10.2 landforms in arid and semi-arid environments Flashcards
(48 cards)
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
