1. Desert environments in Namibia
The physical processes and landforms of deserts
The example of the Namib and Kalahari Deserts

2. Key Question 1.1 What are the characteristics of a desert environment that make it
extreme?
* The climatic, biotic and soil characteristics of a desert environment.
* The links between climate, biotic and soil characteristics.
Key Question 1.2 How is human activity causing pressures on the desert environment?
* The threats that are posed by
(i) population growth;
(ii) mineral exploitation;
(iii) farming;
(iv) tourism.
* The positive and negative outcomes of human activity.
Key Question 1.3 What are the strategies that can be used to manage human activity in deserts?
* Strategies that attempt to
(i) conserve the desert environment;
(ii) alleviate the impacts of human activity;
(iii) control the use of the desert environment
(iv) monitor the impacts of human activity.
* The role of local, national and international groups in the management of the desert environment.

3. LIST OF LANDFORMS IN DESERT AREAS
Sandy desert - ergs/sand seas
Gravel desert - lag deposits/reg
Rocky desert - hamada
Desert pavements
Ventifacts (dreikanter)
Yardangs
Zeugens
Rock lattice (tafoni)
Pedestal rocks (mushroom blocks)
Deflation hollows
Gibber plains
Duricrusts
Mesas and buttes
Canyons
Wadis
Pediment
Bajadas
Dunes
Ripples
Draa (megadune)
Barchans
Star dunes (rhourd)
Seif dunes (linear dunes)
Lunette dunes
Nebkha dunes (shrub coppice dunes)
Lee and fore dunes
Echo dunes
Rhourds
Zibar
Pans (playas/sebkas/chotts)
Inselbergs
Bornhardts
Kopjes
Badlands
Hoodoos
Natural arches
Piping

4. The main processes at work on landforms in arid areas

5. Mechanical weathering
Thermal fraction, i.e. The breaking down of rocks as a result of temperature change is the most important type of mechanical weathering in arid environments. Diurnal temperature ranges may be as much as 25° - 30°C in deserts during summer months and rock temperatures may reach 75°C, The expansion and contraction so produced help disintegrate rocks. Geological structure (joints and bedding planes), surface colour and chemical composition of rocks all influence the way they disintegrate.
The four main processes are as follows:

6. Granular disintegration. Grainy rocks such as coarse sandstones and
granite break down into grains of sand. Granite disintegrates quickly
because it contains both black mica and white quartz crystals which heat
up and cool down at different rates because of their colour
Block separation. Well-jointed sedimentary rocks, particularly limestone
and types of sandstone frequently break down along their joints and
bedding planes which are their main lines of weakness. Blocks can
commonly be seen breaking off the hard cap-rocks of features like mesas
and buttes.
Shattering. Some rocks have neither coarse grains nor a blocky
structure, and so shatter into irregular fragments with sharp edges.
Basalt, which is black and highly metallic, absorbs heat and expands
rapidly. It is the most common rock which disintegrates this way.
Exfoliation. The surface of any rock is more exposed to temperature
changes than the underlying rock. As a result some massive rocks,
especially if they have a rounded profile, tend to peel off in layers.
Granite and sandstone frequently disintegrate this way.

7. Other mechanical weathering processes.
Frost shattering / congelifraction
This may occur in higher altitude areas during winter time when the temperature goes below freezing point. Water, especially from dew, is trapped in cracks, expands when freezes and thaws and contracts.
Wetting and drying
Some softer rocks are very vulnerable to being wetted and then drying lout. Clays, mudstones and shales expand when wetted, then dry out and contract. Fragmentation then results.

8. Chemical weathering
Crystal growth – as different minerals are evaporated out of the
groundwater or as salts are deposited by evaporation following rainstorms their crystals grow, leading to rocks being prised apart until they fragment. This depends on the solubility of the mineral and the rate of crystal growth.
Hydration - this process involves the absorption of water by an anhydrous
mineral such as gypsum (calcium sulphate) which then causes
expansion of the mineral and the break up of the rock.
Solution – this is the dissolving of soluble minerals in water leading to the
wearing away or disintegration of rock surfaces.
LANDFORMS OF CHEMICAL WEATHERING
Tafoni - cave like holes in a rock surface
Alveoles – smaller pits like a honeycomb in a rock surface
Rock pillars
Duricrusts such as gypcrete
Desert varnish – a thin hard shiny dark red layer of evaporated iron and
magnesium oxides

9. Mass movement
Water is not usually available so mass movement processes which involve water and lubrication are not active at these times. When rain falls they come into play. The lack of vegetation cover and thick soils make mass movement more likely

10. There are two main processes of aeolian erosion in deserts:
[1] deflation – loose surface material is picked up and carried away
if fine material is all carried away only gravel etc is left forming a
gibber plain or reg
[2] abrasion – surfaces are sand-blasted by particles carried in the wind. Its effectiveness depends on the wind speed, frequency and direction,
the particle size and the lithology of the rock being eroded
Abrasion creates such landforms as pedestal rocks, ventifacts, yardangs and zeugens.
Aeolian erosion

11. Aeolian transport and deposition
Deserts are often very windy. There is little vegetation to slow the wind down.
Material eroded from one area will be transported and deposited in another.
Material can be transported by being
[1] bounced (saltation) – sand grains are carried a few centimetres above
the surface and occasional return to the ground to bounce
into the air again.
[2] suspended (suspension) – lighter material is picked up and carried in the air for hours or even days. This process is at work in
sandstorms. Material may transported for thousands of
miles, quite often to the UK.
[3] rolled (surface creep) – heavier material is rolled along the surface but
they are too heavy to be lifted into the air.
Aeolian deposition creates various forms of sand dune.

12. A sand storm approaching Gobabeb in the Namib Desert

13. Fluvial erosion
Water is surprisingly important in the world’s deserts because when rain arrives it is often intense and there is no surface coverage of soil to protect the surface of the land. The exception is the sand sea of the Namib, because of high surface permeability.
In highland areas water is dominant as an erosional agent, creating wadis and canyons, whilst in the lowlands deposition is more important making pans and bajadas.
Water erodes where it flows quickly, e.g. on steep slopes.
It erodes by means of 5 processes:
[1] cavitation – bubbles of air implode against the rock creating shock waves
[2] corrosion – water may dissolve some rocks, e.g. limestone and chalk
[3] corrasion – material rubbed against the banks and bed may scrape away
particles
[4] hydraulic action – the force of the water washes away loose material
[5] attrition – particles may rub against each other, getting smaller and
rounder

14. Fluvial transport and deposition
Material may be washed away in rivers, perhaps in flash flood on the rare occasions it rains in the desert until the energy level of the river drops. This is usually at the base of the slope.
Here an alluvial fan may be formed. If alluvial fans join together along the base of a slope a pediment is formed. Further away and with an even lower angle of slope are the bajada and playa (or chott).
Fluvial transport and deposition

15. Types of desert landform
Gravel desert
Sandy desert
Rocky desert
Types of desert landform

16. Location of Kalahari Desert

17. Erg
An erg (also sand sea or dune sea) is a large, relatively flat area of desert covered with wind-swept sand with little to no vegetation cover

18. The dune sea of the Namib Desert
The dune sea of the Namib Desert. Note how the crests of the dunes are aligned in a marked northwest-southeast orientation. These crests form transverse to the formative wind (i.e. crosswise). The dunes act as obstacles, and obstacles cause winds to be deflected significantly to the right, in the southern hemisphere—in effect reorienting the southerly wind as a southwesterly wind. A strand plain lies between the dunes and the ocean.

19. Linear (seif dunes)
Rhourds (star dunes)
Pan
Barchans
Inselberg

20. Reg (gravel desert / gibber plain)
Many deserts are covered with desert pavement, created when wind blows fine material away, leaving coarse material behind. A deposit of heavy material left behind when light material is carried away is called a lag deposit. Lag deposits can be created by wind, running water, and wave action.
Reg A vast stony plain in a desert. (also called a gibber plain or gobi)

21. Hamada (rocky desert)
A hamada (Arabic, حمادة ḥammāda) is a type of desert landscape consisting of largely barren, hard, rocky plateaus, with very little sand.
Hamada Barren rocky highlands in a desert.

22. Ventifacts (dreikanter)
Wind-abraded rocks or ventifacts are often called dreikanters from a South African word meaning "three corners."

23. Hunks of bedrock sticking up above the average surface level tend to be eroded into characteristic shapes called yardangs. A yardang is generally sloping on its stoss side and steep on its lee side.
Wind direction
Yardangs

25. Zeugens
Zeugens occur in areas with resistant and less resistant rocks occurring in horizontal layers. Joints are exploited and the less resistant rock is more eroded than the more resistant rock producing a ridge and furrow landscape.

26. Tafoni or rock lattice
Tafoni are holes or, if larger, cave-like features found in some deserts; of uncertain origin, they have arch-like entrances, smooth, concave walls, and flat floors covered with rock debris. They often occur in groups that can riddle a hillside, cliff, or other rock formation. The currently favoured explanation for their formation is salt weathering, which would be facilitated by the slightly cooler and more humid environment inside them. Sometimes the smaller holes are called alveoles.

27. Pedestal rocks pedestal rock
An isolated, residual or erosional mass of rock supported by or balanced on a pedestal. Isolated masses or rock above and resting on a smaller base or pedestal.
The Mukurob (Finger of God), near Asab in Namibia, collapsed on 04 December 1988 as a result of earth tremors produced by the earthquake in Armenia.

30. Deflation hollows
Deflation basins, called blowouts, are hollows formed by the removal of particles by wind. Blowouts are generally small, but may be up to several kilometres in diameter.

31. Dunes
Sand dunes are formed by aeolian deposition (by the wind). They take on a wide variety of forms and sizes depending upon the strength and variety of wind directions.
Sand ripples are the smallest scale of sand dune measuring only a few centimetres in height

32. The largest size of sand dune, although often without a clear shape (then called complex dunes). The pattern of winds is variable.
Draa (megadunes)
Dune 7 at Sossusvlei, 383 m high
An area of megadunes in the Eastern Namib

33. Crescent-shaped mounds are generally wider than they are long
Crescent-shaped mounds are generally wider than they are long. The slipface is on the dune's concave side. These dunes form under winds that blow from one direction, and they also are known as barchans, or transverse dunes. Some types of crescentic dunes move faster over desert surfaces than any other type of dune
Barchan
They are crescent in shape with their arms called horns. Horns always point along the direction of wind where they move. Ripples form as usual at their backs where winds blow. Barchans form when wind blows in one direction all year long with no change.

34. Star dunes (rhourds) Star
Radially symmetrical, star dunes are pyramidal sand mounds with slipfaces on three or more arms that radiate from the high centre of the mound. They tend to accumulate in areas with multidirectional wind regimes. Star dunes grow upward rather than laterally. They dominate the Grand Erg Oriental of the Sahara. In other deserts, they occur around the margins of the sand seas, particularly near topographic barriers

35. Seif dunes (linear dunes)
Straight or slightly sinuous sand ridges typically much longer than they are wide are known as linear dunes. They may be more than 160 kilometres long. Linear dunes may occur as isolated ridges, but they generally form sets of parallel ridges separated by miles of sand, gravel, or rocky inter-dune corridors. Some linear dunes merge to form Y-shaped compound dunes. Many form in bidirectional wind regimes. Mostly the wind blows from one direction but there is a secondary direction. Which helps create the landform, The long axes of these dunes extend in the resultant direction of sand movement
A linear dune near Gobabeb

36. Lunette dunes
Lunettes are accumulations of semi-consolidated fine sand, silt, and clay-pellet aggregates that form rounded, low (meters high) dunes on the downwind sides of playas. Like ordinary parabolic dunes, these dunes are transverse to the effective wind, and their arms point upwind. The arms, however, are much shorter than those of parabolic dunes because lunette dunes are composed of cohesive materials deflated from the adjacent playa floor.
U-shaped mounds of sand with convex noses trailed by elongated arms are parabolic dunes. Sometimes these dunes are called U-shaped, blowout, or hairpin dunes, and they are well known in coastal deserts. Unlike crescent shaped dunes, their crests point upwind. The elongated arms of parabolic dunes follow rather than lead because they have been fixed by vegetation, while the bulk of the sand in the dune migrates forward.

37. Shrub coppice dunes (nebkha dunes)
The strength and direction of desert winds and the amount of sand available determine the shape and size of dunes. Apart from dunes anchored to vegetation, known as shrub-coppice dunes, all dunes are mobile to some extent, the crescent-shaped barchans most of all. Shrub-coppice dunes, usually only a metre or two high, are common near the sea. They form around a bush or clump of grass when windblown sand lodges among the stems and leaves to form a hump-like mound. As the mound grows the roots lengthen to keep the plant above the surface.
A small dune formed on the leeward side of bush-and-clump vegetation.

38. Zibar dunes WHALEBACKS OR ZIBAR DUNES
They've no slipface and look from far exactly like a whaleback. They're the gigantic in size like typical dunes and are the most suited to cruise on with your 4x4. If you ask a British how to describe them they'd say, they're 'rolling' hills of sands because of their smooth surfaces with no crests.
They're called whalebacks by the famous British Sahara expert Bagnold active in Egypt at the first half of the 20th century (and the father of the scientists working in desert geomorphology) but Zibar now is becoming more used in science. The word Zibar is 'borrowed' from Arabic by D. A. Holm in 1953 when he was discussing dunes in Nejd, Saudi Arabia.
Amorphous dunes with no major recognisable shape

39. Duricrusts
Duricrust refers to a thin hard layer on or near the surface of soil, usually a few millimetres to a few centimetres thick.
It is a general term for a zone of chemical precipitation and hardening formed at or near the surface of sedimentary bodies through pedogenic and (or) non-pedogenic processes.
It is typically formed by the accumulation of soluble minerals deposited by mineral-bearing waters that move upward, downward, or laterally by capillary action, commonly assisted in arid settings by evaporation. Minerals often found in duricrust include silica, iron, calcium, and gypsum.
These duricrusts are known as silcrete, ferricrete, calcrete and gypcrete. Gypcrete is especially common in arid environments.

40. Mesas and buttes The Waterberg plateau
A mesa (Spanish and Portuguese for "table") is an elevated area of land with a flat top and sides that are usually steep cliffs. It takes its name from its characteristic table-top shape. It is a characteristic landform of arid environments.
The Waterberg plateau
A butte is an isolated hill with steep sides and a small flat top, smaller than mesas and plateaus.

42. Canyons
A canyon (rarely cañon) or gorge is a deep valley between cliffs often carved from the landscape by a river. Most canyons were formed by a process of long-time erosion from a plateau level. The cliffs form because harder rock strata that are resistant to erosion and weathering remain exposed on the valley walls. Canyons are much more common in arid areas than in wetter areas because weathering has a lesser effect in arid zones. Canyon walls are often formed of resistant sandstones or granite.
Fish River Canyon

46. (Fig 8.10).

49. (Arabic: وادي wādī) is traditionally a valley
(Arabic: وادي wādī) is traditionally a valley. In some cases it can refer to a dry riverbed that contains water only during times of heavy rain. Compare arroyo (creek), canyon, gulch, wash and gully.
Wadis tend to be associated with centres of human population because sub-surface water is sometimes available in wadis.
Crossing wide wadis at certain times of the year can be dangerous, because of unexpected flash floods.
Wadis
Usually a lot smaller than a canyon without the stepped sides.

50. Pediment
The pediment area is at the foot of the mountains. It is composed of alluvial fans and bajadas. An alluvial fan is a fan-shaped deposit formed where a fast flowing stream flattens, slows, and spreads typically at the exit of a canyon onto a flatter plain. A convergence of neighbouring alluvial fans into a single apron of deposits against a slope is called a bajada, or compound alluvial fan.

51. Namib Sand Sea (looking south) from the NASA Space Shuttle
Namib Sand Sea (looking south) from the NASA Space Shuttle. Atlantic Ocean is on right.
The north end of the Namib Sand Sea (nearest the camera) is distinctly marked by the course of the ephemeral Kuiseb River, which flows out of the Namibian highlands (left) and down to the Atlantic Ocean (right). The general overall sand movement in the Namib Sand Sea is to the north. Yet the dunes do not succeed in crossing the Kuiseb, though they do encroach into the river bed between flows.
This shows that, even though floods in the Kuiseb are infrequent (once a year or less), water action is dominant over wind-driven sand movement. The exception is right near the coast (lower right of picture), the part of the Kuiseb which receives the least water, and the most infrequent flow (the Kuiseb reaches the sea roughly once every 8-10 years).
Not all rivers can achieve this - you can see south of the Kuiseb two more epehemeral rivers entering the Namib Sand Sea from the east (left), but their paths are blocked by dunes. Their drainage catchments are much smaller than that of the Kuiseb.

53. Perfectly planar pediment surface north of Gross Spitzkoppe, with episodic watercourses marked by linear stripes of vegetation.

54. Pans / playas
A playa; also known as an alkali flat, sebkha, chott, saltpan or flat; is a dry lake bed. Such flats consist of fine-grained sediments infused with alkali salts. Their surface is generally very dry, hard and smooth in the summer months, but wet and very soft in the winter months. While the playa itself will be devoid of vegetation, they are commonly ringed by salt-tolerant plants that provide critical winter fodder for livestock and other herbivores Natural salt pans are flat expanses of ground covered with salt and other minerals, usually shining white under the sun. Salt pans can be dangerous. The crust of salt can conceal a quagmire of mud that can engulf a truck.
Etosha Pan

55. Inselberg
A monadnock or inselberg is an isolated hill, knob, ridge, or small mountain that rises abruptly from a gently sloping or virtually level surrounding plain. Inselberg means island mountain
Spitzkoppe

56. Kopjes
In southern and southern-central Africa, a similar formation of granite to an inselberg is known as a kopje from the Afrikaans word: koppie. Kopjes are often topped by a jumbled mass of rounded boulders

57. Bornhardts Mirabib near Gobabeb
bornhardt - A dome-shaped rock outcrop more than 30 m high, and sometimes several hundred metres in width. They commonly rise above erosional plains in the tropics, but also develop in unglaciated uplands in high latitudes. Most are formed in granites and gneisses; some in sandstones and conglomerates.

58. Badlands
Badlands are a type of arid terrain with clay-rich soil that has been extensively eroded by wind and water. Canyons, ravines, gullies, hoodoos and other such geological forms are common in badlands. They are often difficult to walk through. Badlands usually have a spectacular color display that alternates from dark black/blue coal stria to bright clays to red scoria.
Valley of the Moon, Swakop River area

59. Natural arches and Hoodoos
A natural arch or natural bridge is a natural formation (or landform) where a rock arch forms, with a natural passageway through underneath. Most natural arches form as a narrow ridge, walled by cliffs, become narrower from erosion, with a softer rock stratum under the cliff-forming stratum gradually eroding out until the rock shelters thus formed meet underneath the ridge, thus forming the arch. Natural arches commonly form where cliffs are subject to erosion from the sea, rivers or weathering (sub-aerial processes); the processes "find" weaknesses in rocks and work on them, making them bigger until they break through.
Hoodoos are tall thin spires of rock that protrude from the bottom of arid basins and badlands. They are composed of soft sedimentary rock, and are topped by a piece of harder, less easily-eroded stone that protects the column from the elements
A natural arch at Spitzkoppe

60. Development of a natural arch
Rock is faulted or jointed
Chemical weathering occurs along the joints widening them
Weathering continues completely eating away some areas of rock and creating holes on other areas.
The holes and caves are exploited further to form a natural arch