10.2 landforms in arid and semi-arid environments Flashcards
salt weathering
- 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
thermal fracturing/insolation weathering
- 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
block/granular disintegration
- 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
hydration
- minerals absorb H2O:expand:change shape
- gypsum becomes anhydrate
- clay minerals can expand hundreds in size with absorption of water
freeze thaw
- 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
what is wind erosion?
-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.
what is wind transportation?
- 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.
what is wind deposition?
-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
Hammada Desert
Bare Rock Desert (e.g. plateaux areas such as Ksar Plateau in Tunisia)
Reg Desert
Gravelly or Stony Desert as fine material has been deflated (e.g. the basalt lava flow areas in the Syrian-Jordanian Desert)
Desert Pavement
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
Erg Desert
Sandy Desert (e.g. sand seas areas like the Great Erg Oriental and the Great Erg Occidental in the Sahara)
Mountain Deserts
mountain areas within deserts (e.g. Tibesti and Hoggar ranges in the Sahara)
Intermontane basins
-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.
Badlands
- 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
pedestal rock
- 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.
rock lattice
Very localised abrasion can cause a pitted surface that resembles chemical weathering with small depressions in the rock
ventifacts
-Small rocks such as pebbles scattered over the ground
-distinctive facets that have been eroded by the prevailing wind.
- three distinctive faces: Dreikanter.
yardangs
- 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.
zeugens
- 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.
Equifinality
(“same final result”) or convergence, that a given landform can have different origins.
evidence for pluvial periods in deserts
- 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.
evidence for previous aridity
-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.
wadis/arroyos
- 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
canyons/gullies
- 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
mesas
- 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
buttes
- -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
inselbergs / tors /kopjes
- 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
rivers
- 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)
pediments
-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
alluvial fans / Bahadas / Bajadas
- 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
chotts / playas / salt pans / salt laked
- lowest elevations of deserts: water previously ran into a depression and has evaporated leaving salts
- sites of former or occasional lakes
- Great Salt Lake
intermontane basins
- 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
Dome Dunes
- Dunes that are simple accumulations of sand
- few m
- often in the process of becoming other features
Nebkha Dunes
- behind tree or shrub: leeward side of vegetation
- west of an obstacle
Lunette Dunes
- lee of a depression
- may reach 10 m
- shape of wide C
- asymmetric in cross section, steeper side facing wind
Parabolic Dunes
- crescent-shaped but point downwind: inverted
- areas of limited soil moisture or vegetation
- semi-circular
- anchoring on the horns of parabola by vegetation
Blowout (Dunes) and Hollows
-wind erodes a depression on already vegetation stabilised dunes
Barchan Dunes
- 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
Barchanoid Ridges
- 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
Transverse Ridges
- 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
Zibar Dunes
- Loosely-formed Transverse Ridges but on a smaller scale and often have movement impeded by vegetation
Linear Dunes
- 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.
Reversing Dunes
- 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
Star Dunes (Rhourd)
- 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
Climbing/Falling Dunes
- 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
Seif Dunes
- Linear Dunes however they have serrated ridges due to local eddies in the wind
- e.g. SE Libya and SW Algeria
factors included in dune formation
- 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
