Hot Deserts: The role of wind and water in shaping the landscape Flashcards
What is the main driver of arid geomorphological processes?
TEMPERATURE is the main driver (thermal fracture, exfoliation, block disintegration, granular disintegration as a main driver and crystal growth and hardpan as minor driver)
Water is the second largest driver of arid processes (main driver in hydrolysis, desert varnish, hardpan and oxidation)
Salts is the smallest driver (main driver in crystal growth)
What is thermal fracture?
- mechanical weathering
- main driver is temperature
- rapid heating and cooling of rock causing degradation
- amplified by different parts of rock expanding and contracting at different rates = fractures
What is exfoliation?
- mechanical weathering
- main driver is temperature (minor with moisture and salts)
- exterior of the rock heated to a greater extent than the interior, tension creates cracks or flakes
- salt-rich water can be drawn up and deposit salts enhancing process by adding pressure in hydration
What is block disintegration?
- mechanical weathering
- main driver is temperature with minor driver of water
- igneous rocks have regular spaced fractures in them
- erosion of rock brings them closer to the surface and water can enter fractures
- heating and cooling of water causes large blocks of rock to break off (freeze-thaw weathering)
Q: Outline the impact of salts on weathering processes in hot deserts. [4 marks]
- crystal growth is a large scale weathering process that requires salt crystals
- salt crystals in the rock expand due to temperature change and weakens the rock over time due to pressure
- Hardpan is another weathering process requiring salts
- salts are drawn up by water and hardens on the rock surface due to insolation exposure
What is crystal growth?
- type of chemical weathering
- salts are the main driver and temperature is the minor driver
- water creates crystal deposits in rocks and they thermally expand and contract which physically breaks down rock
What is hydration?
- type of chemical weathering
- water is the main driver
- some rock minerals absorb water causing them to expand and become susceptible to further breakdown
- for example gypsum
What is hydrolysis?
- chemical weathering
- main driver is water as it reacts with/dissolved minerals in rock and salts are a minor driver as they exaggerate reaction
- this process in itself degrades the rock, but both are likely to be weaker than the parent rock making it more susceptible to further degradation
What is oxidation?
- chemical weathering
- water is the main driver as water oxidises minerals of iron
- rock containing iron is exposed to oxygen or water is oxidised turning a reddish-brown colour
What is hardpan?
- available moisture drawn to surface by capillary action causing an accumulation of salts due to water being evaporated
- water and temp are the main drivers
- salt is a minor driver as it hardens over the surface over time
What is desert varnish?
- water and temp are main drivers as this is driven by capillary action
- salt is minor driver as it only oxidises rock
- Fe and Mn oxides been weathred from rocks at depth and drawn to surface in solution and then evaporates leaving a deep red stain
Why are aeolian processes and landforms common in hot desert environments?
- barren environment allows wind to blow for miles unimpeded, no friction from land
- high pressure area, wind is air moving to low pressure margins
- localised air movements caused by a wide diurnal range
What are the two aeolian erosional processes?
- deflation
2. abrasion
What is deflation?
- aeolian erosional process
- removes unconsolidated sand or fine clay particles from the surface
- creates a reg desert or a desert pavement (surface covered in coarse/fine pebbles
What is abrasion?
- aeolian erosional process
- material being carried by the wind hits an exposed rock surface and erodes it
- forms erosional features
What are the three aeolian transportation processes?
- Suspension -> smallest particles held indefinitely = sand storms or hazes of dust
- Saltation -> sand-sized particles transported by bouncing/hopping along surface for short distances
- Surface creep -> larger particles slowly rolling or sliding across surface
What is aeolian deposition?
- occurs when velocity of the wind decreases that it can no longer transport sediment it’s carrying
- can form dunes (has to meet an obstacle)
What is a ventifact?
AEOLIAN EROSIONAL LANDFORM
- individual exposed rocks with one of more smooth sides that have been abraded by wind-blown sediment
- side on prevailing wind side is abraded the most
- characterised by facets and sharp edges
What is a yardang?
AEOLIAN EROSIONAL LANDFORM
- long streamlined parallel ridges of hard and soft rock, aligned in the direction of prevailing winds
- separated by wind-sourced groove (soft rock)
- made through abrasion
- more resistant rock creates the standing yardang
- for example in the west if the Tibesti Mountains in Northern Chad