Rock
Cycle
Sedimentary Processes
1) 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 Processes
• Mechanical Weathering Disintegration of rock without change in
chemical composition
• Chemical WeatheringDecomposition of rock as the result of
chemical attack. Chemical composition
changes.
Mechanical Weathering
•Frost wedging
•Alternate heating and cooling
•Decompression
causes jointing
Chemical 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
Oxidation can affect any
iron bearing mineral, for
example, ferromagnesian
silicates which react to
form hematite and limonite
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 silica
Clay 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 classification
1) 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 classification
2) 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 & mollisols
Red – Oxisols
Tan - Aridosols
Minerals formed by weathering:
(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