Weathering, Erosion, and Mass Movement

Chemical weathering

The minerals in rocks formed beneath the surface are in equilibrium with the temperature and pressure conditions at time of their formation and thus are quite stable. However, many minerals are no longer in equilibrium with their environmental conditions when exposed at the surface and are susceptible to weathering. Chemical weathering results in the formation and retention of minerals in equilibrium with environmental conditions at the Earth's surface.  The least stable minerals in igneous and metamorphic rocks are olivine and plagioclase, the most stable is quartz. 

The interlocking and spacing of mineral grains controls the tendency towards weathering. Rocks with loosely interlocking mineral grains allow agents of chemical weathering to penetrate, thus speeding their decomposition. Limestone is primarily composed of calcite, a mineral that is quite soluble under surface conditions and easily dissolves in humid environments. In dry regions, the tight texture of limestone prevents it from disintegration and thus is a relatively resistant rock when found in deserts. 

The process of chemical weathering tends to: 

• increase bulk creating stress within rocks

• lower the density minerals

• decreased particle  size resulting in increased surface area 

• creates more mobile materials

• creates more stable minerals

Chemical weathering processes

Oxidation takes place when oxygen reacts with earth materials. Oxygen dissolved in water combines with atoms of metallic elements abundant in silicate minerals.  Attacking metals in the soil, oxidation causes them to rust leaving the soil a brownish red to red color. When oxygen combines with iron, the reddish iron oxide hematite (Fe₂O₃) is formed:

4Fe⁺³ + 3O₂ -> 2Fe₂O₃

Hydrolysis is an exchange reaction involving minerals and water. Free hydrogen (H⁺) and hydroxide (OH)^- ions in water are able to replace mineral ions and drive them into solution. As a result, the mineral's atomic structure is changed into a new form. It is a process whereby silicate minerals like potassium feldspar are weathered and a clay mineral is formed.

2KAlSi₃O₈ + 2H⁺ + 9 H₂O -> Al₂Si₂O₅(OH)₄ + 4H₄SiO₄ + 2 K²⁺

Hydration involves the absorption of water like which occurs during the conversion of hematite to limonite:

2Fe₂O₃ + 3H₂0 -> 2Fe₂O₃ ^. 3H₂0 

Some geoscientists question whether hydration is a true chemical weathering process because the process is readily reversible and the new product is not chemically different from its precursor. Some would rather call hydration a physical weathering process.

Carbonic acid action involves combination of carbon dioxide and water. Though present in pure water, carbon dioxide dissolved in water provides ions that produces free hydrogen. Carbon dioxide in the atmosphere combines with rain water to form carbonic acid (H₂CO₃): 

H₂O + CO₂ -> H₂CO₃  

Though weak, when carbonic acid is combined with a mineral like calcite (CaCO₃) common to limestone, calcium and bicarbonate ions are released and carried off by groundwater.

CaCO₃ + H₂CO₃  -> Ca⁺²  +  2 HCO_-3

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