weathering vs. erosion
weathering in deserts
Salt crystallisation is a weathering process in deserts that breaks down rock through the formation of salt crystals.
This can occur in two ways, both of are common in hot desert regions where low precipitation and high temperatures cause salt to form below the surface of the rock.
- Temperatures fluctuate around 26-28˚C
- Sodium sulphate and sodium carbonate minerals in rock expand by around 300%
- Expansion of salt crystal formation forces joints in rock to crack
- Sun heats water causing evaporation
- Salt crystals left behind on rock
- Temperatures rise and salt expands, applying pressure to joints in rock
Disintegration is a process of weathering found in deserts caused by repeated heating and cooling of rock.
Deserts have diurnal temperature ranges of up to 40˚C (difference between day and night temperatures). During the day, rocks heat up and the minerals in the rock expand. During the night, the rock contracts, causing stresses in the outer layers. This causes peeling or exfoliation to occur. Water is a key part of the process, without water the expansion of salt minerals in rock would not occur.
WIND EROSION in deserts
Wind performs two kinds of erosional work: abrasion and deflation.
Abrasion is a process of erosion where loose particles lying on the ground surface may be lifted into the air or rolled along the ground by wind action. In the process of wind abrasion, wind drives mineral particles against an exposed rock or soil surface, wearing down the surface.
The sandblasting action of wind abrasion is usually limited to the bottom meter or two of exposed rock above a flat plain. That’s because sand grains don’t rise much higher into the air. Wind abrasion produces pits, grooves, and hollows in the rock. You’ll often see that wooden utility poles on windswept plains have a protective metal sheathing or a heap of large stones placed around the base. Without this protection, they would quickly be cut through at the base.
Deflation is the removal of loose particles from the ground by wind. Deflation acts on loose soil or sedi- ment, and so dry river courses, beaches, and areas of recently formed glacial deposits are susceptible. In dry climates, much of the ground surface can be deflated because the soil or rock is largely bare of vegetation.
The finest particles, those of clay and silt sizes, are lifted and raised into the air—sometimes to a height of a thousand meters (about 3300 ft) or more. Sand grains are moved by moderately strong winds and usually travel within a meter or two (about 3 to 6 ft) of the ground. Gravel fragments and rounded pebbles can be rolled or pushed over flat ground by strong winds, but they don’t travel far. They become easily lodged in hollows or between other large particles. If there’s a mixture of particles of different sizes on the ground, deflation removes the finer sized particles and leaves the coarser particles behind.
- Complete the A3 writing frame: describe, explain and illustrate the processes of weathering and erosion in deserts.
Mesa - Butte - Plateau - Canyon - Wadi - Pediment - Yardang - Zeugen - Salt Pan - Inselberg
Mystery - How did this happen?
drainage basins in hot arid areas
In order to understand how a flash flood like the one above can happen in hot, arid areas, we must first understand how drainage basins work.
Look at the diagram below. How would the drainage basin system and the processes responsible for transferring water be different in hot arid areas?
Flooding occurs when rainfall intensity exceeds infiltration capacity generating surface runoff/overland flow leading to flooding.
Causes of flooding in deserts:
- summer convection rainfall bringing high amounts of precipitation
- unvegetated desert surfaces reduce interception and infiltration, increasing overland flow
- high concentrations of water in wadis, gullies or channels
- presence of desert crusts (duricrusts) and other barriers to infiltration (rocks in desert pavement etc).
infiltration and surface runoff
- Draw a labelled diagram to illustrate how flash floods happen in hot, arid environments.
- Use all of the information above to help you.
RIVERS in deserts
- Name and describe the three main types of river in desert environments
- Explain why rivers can have considerable discharges in arid environments.