Weathering

1
Weathering

• Types
• Physical or mechanical (increases surface area)
• Chemical (chemical transformations)
• Congruent (complete dissolution)
• Incongruent (solids dissolved products)

2
Factors Controlling Chemical Weathering Rates

• Mineralogy
• Surface area
• pH
• Temperature
• Organic acids
• Rate d(mineral mass)/dt -KA
• Where A surface area and Kparameter reflecting
  all other controls

3
Effect of Mineral Type

• In general, rate decreases with
• Increasing tetrahedral linkages
• 2. Less Al for Si.

Faster than olivine
Slower than olivine
4
Measuring Weathering Rates

• Streams
• Instantaneous measures
• Integrates area
• Soils
• Integrates time
• Site specific

5
EBMUD Water Chemistry and Mineral Weathering
6
Sierran Spring Chemistry and EBMUD Water

• Lafayette 2.4 1.1 0.4 0.13 2.8
• Orinda 3.3 1.8 0.6 0.15 5.4

7
Typical Weathering Reactions

• Reaction medium consists of water and CO2
• Products kaolinite and gibbsite plus dissolved
  products

8
Sierran Spring Chemistry and EBMUD Water

• Lafayette 2.4 1.1 0.4 0.13 2.8
• Orinda 3.3 1.8 0.6 0.15 5.4
• Subtract contribution from rain (i.e. assume all
  Cl and SO4 from rain - balanced by Na)
• Assume all Mg derived from biotite
• .15(B).15(H2CO3).15(.5H20).15(K).15(3Mg).15(7
  HCO3).15(2H4SiO4).15(.5 Kaol)
• Assume all K from K spar

9
Chemical vs. Mechanical Weathering Role of
Climate and Geology

• Recently glaciated, tectonically active areas
  have high chemical weathering rates
• Areas of long term stability (and long weathering
  history have lower rates (Aust, Africa)
• Chemical weathering an important means of
  denudation

10
Soil Based View of Weathering Chemical Mass
Balance (Brimhall and others)

• Give soil profile based perspectives
• Depth variations
• Climate variations
• Age variations
• Gives us view of both physical and chemical
  changes in soils during development
• Soils commonly undergoe initial expansion
  followed by collapse
• Soils, given enough time, can become chemically
  depleted and must rely on atmospheric inputs