Arid Landforms
Barchan Dune
- Aeolian Deposition - Large areas of sand in arid regions are called ERGS - Common type of dune resulting from wind deposition - Crescent shaped with horns facing downwind; horns move faster because less sand to move - Migrate up to 30m per year. Up to 30m high (but other dunes vary from few mm to 1000m). - Sand moved up windward slope by saltation and surface creep - Eddying wind helps maintain steep slopes on leeward slope - Continual avalanches on leeward slope - Coarsest grains are at the top - Bottom of leeward slope has sand ripples
Deflation Hollow
- Aeolian Erosion - Large amounts of sand are removed by deflation leaving a hollow - From 3m wide and 1m deep to several kms wide and 100s of ms deep - May form due to faults in rocks - May reach water table and produce an oasis or salt lake
Rock Pedestal
- Aeolian Erosion - Mushroom shaped rocks - Abrasion undercuts a mass of resistant rock. - Less resistant layers will be eroded faster - The bottom is eroded more because it may be softer rock - Also abrasion is most effective up to a height of 1.5m from the ground
Ventifact
- Aeolian Erosion - Rocks lying on the desert floor that have been shaped by the wind (abrasion) - They have sharp edges and smooth sides - They may have more than one smoothed edge if there is more than one wind direction or they get moved by the wind.
Yardang
- Aeolian Erosion - Vertical rock strata (soft/hard) parallel to prevailing wind - 5 to 100m high - Up to 1000m long - Shaped by deflation and wind abrasion - Some undercutting may occur depending on the angle of the rock strata
Desert Pavement
- Aeolian Erosion - Sand is removed by deflation and carried away in suspension by the wind. - Rocks are too large to be moved so get left behind - More and more sand is removed until the surface is covered by closely packed rock fragments.
Zeugen
- Aeolian Erosion - Horizontal rock strata (soft/hard) - 3 to 30m high - Joints in resistant rock widened by weathering - Abrasion cuts through resistant rock and develops deep furrows in softer rock underneath - Undercutting (abrasion and saltation) may occur like with a rock pedestal
Alluvial Fan
- Fluvial Deposition - A stream exits the foot of a mountain meeting gentle lowland slopes - Rapid energy loss leads to rapid deposition. Deposits spread out in a fan shape - Material is graded with the coarsest load deposited first - Vary in size from a few ms to several kms in length
Salt Lake
- Fluvial Deposition - Water collects in deflation hollows and rapidly evaporates due to high temperatures - Capillary action draws salts to the surface creating a thick crust, which cracks into polygonal shapes - Sodium chloride is the most common salt. These salts may be commercially exploited. - Also known as Chotts or Playas
Mesa, Butte, Spire
- Fluvial Erosion - Form in sedimentary rock with horizontal bedding planes. Mesa becomes a butte when it is taller than it is wide. - Lower slopes covered in scree from weathering and rockfall. Eroded by water. Good example of parallel retreat as the sides are eroded uniformly all over. - May have a resistant igneous cap rock. Steep slopes fall away to a wadi or canyon.
Inselberg
- Fluvial Erosion - Rounded isolated hill - Found in semi-arid regions as well as other climates - Much debate about their formation - Believed to be relic features formed when rainfall was high by deep chemical weathering - Crystalline rock e.g. granite
Wadi
- Fluvial Erosion - Dry river channels carved out by water erosion during flash floods/ephemeral rivers. - Vary from a few ms in length to over 100km long - Steep sided with broad flat bottom - Thick sediment leads to braiding - Patterns change after each flood
Badlands
- Fluvial Erosion - Found in semi-arid environments. - Soft and relatively impermeable rock is shaped by rapid runoff from heavy but irregular rainfall - Landscape comprises both erosional and depositional landforms e.g. wadis, pipes, arches, hoodoos, alluvial fans.
Pediment
- Fluvial Erosion - Wide gently sloped regions at the base of more steeply sloped desert mountain ranges. - Often there is little or no soil on top of the bedrock - May be hidden by a bajada or alluvial fan - May be at the base of a Mesa/Butt
Aeolian Erosion
Deflation of unconsolidated material. Corrasion of sand against the surface of the rock (abrasion) Attrition as sand grains collide and become smaller
Mechanical weathering
Exfoliation, Pressure release, Freeze-thaw, Crystal Growth
Fluvial Processes
From; flash floods, exogenous rivers, historic water, underground water
Chemical Weathering
Oxidation, Acid action, Solution
Biological Weathering
Plants and animals
Aeolian Deposition
Wind loses energy and deposits material
Fluvial
What kind of erosion forms mesas, buttes and spires?
Fluvial
What kind of erosion forms pediments?
Fluvial
What kind of erosion forms wadis?
Soft, impermeable
What kind of rock forms badlands?
Aeolian Transportation
Suspension of sand < 60um Saltation as sand (60-500um) bounces along ground Surface creep (>500um) sand roll along surface
Aeolian
What kind of deposition forms a barchan dune?
Fluvial
What kind of deposition forms alluvial fans?
Fluvial
What kind of deposition forms salt lakes?
Aeolian
What kind of erosion forms a deflation hollow?
Aeolian
What kind of erosion forms a rock pedestal?
Aeolian
What kind of erosion forms a ventifact?
Aeolian
What kind of erosion forms a yardang?
Aeolian
What kind of erosion forms a zeugen?
Fluvial
What kind of erosion forms badlands?
Aeolian
What kind of erosion forms desert pavement?
Fluvial
What kind of erosion forms inselbergs?
