starts outside, ends in the desert

ephemeral: starts inside the desert whenever there is rainfall.

Front 21

desertification

Back 21

when land in a semi arid area becomes into desert, due to human activities and changes in climate, leaving land unproductive

1/3 land worldwide, 110 countries have areas at risk

Africa (46% at risk) and Asia worst affected areas

Front 22

desertification causes

Back 22

climate:
-lower rainfall -> lower surface and groundwater storage -> less available water for plants
-less rain means less trees, which bind soil together, leaving it bare for erosion

human:
-overgrazing (trampling of large animals break down the structure of soil so more erosion, meaning less vegetation, so more bare ground)
-overcultivation (over planting takes all of the nutrients out of the soil reduces its productivity)
-deforistation for fuel and building (soil is left bare)
-irrigation (poor irrigation can wash soil away, or if too much water is added, this water sinks then raises the groundwater in aquifiers - rocks containing water - which are normally saline, which kills the land)

Front 23

desertification impacts

Back 23

land is less productive, so less can be grown

less plants means less animal life (e.g they have no food)

farmers can't feed their family or earn a living or they have to migrate, increasing pressure on land they migrate to

if people can't migrate, famine can arise

Front 24

arid land management

Back 24

(external aid is needed to help this)
it can be prevented by:
-reduce overgrazing, overcultivation, improve fertility of soil (leave areas of land untouched, use nomadic farming - moving cattle on the whole time to reduce overcultivation, rotating crops - different crops need different nutrients, planting legumes - such as clovers as they provide nutrients).

make water use sustainable (use little water requirement crops and use drip irrigation)

maintain the level of vegetation (plant trees or use alternative fuel sources other than wood)