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A-Level AQA Geography > Hot Deserts: The role of wind and water in shaping the landscape > Flashcards

Hot Deserts: The role of wind and water in shaping the landscape Flashcards

1
Q

What is the main driver of arid geomorphological processes?

A

TEMPERATURE is the main driver (thermal fracture, exfoliation, block disintegration, granular disintegration as a main driver and crystal growth and hardpan as minor driver)

Water is the second largest driver of arid processes (main driver in hydrolysis, desert varnish, hardpan and oxidation)

Salts is the smallest driver (main driver in crystal growth)

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2
Q

What is thermal fracture?

A
  • mechanical weathering
  • main driver is temperature
  • rapid heating and cooling of rock causing degradation
  • amplified by different parts of rock expanding and contracting at different rates = fractures
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3
Q

What is exfoliation?

A
  • mechanical weathering
  • main driver is temperature (minor with moisture and salts)
  • exterior of the rock heated to a greater extent than the interior, tension creates cracks or flakes
  • salt-rich water can be drawn up and deposit salts enhancing process by adding pressure in hydration
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4
Q

What is block disintegration?

A
  • mechanical weathering
  • main driver is temperature with minor driver of water
  • igneous rocks have regular spaced fractures in them
  • erosion of rock brings them closer to the surface and water can enter fractures
  • heating and cooling of water causes large blocks of rock to break off (freeze-thaw weathering)
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5
Q

Q: Outline the impact of salts on weathering processes in hot deserts. [4 marks]

A
  • crystal growth is a large scale weathering process that requires salt crystals
  • salt crystals in the rock expand due to temperature change and weakens the rock over time due to pressure
  • Hardpan is another weathering process requiring salts
  • salts are drawn up by water and hardens on the rock surface due to insolation exposure
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6
Q

What is crystal growth?

A
  • type of chemical weathering
  • salts are the main driver and temperature is the minor driver
  • water creates crystal deposits in rocks and they thermally expand and contract which physically breaks down rock
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7
Q

What is hydration?

A
  • type of chemical weathering
  • water is the main driver
  • some rock minerals absorb water causing them to expand and become susceptible to further breakdown
  • for example gypsum
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8
Q

What is hydrolysis?

A
  • chemical weathering
  • main driver is water as it reacts with/dissolved minerals in rock and salts are a minor driver as they exaggerate reaction
  • this process in itself degrades the rock, but both are likely to be weaker than the parent rock making it more susceptible to further degradation
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9
Q

What is oxidation?

A
  • chemical weathering
  • water is the main driver as water oxidises minerals of iron
  • rock containing iron is exposed to oxygen or water is oxidised turning a reddish-brown colour
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10
Q

What is hardpan?

A
  • available moisture drawn to surface by capillary action causing an accumulation of salts due to water being evaporated
  • water and temp are the main drivers
  • salt is a minor driver as it hardens over the surface over time
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11
Q

What is desert varnish?

A
  • water and temp are main drivers as this is driven by capillary action
  • salt is minor driver as it only oxidises rock
  • Fe and Mn oxides been weathred from rocks at depth and drawn to surface in solution and then evaporates leaving a deep red stain
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12
Q

Why are aeolian processes and landforms common in hot desert environments?

A
  • barren environment allows wind to blow for miles unimpeded, no friction from land
  • high pressure area, wind is air moving to low pressure margins
  • localised air movements caused by a wide diurnal range
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13
Q

What are the two aeolian erosional processes?

A
  1. deflation

2. abrasion

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14
Q

What is deflation?

A
  • aeolian erosional process
  • removes unconsolidated sand or fine clay particles from the surface
  • creates a reg desert or a desert pavement (surface covered in coarse/fine pebbles
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15
Q

What is abrasion?

A
  • aeolian erosional process
  • material being carried by the wind hits an exposed rock surface and erodes it
  • forms erosional features
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16
Q

What are the three aeolian transportation processes?

A
  1. Suspension -> smallest particles held indefinitely = sand storms or hazes of dust
  2. Saltation -> sand-sized particles transported by bouncing/hopping along surface for short distances
  3. Surface creep -> larger particles slowly rolling or sliding across surface
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17
Q

What is aeolian deposition?

A
  • occurs when velocity of the wind decreases that it can no longer transport sediment it’s carrying
  • can form dunes (has to meet an obstacle)
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18
Q

What is a ventifact?

A

AEOLIAN EROSIONAL LANDFORM

  • individual exposed rocks with one of more smooth sides that have been abraded by wind-blown sediment
  • side on prevailing wind side is abraded the most
  • characterised by facets and sharp edges
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19
Q

What is a yardang?

A

AEOLIAN EROSIONAL LANDFORM

  • long streamlined parallel ridges of hard and soft rock, aligned in the direction of prevailing winds
  • separated by wind-sourced groove (soft rock)
  • made through abrasion
  • more resistant rock creates the standing yardang
  • for example in the west if the Tibesti Mountains in Northern Chad
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20
Q

What is a zeugen?

A

AEOLIAN EROSIONAL LANDFORMS

  • Collective term for rock pillars, rock pedestals and rock mushrooms
  • All have a layer of less resistant rock underneath a layer of more resistant rock that has been eroded by abrasion and other weathering processes
21
Q

What is a sand dune?

A

Main AEOLIAN DEPOSITIONAL landform in hot desert environments

22
Q

What are the conditions for sand dunes to form?

A
  • ready supply of sand
  • strong prevailing winds
  • an obstacle to encourage deposition
23
Q

What does the size and shape of dunes depend on?

A
  • direction and strength of prevailing winds
  • the supply of sediment
  • the morphology of the landscape where the dunes form
  • presence of any other winds
24
Q

Name and describe the 2 main types of sand dunes?

A
  1. Sief dune: knife-edged ridge of sand, separated by wind-scoured depressions (wiggly ridge), made from barchan dunes if wind changes direction
  2. Barchan dune: crescent shaped dune with convex windward side and lateral wings curving in the downwind direction
25
Q

What are the 3 less common types of sand dune?

A
  1. Transverse dune: field of large scale sand in ripples
  2. Parabolic dune: horseshoe shaped dune where open end faces upwind
  3. Star dune: many faces, often massive dome like features (hundreds of metres high and wide)
26
Q

Name and describe the 2 main categories of aeolian landforms?

A
  1. Ventifacts: exposed rocks lying on the desert surface that are eroded
  2. Sand Dunes: depositional features shaped by aeolian processes
27
Q

Name and describe the 2 main types of ventifact?

A
  1. Yardang: streamlined parallel ridge of rock
  2. Zeugen: collective term for all types of rock pillars, pedestals, mushrooms and yardangs that exhibit considerable undercutting
28
Q

What are the 3 types of water sources in hot deserts?

A
  1. exogenous
  2. endoreic
  3. ephemeral
29
Q

What is an exogenous source of water?

A
  • a source of water external to that hot desert environment
  • e.g. The River Nile, source in the Ethiopian Highlands/Central Africa and flows through areas of North Africa into Mediterranean
  • volume of rivers are large enough to maintain all year (respond to seasonal rainfall)
30
Q

What is an endoreic source of water?

A
  • rivers that occupy the drainage basin that are closed off/drainage basins that flow towards an inland lake of swap
  • for example the River Jordan flowing into the dead sea
  • normally in deserts these are saline or ephemeral
31
Q

What is an ephemeral source of water?

A
  • rivers that flow on the surface periodically, usually appear after heavy rainstorms
  • following periods of drought and removal of material by aeolian processes the surface is baked so rainfall will exceed infiltration capacity
32
Q

Why is water in deserts described as episodic?

A
  • deserts remain arid most of the time

- parts of western Algeria or SW Egypt can go a few years without receiving more than 0.1 mm in one 24 hour period

33
Q

What does episodic water cause?

A

Sheet floods, dislodge and move loose material across the surface (sheet erosion)
Channel flash flooding, move significant loads of sediment (significant depositional features)

34
Q

Why do desert storms lead to overland flow and sheet flows?

A
  • rainfall is very intense
  • sparse vegetation = little interception
  • no O horizon means no soil capture
  • ground is hardened by prolonged exposure to insolation = little infiltration
35
Q

What are wadis?

A

FLUVIAL EROSIONAL LANDFORM

  • steep-sided, wide bottomed, gorge-like valleys
  • often covered with thick layers of weathered material
  • when rivers do occur they are often braided to find a path through deposited material (braided streams)
36
Q

What are pediments?

A

FLUVIAL LANDFORM (EROSIONAL AND DEPOSITIONAL)

  • gently sloping areas of bare rocks and debris when the highland meets the gently sloping lowland
  • caused through sheet floors alongside deposition of material washed down from the uplands
37
Q

What are alluvial fans?

A

FLUVIAL DEPOSITIONAL LANDFORM

  • streams of water running through valleys or wadis depositing material on the pediment/mouth of wadi (loss of energy meeting gentler slopes)
  • the coarse material left upstream at the mouth and finest material downstream (few km)
38
Q

What is a bajada?

A

If an area has several close parallel wadis the alluvial fans can coalesce together forming a bajada

39
Q

What is a playa?

A

FLUVIAL DEPOSITIONAL LANDFORM

  • Intense periods of rainfall form ephemeral bodies of water from ephemeral streams flow into depressions which evaporate quickly leaving behind a dry lake bed
  • can also be called salt lakes or chotts
  • sodium chloride most common salt precipitate in the lake bed
40
Q

What geology is required for inselbergs, mesas and buttes to form?

A
  • Bedrock needs to be sedimentary with horizontal bedding planes with resistant rock cap
  • landforms are thought to be the remains of heavily dissected plateau where water has eroded all but a thin isolated rock pillar
41
Q

What are mesas?

A

Isolated flat topped plateau with steep slopes/cliffs on at least one side

42
Q

What are buttes?

A

Smaller mesas, heavily eroded by fluvial erosion

  • lower slopes covered by scree from rock fall and mechanical weathering
  • Monument Valley National Park in Arizona (mesas and buttes)
43
Q

What are inselbergs?

A
  • rounded steep-sided hills that rise abruptly from a lowland plain
  • generally composed of solid crystalline rocks (granite)
  • (theory of formation - surrounding slopes retreated in parallel as pediments encroached into highlands, pediplanation)
44
Q

What is the relationship between process, time, landforms and landscapes in mid and low latitude desert systems?

A
  • No 2 landscapes are the same due to complex range of factors…
    • speed and nature of weatherings, rocks are broken down quicker in areas with greater diurnal ranges (rapid heating and cooling cycles)
    • presence of moisture, amount of water will lead to formation of different landscapes
    • where moisture is available landscape is more likely to contain biological features
    • dominant of erosional/deposition processes
    • geological history
45
Q

How do landforms and landscapes intertwine?

A
  • landforms make up landscapes
  • some desert landscapes can be dominated by a particular type of landform (e.g. desert pavements/harmadas) others a combination (e.g. Monument valley)
46
Q

What are the features of Badlands?

A
  • Wadis of all sizes/shapes with steep sides
  • Unstable slopes, regular collapse
  • Regular mass movement, slope failure and slumping common
  • Alluvial fans and bajadas common with wadis emerging on lowlands
  • Caves and arches emerge as surface water flows into cracks
47
Q

What are the causes of the Badlands landscape?

A
  • Relatively impermeable, less-resistant geology
  • Run-off from heavy and sporadic rainfall
  • Aridity leads to little vegetation establishing, little holding regolith and bedrock together
  • Flowing water creates erosional and depositional features
48
Q

What are place examples of a Badlands?

A
  1. Southern Tunisia

2. South Dakota USA

A-Level AQA Geography (13 decks)

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