Past & Present Desert Landscapes (AQA A Level Geography)

Desert Development Over Time

• Each desert landscape is unique due to past and present interactions of rocks and processes operating on them

• Wind and water action are the most important processes, although weathering, mass movement and vegetation also play a role

• Processes operating in desert systems can create new landforms or change existing landforms

• This means that desert landscapes change over time
  

  • Changes in inputs change the landscape, e.g. an increase in sediment input may increase deposition

  • Over time the landscape would then change from an erosional landscape to one dominated by landforms of deposition

• Landforms of past processes can undergo further change

  • A wadi formed by past river flow will be further eroded by the episodic sheet and channel flooding

• Hot desert landscapes are, therefore, made of a mixture of characteristics and landforms that reflect different periods of time

  • Canyons formed when the climate was wetter (interglacial period ≈8000 years ago) and present-day landforms such as pedestals

• Changes occur over a range of spatial and temporal (time) scales

  • Changes can be short and episodic such as flash floods transporting material in wadis

  • Too long and gradual, such as wind eroding rock over thousands of years to form a pedestal rock

• Changes to desert landscapes occur slower than in other regions because many weathering and erosional processes need water

  • The Atacama Desert can go years without rain, therefore, processes operate slowly and episodically there

Basin and range

• This topography develops in landscapes where past tectonic faulting has occurred through extension, deformation and sedimentation

• The horizontal extension of crustal plates stretches the area and faults appear

• Vertical deformation (displacement) takes place along these fault lines

• Creating a rising 'range' of mountains and a dropped valley 'basin' 

• Mountains continue to erode, with streams carrying sediment down to the valley floor

Case study

• The Western US Basin and Range extends to the Rocky Mountains, South Central and Southwestern states

• It includes all of Nevada, south-eastern California and Oregon, with high mountain ranges running mostly north-south with alternating low valleys

• 20 million years ago, the crust along the Basin and Range stretched, thinned, and faulted into approximately 400 mountain blocks

• Upward pressure from the mantle, lifted some blocks, forming stretched peaks, whilst leaving the lower blocks to form down-dropped valleys

• Boundaries between the basins and ranges are acute (sharp) because the faults are straight between them

• Also, many of the faults are still active

Desert Extent & Distribution of Change

Pleistocene pluvials

• The Pleistocene is the geological epoch that lasted from about 2,588,000 to 11,700 years ago, spanning the world's most recent period of repeated glaciation

• The end of the Pleistocene corresponds with the end of the last glacial period and also with the end of the Palaeolithic age used in archaeology

• A pluvial refers to the characteristic of rainfall

• The evolution of arid and semi-arid landforms is often affected by events that occurred long ago

• Many current desert landforms, began to develop during pluvial periods several thousand years ago

• The sporadic activity takes place with sudden downpours/deluges where a lot of erosional activity takes place in limited locations such as wadis, but this would not be enough to create the range of water-eroded features

• Therefore, pluvial periods in the Pleistocene and beyond are seen as the likely period for most landform formation of deserts

• Some do argue that wind erosion and deposition are still actively giving rise to depressions, yardangs, zeugens and dune systems

• But, the scale suggests that water action and past weathering have been a significant factor

  • Wadis and integrated valley systems are unlikely to have formed under present conditions

  • Piedmont zone landforms show evidence of having been formed by intense water action

  • Dunes are the reworking of vast sand seas, a result of alluvial deposition in a wetter climate

The Changes to Hot Desert Climate Over Time