RockCycle

Sedimentary Processes1) Weathering & erosion

2) Transport & 3) deposition

4) Lithification

Weathering: decomposition and disintegration of rock

Product of weathering is regolith or soil

Regolith or soil that is transported is called sediment

Movement of sediment is called erosion

Weathering ProcessesWeathering Processes

• Chemical Weathering-

Decomposition of rock as the result of chemical attack. Chemical composition changes.

• Mechanical Weathering -

Disintegration of rock without change in chemical composition

Mechanical Weathering

•Frost wedging•Alternate heating and cooling

•Decompression causes jointing

Chemical Weathering ProcessesChemical Weathering Processes

• Hydrolysis - reaction with water (new minerals form)

• Oxidation - reaction with oxygen (rock rusts)

• Dissolution - rock is completely dissolved

Most chemical weathering processes are promoted by carbonic acid:

H2O +CO2 = H2CO3 (carbonic acid)

Oxidation can affect any iron bearing mineral, for example, ferromagnesian silicates which react to form hematite and limonite

Oxidation

Oxidation of pyrite and other sulfide minerals forms sulfuric acid which acidifies surface water and rain

Pyrite + oxygen + water = sulfuric acid + goethite(iron sulfide) (iron oxide)

Hydrolysis

Feldspar + carbonic acid +H2O= kaolinite (clay) + dissolved K (potassium) ion + dissolved bicarbonate ion+ dissolved silicaClay is a soft, platy mineral, so the rock disintegrates

Products of weathering

Clay minerals further decompose to aluminum hydroxides and dissolved silica.

Controls on rates of weathering

• Relative resistance of primary minerals to chemical weathering

• Climate – especially:1. availability of water (for chemical weathering), 2. temperature (increases reaction rates), and 3. vegetation (which increases the availability of

carbonic acid)

A soil profile develops slowly over time

Soil classification1) Laterite or oxisol Forms in a hot, humid climate. Soil is deep red, hard and infertile.

Plants recycle nutrients in a thin A and O horizon.

Deforestation removes the fertile organic layer. The underlying soil is infertile, dries to brick-like hardness when it dries out, and is difficult to cultivate. Aluminum (from bauxite) and iron (from limonite) can be mined from these soils.

Soil classification2) Alfisols and mollisols: Form in warm or cool, temperate climates. Soil is grey and clay-rich, fertile.Alfisols are forest soils, mollisols form in grasslands

Soil map of world

Greens – Alfisols & mollisolsRed – OxisolsTan - Aridosols

Minerals formed by weathering:Minerals formed by weathering:(pages 191-192)(pages 191-192)

Clay minerals: kaolinite, smectite (Na, Ca), illite (K) (pages 255-256)

Aluminum oxides and hydroxides: gibbsite, diaspore, boehmite (consituents of bauxite) (pages 371-374)

Iron oxides and hydroxides: hematite, goethite, lepidocrosite, limonite

Mn oxides and hydroxides (bog ores or wad): pyrolusite, romanechite, manganite, others