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geo:hot-arid and semi-arid environments > 10.2 landforms in arid and semi-arid environments > Flashcards

10.2 landforms in arid and semi-arid environments Flashcards

1
Q

salt weathering

A
  • T around of 26-28C
  • salt crystals expand up to 300%
  • water evaporates and salt crystal growth occurs
  • common in hot deserts with low rainfall and high Ts
  • sodium sulphate appears most effective while common salt and sodium carbonate less so
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2
Q

thermal fracturing/insolation weathering

A
  • T changes
  • heated rock during the day and cooled at night
  • rock is poor conductor, only top layers start to break off
  • peeling/exfoliation
  • moisture is needed
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3
Q

block/granular disintegration

A
  • high T fluctuations
  • rocks are broken down along joints and bedding planes
  • block: larger sections will become detached
  • granular: caused by uneven heating of grains which result in them breaking the grains next to them off
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4
Q

hydration

A
  • minerals absorb H2O:expand:change shape
  • gypsum becomes anhydrate
  • clay minerals can expand hundreds in size with absorption of water
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5
Q

freeze thaw

A
  • water expands 10% when it freezes
  • ability to exert pressure as high as 2100 kg/cm3 at -22C
  • high fluctuations in T and lots of moisture availability
  • high altitude deserts and coastal interior
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6
Q

what is wind erosion?

A

-Wind blows away loose unconsolidated material
-deflation: lowering the surface over time
-Most movement is very close to the surface and abrasion will occur as particles hit into one another.

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7
Q

what is wind transportation?

A
  • Most winds are not strong enough to pick up pebbles but they can easily pick up sand-sized particles.
    -Particles may move in a fashion similar to traction: Surface Creep.
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8
Q

what is wind deposition?

A

-Sediments will be deposited as the wind loses speed.
- permanent or temporary landforms influenced by the landscape they encounter.
- Sand Dunes, Sand Drifts and Sand Sheets

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9
Q

Hammada Desert

A

Bare Rock Desert (e.g. plateaux areas such as Ksar Plateau in Tunisia)​

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10
Q

Reg Desert

A

Gravelly or Stony Desert as fine material has been deflated (e.g. the basalt lava flow areas in the Syrian-Jordanian Desert)

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11
Q

Desert Pavement

A

Pebbles are collected in an area (e.g. during a flash flood event) then wind erosion will flatten the tops of these pebbles leaving a relatively even, varnished surface​
​

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12
Q

Erg Desert

A

Sandy Desert (e.g. sand seas areas like the Great Erg Oriental and the Great Erg Occidental in the Sahara)​

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13
Q

Mountain Deserts

A

mountain areas within deserts (e.g. Tibesti and Hoggar ranges in the Sahara)​

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14
Q

Intermontane basins

A

-drainage basins areas with chotts (salt lakes) in the centre and other drainage features such as inland deltas and wadi floodplains (Chott-el-Djerid in Southern Tunisia). Sebkhas are coastal salt flats. ​

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15
Q

Badlands

A
  • Extensive tracts of heavily-eroded terrain in semi-Arid areas
  • less resistant impermeable rock is moulded by rapid run-off and can lead to gully-formation
  • not enough veg to hold regolith and bedrock together
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16
Q

pedestal rock

A
  • Mushroom Blocks
  • Sand Blasting is most effective within 1.5m of the ground and so there is considerable undercutting of the rock which leaves a top-heavy formation.
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17
Q

rock lattice

A

Very localised abrasion can cause a pitted surface that resembles chemical weathering with small depressions in the rock

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18
Q

ventifacts

A

-Small rocks such as pebbles scattered over the ground
-distinctive facets that have been eroded by the prevailing wind.
- three distinctive faces: Dreikanter.

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19
Q

yardangs

A
  • where rock is laid in parallel bands to one another and the prevailing winds
    -The wind scours away the less resistant rock leaving ridges of more resistant rock.
    -The ridges may then be undercut by wind erosion.
    Mega-Yardangs are hundreds of metres high and kilometres long, Meso-Yardangs are usually only a few metres high and Micro-Yardangs are just a few centimetres high.
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20
Q

zeugens

A
  • rock is laid in alternating LR and MR bands parallel to the surface
  • They resemble Pedestal Rocks however they are longer ridges with MR rock capping the feature.
21
Q

Equifinality

A

(“same final result”) or convergence, that a given landform can have different origins.

22
Q

evidence for pluvial periods in deserts

A
  • shorelines marking higher lake levels around dry, salty basins
  • fossil soils of more humid types, including horizons containing laterite
  • river systems now blocked by sand dunes
  • animal and plant remains in areas that are now too arid to support such species
  • evidence of human habitation, including cave paintings.
  • Sahara, Lake Chad may have been 120 metres deeper than it currently is, and may have extended hundreds of kilometres north of its present position.
23
Q

evidence for previous aridity

A

-Dunes can only develop in continental interiors when the vegetation cover is sparse enough to allow sand movement.
-If the rainfall is much over 150 millimetres, this is generally not possible.
-Satellite imagery and aerial photographs have shown that some
areas of forest and savanna, with 750–1500 millimetres of rain, contain areas of ancient degraded dunes.
-Today about 10 per cent of the land area between 30 °N and 30 °S is covered by active sand deserts, but about 18 000 years ago this area was about 30 per cent sand desert.

24
Q

wadis/arroyos

A
  • dry creek, stream bed that temporarily fills and flows after sufficient rain
  • steep sided, flat bottomed, enlarged by flooding
  • formed by intermittent flash floods and during wetter pluvial periods in the Pleistocene
  • as sediment accumulated the gradient changes, entrenchment triggered when slope reaches critical point
  • surface runoff in the form of sheet flow (flow evenly over land) may become concentrated
  • Arroyo Seco in Southern California
25
Q

canyons/gullies

A
  • deep and narrow valley
  • cut by a river through rock
  • canyon: deep gorges, dry
  • gully: erode headwards, eventually collapse
  • steep sides created by weathering and erosion by rivers, wind, rain and tectonic activity: equifinality
  • canyons are deeper Wadis: associated with downward incision of perennial rivers
  • Grand Canyon cut by the Colorado River
26
Q

mesas

A
  • relic hill
  • flat-topped tableland (plateau-like mountains or hills) with one or more steep sides
  • horizontal bedding planes left with MR cap rocks that determine their shape
  • formed by horizontal stratification of rock pushed upwards by tectonic force - erosion/weathering acts on rocks leaving behind MR - becomes elevated due to differential erosion
  • lava is very resistant: often forms flat top on mesa
  • common in the Colorado Plateau regions of US
  • usually covered in scree from rockfall/mass movement due to mechanical weathering
27
Q

buttes

A
  • -pillar like mesas
  • tall, flat-topped, steep sided rocks
  • isolated from surrounding plateau
  • once part of elevated areas (mesas)- erosion
  • hard rock over soft rock - LR eroded,
  • Arches National Monument Utah
28
Q

inselbergs / tors /kopjes

A
  • small rounded ridge or hill, isolated, steep sided hill
    -rises abruptly from relatively flat surroundings
  • weaker less resistant rock is washed away by erosion: domes of MR
  • deep chemical weathering during pluvial periods, overlying sediment removed
  • pediplanation
  • exhumation
  • Uluru or Ayer’s rock inselberg in Australia
29
Q

rivers

A
  • episodic (sporadic) precipitation:often intense, leading to flash or sheet flooding
  • exogenous: originating elsewhere (eg Nile)
  • endorheic: start outside a desert but flow into an inland sea or lake (River Jordan into dead sea)
  • ephemeral: flow intermittently as a result of periods of sudden rainfall, short lived (appears after rainstorm)
30
Q

pediments

A

-gently sloping areas of bare rock, weathering causes fine material, removed by sheetfloods and wind if no vegetation cover
- shallow slopes formed at the base of a cliff or steep hill/mountain areas
- between 1-7 degrees in elevation
- they are often covered din depositional debris
- denudation: weathering away of terrestrial surface by processes including weathering erosion
- sheet runoff, wadi flooding: weathering in combination with running water
- may be thinly covered with fluvial gravel that has washed over it from the foot of mountains produced by cliff retreat erosion
- water passes across the pediment by laminar sheet flow, but if this is disturbed it becomes turbulent and gullies develop

31
Q

alluvial fans / Bahadas / Bajadas

A
  • triangle-shaped deposit of alluvium
  • stream flows down the hill, picks up material, carries alluvium to a flat plain where it leaves channel to spread out: alluvium is deposited as the stream fans out: loss of energy as they leave mountain channels and enter onto a plain)
  • Bajada: convergence of many alluvial fans: flash floods wash alluvium down from nearby hills
  • Koshi River in Nepal built up alluvial fans more than 15 000 km2 wide
32
Q

chotts / playas / salt pans / salt laked

A
  • lowest elevations of deserts: water previously ran into a depression and has evaporated leaving salts
  • sites of former or occasional lakes
  • Great Salt Lake
33
Q

intermontane basins

A
  • wide valley between mountain ranges that is partly filled with alluvium
  • lowest point on arid landscape
  • oases: areas of exposed water-bearing rock often due to deflation: uncovers underground water that fell as rain long ago or an be formed by an aquifer or river that creates enough pressure for water to steep to surface
  • desert piedmont: foot of mountain: deflation, tectonic uplift, solution of limestone, crustal warping and barrier cutting off the drainage basin
34
Q

Dome Dunes

A
  • Dunes that are simple accumulations of sand
  • few m
  • often in the process of becoming other features
35
Q

Nebkha Dunes

A
  • behind tree or shrub: leeward side of vegetation
  • west of an obstacle
36
Q

Lunette Dunes

A
  • lee of a depression
  • may reach 10 m
  • shape of wide C
  • asymmetric in cross section, steeper side facing wind
37
Q

Parabolic Dunes

A
  • crescent-shaped but point downwind: inverted
  • areas of limited soil moisture or vegetation
  • semi-circular
  • anchoring on the horns of parabola by vegetation
38
Q

Blowout (Dunes) and Hollows

A

-wind erodes a depression on already vegetation stabilised dunes

39
Q

Barchan Dunes

A
  • wind primarily from one direction
  • crescent shaped with gentle windwards slope and steep leeward side
  • sand limited, wind supply constant
  • dominated by flows, slumps and slides
  • horns are mobile
  • up to 30 m
  • El Kharga oasis in Egypt​
40
Q

Barchanoid Ridges

A
  • transition of Barchan dune to transverse ridge
  • align laterally to the prevailing wind direction and move forward
  • gentle windward, steep leeward
  • the Dunhuang oasis in China
41
Q

Transverse Ridges

A
  • transition of Barchan dune
  • sand is abundant
  • wind flow is checked by a topographic barrier, or increased vegetation cover
  • combination of steep and gentle slopes
42
Q

Zibar Dunes

A
  • Loosely-formed Transverse Ridges but on a smaller scale and often have movement impeded by vegetation​
43
Q

Linear Dunes

A
  • Dunes that form parallel to the prevailing wind between 5-30m high
  • They may be hundreds of kms in length
  • usually found where there is a seasonal change in the direction of the wind.
44
Q

Reversing Dunes

A
  • Snaking ridges that result from two well-marked seasonal winds blowing in opposite directions
  • Sousvlei dunefield, Namibia where easterlies dominate in the summer and westerlies in the winter
45
Q

Star Dunes (Rhourd)

A
  • winds come from many directions, lack of dominant wind
  • limbs may extend from a central peak.
  • can be up to 150 m high and 2 km wide
  • Namib Desert around Walvis Bay
46
Q

Climbing/Falling Dunes

A
  • cross rocky barriers such as a rock plateau
  • These are found in areas with mostly rocky plateaux but some shifting sand
  • e.g. the Sinai Peninsula and in Arizona
47
Q

Seif Dunes

A
  • Linear Dunes however they have serrated ridges due to local eddies in the wind
  • e.g. SE Libya and SW Algeria
48
Q

factors included in dune formation

A
  • processes of aeolian transport and deposition
  • relationships between wind speed and sand particle size
  • sand availability
  • variability or constancy of wind strength and direction
  • initial obstruction to begin the development

geo:hot-arid and semi-arid environments (5 decks)

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