Weathering and Erosion

• Weathering – breakdown of rocks to particles
 

 - chemical weathering (dissolution)
 - physical weathering (breaking)

• Erosion
 

- transportation of sediments from their place of formation

• Two processes are linked

 
• controls on weathering
 

 - properties of rock
 - climate
 - soil and vegetation
 - time: more time = more weathering

 
• Properties of rock

 
 - solubilities of minerals
 - some dissolve more easily
 - granites vs. limestone, gravestones
- granites also weather, e.g. feldspars but not quartz
-internal structure allows breaking, e.g. sed. rocks break along planes

 
• Climate: Rainfall & Temp.
 

 - amount of rainfall and T
 - in part from control of organisms
 - also on amount of water available
 - freezing important for breaking rocks

• Soil
 

- soil is bits of bedrock, plus organic matter from organisms
- provides feedback for more weathering
- retains rain water enhancing weathering
- organisms create acidic environment
- burrowing create fractures

Chemical Weathering

• Two products of weathering reactions:
 

 - dissolved minerals (solutions)
 - new minerals (solids)

• Use feldspar as an example

- a very important (i.e. common) mineral
- a very important reaction

Feldspar to Kaolinite clay

• Hydration reaction:

 - major component of granite is feldspar
- if sufficiently weathered, feldspars convert to kaolinite clay
- kaolinite is hydrous, consequently reaction occurs mostly in humid environment
- feldspar also loses some components, particularly K and Si

• Reactions occur on surfaces of mineral grains

- the more the surface area, the more reactive the mineral
- breaking grains increase surface area

 
• The role of CO₂
 

 - dissolution in pure water is very slow
 - speeds up with acidic solution

•Carbonic acid:
 

 - most common natural acid
 - forms from dissolving CO₂ in water
 

CO₂ + H₂O  -> H₂CO₃  -> 2H⁺ + CO₃^2-
 

 - the more CO₂, the more acidic

 
• CO₂ is 4th major component of atmosphere
 

 - makes up 0.03 %
 - sufficient to make rain acidic, pH ~5
 - note “acid rain” from sulfur and nitrogen
 - the acidity is sufficient to dissolve feldspars:

feldspar + carbonic acid + water à
 kaolinite + dissolved silica + dissolved K + dissolved bicarbonate
 
• Link to climate
 

- increase in CO₂ increases weathering (Himalayas)
- weathering decreases CO₂

 
• Other silicates form clays

- e.g. amphibole and mica
- follow similar reaction: hydration and lose Si and cations, Na, K, Ca, Mg
- type of clay depend on parent silicate and climate

• some silicates dissolve without converting

 - olivine, pyroxenes, quartz

• Continued weathering of clays forms bauxite (aluminium hydroxide)

 
• Carbonates also dissolve by acid reactions
 

 - similar to silicates
 - very fast
 - leave behind caverns
 - stalactites and stalagmites
 - used to reduce acidity of soils

 

Iron Silicates to Iron Oxides

• Oxidation reaction - controls oxidation state of iron
 

 - metallic iron: no charge
 - ferrous iron: 2+ valence
 - ferric iron: 3+ valence

 
• Dissolution of pyroxene
 

 - releases Si and Fe²⁺ to solution
 - Fe²⁺ oxidizes to Fe³⁺
- combines with oxygen to form hematite: Fe₂O₃
- very insoluble, comes out of solution
- form distinctive red soils (rust colored)

 
• Mineral stability- measure of tendency for mineral to dissolve
 

- solubitily: the amount of material that will dissolve in given amount of water
- rate of dissolution: how much time it takes to dissolve given amount of material

 Physical Weathering

• Rocks break for many reasons:
 

• Natural zones of weakness, e.g.

 - sed. rocks: bedding planes in sed. rocks
 - metamorphic rocks, planes of fractures
 - tectonic forces fracture into “joints”

• Organisms:

 - rocks forced apart by growing plants
 - burrowing organisms

 
• Frost wedging

 - increase in volume from water to ice

• Mineral crystallization

 - precipitation of minerals force rocks apart

• Transportation
 

- moving rocks hit other rocks and break them apart
 - holes common, beach rocks

Soil

• Layers of broken rock at surface of earth

- regolith: that portion without organic material (Moon)
- soil: the uppermost portion with organic material
- organic material called humus

• Important material

 - forms slowly (1000’s years)
 - not replaceable on human time scale

 
• Many types of soils
 

- identified on basis of their profile: compositional variations with depth

• Profiles broken into “horizons”
 

- A horizon: top most layer, contains organic material, may have soluble/insoluble minerals
- B horizon: layer where organics sparse, contain insoluble oxides
- C horizon: slightly altered bedrock

 
• Soil groups
 

- many different groups because of importance
- three major groups, illustrate variety of soil groups

(1) Laterites
 

 - warm humid climates
- intense weathering, Silica and carbonates gone
 - iron oxides remain
 - little organics: most in vegetation
- they are not productive, problem with logging rain forest

 
(2) Pedalfers
 

 - temperate climates
- upper layers contain abundant insoluble minerals; quartz, clay, iron oxides
- composition of young soil depends on precursor rock, weathering less extreme
- e.g. granite & limestone very different soils
- with longer time, soils are similar

 
(3) Pedocals
 

 - dry climates
 - thin
 - A horizon contains many soluble minerals
 - precipitates of calcium carbonate

(4) Paleosols
 

 - occasionally soils preserved in rock record
- useful for understanding climate if soil type can be identified