Desert Landforms

According to relief type, two general groups of desert landforms can be distinguished: (1) shield-platform deserts and (2) mountain and basin deserts. The shield-platform deserts are most common in Africa, the Middle East, India, and Australia and are characterized by tablelands and basin lowlands. Mountain and hill slopes in this type of deserts are restricted to ancient mountains or areas with more recent volcanic activity. The geologically younger mountain and basin deserts (also called mountain and range) are predominant in the Americas and Asia and consist typically of mountain ranges separated by broad alluvially filled valleys. Within the two groups of desert landforms, there are several dominant geomorphological landscape types that are described here briefly.

Desert mountains consist chiefly of sheer rock outcrops and tend to rise abruptly from desert plains. The slopes of these mountains differ according to geological origin of the parent material. Igneous rock mountains tend to be characterized by large debris (boulder fields), while softer sedimentary rocks tend to lack these. Desert mountains (Figure 2) dominate the desert of the USA (38% of desert area), the Sahara (43%), and Arabia (45%).

Piedmont bajada formations (Figure 3) cover roughly a third of the arid Southwest USA but do less so in other desert areas of the world. These formations are built up from alluvial material that tends to accumulate in fans at the mouth of mountain canyons. Individual alluvial fans often coalesce and form large-scale graded slopes called piedmont bajadas (often only 'bajadas'). Depending on deposition age and location along the bajada, the fill material is very diverse and differs strongly in alluvial particle size and soil structure, thus creating complex gradients and mosaics of distinct geological landforms. These gradients have been studied extensively in the Sonoran and Mojave Deserts and it had been shown that predominantly, the age and consequent erosion of the alluvial material within these mosaics determine the biological communities that can establish on it.

Desert flats (basins) are another common landscape type (about 20% in the USA and 10-20% in other regions). Often these flats have rather fine-textured soils and with sufficient rainfall vegetation is diffuse and rather evenly spaced across the landscape (Figure 4). In more arid regions and when rainfall redistribution is patchy due to minor relief differences, distinct banded vegetation patterns can arise. These bands exist mostly in Africa and Australia but are also present in restricted areas in the Middle East and North America. The open areas produce the runoff of rainfall that accumulates in the bands, supporting the growth of vegetation. Another type of flat desert region can be differentiated as hammadas (bedrock fields). These bedrock fields develop in situ

Table 1 List of the major desert areas of the world (larger than 50 000 km2)a

Size

Table 1 List of the major desert areas of the world (larger than 50 000 km2)a

Size

Name

(km2)

Type

Temperature

Countries

Sahara Desert

8600000

Subtropical

Hot

Egypt, Libya, Chad, Mauritania, Morocco, Algeria Tunisia.

Kalahari Desert

260000

Subtropical

Hot

Botswana, Namibia, South Africa

Namib Desert

135 000

Coastal

Hot

Namibia

Arabian Desert

2 330000

Subtropical

Hot

Saudi Arabia, Jordan, Iraq, Kuwait, Qatar, United Arab

Emirates, Oman, Yemen, Israel

Syrian Desert

260000

Subtropical

Hot

Syria, Jordan, Iraq

Kavir Desert

260 000

Subtropical

Hot

Iran

Thar Desert

200 000

Subtropical

Hot

India, Pakistan

Gobi Desert

1 300 000

Continental

Cold

Mongolia, China

Taklamakan

270000

Continental

Cold

China

Karakum Desert

350 000

Continental

Cold

Turkmenistan

Kyzyl Kum

300 000

Continental

Cold

Kazakhstan, Uzbekistan

Great Victoria Desert

647 000

Subtropical

Hot

Australia

Great Sandy Desert

400000

Subtropical

Hot

Australia

Gibson Desert

155 000

Subtropical

Hot

Australia

Simpson Desert

145 000

Subtropical

Hot

Australia

Great Basin Desert

492 000

Continental

Cold

United States

Chihuahuan Desert

450000

Subtropical

Hot

Mexico, United States

Sonoran Desert

310000

Subtropical

Hot

United States, Mexico

Mojave Desert

65000

Subtropical/rain

Hot (cold)

United States

shadow

Atacama Desert

140000

Coastal

Hot

Chile, Peru

Patagonian and

673000

Rain shadow

Cold

Argentina

Monte Deserts

Antarctic Desert

1 400 000

Polar

Very cold

Antarctica

aVarious sources.

Figure 2 Desert mountains: the Cambrian sandstone formations rise almost vertically from the valley floor filled deeply by sands that locally eroded from the mountain fronts. Wadi Ram, Jordan, October 2003. Photograph by C. Holzapfel.

and depending on the size of rock fragments, can build dense pavements (regs) consisting of densely packed surface stones resting on finer-textured subsoil. This desert landscape type is common in the Sahara and the Middle East and accounts for 40% of the area.

Sand dunes (Figure 5), known as 'ergs' in Arabic-speaking countries, are dominant desert landscapes

Landforms The Gobi Desert

Figure 3 A piedmont bajada in the Mojave Desert: alluvial fan deposits stemming from a nearby mountain range vary in age and structure (here a mixture of Pleistocene and Holocene deposits). The position along the fan and the composition and structure of the deposits determine hydrology and plant growth (here the common desert shrubs Larrea tridentata and Ambrosia dumosa). Fremont Valley, California, USA, March 2006. Photograph by C. Holzapfel.

Figure 3 A piedmont bajada in the Mojave Desert: alluvial fan deposits stemming from a nearby mountain range vary in age and structure (here a mixture of Pleistocene and Holocene deposits). The position along the fan and the composition and structure of the deposits determine hydrology and plant growth (here the common desert shrubs Larrea tridentata and Ambrosia dumosa). Fremont Valley, California, USA, March 2006. Photograph by C. Holzapfel.

only in extreme arid desert areas (25% of Sahara and Arabian Deserts, less than 1% in arid Southwest USA). Characterized by moving sands, they depend on sources of sand, sufficient wind energy, and favorable

Figure 4 Desert flats: this large desert basin in the Atacama Desert has very little surface dynamics and fine-textured soil materials are overlain by rocks forming a partial pavement. Among the harshest deserts on Earth, the Atacama receives very little to no rainfall and plant growth is lacking in most years. South of Antofagasta, Chile, October 1994. Photograph by C. Holzapfel.

Figure 4 Desert flats: this large desert basin in the Atacama Desert has very little surface dynamics and fine-textured soil materials are overlain by rocks forming a partial pavement. Among the harshest deserts on Earth, the Atacama receives very little to no rainfall and plant growth is lacking in most years. South of Antofagasta, Chile, October 1994. Photograph by C. Holzapfel.

Figure 5 Sand dunes: ergs are seas of sands that are constantly on the move. Vast sand deserts are typical for the Sahara (shown here) and Arabian Desert. Douz, Southern Tunisia, March 1986. Photograph by C. Holzapfel.

accumulation areas. Depending on these factors and prevailing wind direction, different dune types arise. Crescent-shaped dunes (barkhan dunes) form perpendicular to the prevailing wind direction and tend to be highly mobile. Linear dunes form in the direction of the wind and therefore do not move over the desert landscape. This distinction is of biotic importance as the edges of dunes are favorable to plant growth, while the dune crest and upper slopes, due to sand drift and fast erosion, are usually devoid of vegetation. In sandy flats, individual shrubs tend to accumulate sand deposits and eventually form phytogenic hummocks (so-called nebkhas).

Playas (Figure 6) are depressions with very fine-textured, often saline soils. Playas are the beds of former lakes that can be flooded in years of abundant rainfalls. These depressions are known under various names (North Africa and Middle East: chotts, sebkas). Even

Figure 6 Desert playas are often prehistoric lake beds with fine-textured, alkaline soils. Depending on current rainfall and temperature, playas can be flooded and then resemble the former lakes, as this playa on the altiplano of the Andes at an elevation of 4400 m. Plant life on playas is typically sparse but when microorganisms and invertebrates are active, birds such as these Andean flamingos (Phoenicopterus andinus) assemble in large numbers. East of San Pedro de Atacama, Chile, November 1994. Photograph by C. Holzapfel.

Figure 6 Desert playas are often prehistoric lake beds with fine-textured, alkaline soils. Depending on current rainfall and temperature, playas can be flooded and then resemble the former lakes, as this playa on the altiplano of the Andes at an elevation of 4400 m. Plant life on playas is typically sparse but when microorganisms and invertebrates are active, birds such as these Andean flamingos (Phoenicopterus andinus) assemble in large numbers. East of San Pedro de Atacama, Chile, November 1994. Photograph by C. Holzapfel.

Figure 7 Ephemeral stream: in arid areas vegetation concentrates along and in the bed of temporal stream beds. Due to available water in the subsoil that is plant extractable even long after temporal surface flow ceased, most of the primary production and species diversity in extreme deserts is restricted to these habitats. Nahal Zin, Negev Desert, Israel, April 1987. Photograph by C. Holzapfel.

Figure 7 Ephemeral stream: in arid areas vegetation concentrates along and in the bed of temporal stream beds. Due to available water in the subsoil that is plant extractable even long after temporal surface flow ceased, most of the primary production and species diversity in extreme deserts is restricted to these habitats. Nahal Zin, Negev Desert, Israel, April 1987. Photograph by C. Holzapfel.

though individual playas can be large, worldwide they cover only 1% of desert.

Badlands form in areas with clay-rich soils and are typically located at the margins of arid lands, although they are found locally in arid regions as well. Depending on the strength of water forced erosion, badlands are areas with extremely high surface relief, typically forming fantastic 'lunar landscapes.'

Dry river beds of ephemeral streams (Figure 7) are of little importance with respect to land cover in deserts (only 1-5% worldwide), but are of immense biological importance. In extreme arid regions, these are the only places with vascular plant growth, and almost all animals depend at least at times during their life on the primary production here. This biological importance of ephemeral streams is therefore a foremost feature in deserts, and even though small in size, these landmarks were always distinctly named by human desert dwellers (washes in North America, wadi/o«ed in Arabic-speaking regions, arroyo seco in Spanish-speaking regions).

Was this article helpful?

0 0
Renewable Energy Eco Friendly

Renewable Energy Eco Friendly

Renewable energy is energy that is generated from sunlight, rain, tides, geothermal heat and wind. These sources are naturally and constantly replenished, which is why they are deemed as renewable.

Get My Free Ebook


Post a comment