B. Desert Landforms Flashcards
Which characteristics of arid areas influence processes that form landforms?
- High pressure and clear skies
- Low rainfall and little vegetation
- Flash floods and seasonal / episodic rainfall
- Large diurnal temperature range
- High wind energy environment
How fast are rates of weathering in deserts?
What does this mean?
Rates of weathering in deserts are the slowest on Earth.
- Regolith is often very thin / nonexistent
- Underlying geology stands out clearly
What is the result of granular disintergration in deserts?
Granular disintergration leads to the development of sandy deserts
What is the result of block disintergration in deserts?
Block disintergration leads to the development of bare rock / stony deserts
Why is chemical weathering so slow in deserts?
Lack of moisture
What is the main chemical process in deserts?
Oxidation - colours surfaces red
What is Exfoliation?
The ‘Onion Skin Effect’ - thick layers of rock peel parallel to surface
Produces rounded, bare rock surfaces, known as exfoliation domes (convienient!)
- Requires some kind of rainfall to occur e.g. episodic
- Especially effective on granites and gneisses
Where do cracks occur in exfoliation?
- Rocks are poor thermal conductors → during the day, heat is concentrated on outermost layers - cracks occur parallel to surface
- During the night, cold causes rock to contract and crack perpendicular to the surface.
What is corrasion?
Corrasion is a type of abrasion (aeolian erosion) where the wind picks up loose sand and hurls it against rock surfaces, with the impact breaking away small fragments
Because of this, the effect is greatest at ~30cm from the surface (and no more than 2m) as most sand is blown to this height
What landforms does corrasion form?
Landforms of corrasion (aeolian erosion)
- Mushroom rocks
- Yardangs
- Zeugen
What are exogenous rivers?
Exogenous rivers are rivers that start outside desert areas and flow through them
What are epheremal / episodic rivers?
Epheremal / episodic rivers are rivers that only flow seasonally after an intense rainstorm / in the wet season
What are endorheic rivers?
Endorheic rivers are rivers that end as an inland sea / delta and never reach the sea
What are the characteristics of desert flash floods?
- Little vegetation to prevent flow
- Rain splash and sheet wash are common
- Surface is easily eroded leading to sediment charged rivers which have a lot of erosive power (especially abrasion)
What are arroyos?
Arroyos are the dry riverbeds of episodic rivers
Sometimes there are shrubs / bushes along the edges of arroyos which pooint to a possible underground water source
How do arroyos form?
Fluvial erosion
Most arroyos form as a result of flash flooding from seasonal rainfall, which flows over land in a consistent path, eroding a dry channel in the ground which becomes a river during rainfall
These form in soft materials: beds are covered in dry mud / sand
Give two examples of areas where arroyos can form:
Gobi Desert, Mongolia
San Lorenzo Canyon, New Mexico, USA
What are wadis?
Wadis are deep channel systems which fill with rainfall during periods of seasonal rainfall (ethemeral / episodic rainfall)
They usually have steep sides with a flat channel floor showing channel braiding from water flow
How do wadis form?
Fluvial erosion
Wadis form as a result of heavy seasonal rainfall which erodes a deep channel in rock over time
Wadis form as valleys cut into rock / more resistant materials
Give two examples of wadis:
Wadi Rum, Jordan
Wadi Deyla, Egypt
How does a slot canyon form?
Fluvial erosion
A salt canyon is formed in the same way as a wadi (…) but due to rock formation appears much more narrow, and only contains water during episodic rainfall.
What are alluvial fans?
Alluvial fans are cones of debris found at the foot of mountains. They are not restricted to arid climates but are characteristic of them.
They are made up of coarse sand, gravel and large cobbles which form a gentle surface slope (usually < 10°) away from mountains.
Temporary rivers on the fan have braided patterns
How do alluvial fans form?
Fluvial deposition
When a flash flood emerges from a mountain valley onto the surrounding plain, deposition is triggered by:
- the sudden drop in gradient and energy
- the lateral spreading of the water
- evaporation rapidly causing the river to dry up completely
- water percolating into the earlier, highly permeable gravel deposits
Give an example of an area where alluvial fans form:
Death Valley, California, United States
What is a bahada?
A bahada is a continuous gentle slope fringe of scree, gravel and coarse sand along the base of a mountain range in a semi-arid area:
How does a bahada form?
Fluvial deposition
They are formed by the coalescence of a series of alluvial fans
What is a playa?
A playa is the flat-floored bottom of an undrained desert basin that becomes a shallow lake during periods of episodic rainfall.
Their beds form some of the flattest surfaces on Earth
Give two examples of playas / salt lakes in the USA:
Bonneville Salt Flats, Utah
Groom Lake, Nevada
How do salt lakes form?
Fluvial deposition
Salt lakes are formed as ethemeral / episodic rivers deposit their coarser loads before reaching the lake, leaving only fine salts to be transported to the lake / playa
There, evaporation results in their crystallisation, causing extremely high salinity levels with extensive salt crusts surrounding the remaining water.
What is a piedmont zone?
A piedmont zone is the area at the base of a mountain slope, including the pediment and bahada.
What is a pediment?
A pediment is a very gently sloping surface, sloping down from the base of a steeper retreating desert cliff / escarpment.
How does a pediment form?**
Fluvial deposition / erosion
A pediment develops when sheets of running water (sheet wash) wash over it in intense rainfall events
It may be thinly covered with fluvial gravel that has washed over it from the foot of the mountains produced by cliff retreat erosion
What is abrasion?
(In a desert)
Abrasion is where material carried by the wind collides with exposed surfaces of materials, wearing them down in a sandpaper-like effect
What is deflation?
(In a desert)
The lifting and removal of loose material from the surface by aeolian processes.
Which landforms are formed by deflation?
- Desert pavements - formed when all small materials / sand have been removed, leaving a thin coating of stones
- Deflation hollows - low depressions formed by the removal of sand and light material by a prevailing wind
Which are taller - dust storms or sandstorms?
Dust storms - can extend thousands of feet high*
Sandstorms only exist very near the surface
* all Hollywood ‘sandstorms’ are actually these
How do mushroom rocks form?
Aeolian erosion
Winds carrying fine sand particles act as an abrasive and erode exposed surfaces of the rock (abrasion)
The softer, less resistant rock is eroded faster than the more resistant upper cap, eventually creating a mushroom-like structure
Continued erosion leads to the collapse of the pedestal
Give an example of an area with a famous mushroom rock:
Wadi Rum, Jordan
What are yardangs?
Yardangs are elongated, streamlined ridges, often less than 10m deep and more than 100m long.
How are yardangs formed?
Aeolian erosion
Formed when vertical layers of resistant and less resistant rock are aligned to the direction of the prevailing wind
The less resistant rock is eroded by abrasion, forming deep troughs and leaving behind vertical yardangs of resistant rock
How do Zeugen form?
Aeolian erosion
Zeugen form in the same way as yardangs but the layers of resistant and less resistant rock lie horizontally
Resulting ridges are 3-30m high
- Joints in the resistant rock widen through weathering
- Abrasion deepens the furrows down into the less resistant rock beneath
Which processes can affect the formation of zeugen?
- Undercutting of the furrows may also occur, to give them a pedestal-like shape with a flat cap rock which protects the underlying, less resistant rock.
- As abrasion is the primary process here, concentrated within 2m of the desert floor, zeugen often have an eroded, narrower base.
How do sand dunes form?
Aeolian deposition
Wind blows sand into a network of troughs, crests and ripples perpendicular to the wind direction as a consequence of saltation (transport process)
These accumulations develop into mounds and ridges
Dunes grow as sand particles move up the gentler, windward slope by the process of saltation and surface creep
What is the minimum height and slope angle of a sand dune?
Dunes count as dunes when the crest is a minimum 30cm tall and the slip face’s angle of response is 34°
What are the 5 types of dunes?
- Barchans
- Parabolic dunes
- Transverse dunes
- Seifs / longitudinal dunes
- Star dunes
What are barchans (dunes)?
A migrating crescent dune with horns pointing downwind from where strong winds blow in a consistent direction
- They move quickly
- They are common in central Asia and the Sahara
- They can reach hundreds of feet in height
What are parabolic dunes?
Parabolic dunes are barchan dunes where the horns have become fixed, meaning they now point upwind
What are transverse dunes?
Transverse dunes are parallel waves of crescent dunes perpendicular to wind direction
- They are found in areas with large supplies of sand
- They migrate downwind
What are seifs / longitudinal dunes?
Seifs / longitudinal dunes are multiple very long parallel dunes.
- May be created in areas with at least two dominant wind directions
- Can be tens of miles long
- Common in deserts outside of the USA
What are star dunes?
Star dunes are formed when wind blows from many directions
- Have multiple steep faces
How can old vegetated dunes be reactivated and begin to move?
Old vegetated dunes can be reactivated if vegetation is removed by farming, drought or fire.
How can dunes be managed?
- Controlling grazing
- Fences
- Planting vegetation
- Direct removal
To summarise, what were the effects of the Miocene?
Generally, the Miocene made everything higher, drier and colder.
In more detail now, what were the effects of the Miocene?
Higher:
- Uplift of Rockies caused increasing aridity on the Great Plain, the uplift of the Andes had a similar effect on the Atacama desert and Patagonian grasslands
- Uplift of Himalayas and Tibetan plateau blocked petration of moisture-bearing monsoons into central Asia, creating the Gobi desert
Drier!
- Increased aridity in continental Australia, Asia, Africa and the Americas
- Tropical easterly jet stream became stronger, bringing dry stable air to the Sahara
- Tethys Sea closed up increasing continentality and aridity of North Africa
Colder
- Antartic sheet ice developed in southern Hemisphere
- Southern Ocean cooled and cold Benguela current formed
What geographical evidence suggests the existence of the Miocene period?
- Creation of the Gobi Desert from uplift of Himalayas, Mojave desert from uplift of Sierra Nevada range.
- Many modern deserts created from increased aridity
- Emergence of Great Plains and Patagonian Grasslands coincides with evolution of horse in North America
What are interglacials?
There have been as many as 11 interglacials in the past million years
- Glacials = global cooling and ice sheets
- Interglacials = global warming and ice melt
- Pluvials = period of increased moisture availability due to increased precipitation
Present day deserts were probably wetter, but the area around the Equator was possibly drier.
In more detail now, what were the effects of the Pleistocene?
Drier
- Marginal desert areas such as the Sahel, Kalahari and parts of Australia were far more arid
- Sahara may have advanced 500km South and Kalahari extended north - dune systems now covered by vegetation
Wetter
- Wetter periods caused deserts to shrink (though hyper-arid Sahara and Namib were not affected)
- Major river systems may have existed in the Sahara
How do pluvial lakes demonstrate the importance of the Pleistocene pluvials?
- Pluvial lakes experienced large fluctuation in volume due to changes in rainfall and evaporation
- Former beds of pluvial lakes are marked by salt deposits, former lakesides marked by cliffs and land features
- Lake Chad reached maximum extent in 5000 BCE
- Lake Bonneville, today only 5% of its maximum size of 51 000 kilometers, roughly 14.5-32 thousand years ago.
- Lake Bonneville was formed by tropical storms
How does fluvial erosion demonstrate the importance of the Pleistocene pluvials?
- In 2013, a team of British and German researchers found evidence for three major river systems previously in the Sahara (others identified by river sediments)
- Plateaus, mesas, buttresses formed by water removing rock sto that it now has cliffs on all sides e.g Wadi Rum
- Large sand seas such as Erg Chebbi in Morocco are too large to have been blown by winds → deposits from large river basin shaped by current winds
What archeological evidence demonstrates the importance of Pleistocene pluvials?
- Wadi Rum, Jordan - 25,000 rock carvings and 20,000 inscriptions demonstrate 12,000 years of human occupation, including in wetter periods. Some carvings depict animals associated with wetter climates
- Evidence of ancient Garamantes civilisation, pastoralists who left cave paintings
- Significant fossil evidence found of tree pollen and even crocodiles demonstrates a wetter climate
What evidence outside of deserts demonstrates the impact of pleistocene pluvials?
Vegetated sand dunes in the Kalahari, a semi-arid area, demonstrate previous drier climates
Which pieces of evidence demonstrate the importance of modern desertification?
- Ancient Roman granary found on edge of Sahara
- Some early 20th centrury forests on edge of Sahara are now desert
