Weathering

Types:

•Physical or mechanical (increases surface area)

•Chemical (chemical transformations)

–Congruent (complete dissolution)

–Incongruent (solids + dissolved products)

Factors Controlling Chemical Weathering Rates

•Mineralogy

•Surface area

•pH

•Temperature

•Organic acids

Rate = d(mineral mass)/dt = -KA

Where A = surface area and K=parameter reflecting all other controls

Effect of Mineral Type

In general, rate decreases with:

1. Increasing tetrahedral linkages

2. Less Al for Si.

Faster than olivine

Slower than olivine

Measuring Weathering Rates

1. Streams

• Instantaneous measures

• Integrates area

2. Soils• Integrates time• Site specific

EBMUD Water Chemistry and Mineral Weathering

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

Typical Weathering Reactions

•Reaction medium consists of water and CO2

•Products kaolinite and gibbsite plus dissolved products

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 5.4Subtract 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(7HCO3)+.15(2H4SiO4)+.15(.5 Kaol)

Assume all K from K spar

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

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