5. Weathering processes5. Weathering processes Local geology and climateLocal geology and climate Mineral-water reactionMineral-water reaction Mechanical weatheringMechanical weathering Slope stability and unconsolidated Slope stability and unconsolidated

debrisdebris

Rocks and mineralsRocks and minerals

Climate and weatheringClimate and weathering

Weathering processesWeathering processes Weathering encompasses the Weathering encompasses the

breakdown of rock into smaller breakdown of rock into smaller components by a combination of components by a combination of physical, biological, and chemical physical, biological, and chemical processes processes

Processes of Processes of decompositiondecomposition

Chemical processes lead to release Chemical processes lead to release of compounds in solution AND of compounds in solution AND creation of new mineral productscreation of new mineral products

Chemical weathering reflects Chemical weathering reflects tendency for new minerals to be tendency for new minerals to be formed which are stable under formed which are stable under conditions prevailing at the earth’s conditions prevailing at the earth’s surfacesurface

Magnitude of chemical weathering Magnitude of chemical weathering depends on:depends on:. Composition and structure of minerals. Composition and structure of minerals. How they break apart. How they break apart. How mobile the constituents are. How mobile the constituents are

Principal reacting agents are:Principal reacting agents are:. Water. Water. Carbon dioxide. Carbon dioxide. Oxygen. Oxygen

Most common processes:Most common processes:. Oxidation. Oxidation. Hydrolysis . Hydrolysis . Carbonation. Carbonation

Mineral – water reactionsMineral – water reactions1) Mineral + water 1) Mineral + water solution solution

. Reaction in which the same phases . Reaction in which the same phases (mineral and water) are present (mineral and water) are present before, during and after the reactionbefore, during and after the reaction

. No alteration products – many . No alteration products – many minerals dissolve in rain water (e.g. minerals dissolve in rain water (e.g. calcite, dolomite)calcite, dolomite)

Mineral – water reactionsMineral – water reactions2) Mineral A + water = Mineral B + solution2) Mineral A + water = Mineral B + solution

. A new phase (mineral B appears during . A new phase (mineral B appears during the reaction. Water is here a reactant, not the reaction. Water is here a reactant, not merely a solventmerely a solvent

3) Mineral (1) + solution (A) = Mineral (2) + 3) Mineral (1) + solution (A) = Mineral (2) + solution (B)solution (B). Reaction involving ion exchange. Reaction involving ion exchange. Change in the composition of both phases. Change in the composition of both phases

Processes - oxidationProcesses - oxidation Oxidation: oxygen dissolved in soil and Oxidation: oxygen dissolved in soil and

subsurface water can oxidize mineralssubsurface water can oxidize minerals Oxidation occurs when oxygen is added in a Oxidation occurs when oxygen is added in a

chemical reactionchemical reaction Oxidation is “normal” sate of most silicate Oxidation is “normal” sate of most silicate

minerals at the surfaceminerals at the surface With oxidation, minerals converted to oxidesWith oxidation, minerals converted to oxides Metallic elements are: calcium, magnesium, Metallic elements are: calcium, magnesium,

iron.iron.

Processes - hydrolysisProcesses - hydrolysis Water reacts directly with minerals Water reacts directly with minerals

through hydrolysis (to produce a new through hydrolysis (to produce a new compound nearly always softer and compound nearly always softer and weaker than the original)weaker than the original)

Igneous rocks particularly susceptible to Igneous rocks particularly susceptible to hydrolysis because their silicate minerals hydrolysis because their silicate minerals combine readily with water.combine readily with water.

Hydrolysis invariably increases the volume Hydrolysis invariably increases the volume of the mineral (which contributes to of the mineral (which contributes to mechanical weathering) in addition to the mechanical weathering) in addition to the chemical changechemical change

Processes - carbonationProcesses - carbonation Process by which carbon dioxide in water Process by which carbon dioxide in water

reacts with carbonate rocks (such as reacts with carbonate rocks (such as limestone and dolomite) to produce a limestone and dolomite) to produce a solution – calcium bicarbonate solution – calcium bicarbonate

Calcium bicarbonate can readily be Calcium bicarbonate can readily be removed by runoff or percolation. Can removed by runoff or percolation. Can also be deposited in crystalline form if also be deposited in crystalline form if water is evaporated.water is evaporated.

Factors influencing Factors influencing chemical weatheringchemical weathering

Mobility: relative mobility of dissolved Mobility: relative mobility of dissolved constituents. Role played by amount of constituents. Role played by amount of water “leaching” weathering zone water “leaching” weathering zone

Leaching: downward movement of Leaching: downward movement of water through the weathering zone water through the weathering zone which leads to the removal of soluble which leads to the removal of soluble productsproducts

Significance of leaching: provides Significance of leaching: provides mechanisms by which constituents are mechanisms by which constituents are transferred to lower levels.transferred to lower levels.

Mechanical weathering Mechanical weathering processesprocesses

Force needed to accomplish disintegration Force needed to accomplish disintegration provided by:provided by:. . ExpansionExpansion of rocks and minerals of rocks and minerals. . GrowthGrowth of foreign substances in spaces within the of foreign substances in spaces within the

parent materialparent material Growth: Growth:

. freeze-thaw action. freeze-thaw action

. Salt wedging. Salt wedging

. Biotic activity. Biotic activity Expansion: Expansion:

. Temperature changes. Temperature changes

. Unloading. Unloading

. Hydration and swelling. Hydration and swelling

Salt wedgingSalt wedging Caused by crystallization of salts Caused by crystallization of salts

from evaporating water (dry-arid from evaporating water (dry-arid climates)climates)

Water often drawn upward in rock Water often drawn upward in rock openings by capillary actionopenings by capillary action. This water carried dissolved mineral . This water carried dissolved mineral

salts. Evaporation salts. Evaporation salts left behind as salts left behind as tiny crystalstiny crystals

With time, crystals grow and expand.With time, crystals grow and expand.

Expansion of parent Expansion of parent minerals or rocksminerals or rocks

T° changesT° changes. Change sin the volume of . Change sin the volume of mineral particles. Weakens the coherence mineral particles. Weakens the coherence of mineral grainsof mineral grains

UnloadingUnloading. Expansion of large segments of . Expansion of large segments of rock masses which occurs when pressure rock masses which occurs when pressure is released by erosion.is released by erosion.

Hydration and swellingHydration and swelling: Expansion which : Expansion which occurs when minerals are formed or when occurs when minerals are formed or when they are altered by the addition of water they are altered by the addition of water to their structureto their structure

Slope stability and Slope stability and unconsolidated debrisunconsolidated debris

Whether debris created by Whether debris created by weathering will resist erosion weathering will resist erosion depends on balance between internal depends on balance between internal resistance of materials and magnitude resistance of materials and magnitude of external forces acting on themof external forces acting on them

Slope stability: Slope stability: . Internal resistance: shear strength. Internal resistance: shear strength. External forces: shear stress. External forces: shear stress