Mechanical weathering: Rock is physically broken into smaller pieces in situ.
- Thermal fracture:
- Extreme temperatures in deserts (0-50OC desert air can’t block sunlight or trap heat)
- During the day (when it’s hot) rock expand and at night (when it’s cold) they contract.
- Thermal expansion & contraction occur at different rates on different parts = fractures
- Salt weathering caused by saline (salty) water, which comes from rainfall or groundwater that’s drawn up to the desert surface by evaporation.
- Saline water enters pores in desert rocks
- High temperature causes water to evaporate, forming salt crystals. As the salt crystals form, they expand, exerting pressure on the rocks.
- More evaporation, salt crystals expand more= increases pressure in rocks= pieces to fall off
- Frost shattering (freeze-thaw weathering) occurs in areas where there’s moisture and temperatures that fluctuate above and below freezing.
- Water from rainfall enters the joints and crevices in desert rocks.
- At night, if temp drops below 0 degrees, the water in the cracks freezes and expands.
- Over time, repeated free-thaw action weakens the rocks and causes pieces to fall off.
Chemical Weathering:
- Hydration:
- When moisture combines with minerals in rocks, especially rocks containing salt, and causes them to swell. This puts pressure on the rock that can cause it to crack.
- Oxidation:
- When rocks containing iron are exposed to oxygen in air or water. The iron can react with oxygen to form iron oxide, which is quite weak – this makes the rock crumble more easily.
- Carbonation:
- When carbon dioxide in the atmosphere dissolves in rainwater, forming a weak carbonic acid. This acid reacts with rock that contains calcium carbonate, e.g. carboniferous limestone, so the rocks are gradually dissolved.
Rock Breakdown:
- Exfoliation: The peeling off of the outer layers of a rock,
- Granular disintegration: The breaking off of individual grains of sand
- Block disintegration: The breaking off of larger chunks of rock.
Wind Erosion:
- Deflation: The removal of fine, loose particles from the ground surface.
- Abrasion: Small particles being carried by the wind scrape off particles from the rock surface.
Wind Transport:
- Suspension: When very small particles (less than 0.15mm) are picked up and carried by the wind.
- Saltation: When small particles (0.15-0.25mm) are temporarily lifted form ground and bounce along
- Surface Creep: Large particles are hit & pushed along ground by particles moved by saltation.
Types of Rivers:
- Exogenous rivers:
- Source outside the desert margin
- Flow throughout the year despite evaporation reducing their volume.
- g. the source of the Colorado River (USA) is in the Rocky Mountains. It flows through the Sonoran Desert and the Grand Canyon to the sea.
- Endoreic rivers:
- Terminates inland (does not flow to sea) in the form of an inland sea or delta.
- g. the River Jordon terminates in the Dead Sea – an inland sea in the Middle East
- Ephemeral rivers:
- These flow intermittently or seasonally after rainstorms.
- g. Todd River in the Simpson Desert (Australia) only flows a few days a year and remains a dry river bed for the rest of the year.
Types of flooding:
- Channel flash flooding:
- Sudden, strong and rapid flow of water through a channel.
- Heavy rainfall can’t be absorb by desert soil – so runoff collect in channels and flows downhill
- Enough energy to transport large pieces of desert rock by traction (when very large particles are pushed along the river bed by the force of the water).
- Transport pebbles, gravel and sand by suspension and saltation -eroded into smaller fragments by attrition (rocks smashing into each other).
- The material carried erodes the channels by abrasion, making them deeper
- Mouth of channel, the flash flood waters spread out, slow down and soak into the ground (
- Sheet flooding:
- Slow-moving, even flow of water over land (i.e it isn’t confined to a channel).
- Occur after a period of intense rainfall, where water collects across the dry, impermeable desert floor and flows down gentle slopes as a sheet of water.
- Sheet floods have less energy but can still transport pebbles, gravel and sand
- The material carried by sheet floods erodes the desert surface by abrasion.
Deposition:
- Occurs when sediment load exceeds the ability of the wind/water to carry it.
- This could be because there’s a reduction in the speed of the wind/water flow:
- Wind/water slow down when they hit an obstacle or roughness of ground surface increases
- If water leaves a channel and spreads out (e.g. to form a sheet flood), or gradient decreases
- Wind/water slows down, loses energy so can’t carry as much material, so some gets dropped.
Mass Movement:
- The movement of material down a slope due to gravity. It’s common on landforms with steep slopes
- Rockfall are when material breaks up and moves rapidly down a steep slope
- Debris flows: material ‘flows’ downslope – usually during or after rainfall. Water increases weight of material and decreases friction between particles = material more likely to collapse