The Processes


Understanding the differences between EROSION & WEATHERING:

Its important to understand that both EROSION & WEATHERING are different processes, the extract below taken from Wikipedia sums up the differences quite nicely:

"Weathering is the breaking down of rocks, soils and minerals as well as artificial materials through contact with the Earth's atmosphere, biota and waters. Weathering occurs in situ, or "with no movement", and thus should not be confused with erosion, which involves the movement of rocks and minerals by agents such as water, ice, snow, wind and gravity."

SOURCE: http://en.wikipedia.org/wiki/Weathering, accessed on 17/03/2012 @ 15:39pm

Where does salt weathering occur?

Its not confined to any set types of location. Often you tend to find the best examples of salt weathering in 'dessert' areas and 'coastal' areas. Primarily due to the temperatures and abundannce of salts. Tschang, 1974 provides a wide spectrum of locations where good examples of salt weathering can be found. He particularly makes reference to 'tafoni' weathering (see later).

Salts.....What are they?

Salt is comminly misinterpreated as just being common table salt, NaCl, but for the purpose of this site we mean any chemical salt. Salt is defined as being "
A chemical compound formed by replacing all or part of the hydrogen ions of an acid with metal ions or electropositive radicals." (Bender & Bender, 2004).

The main salts often deemed as causing the most damage in salt weathering are Calcium, Sodium or Magnesium based. Some examples of these are: Calcium Carbonate, Sodium Sulphate, Sodium Carbonate, Magnesium Sulphate and Sodium Chloride.

Furthermore salts can take the form of 'hydrous' states and integrate more 'ion' based salts such as sulphate ions. Evaporites can be a source of these salts as well.

 Mineral            
 Composition Type of Occurence
 Gypsum    CaSO4.2H2O  Dripstone, wall crusts, flowers, bladed crystals, pore cement in cave soils
 Halite       NaCl          Dripstone, wall crusts, flowers, pore cement in cave soils
 Epsomite  MgSO4.7H2O  Dripstone, flowers, pore cement in cave soils
 Hexahydrite     Na2SO4.6H2O  Loose crusts on cave floor
 Mirabilite     Na2SO4.10H2O  Dripstone, flowers, loose tufts of crystals
 Bloedite          Na2SO4.MgSO4.4H2O  Loose crusts on cave floors
 Celestite          SrSO4  Loose crusts on cave floors






Figure 1  (Warren, 2006) - Shows the influence of some of these evaporites in cave geomorphology (particularly speleothems)

A speleothem is a 'secondary mineral deposit' which induces growth of cave features such as columns, flow stones, stalagmites, stalagtites, straws, columns & drapery. It relies on mineral rich flows of water at slow enough speeds for deposition to occur. Over many years (often millions) defineable growth of one of these named structures can occur.
 
What are the 'source' of these salts?
 
Three main sources of salts are:
 
1) The Sea
 
- The sea is a massive store of 'salts' such as Magnesium Chloride, Sodium Chloride and similar. Simply the direct movement of sea water over coastal rocks, sea breezes blowing the spray onto land, or flood/high tide breaches of coastal landforms/defences can be the source of salts for salt weathering to occur.
 
2) Contaminated groundwater seepage
 
Sea water naturally intrudes the water table at coastal locations or inland via salt rich lakes.
 
Figure 2 - A diagram to show salt water intrusion
 
3) Decay/weathering of rocks
 
Rocks are comprised of different minerals. For example the standard form of the rock 'Granite' is comprised of:
 
  • SiO2 — 72.04% (silica)
  • Al2O3 — 14.42% (alumina)
  • K2O — 4.12%
  • Na2O — 3.69%
  • CaO — 1.82%
  • FeO — 1.68%
  • Fe2O3 — 1.22%
  • MgO — 0.71%
  • TiO2 — 0.30%
  • P2O5 — 0.12%
  • MnO — 0.05%

  • (SOURCE: Wikipedia, Granite, accessed on 17/03/2012 @ 15:44pm)
  •  
    As rocks are weathered by either MECHANICAL, CHEMICAL or BIOLOGICAL factors they break down into their basic minerals then their basic chemical properties. This weathering can cause the release of 'salts' which can then partake in further weathering of the rock or other materials.
     
    How does Salt Weathering actually occur?
     
    The three main methods by which salt actively erodes rock/material are: 'Abrasive Action,' 'Hydration & Expansion' & 'Crystallisation.' Each of the sub headings below provide more information on each of these topics:

    Abrasive Action:

    Simply, this is the process by which the precipitate format of a salt (e.g. solid form) collides with rock/material and in doing so causes damage. Usually the salts are transported by aeolian methods (wind).
    Furthermore this process can then
     
    Crystallisation:

    This process is known as 'haloclasty' and is considered to be the most active in terms of salt weathering. It relies on salt rich waters/fluids entering fissures and cracks in rocks. Subsequent evaporation leaves behind crystals of salt which take up a greater volume than the liquid form. This 'expansion' creates pressure which in turn causes the breakdown of the rock.
    Figure 3 looks at the factors involved as well as the processes involved in salt weathering.


    Figure 3 - A flow chart showing the processes involved & factors that form salt weathering. (SOURCE: Pg 53 Siegesmund et al, 2002)
     
    Chemical breakdown:

    This encompasses other chemical interactions with the rock. Usually via the hydration of salts they become 'activated' and can chemically interact with the host material.
    This could include processes such as carbonation, disolution, hydrolysis and oxidation.

    What is 'Tafoni'?

    "Tafoni (singular tafone) are large weathering features that take the form of hollows or cavities on a rock surface [...] The tend to form in vertical or near vertical faces of rock. They can be as little as 0.1m to several metres in height, width and depth. Some tafoni cut right through boulders or slabs or rock to form rounded shafts or windows."
    (Huggett, 2007)

    The HOME page of this site has a slideshow showing examples of 'tafoni' weathering structures.  


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