WEATHERING PROCESSES

A. Introduction

• weathering produces unique landforms

• weathering produces regolith or a weathering mantle, that may eventually become soil

• physical weathering & chemical weathering

B. Physical Weathering

1. Resistance to weathering depends on:

• internal resistance of material

• magnitude of external forces
  

2. Growth in voids

a. freeze-thaw

• high latitudes & high altitudes

• produces angular rubble; talus & felsenmeers

b. crystal growth; salt weathering

• tafoni
  

3. Wetting & drying

a. hydration: addition of water to mineral structure causes structure to expand

• chemical reaction with physical results

b. susceptible soils & rocks:

• soils with 2:1 layered clays (e.g. montmorillonite)

• shale, clayey siltstones and sandstones, granite

c. result:

• expansive soils (vertisols)

• small scale - spheroidal weathering

• large scale - exfoliation
  

4. Thermal expansion

• different minerals have different coefficients of thermal expansion

• e.g. quartz is about 3 times that of feldspar
  

5. Unloading

• exfoliation fractures

• rock bursts in deep mines

C. Chemical Weathering

1. Introduction

• progression from less stable minerals to more stable minerals

  • primary minerals - secondary minerals - new secondary minerals

• water is critical

• geochemical weathering: driven by inorganic processes; produces "rotten" rocks or saprolites

• pedochemical weathering: controlled by biologic processes; leads to formation of soil from saprolites
  
  

2. Solution

a. virtually all chemical weathering involves some solution

b. most common minerals are soluble to some degree in normal waters

except:

• silica when contained in quartz

• aluminum oxides - virtually insoluble under normal conditions

• ferric iron - requires very acidic fluids
  
  

3. Oxidation & reduction

a. an element in a mineral structure loses electrons to an oxygen ion; forms oxides and hydroxides

• e.g. ferrous iron (Fe⁺²) oxidizes to ferric iron (Fe⁺³)

  4Fe⁺² + 3O₂ = 2Fe₂O₃

  iron + oxygen = iron oxide

b. most elements at earth's surface exist in an oxidized state

c. reduced form of elements are more mobile than oxidized because they're more soluble

4. Hydrolysis

a. water dissociates into H⁺ and OH^-

b. H+ displaces other cations in mineral structure

c. important mechanism for breaking apart silicate minerals

5. Ion exchange

a. substitution of ions in minerals (usually cations) by ions in solution without rearrangement of mineral structure

b. most effective in clays

• cations held to clay surface by adsorption; not held too tightly

• H⁺ and Ca⁺² are most readily adsorbed; Na⁺² is most readily released

c. cation exchange capacity: propensity for adsorbing cations

 

D. Summary

• physical and chemical weathering are separate but not independent

• water is critical

• extent of physical weathering dependent on internal strength of material and strength of external forces; growth in voids, wetting & drying, thermal expansion, unloading

• extent to which chemical weathering alters parent mineralogy depends on removal of elements from mineral structure (solution, oxidation, hydrolysis, ion exchange) and the relative mobility of constituent ions (see text)

• result: various landforms, regolith, saprolites and clays