HKDSE Geography/E1/Chemical Weathering

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Chemical weathering - the in situ distintegration of rocks into smaller fragments by chemical means involving change in chemical composition.


Five Processes[edit]

Favourable conditions: Waterlogging (for hydration and reduction), exposure to air (for oxidation), rocks made of soluble minerals (for solution and carbonation), hot and wet climate

There are five processes involved in chemical weathering, which may appear simultaeneously:

Name Description Example
Hydration Water seeps into rocks and wets it. This weakens the rock's structure. Feldspar in granite
Hydrolysis Hydrogen in water reacts with rocks. This weakens the rock's structure.
Oxidation Oxygen in air reacts with metals in rocks to form oxides (generally reddish brown). This weakens the rock's structure. Iron oxide, aluminium oxide
Reduction The inverse process of oxidation. It occurs in waterlogged areas. Reddish brown rocks are discoloured into grey or blue. The amount of oxygen is reduced in the rock.
Solution
  • Sulphur and nitrogen react with rainwater to form acids. The acids attack rocks.
  • Vegetation secretes organic acids during growth. The acids attack rocks.
  • Vegetation produces humic acid when it dies and is decomposed. The acid attacks rocks.
Sulphuric acid, nitric acid
Carbonation Carbon reacts with rainwater to form acids. The acids attack rocks. Carbonic acid

Two Forms[edit]

There are two important 'forms' of chemical weathering - i.e. the shape that the rocks look like after weathering.

Honeycomb weathering[edit]

Favourable conditions: Found in rocks with minerals of different resistance.

  1. Salt sprays from the sea dissolve minerals in rocks.
  2. The minerals are removed by solution on the rock face, weakening the rock to produce pits.
  3. This creates a honeycombed effect.

Spheroidal weathering[edit]

Favourable conditions: Well-jointed and exposed rocks (e.g. granite), heavy rainfall

  1. Rainwater seeps into cracks and joints.
  2. Chemical weathering takes place mainly on the surface and along the joints.
  3. Gradually, the rainwater works its way to the interior part of the rock.
  4. The rocks are reduced to small and round boulders surrounded by concentric weathering layers. The boulder in the centre is a corestone.
  5. The corestone gets smaller and rounder as time goes by.
  6. When the weathered layers are removed by wind or rain, corestones will be exposed on the ground. They pile up to form tors.

Note: When the bedrock is covered by a very thick layer of regolith already, furthering weathering will be restricted.

Effect of Climate on Weathering Occurence[edit]

TODO: The rectangle with four corners diagram