3.2 How are landforms of mid- and low-latitude deserts developed? Flashcards
3.2 How are landforms of mid- and low-latitude deserts developed?
Key idea ➡ Dryland landscapes develop through the interaction of processes such as weathering, mass movement, erosion, transport and deposition, with rock formations in drylands landscape systems.
Desert landforms develop through
The interaction of processes such as weathering, mass movement, erosion, transport and deposition, with rock formations in drylands landscape.
Geomorphic processes
-Weathering processes (Mechanical / physical, chemical, biological)
-Mass movement
-Fluvial processes
-Aeolian processes (Aeolian erosion and transport - Creep; Saltation; Suspension, Aeolian corrasion, Aeolian attrition, Aeolian deposition)
Mechanical physical (Weathering processes (Geomorphic processes))
Weathering is the in situ breakdown of rocks by mechanical, chemical and biological processes. The dominant weathering processes in mid- and low-latitude deserts are mechanical, where changes in temperature and moisture cause rocks to break down into smaller particles.
Mechanical breakdown is rapid because absence of soil and vegetation means that bedrock is widely exposed at the surface. Most types of weathering require moisture, and even the driest deserts receive some rain. In addition, moisture is often available from dew at night.
Chemical (Weathering processes (Geomorphic processes))
Chemical weathering processes rely on water; therefore the chemical breakdown of rocks in deserts is extremely slow. Nonetheless, chemical weathering is important: deserts are not completely dry; moisture is available from convective downpours, dew and fog.
The main chemical weathering processes are hydration, oxidation and solution.
Biological (Weathering processes (Geomorphic processes))
Biological weathering is limited in desert environments owing to the sparse vegetation cover. However, trees and shrubs have long root systems that penetrate and widen joints and dislodge rock particles. Lichen and algae growing on rock surfaces release CO₂; combined with water and organic matter, chemical weathering processes such as solution and chelation take place.
Mass movement (Geomorphic processes)
Mass movement is the downhill transfer of slope materials as a coherent body.
In desert environments most mass movements are debris flows and rockfalls. Debris flows develop during periods of heavy rainfall, particularly on saturated slopes with sparse vegetation cover and rapid run-off. They comprise large quantities of rock fragments, mud, soil and other debris (e.g. timber) which move at speeds of up to 50 km/ hour.
Even on gentle slopes, debris flows can travel long distances across alluvial fans and desert basins. Steep, angular slopes and rocky outcrops are prominent in desert landscapes. Where resistant rocks such as sandstone rest on weaker beds like shale, undercutting at the base of a slope by erosion and weathering can create rockfalls and rock slides. Mechanical weathering may also remove smaller rocks particles which fall under gravity to form talus slopes.
Fluvial processes (Geomorphic processes)
Paradoxically many characteristic drylands landforms are the result of fluvial processes. Most drylands streams and river are ephemeral and only flow intermittently during and shortly after prolongs or intense rainfall events. Yet, while surface run-off is short-lived, streams and rivers in drylands have tremendous power. This is explained by:
-Sparse vegetation cover, with minimal interception flow to slow the movement by surface wash to stream and river channels.
-Ground surfaces baked hard by the sun, which limits infiltration.
-Rainsplash on unvegetated surface which quickly fills soil spores and reduces soil permeability
-Shallow soils, which allow little water storage and throughflow.
Aeolian processes (Geomorphic processes)
In desert environments wind is an active agent of erosion, transport and deposition. The sparse vegetation cover and dry conditions make aeolian (or wind) processes more effective than in humid and sub-humid environments.
Aeolian erosion and transport (Aeolian processes (Geomorphic processes))
The main erosional effect of the wind in deserts is the removal of fine particles - a process called deflation. Tiny silt and clay particles are transported thousands of kilometres by the wind. In the UK, silt and clay exported from the Sahara is frequently washed out of the atmosphere by rain, coating windows and windscreens in red dust.
It has also been suggested that Saharan dust, transported across the Atlantic, provides Amazonian soils with essential minerals that sustain the rainforest. Locally, deflation is responsible for dust storms and for surface erosion which creates shallow depressions littered with coarse lag particles. The wind transports sand and dust in three ways:
Creep (Aeolian erosion and transport (Aeolian processes (Geomorphic processes)))
When sand grains slide and roll across the surface. Creep is caused by drag and small differences in wind pressure on sand grains that create lift.
Saltation (Aeolian erosion and transport (Aeolian processes (Geomorphic processes)))
The downwind skipping motion of sand grains. It is confined to within 1 or 2 metres of the surface. When saltating grains hit a hard surface they have a ballistic effect, setting other grains moving in the direction of the wind. Saltation is the main process of wind transport in desert areas.
Suspension (Aeolian erosion and transport (Aeolian processes (Geomorphic processes)))
Small dust particles (less than 0.15 mm in diameter) are entrained by the wind and transported aloft, sometimes vast distances and beyond desert areas.
Aeolian corrasion (Aeolian processes (Geomorphic processes))
The abrasive action of wind-blown sand against rocks. Over time corrasion carves rocks into a variety of shapes. However, because sand grains are transported by saltation, the sand-blasting effect is confined to just a couple of metres above the ground. Unlike deflation, corrasion is a slow process. Depending on the strength of the rock and wind speeds, it take may a century or more to erode a layer of rock 1 mm thick.
Aeolian attrition (Aeolian processes (Geomorphic processes))
Attrition takes place as grains of sand carried by the wind collide with each other and impact solid rock outcrops. As a result sand grains become smaller and rounder.
Aeolian deposition (Aeolian processes (Geomorphic processes))
As winds subside and energy levels fall, the transport of sand, silt and clay ceases and deposition occurs. Sand dunes are the most obvious evidence of deposition. Often sand accumulates in areas of reduced wind speed as vast sheets or ‘sand seas’. In the Sahara these areas are known as ergs.
Once deposited, sand attracts further deposition. This is because saltating grains are less able to rebound off soft sand compared with hard rocky surfaces.
Surface wash
The process which occurs when infiltration capacity is exceeded or saturation is reached. Gullies are sometimes formed. Also occurs when ground is frozen.
Infiltration capacity
The maximum rate at which rain can be absorbed by a soil in a given condition.
Erosional desert landforms
-Wadis
-Canyons
-Pedestal rocks
-Ventifacts
-Desert pavements
Erosional desert landforms definition
The erosional processes operating in mid- and low- latitude deserts give rise to distinctive landforms. Among these desert landforms are wadis, canyons, pedestal rocks, ventifacts and desert pavements.
Wadis (Erosional desert landforms)
Wadis are stream and river channels which are dry for most of the time. Although run-off in deserts is short lived, temporary streams and rivers have abundant surplus energy for erosion. This is partly due to the nature of the rainfall, which is often convective and intense; and to rapid run-off and high peak flows.
Rapid run-off is due to limited water storage because of minimal soil and vegetation cover, and ground surfaces baked hard by the Sun. The result is powerful flash floods.
Convective
Surface air that is heated and, thus, is rising
Canyons (Erosional desert landforms)
Canyons are narrow river valleys with near vertical sides, cut into solid rock. They are a common feature of desert mountains and plateaux and evidence of the power of fluvial erosion are formed by the scouring action of coarse sediment transported by rivers.
The shape of canyons in cross profile is mainly determined by rock type. Where rocks are highly resistant and homogeneous, narrow slot canyons with vertical rock walls develop. Antelope Canyon, near Page in Arizona, is a classic example. Other canyons (including the Grand Canyon) where rocks of variable resistance crop out have stair-like sides.
Canyons (Erosional desert landforms) (Processes)
Erosion is vertical rather than lateral because
1. Solid rock walls allow little sideways movement of river channels
2. Mass movements processes which would lower valley slopes in more humid environments (e.g. soil creep, mudlows, landslides), are absent in deserts. deserts. Canyons
Pedestal rocks (Erosional desert landforms)
Pedestal rocks are isolated, mushroom-shaped rocks which belong to a larger group of wind-eroded rocks known as zeugens. Their curious shape is explained by the undercutting erosive effects of saltating sand grains confined to within 1.5 m of the ground.
This process may be aided by weathering concentrated at the base of rocks where moisture is more freely available.
Sometimes called hoodoo rocks.
Ventifacts (Erosional desert landforms)
Ventifacts are small rocks that have been abraded or shaped by wind-blown sand. The facets, formed on the upwind side, are separated from the protected lee side by sharp edges.
Depending on the number of facets, ventifacts are described as einkanter (one facet), zweikanter (two facets) and dreikanter (three facets). Ventifacts with multiple facets may have been moved by the wind, or form where there are seasonal reversals in prevailing wind direction.
Desert pavements (Erosional desert landforms)
Deflation lowers the ground surface through the selective removal of fine-grained particles by the wind. As a result tightly packed, coarse-grained particles are left at the surface. Eventually this process creates extensive surfaces of coarse, rocky particles or lag deposits that protect the underlying material from deflation. Such a surface is known as a desert pavement.
Depositional desert landforms
-Dunes (Barchans, Linear dunes, Star dunes
-Alluvial fans and Bajadas
Depositional desert landforms definition
Distinctive depositional landforms comprising sediments transported and deposited by wind and water include dunes, alluvial fans and bajadas.
Dunes (Depositional desert landforms)
Dunes are mounds and ridges of wind-blown sand. Two conditions are needed for dune formation: an adequate supply of sand and winds strong and persistent enough to transport sand.
A typical dune has a windward slope of 10-15°, a sharp crest and a steeper leeward slope of 30-35°. The slip-face stands at the angle of repose, i.e, the maximum angle at which loose sand is stable. Creep and saltation transport sand up the windward slope. As sand accumulates on the crest it eventually exceeds the angle of repose, causing miniature avalanches down the slip-face which restore equilibrium. In this way, dunes advance in the direction of the prevailing wind.
Types of Dunes (Depositional desert landforms)
There are three common types of dune: barchans, linear dunes and star dunes. However, many dunes have more complex forms than these basic types, with one form superimposed on another.
Barchans (Dunes (Depositional desert landforms))
Barchans are crescentic in planform, with two horns facing downwind. They form where winds blow predominantly from one direction. The horns move faster than the main body of the dune. The windward slope is much gentler than the leeward slope. Barchans are highly mobile and move across the desert surface at speeds of up to 30 m/year.
Transverse dunes form in a similar way to barchans but develop at right angles to the prevailing wind. They have steep and less steep sides and are associated with erg environments.
Linear dunes (Dunes (Depositional desert landforms))
Linear dunes are straight or slightly curved. They are much larger features than barchans: sometimes more than 100 km long with steep slip faces on alternate sides. Again, in contrast to barchans, they form parallel to the prevailing wind direction.
They occur either as isolated or parallel ridges and cover a larger area than any other dune type.
Star dunes (Dunes (Depositional desert landforms))
Star dunes are pyramidal in profile with slip faces on three or more arms that radiate from a dome-like summit. They form in areas where the wind direction is multidirectional. Because they tend to build upwards they include some of the tallest dunes in the world.
Alluvial fans (Depositional desert landforms)
Alluvial fans are cones of river-deposited sediment that accumulate at the foot of steep slopes, often along mountain fronts. They form where river channel gradients change abruptly, as a river leaves a steep mountainous course.
This may occur where a river confined by narrow rock walls enters an adjacent lowland or basin. The sudden loss of energy, coupled with a huge sediment load, results in the disposition of alluvium and the main channel splitting into hundreds of smaller channels. This creates a delta-shaped mound of debris, with a concave profile in section.
Bajadas (Depositional desert landforms)
Where several alluvial fans develop at intervals along a mountain front they often merge to form a continuous alluvial apron called a bajada. Alluvial fans and bajadas are formed by ephemeral streams, so are dry features for most of the time.
