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2.5 Erosion Processes in the Tropics (Water Erosion (Defined as flow of…
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Aeolian Transport
Suspension
(clay & silt)
- Fine particles are lifted into the atmosphere and suspended at high elevations for long distances
- Constant & large scale process that can cover the entire arid tropics
- Most effective in transporting fine clay & silt in turbulent wind eddies + carrying them for a long time and distance
- terminal velocity < friction velocity of the turbulent wind
- Larger sands get lifted closer to ground but falls out near the point of takeoff due to greater mass
- Produces erosion: deflation
Saltation
(sand)
- Occurs when friction velocity is much < terminal fall velocity
- Large sand grains bounce, land and rebound impacting other grains in a series of downwind jumps
- Confined to short distances and heights of about 2m
- Powers creep and reptation processes
- Produces erosion: abrasion
Reptation & Surface Creep
(gravel & pebble)
- Coarse sand / small pebbles inch forward by rolling and sliding along the surface with momentum gained from saltating grains as they are too large to be lifted by the wind itself
- Continuous rain of saltating grains come into contact with stationary grains, dislodging them & causing entrainment at a velocity lower than threshold velocity (impact threshold)
- Gets progressively more significant as size increases
- One saltating grain can cause a large number of grains to move at slower velocities
- Entrainment is self-generating when grains move at friction threshold
- Involves fine to medium sand, or larger particles during extremely strong winds
- Movement is slow when wind starts
- Responsible for building dunes
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- Distance transported
- Elevation reached
- Affected by size of particle
- Removal of loose , fine material (silt and clay) from a surface (often pre-weathered from salt weathering), leading to aeolian transport
- Occurs when the upward friction velocity exceeds > terminal velocity
- Able to lift particles to high elevations and the sediments often settle back to the surface as part of fine-grained aeolian deposits
- Progressively removes small material until only the larger material is left behind, forming a stony desert surface
- Active in arid areas due to sparse vege and thin soil cover
- High evpt and dry conditions suggests unconsolidated sand
- Erodes softer rocks and poorly unconsolidated sediments , does not require elevation
- Salt weathering and wetting/drying processes are precursors to deflation
- Mechanical wear of upstanding rocks and boulders by the impact of particles in saltation
- Most effective from surface to 1-2m of height (above it is ineffective)
- Weak erodible rock surfaces (clay and silt) are highly erodible and susceptible to abrasion from sand grains transported by winds that are used as grinding tools
- Hard upstanding bedrock (sandstone) undergoes abrasion at its base, resulting in polished, smooth rock surfaces and yardangs
- At greater heights, particles have higher velocities but there is also lesser number of particles
- As wind velocity ↑ , height of max abrasion ↑
- Winds are accelerated in valleys where they ascend over hills promoting erosion
- Magnitude of erosion depends on susceptibility to abrasion and properties of impacted particle
- Slower than deflation , 100 years for 1mm thickness of rock to be eroded