Weathering

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  • Created by: bibibibra
  • Created on: 17-01-19 15:34

Weathering

Weathering - the decay and disintergration of rocks in situ.      (Does not transport material away)

Physical weathering - disintergration of rock (broken)

Chemical weathering - decomposition of rock (changed)

Biological weathering 

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Freeze-thaw (frost shattering

PROCESS Physical

  • Water trickles into rock during day and freezes during night

  • Expands and widens by 10%

  • Repetition results in scree and felsenmeer

  • Scree  = angular rock fragments, falling from a cliff and being deposited as a cone or fan shape

FACTORS

  • Not active in winter where there's constant frost

  • Shape and size dependant on nature of rock (direction and angle of cracks)

 

Critical feature : number of freeze-thaw cycles rather than intensity of frost.

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Thermal fracture : Granular & Block disintegration

PROCESS                                                                                                             Physical 

  • Expansion (day)  and contraction (night) of rocks in deserts

  • Granular : Mineral grains produced create a sand sea (ergs)

  • Block : rock fragments result in stony desert (reg)

FACTORS

  • Hot temperature, desert

  • Different minerals

  • Different axes of crystal

  • Different sizes

  • Colouring of rock - pale colours emit and absorb more heat than a darker colour

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Salt Crystal Growth

PROCESS                                                                                                                     Physical

  • Salt solutions in pores or joints of rock crystalise

  • Crystal expands, forcing rock apart

FACTORS

  • Most effective : sodium sulfate, magnesium sulfate, calcium chloride,

  • Temp around 27oC where fluctuations produce expansion rate up to 300%

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Pressure release (dilatation) / Unloading

PROCESS                                                                                                                physical

  • Removes weight from previously buried rocks (very deep)

  • Leads to expansion of upper parts of the rock, allowing cracks to occur parallel to ground surface  (pseudo-bedding planes)

  • Horizontal pressure released by rock falling off cliffs allow cracks to grow vertically

FACTORS

  • Horizontal pressure significant after glaciation caused by quarrying

  • Areas where ground surface is lowered by erosion

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Hydrolysis

PROCESS                                                                                                         Chemical

  • Minerals broken down by a reaction including water

  • Can include some carbonation

  • Produces a clay residue and various other solutions, removed in groundwater

FACTORS

  • Important in the silicate minerals that form most rocks

  • Acid conditions

  • Encouraged by decaying vegetation

  • Carbonation encouraged by carbon dioxide from plants

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Hydration & Dehydration

PROCESS                                                                                                           Chemical 

  • Wetting and drying

  • Addition or removal of water causes expansion or contraction which assists disintegration

  • Anhydrite and Gypsum affected

FACTORs

  • If water is applied or removed from the rock
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Carbonation

PROCESS                                                                                                              Chemical

  • Weak carbonic acid which is in rainwater forms as the water absorbs carbon dioxide in the atmosphere

  • Carbonic acid attacks carbonate minerals in limestone

  • Calcium hydrogen removed and washed away down river, muddy insoluble impurities left as cley residue

  • Process is responsible for limestone scenery characteristics (karst)

FACTORS

  • Accelerated by pollutants such as sulfur dioxide and oxides of nitrogen
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Organic action

PROCESS                                                                                                                     biological

  • Physical effect on rocks

  • Seeds and roots in shallow soil find their way into cracks in bedrock

  • As seeds germinate and roots grow, the cracks widen and deepen, eventually breaking bedrock

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Biological weathering that assists chemical proces

Biological weathering that assists chemical processes

  • Humanic acids from decaying vegetation encourage hydrolysis
  • Carbon dioxide from plants encourages carbonation

  • Vegetation traps water, encouraging variety of chemical processes

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