3.2 How are Landforms of Mid- and Low-Latitude Deserts Developed? Flashcards
(81 cards)
1
Q
What processes interact to develop desert landforms?
A
Desert landforms develop through the interaction of weathering, mass movement, erosion, transport, and deposition with rock formations in dryland landscape systems.
2
Q
What is weathering?
A
Weathering is the in situ breakdown of rocks by mechanical, chemical, and biological processes.
3
Q
What type of weathering is dominant in mid- and low-latitude deserts?
A
Mechanical weathering is the dominant process in mid- and low-latitude deserts.
4
Q
What causes mechanical breakdown of rocks in deserts?
A
Mechanical breakdown is rapid due to the absence of soil and vegetation, exposing bedrock at the surface.
5
Q
How does moisture contribute to weathering in deserts?
A
Moisture from rain and dew can lead to salt weathering, which is a major cause of rock disintegration in desert areas.
6
Q
What is insolation weathering?
A
Insolation weathering is caused by intense solar heating of rocks, leading to expansion and contraction that weakens rocks.
7
Q
What is freeze-thaw weathering?
A
Freeze-thaw weathering occurs when water trapped in rock joints freezes and expands, breaking up rocks.
8
Q
What is the outcome of mechanical weathering?
A
The outcome of mechanical weathering is rock particles of varying size and shape, often resulting in granular disintegration.
9
Q
What is chemical weathering?
A
Chemical weathering relies on water for the breakdown of rocks, which occurs very slowly in deserts.
10
Q
What are the main chemical weathering processes?
A
The main processes are hydration, oxidation, and solution.
11
Q
What is biological weathering?
A
Biological weathering is limited in deserts but can occur through the action of roots and lichen.
12
Q
What is mass movement?
A
Mass movement is the downhill transfer of slope materials as a coherent body.
13
Q
What types of mass movement are common in deserts?
A
Common types include debris flows and rockfalls.
14
Q
What are fluvial processes in drylands?
A
Fluvial processes involve the action of ephemeral streams and rivers that flow intermittently during rainfall events.
15
Q
How does sparse vegetation affect fluvial processes?
A
Sparse vegetation cover minimizes interception and allows for rapid surface runoff.
16
Q
What is aeolian erosion?
A
Aeolian erosion is the process by which wind removes fine particles from the desert surface.
17
Q
What are the main processes of aeolian erosion?
A
The main processes are deflation, corrasion, and attrition.
18
Q
What are dunes?
A
Dunes are mounds and ridges of wind-blown sand formed under specific conditions.
19
Q
What is the Colorado Plateau?
A
The Colorado Plateau is a mid-latitude desert located in the southwest USA, characterized by sparse drainage and significant elevation.
20
Q
What are wadis?
A
Wadis are dry stream and river channels that are typically dry for most of the time.
An area of weakness is found in an impermeable surface and vertical erosion ‘downcutting’ occurs. This weathering on exposed wadi sides provides sediment for mass movement and erosion.
Are empty most of the year apart from after large amounts of rainfall.
21
Q
How are wadis formed?
A
- Regions of low and irregular rainfall patterns, this is needed dur to that fact that when rainfall does fall then it falls in heavy intense short bursts.
- Rock regions must be hard with soft rock in-between that can be eroded, there also needs to be impermeable rock so that water will not flow into the rocks.
- Sediment is carries by the rushing water through the wadi, as it is carries it is eroding at the base of the wadi making the walls deeper.
- After the water has stopped flowing it the leaves sediment along the wadi floor leading to the characteristically rocky and flat wadi floor.
- The wadi appearance is changed all the time with each rainfall in the area.
- Mechanical weathering is the most prominent type of weathering at this place
Wadi rum located in south Jordan, also known as the valley of the moon has been used in many movies as a scenic backdrop.
Wadi Hadramawt, it is characterized by its deep walls and a lot of vegetation, second biggest wadi in Yemen.
22
Q
What are canyons?
A
Canyons are narrow river valleys with near vertical sides, formed by the scouring action of rivers.
Canyons are much deeper, steeper sided versions of wadis, they are associated with the downcutting of flowing water into rock surfaces.
A canyon is a deep, narrow valley with steep sides or cliffs, often with a river or stream running through it.
23
Q
What are the different types of canyons?
A
Box: A narrow, steep sides canyon with high walls and closed or dead end structure, often formed through geological processes and sometimes containing watercourses e.g. Dead Horse Point
Slot: Narrow, deep canyon with steep, towering walls that are often formed by the erosional action of water, creating a striking and confined geological feature e.g. Antelope Canyon
Submarine: Deep, V-Shaped underwater channel carved by underwater currents and sediment flows, resembling terrestrial canyons but hidden beneath the ocean’s surface e.g. Monterey Canyon
24
Q
How do canyons form?
A
Tectonic Activity: Tectonic forces or volcanic activity can uplift the land, creating elevated areas with varying topography.
River/Water Erosion: Erosion from different water bodies plays as significant role. Water carries sediments and debris, which act as abrasives, gradually wearing away the land.
Downcutting: The flowing water cuts vertically into the terrain, deepening the valley over time. This process known as downcutting is crucial for canyon formation.
Differential Erosion: Different rock types are more resistant to erosion. Softer rocks erode more quickly, creating steep cliffs and rugged canyon walls.
Natural Geologic Features: Geological features such as faults, joints or fractures can provide paths of least resistance for erosion, influencing the canyon’s formation.
Time: Canyon formation is a slow, long term process that can take millions of years, shaping the landscape and creating deep, steep-sided valleys.
25
What are pedestal rocks?
Pedestal rocks are isolated, mushroom-shaped rocks formed by wind erosion.
26
How are pedestal rocks formed?
The process begins with the presence of a resistant rock outcrop which acts a protective cap shielding the underlying material from weathering and erosion.
The arid or semi-arid environment is crucial as it contributes to slow rates of weathering due to the limited presence of water.
As the landscape is subject to weathering the unprotected rock or sediment is eroded at a faster rate than the protected area beneath the caprock.
Over time this results in the pedestal with the caprock acting as a natural shield.
The erosion of the unprotected material may occur through abrasion or thermal stress.
There is an overhang between the caprock and the less resistant rock due to undercutting from eddying (circular movement of wind) which causes the base to have the highest amount of erosion. The less resistant rock is usually 2.5m with abrasion being the most powerful erosion agent.
27
What are ventifacts?
Ventifacts are small rocks shaped by wind-blown sand, characterized by facets on their surfaces.
A ventifact is a rock that has been abraded, pitted, etched, grooved, or polished by wind-driven sand or ice crystals. These geomorphic features are most typically found in arid environments where there is little vegetation to interfere with aeolian particle transport, where there are frequently strong winds, and where there is a steady but not overwhelming supply of sand.
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What is a facet on a ventifact>
A facet is a side of a ventifact that has been eroded down to become a flat surface, often very smooth.
29
What is needed for the formation of ventifacts?
Aeolian particle transport-
As the sand moves, it hits other grains which cause them to bounce up in the air. The wind then picks these airborne particles up and carries them. Gravity causes them to fall back down. If sand lands on a hard surface (e.g., rock), the sand particle will bounce off again, being carried further.
This occurs because, even in strong winds, sand grains can't be continuously held in the air. Instead, the particles bounce along the ground, rarely reaching higher than a few feet above the Earth.
A ventifact is a stone that has received one or more highly polished, flattened facets as a result of Erosion by windblown sand. The facets are cut in sequence and correlate with the dominant wind direction. As one surface is cut, the stone may become out of balance and may turn to expose another surface to the wind.
Ventifacts are produced under arid conditions and are generally formed from hard, fine-grained rocks such as obsidian, chert, or quartzite. Ventifacts can be abraded, pitted, etched, grooved, or polished by sand or ice particles.
30
What is desert pavement?
Desert pavement is a surface formed by the selective removal of fine particles, leaving a layer of coarse materials.
31
How are desert pavements formed?
1. Initial Surface Exposure: The process begins with the exposure of a relatively flat or gently sloping area of bedrock or sedimentary material in an arid or semi-arid region.
2. Weathering and Erosion: Sun, wind, and occasional rainfall subject the exposed surface to weathering and erosion. This gradually breaks down the surface material into smaller particles.
3. Wind Erosion: Wind is a powerful agent of erosion in arid environments. It picks up and carries away fine-grained particles like sand and silt. These airborne particles can abrade the surface of the bedrock or sediment.
4. Saltation: As the wind blows, it causes larger particles, such as gravel and small stones, to "hop" or "bounce" along the ground. This process, known as saltation, causes these particles to collide with the surface.
5. Abrasion and Sorting: During saltation, the impacting action of the particles causes abrasion. This further breaks down the surface material and contributes to the sorting of particles based on size and weight.
6. Transportation and Settlement: The wind carries the finer, lighter particles (like sand and silt) away from the site. Meanwhile, larger, heavier stones tend to settle closer to their source. Over time, the wind action continually transports and sorts the particles.
7. Accumulation of Coarser Material: As finer particles are removed, the coarser materials (larger stones and pebbles) accumulate and become more concentrated on the surface.
8. Interlocking Arrangement: The accumulated stones and pebbles become tightly packed and interlock with each other. This forms a stable, mosaic-like surface known as the desert pavement.
32
What is desert pavement varnishing?
- Desert varnish forms over long periods of time and thousands of years
- Varnish layer builds up gradually as new material is added and the underlying rock weathers
- Environmental conditions, such as temperature humidity and rainfall pattern influence the rate at which desert varnish forms
- Dust accumulates in the gaps between the stones acting as a natural cementing agent
- Microorganisms, such as bacteria and fungi contribute to the chemical processes by facilitating reactions and aiding the precipitation of minerals
- Chemical reactions between minerals in the rock surface and elements in the surrounding environment lead to precipitation of new minerals.
- Iron and manganese oxidise, binding to the rock’s surface contributing to the dark coloration characteristic of desert pavements
33
What gravitational processes occur on cliffs and scarp slopes?
Rockfalls and rockslides occur frequently due to weathering.
34
What types of landslides are active on debris slopes?
Landslides and slumps are active on debris slopes.
35
What natural events create flash floods on the Colorado Plateau?
Occasional torrential downpours create flash floods.
36
What are the principal desert landforms on the Colorado Plateau?
Pediments, screes, buttes, mesas, canyons, and vines.
37
What are pediments?
Extensive gently sloping rock platforms formed from the parallel retreat of mesas and buttes.
* A shallow slope at the foot of a steep slope/mountain; 0.5°-7° angle
* Found at the base of hills in arid regions where rainfall is irregular.
* Regarded by many as the most common feature of arid lands.
* It is unknown how they form, but the two most accepted theories are:
1. Lawson's recession theory:
The front of the mountain is weathered away and retreats, leaving behind a flat, rocky platform at its base.
2. McGee’s sheet-flood theory:
Sheet flooding occurs across the pediment after rainstorms, eroding the base and causing collapse; this causes the pediment to grow.
Eroded materials are transported downslope and are deposited.
Original theories suggested that pediments were created by wind erosion, but the rainfall patterns in areas with pediment suggest that precipitation also plays a role.
* Can become larger over time as the mountain front retreats.
* Can be created newly or grow due to geographical processes.
Are found in the monument valley in Colorado Plateau and in the Namib.
38
How are pediments formed?
They are covered by sediments derived from rockfall and transported by surface wash and ephemeral streams.
39
What is the role of geographical processes of pediments?
* Weathering: causes the mountain front to retreat, leaving a pediment at its base.
* Mass movement: the pediment will be covered with more sediment after events of mass movement (rockfalls/landslides); still a pediment, just partially covered.
* Deposition: occurs after sheet flooding (when the whole pediment floods).
* Erosion: the base of the mountain is eroded, causing it to collapse and retreat
* A 20th century geomorphologist suggested that arid landscapes undergo gradual erosion until a state of pediplanation is reached.
[pediplanation: formation of pediplains when many pediments join]
40
What is the difference between a pediment and a bajada?
* A pediment is formed at the base of mountains where there is no river channel.
* A bajada is formed when many alluvial fans join at the base of a single mountain.
* Alluvial fans are formed when a wadi/canyon meets the ground; there are river channels that flow over the surface of alluvial fans/bajadas.
41
What are inselbergs?
Isolated flat-topped, steep-sided remnants of the plateau, with larger ones called mesas and smaller ones called buttes.
An inselberg (monadnock)is an isolated hill, ridge, or small mountain that abruptly protrudes out from a virtually level surrounding plain. It is a type of disintegration landform
Eg:
Uluru/Ayer's Rock, a non-granitic inselberg located in Central Australia.
o The Spitzkoppe inselberg, rising to 3,600 feet in the Namib Desert, is the tallest of this landform in Africa.
o the Sandstone Inselbergs are 2625 feet which characterize the landscape in southwest Jordan
42
What must be present for an inselberg to form?
erosion + long term weathering
has to be significant variations in the level of weathering of the land
43
How are inselbergs formed?
o Inselbergs come from rocks which erode at a slower rate than the surrounding rocks.
o The landform consists of an erosion-resistant rock which protects a softer rock (e.g limestone)
o resistant rock remains isolated as ongoing erosion erodes the less resistant rock around it.
o Talus (scree/loose rocks from weathering) accumulates at the base of the slope
o Lithological and structural factors are responsible for the differences in the rate of erosion of material on the inselberg sideslopes, are considered to be the primary source of influence on Inselberg formation.
o During the rainy season deep weathering is more significant
44
How do inselbergs change over time?
o They are relict features
o a relic feature is a landform that was produced as the remains of an ancient landscape but is still being modified today by chemical rainfall. This only occurs when there is heavy rainfall and therefore can change the shape of the inselberg rapidly.
o a relict feature is a feature that was not completely destroyed by a destructive or transformative process
o Overtime the high hills would have risen through many repeated stages of deep weathering and stripping. Therefore overtime their height would have increased through time.
45
What causes the flat surface of inselbergs?
A resistant caprock protects the weaker sandstone beneath.
46
What is a characteristic feature of slope profiles in Monument Valley?
Their similarity, indicating they formed by parallel retreat.
47
What are the best-known canyons on the Colorado Plateau?
The Grand Canyon and Glen Canyon.
48
What role do rivers play in canyon development on the Colorado Plateau?
Rivers have incised their channels by downcutting to create today's canyons.
49
What long-term changes have occurred in the Colorado Plateau?
Uplift of the plateau and dissection by rivers during wetter climatic phases.
50
What are some short-term changes in the Colorado Plateau's landscape?
Weathering from freeze-thaw cycles, minor rockfalls, and sediment transport during thunderstorms.
51
What evidence shows that inselbergs continue to develop?
Extensive talus slopes around their bases indicate recent slope failures.
52
What are star dunes?
A type of sand dune that has three or more radical arms extending in various directions from a central high point. They develop in multidirectional wind regimes, where seasonal changes in the overall sand transporting direction cause sand to accumulate vertically, sometimes through the merging of other dune types.
- Compromise around 8.5% of the world's sand dunes.
- Great Sand Dunes, Colorado- some star dunes found along the northeast and southeast edges of the sand field.
- Namib Sandsea, Namibia- only major sandsea with star dunes in the southern hemisphere. Occur on the eastern flanks of the sandsea.
53
What are the factors required for star dunes formation?
- Multi-directional wind regime- this allows the dune to grow in several directions simultaneously.
- Underlying and surrounding topography- flat land to allow sand to accumulate easily.
- Rich sediment availability
- Sandy desert
- Tallest dunes- sand accumulates from being blown by the wind from multiple direction. This aeolian process builds the dune upwards.
- Development model:
o Barchan dunestransverse ridgesdune networkssimple star dunescompound star dunesstar dunes atop complex linear dunes.
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What are the geographical roles of star dunes?
Deposition:
- Occur in areas that represent depositional centres.
- Products of sand deposition, usually distributed in a unique sedimentary environment.
- Interactions between dune formation and airflow as winds change seasonally leads to deposition of sand in the central parts of the dune, giving a pyramidal shape and the extension of the linear arms.
Mass movement:
-Arm propagation is an important process of mass exchange in dune fields.
Erosion:
-Winds can erode the sand away from the dune.
-Sand avalanches can occur, along with notches, v shaped indents made in the sand, and slumping.
Weathering:
-Biological: animals and vegetation can burrow holes into the sand and create a fast movement of sand, avalanches.
Transportation:
Star dune arms grow by the transport of sediment over a non- erodible ground.
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How do star dunes change over time?
- Little is known about their formation and evolution.
- Is a landform being created today.
- The morphology of the individual arms of sand dunes can vary seasonally in response to sand transport shifts such that they can behave like transverse or linear dunes. Star dunes are sometimes found in chains, and if one transport direction is stronger than the others, slow migration may result.
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What are linear dunes?
Linear dunes also knows as longitudinal dunes or seif dunes are long and follow the direction of the prevailing wind.
They result in linear rows of sand that trend North West- South West
These Dunes can be up to 47 KM in Length
They play a crucial role in shaping desert ecosystems
They have 2 windward slopes which are vulnerable to sand avalanches on both sides depending on the angle of repose.
If you were to take your fingers and run them through sand, you would create ridges that all run in the same direction. That is what linear dunes look like when seen from space. These dunes line up in the same direction that the wind blows.
57
What factors are needed for the development of linear dunes?
Must be located in Desert Regions
Wind pressure must be equal on both sides of the dune
High Availability of Sand
Presence of Prevailing wind
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How do linear dunes form?
1) Linear Dunes follow the direction of the major wind (prevailing wind). The wind must blow around evenly on both sides.
2) The Linear Dunes will transport sediment including sand, gravel, pebbles, rocks etc
3) Obstacles, such as rocks and vegetation cause the winds to converge leading to the sediment being deposited in a linear pattern
4) Ongoing wind moves sand from the windward side to the leeward side. This causes the dune to advance in the wind’s direction
5) Layers of sand called sand beds form. This helps indicates the direction of the prevailing wind.
Cross bending: The process where layers of sand form.
Obstacle : A landscape e.g Vegetation or Rock that causes wind to be deposited to help shape the linear dunes
Dune Migration: The movement of sand from windward to leeward side
Sand sheet : Lots of loose sand that is used for the formation of the dune
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How do linear dunes change over time?
Erosion + Weathering: Rainfall and temperature variation lead to Erosion and Weathering on the surface of the Dune.
Climate change: Climate change affects wind patterns or changes in precipitation that ultimately affects the formation of the dune
Human activities: Human activities, such as mining impact the dunes shape.
60
What are Yardangs?
- Yardangs are sharp, irregular ridges of sand which are aeolian (wind) landforms.
- They can be find in exposed desert regions, and the largest concentration of yardangs can be found in the Sahara.
Similar to headlands and bays but is not a fluvial landform so no water present.
Can also be shortened due to a line of weakness in the rock.
61
What must be present for a yardang to form?
- Scarce amounts of water
- Strong prevailing winds
- Arid regions
- Loose, easily erodible rock e.g. clay- in between hard rock- eroded easily.
62
How do Yardangs form?
- Yardangs are normally formed
By a two process action of wind abrasion and deflation (removal of loose material).
- They are usually formed from areas of harder and softer material, the soft material is eroded and removed by the wind and the harder material remains.
- The Wind carries particles such as sand which collide with rock surfaces
- As the wind particles collide with the rock surface, abrasion occurs.
- Abrasion slowly erodes away the exposed rock.
- Not all rock will erode at same time, depending on the variation in rock hardness and composition. This forms ridges and troughs.
- Leeward side – steeper and less affected by erosion
- Windward side – faces prevailing wind direction and experiences more continuous erosion.
- Deflation also occurs as wind removes loose sediment from the area. This results in the exposure of the more resistant rock formations, contributing to the yardang formation.
63
What are the different types of yardangs?
Mega-Yardangs, Meso-Yardangs, Micro-Yardangs
Mega Yardang = can be several kilometers long and hundreds of meters high and are found in arid regions with strong winds. Most occur near the Tibesti Mountains in Sahara.
Meso- Yardangs = are generally a few meters high and 10-15 meters long and are normally found carved in semi-consolidated playa sediments.
Micro-Yardangs = only a few meters high
64
What is the venturi effect in relation to yardangs?
- Where wind speeds increase and accelerate through narrow passages e.g. in-between Yardangs
- Is responsible for high wind speeds in the Yardang and also helps to speed up erosional processes.
- It can create pillars between the long yardangs and it cuts through.
65
What are Hoodoos?
A hoodoo is a tall, thin spire of rock formed by erosion. Hoodoos typically consist of relatively soft rock topped by harder, less easily eroded stone that protects each column from the elements. They generally form within sedimentary rock and volcanic rock formations.
eg. Bryce Canyon Hoodoos
66
What factors are needed for the formation of hoodoos?
1. Sedimentary Rock Composition:
* E.g. Sandstone, limestone, siltstone
2. Distinctive Rock Layers:
* Varying hardness and erodibility
* Harder layers resist erosion
* Softer layers erode more easily
* Therefore critical in shaping Hoodoos
3. Erosional Forces:
* Wind - prevailing wind
* Water - rainfall
4. Climate:
* Arid and semi-arid regions
* Low levels of rainfall + vegetation - allow for exposure of rock
5. Time:
* Slow and gradual process
6. Weathering
* Shape and reshape these landforms
* Biological, Chemical, Mechanical
67
What determines the size of hoodoos and how do they change over time?
1. Geological Formation: Hoodoos typically form in sedimentary rock layers, such as sandstone, limestone, or volcanic tuff. These rocks are composed of various layers with different hardness and erosion resistance.
2. Erosion Resistance: The formation of hoodoos relies on the contrast between harder, erosion- resistant layers (often composed of denser rock) and softer, more easily eroded layers (usually composed of less dense rock, like clay or silt). The erosion-resistant layers protect the softer layers beneath them.
3. Weathering and Erosion: Hoodoos are shaped over time by various weathering and erosion processes, including wind, water, and frost. These processes remove the softer rock layers, leaving behind the harder, more resistant layers in the form of a spire.
4. Freeze thaw: Freeze thaw, where water infiltrates cracks in the rocks, freezes, and expands, car slowly break down the rock, contributing to the formation of hoodoos.
5. Water Erosion: The flow of water, such as in intermittent streams or flash floods, can carve and shape hoodoos as it removes the softer rock material.
6. Wind Erosion: Windblown sand and particles can erode the exposed surfaces of the rock, smoothing and sculpting the hoodoos over time.
7. Vegetation: The presence of plants and vegetation can affect the size and shape of hoodoos. The roots of plants can infiltrate cracks in the rocks, exerting pressure and potentially accelerating the erosion process.
68
How are hoodoos formed?
1. Process begins with the deposition of layers of sediment on the earths surface over a long period of time
2. Tectonic plate movements uplift the rock layers, exposing sedimentary rock
3. Overtime these sedimentary rock layers accumulate horizontally: mixture of erosion resistant rocks and softer more easily erodible rock
4. Hoodoos begin to be shaped by erosional processes:
* Wind erosion-sand particles in the air impact the rock surfaces, sculpting them over
time
* Water erosion-occasional rainfall leads to the formation of streams and flash floods which carves into the softer layers of rock
* Freeze thaw - water enters the cracks, freezes, expands, exerting pressure and gradually breaking down the rock
5. As the softer layers erode away more rapidly, the harder layers remain, creating the characteristic spire-like shape of hoodoos
6. Overtime, erosional processes continue to reshape hoodoos resulting in a variety of unique forms - they may slowly grow taller as erosion works its way down through the layers or erode away entirely over extreme long time scales
69
What is a salt pan (playa)?
Site of a former or occasional lake.
70
What is an alluvial fan?
An alluvial fan is a cone of sediment found at the foot of steep slopes, often along mountain fronts crossed by many small channels.
Is formed where a wadi or canyon meets the lower ground- a bajada is formed where these fans merge.
eg. Badwater, Death Valley, CA
Desert state park, CA, USA
71
What are the factors needed for the formation of an alluvial fan?
- Lack of vegetation (so channel can shift)
- Occasional heavy thunderstorms causing sediment charged masses of water to flow
- Weathering processes producing considerable amounts of coarse debris- happening in the wadi
- Sharp juxtaposition of mountain and lowland
Alluvial fans are made up of alluviam= sand, silt, gravel and sometimes borders.
72
What determines the size of alluvial fans
- The drainage basin area of the contributing river/stream
- Geology- how resistant the rocks are in the basin
- Higher precipitation-= bigger alluvial fans
- Will shrink if its drier because more sediment gets taken away by the wind
- Rates of weathering- less erosion upstream in the wadi, less weathering
73
What is a bajada?
The convergence of blending of many alluvial fans entering the desert plain from the same area of high ground.
74
What is channel aggradation in alluvial fans?
When the supply of sediment is greater than the amount of sediment that the system is able to erode- channel becomes choked. Is when there is insufficient energy for deposition to occur.
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What is channel incision in alluvial fans?
When the discharge has the velocity and load to erode the channel downwards. When the supply of water is greater than the amount of sediment.
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What are distributaries?
Small channels created when the main channel divides due to obstruction caused by deposition
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What is the apron?
The wide semi-circular/triangular section due to juxtaposition.
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What is the apex?
The narrow point of the top of the fan. Narrow wadi= venturi effect- speeds up in narrower spaces.
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What is a barchan dune?
A crescent dune with two horns facing downwind.
Barchan dunes migrate fast over flat land, moving up to 30 metres a year and it is predicted that they will move faster and further if climate change brings about abnormal, yet optimal, conditions.
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What factors are required for the formation of barchan dunes?
- Obstacles. Sand can get picked up from the wind and will travel along the land until it meets an obstacle. This obstacles, such as vegetation or rocks, will block the path of the sand and sediment will begin to accumulate as wind velocity reduces. However, this only happens with the presence of a high availability of sand and fine sediment, which can easily be moved by wind in a uniform direction.
- Fine sediment. Finer grains of sand, with a smaller angle of repose, 35 degrees, create a gentle apron further away from the steep slope.
- High sediment availability. Saltation and surface creep are active at the windward slope, creating a gentle incline. However, on the leeward slope, sand avalanches are common and wind eddies the sand, creating a circular motion of sediment, and a steeper slope is formed due to unconsolidated material.
- Prevailing wind in a uniform direction. The horns move faster than the main body of sand as there is less sand to move.
- Dry conditions
- Flat land
81
What determines the size of barchan dunes and how do they change over time?
- Prevailing wind strength. Stronger= bigger and faster migrating
- Rates of erosion elsewhere which provide the sediment for dunes.
- Precipitation- more low-intensity storms= less movement of sand= slower migration.
- Removal for sand for human use
- Tourism- dune buggying changes the shape of the dunes.
