Land Based Sub-Aerial Weathering and Mass Movement

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Weathering

Weathering is the disintegration and decomposition of rock in situ. Weathering is the natural breakdown of rock, and is distinguishable from erosion because weathering does not involve movement of material. 

Degree of weathering depends on structure and mineral composition of rocks, local climate, vegetation and time. It is known as sub-aerial as it occurs at the earth's surface (instead of underground/water.)

There are 3 main types:

  • Mechanical/Physical: Disintegration of rock without change in the chemical composition, likely to occur in deserts, high mountains and arctic regions, areas void of vegetation. Likely to form sands
  • Chemical: Decompostion of rock resulting from chemical change, likely to take place in warm, moist climates with vegetation cover. Likely to form clays.
  • Biological: Direct cause is plants and animals, representing a mixture of physical and chemical weathering as a consequence of biological impacts.
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Mechanical Weathering

Freeze Thaw: Occurs in rocks containing crevices where there is little vegetation, and temperatures that fluctuate around 0. Water enters the joints when it's warmer, and at night, it freezes, exerting pressure as it occupies more volume. The alternating freeze-thaw process widens the cracks and pieces of rocks shatter. Most widespread form of weathering.

Sheeting: Rocks developed under pressure, when exposed to the atmosphere, which has lower pressure, expand. Cracks develop parallel to the surface, causing sheeting, resulting in the outer layers peeling away.

Salt Crystallisation: If water entering cracks is salty, when the water evaporates, salt is left behind. As the crystals get larger, they will break up the rock. This is common at the coast where salty water is abundant and in deserts when capillary action draws water to the surface.

Exfoliation: Rocks expand when heated and contract when cooled. In the desert where temperature change between day and night is extreme, the outer layers of rock cool/warm up much faster than the inner ones. This causes stresses in the rock, causing the outer layers to peel off like an onion skin.

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Chemical Weathering

Hydrolysis: Reaction between water and a mineral, where the hydrogen ions in water displace ions in the mineral. Eg. Feldspar and quartz, where potassium ions are displaced.

Hydration: Occurs when minerals absorb water, causing a physical change where the mineral swells, weakening the rock. Eg. Anhydrite forming gypsum.

Carbonation: When rainwater reacts with calcium to form calcium bicarbonate which is soluble and is carried away by water. Happens in chalk and limestone.

Oxidation: When a mineral combines with oxygen from the air or water, weaking the mineral structure and makes the rock more vulnerable to erosion and other weathering types. Eg. Iron.

Chelation: When rocks and soils weather through the action of organic acids, produced by bacteria decomposing vegetation and dead animals. Eg. Humic acid.

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Biological Weathering

Plants: Some plants grow roots below the ground surface, which widens cracks in rocks and forces them apart.

Animals: Burrowing animals open cracks and joints in rocks, exposing the rock to further weathering and allows increased percolation of rainwater. They also mix rotting plant material into the soil which aerates it. When the matter decomposes, acids are formed which weather the fock chemically. Organisms such as slugs eat plants on the rocks, whose slime erode further.

Micro-organisms: Lichens, algae, fingi and bacteria grow directly on the rock, and extract dissolved nutrients which can produce holes in the rocks, exposing it to other forms of weathering. Micro-organisms also absorb water, increasing their volume, exerting pressure in the pores in the rock's surface.

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Mass Movement

Mass Movement is the movement of material under the force of gravity.

Weathered rocks are unstable and exposed to:

  • Rockfalls
  • Landslides
  • Rotational Slumping
  • Soil Creep
  • Mudflows

Cliffs are undercut by the sea and processes like freeze-thaw also weaken the cliff.

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Rockfalls/Landslides

Rockfalls

  • Rockfalls occur on steep cliff faces (>90 degrees) with little or no vegetation on them.
  • The rock face is usually always exposed and suffers from weathering (eg. Salt crystallisation, freeze-thaw) on a regular basis this causes the well jointed rock to become weaker. The cliff face is also undercut by the sea.
  • Eventually the rock becomes detached from the cliff face, and falls quickly with little or no contact with the face itself. The rock gathers as scree at the bottom of the cliff.

Landslides

  • Rockfalls occur on cliff faces less than 90 degrees in steepness.
  • It is the rapid movement of detached slabs of rock (weakened by weathering and erosion), where the material remains in contact with the slide plane.
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Mudflows/Rotational Slumping

Mudflows

  • Mudflows can develop when water saturates the ground, eg. heavy or prolonged rainfall. causing a thick liquid downhill flow of earth in a lobe. The saturated soil and debris forms a stream.
  • Some broad mudflows are rather viscous and therefore slow; others begin very quickly and continue like an avalanche.

Rotational Slumping

  • Occurs where softer material overlies resistant rocks.
  • With excessive rainfall, whole sections of the cliff move downwards with a slide that is concave, producing a rotational movement. 
  • A scarp face is exposed at the head of the slump and a hummocky toe at its foot.
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Soil Creep

Soil Creep

  • Soil Creep is the very slow downward progression of rock and soil down a low grade, vegetated slope.
  • When wet, soil particles increase in size and weight, and expand at right angles. When the soil dries out, it contracts vertically.
  • The rate of soil creep down a slope depends on gradient of the slope, water absorption, type of sediment and vegetation.
  • Results can be seen in step-like terracettes on hillsides with small cliffs at the foot of the slope.
  • Effects on manmade structures include tension gashes in the road and tilted telegraph poles. Trees can also turn downslope.
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Run-Off

Run-Off:

  • Operates on coastlines. The presence of water will assist many mass-movement processes.
  • It is sediment transported from the land to the coast by:
    •  Rivers: Stream cascading over a cliff, excavating a V-Shaped groove
    • Surface Run-off after snowmelt or rainfall.
    • Streams emerging in a bay carrying large quantities of load.
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Comments

Manon Burbidge

The colour isn't working so sorry about the blandness of the cards!

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