Landforms Of Hot Arid And Semi Arid Environments Flashcards
(177 cards)
1
Q
OUTLINE THE EXFOLIATION PROCESS
A
- is a weathering process
- aka ‘onion skin effect’/ ‘thermal exfoliation’
- thick layers of rock peel parallel to surface
- during day: high temps mean that surface rock layers heat up + expand
- during cold nights: rocks cool, contract + crack at right angles to surface
- repeated expansion + contraction of surface layer rock eventually cause it to peel/flake off
- effects have partially been due to pressure release jointing, salt crystal growth + chemical weathering
- experiments have taken place to produce exfoliation in lab
- involved heating + cooling rocks
- process is only effective when water is present. could be from rain/dew
2
Q
EXAMPLES OF WHERE EXFOLIATION HAS HAPPENED
A
- rocks in Northern Cape, S.Africa
- half dome in Yosemite National Park
3
Q
WHAT ARE EXFOLIATION DOMES
A
- process of exfoliation can produce these
- are round, bare rock surfaces
4
Q
REGOLITH DEFINITION
A
- loose material covering Earth’s surface
5
Q
WHAT CAUSES EXFOLIATION WEATHERING
A
- caused by insolation weathering
6
Q
WHAT ROCKS ARE EXFOLIATION WEATHERING EFFECTIVE ON
A
- granites
- gneisses
7
Q
OUTLINE OF THE SALT WEATHERING PROCESS
A
- happens when salt in rocks crystallises out of solution
- high temps draw saline groundwater to surface
- water evaporates + leaves behind salt crystals
- growth of salt crystals between pores + joints in rock creates stresses in rock. causes it to disintegrate
- can lead to granular disintegration/block disintegration
- happens in deserts as salts are dissolved by rainwater + removed in solution by streams + rivers
- BUT: in drier deserts: this doesn’t happen, e.g. Atacama + so accumulate in inland drainage basins
8
Q
WEATHERING DEFINITION
A
- decay + disintegration of rocks in situ
- include physical, chemical + biological
9
Q
SODIUM SULPHATE
A
- can expand by 300% in areas of high insolation
- involved in salt weathering
10
Q
OUTLINE OF GRANULAR DISINTEGRATION PROCESS
A
- physical/mechanical weathering due to repeated heating + cooling due to temp changes
- Due to big temp changes from hot to cold in day + night, rock surfaces expand + contract daily
- occurs due to rock type + colour
- e.g. dark coloured mica in granite absorbs more heat + heats up faster
- e.g. light coloured quartz or feldspar reflects light + is heated up slower
- due to repeated expansion + contraction of minerals in rock, it breaks down into smaller pieces
11
Q
OUTLINE OF BLOCK DISINTEGRATION PROCESS
A
- caused by repeated heating + cooling of rocks that are well-jointed
- eg granite, limestone
- rocks break down along joints + bedding planes as they are main lines of weakness
- effective in areas with a low diurnal range of temp (10-15/more degrees)
12
Q
THERMAL FRACTURE
A
- happens after physical expansion + contraction causes a large diurnal temp range
- this large range causes rocks + minerals to expand during day + contract at night
13
Q
WHAT DOES BLOCK DISINTEGRATION LEAD TO THE DEVELOPMENT OF
A
- development of bare rocks deserts
- aka hamada
- development of stony deserts
- aka serir/reg
14
Q
WHAT DOES GRANULAR DISINTEGRATION LEAD TO THE DEVELOPMENT OF
A
- development of sandy deserts
- eg sandy desert in California, USA
15
Q
COLOUR OF SURFACE AFTER OXIDATION
A
- red
16
Q
WHAT DOES THE RATE AT WHICH WEATHERING OCCURS DEPEND ON
A
- rock type
- climate
- relief
17
Q
WEATHERING ON EARTH
A
- is slow
- means that regolith is v.thin/ doesn’t exist
18
Q
PHYSICAL WEATHERING DEFINITION
A
- aka mechanical weathering
- occurs when rocks are broken down by physical factors in environment such as wind, water + temp change
- doesn’t change chemical composition of the rock
19
Q
CHEMICAL WEATHERING DEFINITION
A
- breakdown of rocks
- due to interaction of air, water or acid with the chemical composition of the rock
20
Q
EXAMPLES OF PHYSICAL WEATHERING
A
- freeze-thawing
- thermal fracture
- salt crystal growth
- dilatation
21
Q
EXAMPLES OF CHEMICAL WEATHERING
A
- hydrolysis
- carbonation
- hydration + dehydration
- oxidation
22
Q
LOCATION OF A BARE ROCK DESERT
A
- Arches National Park, Utah, USA
23
Q
LOCATION OF A STONY DESERT
A
- Namibia
24
Q
WHAT IS AEOLIAN EROSION
A
- are processes of erosion by wind
- are 2 of them
- corrasion (abrasion)
- deflation
25
CORRASION (ABRASION) DEFINITION
- is a process of aeolian erosion
- is the sand blasting effect of wind
- wind picks up loose sand + hurls it against rock surfaces
- impact of this breaks away small fragments
- effect is felt greatest 30cm above surface
- it attacks weaker layers + leaves more resistant ones
26
ANOTHER TERM FOR ROCK PEDESTALS
- mushroom rocks
27
LOCATION OF A ROCK PEDESTAL
- White desert, Egypt
28
WHAT LANDFORMS CAN CORRASION FORM
- rock pedestals
- yardangs
- zeugens
29
WHAT ARE VENTIFACTS
- are the result of corrasion
- aka dreikanter
- are pebbles/boulders found in a desert
- they have a series of 3/4 surfaces (facets) which are worn + polished
30
LOCATION OF A VENTIFACT
- Wright Dry Valley, Antarctica.
| - it's in the form of dolerite/basalt
31
WHAT IS DESERT VARNISH
- involved in corrasion process
- when a surface is abraded by another surface, it could have a coating of desert varnish
- is a film of iron + manganese oxides deposited by evaporation of solutions brought to surface by capillary action
32
DEFLATION DEFINITION
- is a process of aeolian erosion
- is the removal of dry, unconsolidated material ( soil, dust, sand) from surface
- finest material is carried high in air + carried for many miles
33
EFFECT OF DEFLATION ON LANDSCAPE
- 1. wind speed
- wind mainly moves lighter particles
- heavier ones left behind form stony deserts
- sand is moved to form the great sand erg
- v.fine particles can be blown outside desert area + form soils in humid areas
34
SERIR/REG
- another term for a stony desert
35
DEFLATION HOLLOWS
- deflation can result in major, enclosed depressions
| - aka deflation hollows
36
LOCATION OF A DEFLATION HOLLOW
- Qattara Depression, N.W Egypt
- floor of depressions is below sea level + covered with salt pans, sand dunes + salt marshes
- depression covers 19,605 km square
- formed due to salt crystal growth breaking don the rock with deflation removing the weathered debris
37
EFFECT OF DEFLATION ON LANDSCAPE
- 2. on human activity
- in big sandstorms normal life stops
- after it happens, deposited sand needs to be removed from roads etc. this takes time
38
WHAT DOES RAIN NEED TO BE LIKE FOR DEFLATION TO HAPPEN
- needs to be low
- so that soil dries out + is loose
- allows strong winds to blow the soil by deflation
39
LOCATION OF A SANDSTORM
- May 1997: sandstorm swept across Egypt
- caused lots of damage
- killed 12
- storm started in Libya
- blew swiftly northeast up to 60 mph
40
WHAT CAN DEFLATION CAUSE
- 1. oases. where ground is excavated to water table + water is at the surface in the floor of the deflation hollow
- 2. artesian effects. water table in mountains outside desert is higher than ground level within desert. An aquifer is replenished by rain in mountains.
41
WHAT IS AEOLIAN TRANSPORT
- processes of transport by wind
- are 3 of them
- 1. traction
- 2. suspension
- 3. saltation
42
SALTATION
- process of aeolian transport
- small pebbles + stones are bounced along bed
- material is deposited when wind speed drops + energy is lost/ sheltered in the lee of dunes + boulders
43
TRANSPORTATION OF SAND
- in v.strong winds: sand is suspended in air as well as fine silt + clay-sized particles
- most winds don't have enough energy to do this + so sand hops along bed
- sand is constantly picked up by stronger gusts + deposited in more calm conditions
- at low velocities sand can be moved by creep where it is pushed forward without leaving surface
44
TRACTION
- process of aeolian transport
| - large rocks + boulders are rolled along bed
45
SUSPENSION
- process of aeolian transport
- fine light material is carried along in water
- material is deposited when wind speed drops + energy is lost/ sheltered in the lee of dunes + boulders
46
DEPOSITION
- as wind speed drops, theres less energy to carry sand
- so heaviest material is deposited first + then lightest
- obstacles such as plants can encourage material to be deposited
47
SAND DUNES
- have complex forms
- there are 5 types of them
- 1. barchans
- 2. star dunes
- 3. parabolic dunes
- 4. echo dunes
- 5. seifs
48
WHAT 4 WAYS CAN PRODUCE SAND
- 1. marine erosion
- 2. fluvial erosion
- 3. aeolian erosion
- 4. weathering
- after this, sand is transported + deposited by wind to areas where it accumulates
49
BARCHANS DEFINITION
- crescent shaped
- develop transverse to winds that blow from a single direction + where there's little sand supply
- occur in large swarms
50
LOCATION OF BARCHANS
- N.Africa
- Namibia
- Peru
51
BARCHANS PROCESS
- wind causes sand to saltate up gentle windward slope
- then avalanches down steep leeward slope
- as wind blows up windward slope streamlines are compressed. this speeds up wind + erosion
- beyond crest: streamlines are decompressed, velocities decrease + deposition occurs
- transfer of sand from windward-leeward causes dune to move forward
- horns move forward faster than rest of the dune due to less sand at the margins
52
BARCHANOID RIDGES
- develop when there's an abundant sand supply
- but winds are still unidirectional
- are a series of barchans
- horns have merged into a ridge
53
SEIFS
- linear dunes
- lie parallel to main wind direction
- develop where there are winds from 2 directions
- develop when there's limited sand supply
- linear ridges can be 200 m high + 1km apart
- have sharp crests + steep sides with slip faces
- they trap sand from 2 directions with slip faces on either side
54
LOCATION OF A SEIF DUNE
- Sossusvlei, Namibia
55
STAR DUNES
- known as mega-dunes
- v.large
- develop where there are complex winds
- develop when there's lots of sand
- there's a central peak
- has radiating arms
- each arm corresponds to a different wind direction
- can be 400m tall
- no overall lateral movement
56
LOCATION OF A STAR DUNE
- central Sahara
- Namibia
- China
- Iran
57
PARABOLIC DUNES
- are barchans
- but horns are fixed by vegetation
- central part of dune moves forward
- horns are fixed so they face up wind
58
LOCATION OF A PARABOLIC DUNE
- Sossusvlei, Namibia
59
ECHO DUNES
- form in sheltered lee of hills
- sometimes vegetated
- fairly stable
60
EFFECT OF MOVING DUNES
- moving them can swamp houses, roads, oil installations + oases
61
HOW DO YOU MANAGE DUNES
- control grazing
- use fences
- plant vegetation
- simply remove the dunes
62
HOW DO YOU REACTIVATE OLD DUNES
- happens if vegetation is removed by farming, drought or fires
63
WHAT PRODUCES FLASH FLOODS
- torrential episodic rainfall
64
LOCATION OF A FLASH FLOOD
- August 1995
- Imlil flood, Morocco
- torrential downpour dumped 70mm of rain in a few hours near village of Imlil
- in semi-arid foothills of Atlas Mountains
- swept huge boulders down valet of River Reraya + into village
- showed how low frequency high magnitude events cause huge changes to physical + human grog of a desert environment
65
EFFECTS OF MOROCCO FLASH FLOOD
- killed 150 ppl
- crops of maize, alfalfa + grass destroyed
- walnut trees were swept away. Are an important cash crop for local Berber people
- takes 15 yrs for a walnut tree to mature
66
FLASH FLOOD DEFINITION
- brief periods of heavy rain
- if there's a sudden burst of heavy rain, rainwater can't infiltrate quick enough + so water enters rive via surface runoff
- leads to sudden + large increase in river's discharge, leads to flash flood
67
TERM USED TO DESCRIBE A V.LARGE FLASH FLOOD
- Jokulhlaups
- eruption of Eyjafjallajokull, Iceland 2010 triggered this as volcano was capped by a glacier that melted when it erupted
68
EPISODIC RAINFALL
- where rainfall, esp in hyper-arid areas occurs in a torrential storm
- happens every few years
- leads to rivers with v.unusual hydrological regimes
69
SEASONAL RIVERS
- develop when there's summer rainfall
| - e.g. semi-arid areas: Sahel on south side of Sahara, with boundaries with savannah may experience summer rainfall
70
INTERMITTENT RIVERS
- where climate is drier
| - eg. semi-arid areas: parts of Maghreb with boundaries with area of Mediterranean rivers during this season
71
PERENNIAL RIVERS
- where rivers have sources outside the arid area
- have a continuous flow all year round when there's a normal amount of rainfall in that specific area
- ones that flow continuously in arid areas are uncommon as rainfall is unreliable
- described as exogenic rivers
72
EFFECT OF A FLASH FLOOD
- v.dangerous
- to both people + property
- short-lived rainfall + high evaporation rates means that flash floods dry up quite fast.
73
FREQUENCY + MAGNITUDE OF A FLASH FLOOD
- low frequency
| - high magnitude
74
PROCESS OF A FLASH FLOOD
- rainfall intensity exceeds infiltration rate + overland flow occurs
- run-off from sparsely vegetated surfaces is fast
- produces dangerous, short flash floods
- abundance of weathered material on surface
- stream may turn into a mudflow
- mudflow contains material from clay-sized particles to boulders which has huge force
- lots of sediment is moved in a short time
75
WHERE DO FLASH FLOODS NORMALLY HAPPEN
- in dry valleys
76
INLAND DRAINAGE BASINS
- intermittent + seasonal rivers lie within this
| - are common in tectonically active areas
77
LOCATION OF INLAND DRAINAGE BASIN
- basin + range topography in western USA
- eg. basins centred on Lake Chad, Africa
- eg. Okavango Delta, Botswana
78
SHEET FLOOD
- causes erosion
| - occurs when water in OF on gentle slopes isn't concentrated in channels
79
WHY DO RILLS FORM
- form as a result of sheet wash
80
WHAT ARE RILLS
- steep-sided, narrow watercourses
- they carry water during times of high precipitation
- small features
- 5-30 cm deep + 22-100cm wide
- they carry sediment for a short period of time
- are an intermediate stage between water being transported as OF + development of a gulley
81
WHAT ARE GULLIES
- are larger channels
- more permanent than rills
- narrow but relatively deep with steep sidewalls
- when rills join together they form these
82
LOCATION OF RILLS
- Valley of Kings, west bank of the river Nile near Luxor, Egypt
- common in agricultural areas following the removal of vegetation during harvest
- common following deforestation/land-use changes as a result of compaction
83
RAIN SPLASH PROCESS
- raindrops have an erosive effect on hillslopes
- on a 5 degree slope, 60% of movement is downslope
- on a 25 degree slope, 95% is downslope
- amount of erosion depends on rainfall intensity, velocity + raindrop distribution
84
SURFACE RUN-OFF DEFINITION
- occurs when soils' infiltration capacity is exceeded + so water flows downhill due to gravity
- most likely to occur in UK during winter months as water drains across saturated/frozen ground
- in deserts: may occur due to nature of surface, such as impermeable, or hard-baked
85
WHEN IS EROSION MOST EFFECT ON A SLOPE
- when angles are between 33-45 degrees
| - when it's the start of a rainfall event when soil is still loose
86
SHEET WASH DEFINITION
- the unchanneled flow of water over a soil surface
| - there's no stream bed or river channel
87
WHERE DOES SHEET WASH OCCUR
- occurs on footpaths (impermeable)
| - occurs on moorlands (saturated)
88
WHAT MATERIAL DOES SHEET WASH TRANSPORT
- transports material that has been weathered/ dislodged by rain splash
89
DESCRIPTION OF A YARDANG
- is a wind-abraded ridge
- are elongate features that are 3/4 times longer than wide
- resemble hull of a boat from above
- facing wind is a steep, blunt face that slowly gets lower + narrower toward lee end
- some shapes resemble various objects/people
90
EXPLANATION OF A YARDANG
- formed in cohesive material
- wind abrasion concentrates on weak strata
- this leaves behind hard material
- form in environments where water is scarce + prevailing winds are strong, unidirectional + carry an abrasive sediment load
91
LOCATION OF PERENNIAL RIVERS
- Blue Nile, which originates in Ethiopian Highlands outside of Sahara Desert region
- Colorado River in the Rocky Mountains, outside the Great Basin, Sonoran + Mojave deserts
92
WHAT ARE EPHEMERAL RIVERS
- only flow occasionally
- flow during + after periods of heavy rainfall
- flow is unpredictable + irregular
- may not last for a long time
- when they flow they exhibit lots of discharge
- have lots of erosive power when they flow
- have ability to carry + transport large loads
93
WHAT ENVIRONMENT ARE EPHEMERAL RIVERS LIKELY TO BE IN
- arid tropics
94
WHAT ARE ENDOREIC RIVERS
- have a centripetal inward flowing pattern of drainage
| - flow into deserts + then terminate in a lake/inland sea/salt pans
95
LOCATION OF ENDORHEIC RIVERS
- 1. River Jordan drains into Dead Sea
| - 2. where channels flow into Lake Chad + Chott el Djerid, Tunisia
96
EFFECT OF SURFACE RUN-OFF
- rainfall in deserts impact landscape in a short period of time due to lots of runoff being generated
- this is due to:
- low levels of vegetation cover
- sparsity of vegetation cover
- interception is minimal
97
WHAT 7 EROSIONAL LANDFORMS ARE CREATED BY FLOWING WATER IN DESERTS
- rills
- gullies
- wadis
- canyons
- mesas
- buttes
- pediments
98
WHAT 3 DEPOSITIONAL FEATURES ARE CREATED BY FLOWING WATER IN DESERTS
- alluvial fans
- bahadas
- playas
99
ARE RILLS TEMPORARY OR PERMANENT
- can be temporary
| - as they can be destroyed during dry periods
100
HOW DO GULLIES FORM
- when there's a sufficient gap in vegetation cover
| - when there's a small surface depression that encourages water to accumulate
101
ROLE OF RILLS + GULLIES TOGETHER
- may embroider soft, impermeable rock formations with lots of small, ephemeral channels in semi-arid environments
102
HOW ARE RILLS + GULLIES FORMED
- formed by water erosion
103
HOW ARE WADIS FORMED
- formed by water erosion
104
HOW ARE CANYONS FORMED
- formed by water erosion
105
HOW ARE PEDIMENTS FORMED
- formed by water erosion
106
HOW ARE MESAS + BUTTES FORMED
- formed by water erosion
107
HOW ARE ALLUVIAL FANS + BAHADAS FORMED
- formed by water deposition
108
HOW ARE PLAYAS FORMED
- formed by water deposition
109
LOCATION OF A YARDANG
- Sphinx, Giza
110
ZEUGEN DESCRIPTION
- are ridges
- can be 100ft
- a layered structure
111
ZEUGEN EXPLANATION
- formed by wind abrasion where a surface layer of hard rock is underlain by a layer of soft rock into a ridge + furrow landscape
- are undercut + eventually worn away
112
LOCATION OF A ZEUGEN
- Areas in Bahrain
113
DEFLATION HOLLOWS DESCRIPTION
- found in non-mountaineous, arid regions
- don't have a lot of vegetation
- have some scree at the bottom
114
DEFLATION HOLLOWS EXPLANATION
- when sand is carried away over long distances by wind, depressions form
- depressions collect rainwater + hold it for some time . depends upon evaporation rates through stones
- caused by removal of fine particles by wind
115
LOCATION OF A DEFLATION HOLLOW
- Qattara Depression
116
SCREE DEFINITION
- collection of broken rock fragments
- is loose debris/ talus accumulated at the foot of a cliff
- comprises angular stones + boulders
117
MESAS + BUTTES DESCRIPTION
- a form of an inselberg
- are relict hills which have been cut out + isolated by fluvial erosion
- mesas:
- larger masses
- flat-topped
- table-like formations
- a small plateau
- may shrink + retreat on all sides to form smaller buttes, then smaller pillars, then pinnacles
- buttes:
- slimmer masses
- pillars of upstanding rocks
- a v.small plateau
118
INSELBERGS
- from German word meaning 'island mountain'
- means an isolated upland
- insolation of them is due to erosion
- are residual hills ( are a remainder of former landscapes)
- come in 3 different forms
- mesas + buttes
- bornhardts
- kopje
119
CREATION OF MESAS + BUTTES
- created where sedimentary rocks are present
- have horizontal layers of compressed sediments
-
120
LOCATION OF MESAS + BUTTES
- form landscapes of much of southern Africa
| - form scenery of Zion National Park + Monument Valley, Utah, in Great Basin Desert of USA
121
BORNHARDTS
- are domed inselbergs
- rise quickly from plains
- form in coarsely crystalline granites + gneisses
122
LOCATION OF A BORNHARDT
- Uluru, Ayers Rock, Northern Territory, Australia
| - (here, the rock type is sandstone)
123
KOPJE
- are piles of granite boulders
- they rest on bedrock which occur on hilltops
- represent last stages of destruction of an inselberg by erosion
124
WHERE DO YARDANGS FORM
- form where there are vertical layers of hard + soft rock
| - this is aligned to the direction of the prevailing wind
125
WHERE DO ZEUGENS FORM
- form where there are horizontal layers of hard + soft rock
126
HOW ARE WADIS PRODUCED
- by water erosion
127
WADIS DESCRIPTION
- a steep sided rocky ravine/ valley
- located in a desert/ semi desert which is dry
- is an Arabic term
- experience infrequent flows of water
- vary in size
- contain features of upper course river in humid lands
- they may show interlocking spurs or dry waterfalls
128
LOCATION OF WADIS
- Wadi Rum, Jordan, Arabian Desert
| - Wadi Bani Khalid, Oman, Arabian Desert
129
SIZE OF WADIS
- vary in size
- from a few metres
- to several hundreds of km long
130
EXPLANATION OF WADIS
- are filled with water derived from overland flow which can arrive through rills + gullies during irregular flash flood events
- flow of water within it takes place in narrow braided channels
131
WHY ARE WADIS FORMED
- formed due to rapid vertical erosion during short flooding
| - erosion is fast due to power of flash flood which is due to high discharge of water + large load carried
132
ARE WADIS RELICT FEATURES
- suggested that they could be
- due to paucity + infrequency of water entering dry valley
- maybe could have only been formed when the climate was wetter
133
ARROYOS DESCRIPTION
- is a Spanish term
- another name for a Wadi
- a stream bed
- is normally dry except during flash floods
134
ALLUVIAL FANS DESCRIPTION
- are semi- conical of debris
- located at the foot of mountains
- made of coarse sand, gravel + large cobbles what form a gentle surface slope less than 10 degrees away from mountains
- temporary rivers on fan show braided patterns
- are delta- like depositional features
- are between a few metres + up to 20km in length + 300 m thick
135
WHY ARE ALLUVIAL FANS FORMED
- where flash flood waters discharge from the wadis in upland areas + meet the flatter desert plains, alluvial fans may be formed
136
HOW ARE ALLUVIAL FANS CREATED
- created where a Wadi/ smaller perennial stream which is loaded with sediment runs down from a mountain foot onto a gentler lowland slope
137
WHAT IS A KNICK/ PIEDMONT ANGLE
- a v.sudden change in slope angle of an alluvial fan
- it leads to sudden loss in velocity + energy of the flowing water
- leads to deposition of lots of the river’s load into a fan shape
138
LOCATION OF ALLUVIAL FANS
- occur in desert areas such as basin + range province in southwest USA where steep mountains fronts meet downfaulted basins
- eg where Death Valley meets Sierra Nevada Mountains, California
139
WHAT IS THE DEPOSITION OF ALLUVIAL FANS TRIGGERED BY
- sudden drop in gradient + energy
- lateral spreading of water
- evaporation, which causes river to dry up
- water percolating into permeable gravel deposits
140
BAHADAS DESCRIPTION
- continuous gently sloping fringe of scree, gravel + coarse sand along base of a mountain range
- in a semi- arid area
141
HOW IS A BAHADA FORMED
- formed when multiple alluvial fans coalesce as a series of parallel wadis arrive at a mountain front
142
PEDIMENTS DESCRIPTION
- is a gently sloping rock platform
- between 1-7 degrees
- it’s either bare/ has a thin covering of rocks which stretches away from foot of a mountain range
- upper edge forms a sharp angle with the mountain front. Upper edge could be covered by bahada deposits
143
WHERE ARE PEDIMENTS
- are at the foot slopes of receding desert mountain ranges
144
WHAT IS A PEDIPLAIN
- when pediments join together to form vast erosional plains
| - isolated rock outcrops such as mesas + buttes may arise from this
145
LOCATION OF PEDIMENTS
- found in SW USA
| - found in arid lands of Southern Africa
146
2 THEORIES ABOUT HOW PEDIMENTS FORMED
- SHEET FLOODS/RILL WASH. theory suggests pediments are formed due to this. They would have planed down the rock surface. Rill wash would have happened when there’s a removal of surface debris on low gradient slopes by the raindrop impact
- WEATHERING. theory suggests pediments are formed due to this. Lots of water which accumulated at foothills of mountain started this weathering off. This weathering generates fine material which could be washed away by sheet floods/ removed by wind. This creates a knick which marks the sharp change in gradient
147
WHAT IS THE PIEDMONT ZONE
- means ‘mountain foot’
- zone that includes pediment, peripediment, bahada + alluvial fans
- also used as a place name. In USA: it’s a zone lying East of Appalachian mountains. In Italy: it’s a province at the foot of the Alps
148
DESCRIPTION OF A PLAYA
- is a depression at centre of a desert basin
- provides a site of occasional temporary shallow saline lakes which form after episodes of rainfall
- are fed by temporary rivers
149
WHERE ARE PLAYAS FOUND
- commonly found at foot of alluvial fans /bahadas
150
WHERE DO PLAYAS/SALT LAKES EXIST
- in hot deserts
| - where evaporation rates are high + so rivers don’t reach the sea + instead centripetal drainage patterns exist
151
SIZE OF OF PLAYA LAKES
- fluctuate in size
| - will dry up when there’s no rainfall at all
152
WHAT FEATURES ARE PLAYAS
- are ephemeral features
153
TRANSPORTATION PROCESS IN PLAYAS
- only silt, clay + solutions are carried by rivers to playas
- salts are products of weathering of the surrounding rocks + are transported to the playa where evaporation leads to crystallisation
154
PLUVIAL LAKES IN PLAYAS DESCRIPTION
- have v.high salinity levels
- lots of salt crust surrounds water
- salts are mainly halite; sodium chloride
- but gypsum; calcium sulphate, calcite; calcium carbonate + other salts may occur
155
WHAT ARE DESICCATION CRACKS
- appear in playa lakes
| - they create polygonal patterns on bed of former water as its dried out by solar heating + then it shrinks
156
WHAT 5 PIECES OF EVIDENCE SHOW THAT THERE HAS BEEN CLIMATE CHANGE
- 1. Landforms + deposits
- vegetated sand dunes in KALAHARI; semi-arid area show a previous dry climate
- old lake beds, lake sediments + river channels show previous wet climates
- 2. Archaeological evidence
- rock paintings in SAHARA show animals such as elephants. Show previous wetter climates
- human populations in Africa have fluctuated. In wetter periods areas that are now deserts used to be inhabited
- 3. Faunal evidence
- evolution of the horse in Tertiary period in NUSA happened at same time as development of a drier climate
- 4. Ocean sediments
- during dry windy climate, sand was blown out of deserts onto oceans. This sand is found in cores of ocean floor sediment
- 5. Ice cores
- are cylinders of ice drilled out of an ice sheet/ glacier
- oldest continuous ice core records to date go back 123,000 years in Greenland + 800,000 years in Antarctica
- ice contains small bubbles of air that can be used to measure past concentration of gases
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DESCRIPTION OF A PLAYA
- is a depression at centre of a desert basin
- provides a site of occasional temporary shallow saline lakes which form after episodes of rainfall
- are fed by temporary rivers
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WHERE ARE PLAYAS FOUND
- commonly found at foot of alluvial fans /bahadas
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WHERE DO PLAYAS/SALT LAKES EXIST
- in hot deserts
| - where evaporation rates are high + so rivers don’t reach the sea + instead centripetal drainage patterns exist
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SIZE OF OF PLAYA LAKES
- fluctuate in size
| - will dry up when there’s no rainfall at all
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WHAT FEATURES ARE PLAYAS
- are ephemeral features
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TRANSPORTATION PROCESS IN PLAYAS
- only silt, clay + solutions are carried by rivers to playas
- salts are products of weathering of the surrounding rocks + are transported to the playa where evaporation leads to crystallisation
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PLUVIAL LAKES IN PLAYAS DESCRIPTION
- have v.high salinity levels
- lots of salt crust surrounds water
- salts are mainly halite; sodium chloride
- but gypsum; calcium sulphate, calcite; calcium carbonate + other salts may occur
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WHAT ARE DESICCATION CRACKS
- appear in playa lakes
| - they create polygonal patterns on bed of former water as its dried out by solar heating + then it shrinks
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MIOCENE ERA
- happened 5.3-23 millions yrs ago
- in the Neogene period
- Africa: tropical easterly jet stream became stronger, led to dry stable air in Sahara. Tethys Sea, N of Sahara, was closed up during Miocene time. This increased aridity + continentality of N.Africa
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PLIOCENE ERA
- happened 2.6-5.3 millions yrs ago
| - in the Neogene period
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OLIGOCENE ERA
- happened 23-33.9 million yrs ago
| - in the Palaeogene period
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ICE AGES + PLEISTOCENE PLUVIALS
-
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WHAT HAPPENED TO EARTH’S CLIMATE IN THE PAST MILLION YEARS
- has varied between glacial + interglacial periods
- were 11 glacial phases
- during glacials there was cooling + ice sheets developed over poles
- in interglacials: there was global warming + ice sheets retreated
- patterns of global pressure + winds changed + had an effect on deserts
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WHAT DOES PLUVIAL MEAN
- used in 1868
- by Taylor
- refers to a period of increased moisture availability due to increased precipitation + low evaporation
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GLACIAL PERIODS
- during these times the climatic belts moved south so that deserts were wet
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INTERGLACIAL PERIODS
- deserts were dry
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PLUVIAL LAKES DEFINITION
- have experienced large fluctuations in volume due to changes in rainfall + evaporation
- used to interpret palaeoclimates
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LAKE BONNEVILLE
- in SW.USA
- was a massive prehistoric Pluvial lake
- Great Salt lake, Utah lake + Sevier lake show 5% of its former size
- 32,000-14,500 yrs ago: was 330m deep
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CLIMATE SINCE THE ICE AGES
- after last glacial period, semi- arid areas became wetter. Lead to development of savanna vegetation on dunes in KALAHARI
- change since warming started 18,000 yrs ago + has been related to changes in circulation of water in deep oceans. Circulation slowed + colder surface waters led to less evaporation + reduced precipitation in surrounding land masses
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WHEN WAS THE LAST GLACIAL PERIOD
- the Pleistocene epoch
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SALT LAKES DESCRIPTION
- found in basins isolated between mountains
- found where endoreic rivers dry up
- when river water dries, dissolved salts in water are deposited
- salts are commonly halite but some can be more complex. Depends upon nature of geology
- salts are flat + don’t have much vegetation
