Characteristic Desert Landscapes Through Time (AQA A-Level Geography): Revision Notes

Characteristic Desert Landscapes Through Time

Introduction to desert landscape diversity

Sand is often the first thing that comes to mind when picturing desert landscapes. However, hot desert environments contain some of the most spectacular and varied landscapes on Earth. These range from seemingly endless flat plains to enormous fields of sand dunes the size of mountain ranges. Some hot deserts feature remarkable geological formations that attract millions of tourists each year, including famous landmarks like Uluru in central Australia and the Grand Canyon in southwestern USA.

What makes each desert landscape unique

No two hot desert landscapes are exactly the same. This diversity exists because each location has experienced a distinctive combination of geological processes, time periods, and landform features. Every hot desert landscape develops through the processes discussed earlier in this chapter and consists of unique assemblages of the landforms already covered.

The fascinating variety of hot desert landscapes results from several key factors relating to time, process and landform.

Factors influencing landscape formation

The role of weathering and temperature

How quickly and in what way rocks break down varies considerably between different desert regions. The speed of weathering depends on:

• The type of rock present in the area
• Rapid heating and cooling cycles affecting different minerals within rocks
• The extent of daily temperature fluctuations

Rocks break down much more quickly in areas with greater diurnal temperature ranges. Some deserts experience extreme temperature swings on a daily basis, whilst others have more moderate day-night temperature differences.

The influence of moisture

Where moisture is present, hot deserts are more likely to develop landscapes with biological and organic features. These range from simple organisms like lichens and bacteria through to grasses and cacti. The presence or absence of water creates fundamentally different landscape types.

The dimension of time

The speed at which both erosional and depositional processes shape the desert landscape heavily depends on time factors:

• Some deserts regularly have moisture present
• Others only experience occasional seasonal rainfall
• Some remain completely arid for years at a time

The rate of landscape formation varies enormously depending on these temporal patterns of water and wind activity. Wind and flowing water may be constant forces, or seasonal, or sporadic, depending on the climate regime.

Desert landscapes through geological time

Water and wind not only shape the present-day desert landscape, they may also have played a fundamental role in the geological history of the area. Looking at desert landscapes through time reveals fascinating evidence of changing environmental conditions.

Evidence of wetter periods in the past

Some landscapes contain features that record a much wetter past:

• Ancient river systems that once flowed through areas now almost completely arid
• The presence of inselbergs (isolated rock hills)

Recent landscape features

Some desert landscapes are dominated by relatively recent features that have developed over hundreds to a few thousand years:

• Alluvial fans at the mouths of wadis
• Bajadas (merged alluvial fans)
• Recently formed sand dunes

These features show evidence of ongoing or recent geological activity.

Ancient landscape features

Other desert landscapes are truly ancient:

• The great 'shield deserts' of central Australia are dominated by vast, almost featureless plains
• These ancient surfaces have rare outcrops of resistant rock, like Uluru
• These landscapes formed millions of years ago

Badlands

Water action can create one of the most distinctive features of semi-arid regions: badlands.

Key characteristics of badlands

Badlands display a range of distinctive features:

• Wadis of all shapes and sizes with steep sides and debris-covered bottoms
• Unstable slopes that regularly collapse as gullies erode headward into hillsides
• Regular mass movement as slope failure and slumping are common
• Alluvial fans and bajadas form as the many wadis and small gullies emerge onto lowland areas
• Pipes develop as water carves out surface cracks, which then erode into larger caves as surface water flows into them
• Natural arches form as caves erode further over time, aided by weathering processes

Examples of major world deserts

This section demonstrates just how unique all hot desert landscapes are. Different factors combine to create different assemblages of landforms and features, giving each desert its own characteristic appearance. Unlike badlands, not all assemblages of landforms create a named landscape type.

The table shows the dominant features of some of the world's most well-known hot deserts:

• Arabian Desert (Arabian Peninsula) features almost entirely sandy terrain with some of the largest sand dune systems in the world
• Australian deserts (Great Sandy, Simpson, Great Victoria, Gibson and Sturt) display mostly sandy plains or mostly stony surfaces depending on location
• Chihuahuan Desert (Arizona, New Mexico, Texas and Mexico) consists of high flat plateaux with some stony surfaces and sandy soil, broken by mountain ranges and distinctive mesas
• Kalahari Desert (southwestern Africa) contains extensive sand dunes interspersed with gravel plains
• Mojave Desert (Arizona, California, Nevada) shows varied landscape features including sandy soils, gravel pavements and salt flats
• Sahara Desert (Northern Africa) displays vast ranges of dunes amongst mountains and rocky areas, plus gravel plains and salt flats
• Thar Desert (India and Pakistan) features mostly sand dunes with areas of gravel plains