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WEATHERING PROCESSES

A. Introduction

Weathering important because:

weakens resisting forces; makes landscape more susceptible to erosive forces

produces unique landforms

produce regolith (weathering mantle), which may evolve to soil

Weathering: in-situ breakdown of material

chemical weathering

physical weathering

resistance to weathering function of:

internal resistance of material

magnitude of external forces

Learning outcomes: you should be able to:

list different types of physical and chemical weathering;

explain how these weathering processes weaken the resisting framework;

indicate the landscapes or environmental conditions under which the different types ofweathering processes are most likely to occur; and,

describe the characteristics and formation processes of various weathering landforms andidentify examples of these landforms.

B. Physical Weathering

Frost action

(Talus cones , Banff National Park, Alberta. Photo: Marli Miller, University of Oregon.http://marlimillerphoto.com/talus.html)

9% volumetric expansion up freezing

effective only in closed voids that are almost entirelysaturated

effective when temperatures oscillate above and belowfreezing point

ice segregation

frozen water in voids generates a suction force pulling liquid water toward the ice

migrating water generates pressure forces sufficient to enlarge cracks

primary cause of frost heave in soils

only more recently accepted as mechanism working in solid rock

most effective in temperature range -3 to -8C

Landforms: talus, talus cones, scree slopes

Salt weathering

salt crystallization: occurs as saline solutions evaporate

salt crystal expansion: occurs when salt crystals get wet

occurrence:

hot and cold arid and semi-arid environments

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capillary rise brings saline water toward surface

limited liquid water (either due to supply or phase)incapable of washing salts away

hot arid regions: large diurnal changes in temperature andrelative humidity promote repeated wetting and drying

cold regions: cold temperatures encourage saltprecipitation from solutions

rocky coastal areas

rock susceptibility to salt weathering

proportion of micropores

water absorption capacity

surface texture

presence of clay minerals

landforms

tafoni

honeycomb weathering

granular disintegration

spalling

Honeycomb weathering in greywacke sandstone, Golden Gate i lRecreation Area. Photo: National P ark Service,http://www.nps.gov/goga/forteachers/graywacke-sands tone-fa

Photo: K. Segerstrom, USGS Photographic Library image sk00http://libraryphoto.cr.usgs.gov/

Photo: R.C. Moore, USGS Photographic Library, Image mrc000http://libraryphoto.cr.usgs.gov/

Wetting & drying

susceptible soils & rocks:

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

shale, clayey siltstones and sandstones, granite

result: spalling, granular disintegration

Photo: Ma rli Miller, University of Oregion. Earth Science World Ima geBank, photo hhrhuz, http://www.earthscienceworld.org/

Thermal expansion and contraction

different minerals have different coefficients of thermalexpansion

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

effectiveness of insolation debated

result: spalling

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Photo: J.R. Stacey, USGS Photographic Library, Image hcb00980.http://libraryphoto.cr.usgs.gov/

Pressure release

sheet joints and exfoliation

rock bursts in deep mines

Photo: F.E. Matthes, USGS Photographic Library, Image mfe00007.http://libraryphoto.cr.usgs.gov/

Half Dome, Yosemite. Photo: F.E. Matthes, USGS Photographic Library, Image mfe00001.http://libraryphoto.cr.usgs.gov/

Sheet joints, Yosem ite National Park. Pho to: N.K. Huber, USGSPhotographic Library, Image hnk00031. http://libraryphoto.cr.usgs.go

C. Chemical Weathering

Introduction

progression from less stable minerals to more stable minerals

primary mineral stability

progression: primary minerals to secondary minerals to new secondary minerals

secondary minerals: clay minerals

formed primarily by recombination of silica, alumina and metal cations released duringweathering

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2:1 layered clays: smectite (montmorillonite), illite, vermiculite, chlorite

1:1 layered clays (kaolinite)

progression: primary minerals to 2:1 layered clays to 1:1 layered clays to hydrous oxides ofiron and aluminum

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 fromsaprolites

Solution

virtually all chemical weathering involves some solution

solution of calcite (CaCO3) and halite (NaCl)

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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

result: granular disintegration, spheroidal weathering, weathering pits, karst

(Photo: N.K. Huber, USGS Photographic Library, Image hnkb0004. http://libraryphoto.cr.usgs.gov/

Hydrolysis

water dissociates into H+(hydrogen cation) and OH-(hydroxyl anion)

H+ displaces other cations in mineral structure

K+, Na+, Ca2+, Mg2+

may combine with hydroxly anion or be carried away in solution

hydrolysis promoted by:

decreasing pH (increasing H+)

decomposition of organic matter (releases H+)

increased water temperatures (promotes dissociation)

important mechanism for breaking apart primary minerals

example: albite weathers to kaolinite plus some residual silica

albite + water = kaolinite + silica + sodium ion + hydroxyl ion

4NaAlSi3O8+ 6H2O = Al4Si4O10(OH)8+ 8SiO2+ 4Na++ 4OH-

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result: spalling, weathering pits, spheroidal weathering, weathering rinds, production of claymineral

fine grained mafic igneous rock with orange-brown, iron rich weathering rind(Photo: USGS http://pubs.usgs.gov/of/2002/of02-437/gallery.htm )

Oxidation/reduction

oxidation

element in a mineral structure loses electrons increasing their charge

reaction between ions and oxygen results in formation of:

oxides: compounds of metals + oxide ions, O2-

hydroxides: compounds of metals and hydroxide ions (OH-)

occurs above water table

examples:

ferrous iron (Fe+2) oxidizes to ferric iron (Fe+3)

4Fe+2+ 3O2= 2Fe2O3

iron + oxygen = iron oxide (hematite)

olivine weathers through combination of hydrolysis and oxidation to form hematite

olivine + water + oxygen = hematite + silicic acid

2Fe2SiO4+ 4H2O + O2= 2Fe2O3+ 2H4SiO4

reducing agents: react to form cations (oxidize); Fe2+>Al3+>Mg2+>Na+> Ca2+>K+

iron is most commonly oxidized material

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

reduction: opposite reaction

occurs below water table

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

result: weathering rinds

Cation exchange

exchange of ions in minerals (usually cations) with ions in solution

colloids: particles with diameter

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common exchangeable cations: H+, K+, Ca2+, Mg2+, Na+, Al3+

result: production of new, more stable secondary (clay) minerals

D. Patterns To Weathering

Model based on global air temperature patterns and

global precipitation patterns

Implication: air temperature affects type of weathering

Reality:

temperature affects rate, but not type of

weathering

evidence for chemical weathering in hot and colddeserts

air temperature not reflective of ground surfacetemperature

ground surface temperature affected by airtemperature, insolation, albedo, and thermalconductivity

Implication: precipitation affects amount of weathering (more weathering in wetter climates)

Reality:

precipitation isn't only one source of water for weathering

constant flow of soil water and groundwater brings fresh influx of reactants and removessoluble elements

abundant groundwater flow may allow significant chemical weathering in dry climates

sufficient soil moisture in some deserts to allow chemical weathering

Amount or rate of weathering

traditional model

high amounts & thick regolith in humid tropical climates

small amounts & very thin regolith in semi-arid and arid climates

moderate amounts & moderate regolith in humid mid-high latitude climates

small amounts and thin regolith in periglacial environments

reality

climate change impacts reolith thickness; e.g. thick regolith in dry regions of Australia

erosion impacts regolith thickness;

erosion rate affected by topography and vegetation cover

thin regolith in arid environments may be result of high erosion rates, which mask theactual weathering rate

E. Summary

physical and chemical weathering are separate but not independent

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water is critical

physical weathering: frost action, salt weathering, thermal expansion, wetting and drying, pressurerelease

weathering proceeds from less stable to more stable minerals; primary minerals to secondary minerals tomore weathered secondary minerals

chemical weathering: solution, hydrolysis, oxidation/reduction, ion exchange

traditional model relates weathering type and amount to global climate patterns, but reality more complex

surface temperatures also affected by insolation, albedo, and thermal conductivity

regional and local soil and groundwater flow provide other sources of water

evidence of weathering removed by erosion

results of weathering

weakening of resisting framework; increased susceptibility to erosive forces

landforms: talus, honeycombs, tafoni, granular disintegration, spalling, sheet joints, exfoliation,spheroidal weathering, karst, weathering rinds

production of clay minerals and development of soils

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Karen A. Lemke: klemke@uwsp.edu

Last revised October 16, 2013