Weathering Processes

Assignments

    1. film Earth Revealed: Weathering movie. Courtesy of Anneberg Media, URL <http://www.learner.org/resources/series78.html>.  Requires Windows media Player.  Sign in and view #15 Weathering

Suggested:

    • Winkler, Erhard, M., 1998, The Complexity of urban stone decay, Geotimes, September, p. 25-29.
    • Wampler, J. M., 1997, Mythical influences of crystallization temperature and pressure on the susceptibility of minerals to weathering: Journ. Geol. Edu., v. 45, p. 74-76.

Pidwirny: Weathering, Landforms of Weathering, Introduction to soils

FITZNER, B.& HEINRICHS, K. (2004): Photo atlas of weathering forms on stone monuments, site: Working group "Natural stones and weathering" Geological Institute, RWTH Aachen University, Aachen, Germany, URL: http://www.stone.rwth-aachen.de

Images: Weathering: weathering features tutorial

wikiglobe
NPS glossary Terms: physical weathering (disintegration) , chemical weathering (decomposition), primary and secondary joints, unloading (dilation) joints, ice wedging, salt weathering, gypsum, salt, clay mineral, solution, congruent and incongruent, hydrolysis, acid, carbonation, goldich sequence, silicate mineral, ferromagnesium and non-ferromagnesium minerals, hydration, exfoliation, spheroidal weathering, granular disintegration, grus, saprolite

Introduction

Weathering is simply the physical breakup (disintegration) and chemical breakdown (decomposition) of a rock. The two processes work in concert to weaken and tear apart the rocky foundation so it can be more easily eroded. Each processes increases the effectiveness of the other. Mechanical weathering processes, such as unloading, tectonic fracturing, and frost wedging (or ice riving) break bedrock into smaller pieces thereby increasing the surface area along which chemical reactions can occur. Chemical weathering alters hard unstable minerals into softer minerals, such as clay, capable of volume increases and adsorbing water. These alterations further add to centimeter- and millimeter-scale exfoliation and granular disintegration. So mechanical weathering prepares the the rock for chemical weathering which in turn prepares the rock for further mechanical weathering through expansion and hydration. Rock thoroughly altered by weathering is called saprolite or rottenstone. In New England and in other glaciated regions saprolite is quite rare, having been removed by glaciation. 

Erosion is the removal of weathered material.  Where denudation dominates weathering and erosion create landscape by removing the weakest rock and leaving behind the more resistant. Rock can be inherently weak because of poor internal cohesion or easily weathered mineralology. Water penetrating fractures in shattered and faulted rock forms valleys between monoliths of unaffected rock of similar composition.  Such activity, known as differential weathering and erosion, etches the rock creating an infinite variety landforms depending on its lithology, structure, and history of exposure.

 Mechanical/Physical weathering (disintegration)

Fracturing

Joints

Fracturing, which includes jointing and faulting, is often the first step in weathering. Rock fracturing prepares the rock for chemical weathering by creating pathways where water can penetrate, and by increasing the total surface area exposed to the elements. Brittle rock will joint when stresses, either internal or external, exceed the strength of the rock.  External stresses could be tectonic in nature.  Internal stresses are typically related to expansion or contraction driven by thermal or chemical changes, or removal of confining rock.   Joints may also develop along some inherent weakness in a rock such as a bedding, cleavage, or lineation.

old man joints

Types of joints

1. Primary joints: These are joints developed along inherent planes of weakness created during the formation of the rock.

  • Examples: columnar joints and other cooling joints, bedding plane joints, and joints formed along metamorphic foliation or igneous lineation.

2. Secondary joints: Joints created by forces acting on the rock after it's formation.

  • tectonic fractures: Faults and joints (e.g. conjugate shear fractures, tension fractures, pressure solution bands, etc.)
  • unloading (dilation) joints (e.g. sheeting, exfoliation joints, etc.)
Figure 1.   New Hampshire's symbolic Old Man In The Mountain surrendered to weathering and disintegrated in a giant rock fall. In 2001 the profile, located along a steep glaciated valley wall, collapsed in response to frost wedging and chemical weathering along horizontal and vertical dilation joints that weakened the face. Modified from Russell and Demarco,2003, N. H. says "farewell, Old Man" Boston Globe Online.

Figure 2. Types of joints. Images from left to right: Columnar jointed basalt, Devils post pile, CA (source:  ngdc.noaa.gov), tectonic joints in Salem Gabbro-Diorite, Salem, MA; Joints in vertically cleaved slate of the Carrabassett FM, central ME, horizontal bedding-plane joints and vertical dilation (expansion) joints in Wingate Sandstone, Capitol Reef, UT. Which of these are primary and which are secondary?

Organic activity (growth of roots and rootlets, bioturbation, hominidation, etc)

Burrowing and drilling organisms, plants (flg. 3), and people are also responsible for mechanical weathering.  Man is probably the most destructive considering the millions of tons/year blasted for mining, road building, and construction.  Tailings piles from metal and coal mining are leached by rainwater releasing large quantities of chemically active sulfuric acid into the natural system. It took the Colorado River at least 6 million years to create the Grand Canyon and it took man less than 100 years to create the Bigham Canyon Open Pit Copper Mine  and to destroy the rigelines of Appalachia. We are afterall, part of the biosphere.

Ice Wedging (frost wedging, ice riving, frost shattering--it's all the same)

Ice is a forceful agent of weathering in temperate, alpine, and periglacial climates.  Water expands 9° upon freezing exerting a pressure of 2,100 kg/cm2 at -22°C.  This force far exceeds the tensile strength of rock, which is typically less than 250 kg/cm2 (Prick, 2004).  In addition, because water is incompressible, trap water in a closed system is forced into the rock, hydrofracturing the rock at joint tips and grain boundaries.  

 

Plant Wedging

Figure 3. Plant wedging of asphalt driveway.

Thermal expansion and contraction

Observations of shattered rock in deserts has lead to a long standing hypothesis that rock can be shattered in response to daily cycles of heating and cooling.   However, laboratory experiments have been inconclusive.  Nevertheless differential expansion of mineral grains may create enough internal stress to disaggregate certain rocks.  There is no doubt however, that rock can be shattered by the intense heat produced by forest fires (fig. 4).

fire shattered Figure 4.  Thermal shattering of sandstone by forest fire. Here's an unequivocal example of thermal weathering. Matagamon Sandstone, near Harrington Lake, north-central Maine. Click image to enlarge.

Disintegration induced by chemical processes

Salt Weathering

Precipitation of salts (CaSO4.2H2O, Na2SO4, Na2CO3.H2O, NaCl, KCl, NaF, KF)

Elements from rocks, water, and the atmosphere combine to produce soluble minerals that crystallize in rock pores, along grain boundaries, and in fine fractures, ultimately causing disintegration.  This process aids in the weathering of rocks in coastal regions where salt is provided by seaspray (fig. 5).  Salt weathering also attacks urban monuments and buildings.   Sulfur dioxide released by the burning of fossil fuels dissolves in rainwater to produce dilute sulfuric acid.  The acid attacks limestone and masonry and combines with calcium to produce gypsum, hydrous calcium sulfate.  The gypsum in turn crystallizes in cracks and pours causing the rock to mechanically peel and flake.  

Salt weathering by F. J. P. M. Kwaad

salt weathering

Figure 5. Salt weathering along the shores of Misery Island, MA.

Hydration

Hydration is a chemical reaction entailing the adsorption of water by certain mineral. The resulting mineral-grain expansion aids in the disintegration of rocks.  Residual minerals and precipitates such as limonite, clay, and anhydrite-gypsum, adsorb water and expand.   Weathered biotite expands up to 40%, strongly contributing to the granular disitegration of granite and other phaneritic rocks;  the granular debris of detached grains that litters these outcrops is called grus.   Salt will also hydrate and aid in disintegration as evidenced by the slow destruction of Cleopatra's Needle in New York's Central Park.  The obelisk, which was donated in 1881, had previously rested for 3000 years in Egyptian alluvial unaffected by weathering.  However when the salts that had permeated the stone were exposed to the humid New York climate they hydrated precipitating the rapid disintegration of the monument's surface hieroglyphics.

Several times I've tried to gather shale specimen for lab only to have them crumble in my hands. In humid, temperate, and even arid climates shale readily disintegrates into soft flacky aggregates.  This shale slacking is caused by expansion and contraction of clay minerals (e.g. smectite/montmorillinite) by repeated wetting and drying, resulting in the addition of aligned sheets of H20 that exerts internal pressure and break apart the rock. 

Abrasion by geologic agents:

Abrasion is both weathering and erosion at the same time. That which is abraded is removed in the process. Abrasion, which occurs sporatically depending on the agent, exposes fresh material for abrasion and chemical weathering. Abrasion is accomplished by debris carried by some geologic agent:

  • ice: primarily as glaciers
  • wind
  • Water: waves, streams, overland flow, raindrops etc.
  • Mud, debris flows, etc.

 Rates of abrasion of depend on:

  • hardness and strength of abrasive relative to the rock being abraded
  • the agent carrying the abrasive (e.g. wind, water, glacier, etc.)

Chemical weathering (decomposition)

Processes of Chemical weathering

Chemical weathering breaks down minerals into different chemical components.   Rocks, such as rock salt and limestone dissolve entirely, this is known as congruent solutionIncongruent solution occurs when some components are carried off as dissolved ions and a residuum is left behind to form a new mineral.  Some common chemical weathering processes are listed below.

1. Simple solution: accomplished through the polarity of water molecules

2. Hydrolysis: decomposition caused by reaction with H+ and OH- in water

  •  Example:
    • Orthoclase to kaolinite
        2KAlSi3O8 (orthoclase)+ 2H+ + 9 H2O >
        Al2Si2O5(OH)4(kaolinite) + 4H4SiO4 (silicic acid)+ 2 K+
      •  Orthoclase to illite
      • 3KAlSi3O8 (orthoclase) + 2H+ + 12 H2O >
        KAl3Si3O10(OH)2 (illite)+ 6H4SiO4 (silicic acid)+ 2 K+

  Carbonation: decomposition caused by reaction with carbonic acid

      CO2 + H2O > H2CO3 carbonic acid
       H2CO3 + CaCO3 = Ca++ + 2HCO3- (calcium and bicarbonate ions in solution)
    •  Factors that would drive the reaction to the right (dissolve carbonate)
      • decrease in pH (pH<7 is acid) pH=-log10[H+]
      • increase in CO2
      • removal of Ca++
    •  sources of CO2
      • atmosphere .03%
      • humus layer in soil
      • burning of fossil fuels

  Reactions involving other acids

  • organic acid (humic, acetic, etc.)
  • sulfuric acid H2SO4 (byproduct from the oxidation of pyrite)
  • nitric acid HNO3
  • nitrous acid HNO2

3. Oxidation: Decomposition caused by the stripping of electrons from mineral cations is called oxidation. As the name suggests, oxygen is the most common oxidizing agent. Ferrous (Fe++) iron in the ferromagnesium silicates is particularly susceptible.  Oxidation produces ferric (Fe+++) iron and disrupts the mineral structure. An example is given below in the weathering of pyroxene.

4FeSiO3 + O2 + 2H2O > 4FeO(OH) (hydrated iron oxide) and 4SiO2(dissolved silica)

4. Chelation: complex plant-induced hydrolysis combined with chelating compounds that capture and bind cations. This is accomplished by organic acids.

Factors that influence rates of chemical weathering

(controlled by mineral stability, temperature, water, removal of solute)

Characteristics of the parent material

1. Mineralogy

  • Stability of silicate minerals: Silicate minerals are composed of (SiO4) tetrahedra linked directly to each other by the share of oxygen or through the binding ability of an intermediary cation.  A fundamental principle of silicate weathering  is that the Si-O covalent bond is stronger than ionic bond between the (SiO4) and the cation. Therefore:
    • stability increases as tetrahedra become more directly linked orthosilicates (least stable) >> sorosilicates >> cyclosilicates >> inosilicates >> phyllosilicates >> tectosilicates (most stable)
    •  within a structural group stability decrease with increased isomorphous substitution this is one reason why Ca-plagioclase is relatively unstable even though it is a tectosilicate.
      • Plagioclases series:CaAl2Si2O8 >>> NaAlSi3O8
    • also, within a structural group stability decreases with decreasing electronegativity of the metal ions (e.g. wollastonite CaSiO3 >>> FeSiO3 hypersthene )
    • The unsual stability of zircon ZrSiO4 (an orthosilicate) is due to a unique metal ion-oxygen bond that is uniquely strong

differential weathering

Least stable

Olivine and Ca plagioclase feldspar
Pyroxene
Amphibole
Biotite
Na plagioclase feldspar
Potassium feldspar
Muscovite
Quartz
zircon

Most stable

Figure 6. The differential weathering of these mafic and felsic intrusives reflects the relative stability of the minerals composing them. The felsic rock which stands out in high relief above the dark mafic rock is clearly more resistant. This relationship is illustrated in the Goldich Weathering Series on the right.

      •  Non-silicate minerals
        • minerals that are soluble (e.g. chemical precipitates; halite, carbonate, gypsum, etc.) are generally unstable --except in arid climates
        • minerals formed as the residual by-products of weathering are very stable (e.g. oxides, clay minerals, various hydroxides, etc) but are typically soft and susceptible to erosion

    2. Texture

    • In a sediment texture controls the porosity and permeability--solution pathways
    • In a rock texture controls internal integrity as well as porosity and permeability.
      •  degree of fragmentation and inherent structural weaknesses (e.g. fracturing, foliation, fissility, etc)

 Climate: precipitation and temperature

  • Temperature: Rates of reaction double for each 10°C rise in T
  • Water: Universal solvent and catalyst for most reactions
  • organic activity: Organic activity is a variable that is dependent on climate.   Plants increase chemical decomposition by producing acids and chelating compounds.

Additional notes and comments:

Why is water an effective agent of chemical weathering?

  • Water is a polar molecule, which
    • moves by capillarity; can get drawn into rocks
    • forms weak bonds with unsatisfied ions
  • releases H+ which are small and capable of penetrating a mineral structure
  • carriers chemically active ions that attach rock
  • its abundant

The removal is ions (K+, Ca++, NA+, Fe++, etc.) from minerals by dissolution is called leaching.  Factors that influence leaching include:

  1. supply of fresh water : A continuous supply reduced the chances that the solubility equilibrium will be reach, reaction is continuous). This factor is dependent on climate and permeability.
  2. Eh (redox potential, oxidation-reduction) and pH (acidity , H+ concentration)
  3. presence of organic acids and chelating compounds increases leaching

 Such reactions continue until

  1. free H+ are no longer available
  2. replacement cations are no longer present
  3. the solvent has reach saturation with respect to ions being liberated

 *Topography

  • variable topography increases over-all surface area
  • high relief increases potential energy and the rate at which weathered rock is removed
  • topography influences drainage: rates of infiltration and saturation, position of water table, etc.

 *Time:

  • If the weathered product is not removed, rates of weathering generally decrease with length of exposure and development of a weathered zone  

Styles of disintegration

The manner is which a rock breaks up is its style of disintegration.  Some rock crumble, flake, peak or just separate into rubbly blocks. Style typically reflects lithology and weathering processes.

  • block separation
  • Shattering
  • exfoliation: The peeling of rock in concentric layers
  • flaking/slacking (e.g. shales)
  • Granular disintegration (e.g. phaneritic igneous rocks)
  • Spheroidal weathering (e.g. aphanitic igneous rocks and some sandstone)
Figure 6.  Block separation along horizontal and vertical unloading joints on Mount Katahdin, Maine.

 

exfoliation exfoliation
Figure 7. Types of exfoliation.  On the left, chemical weathering of mafic rock produces iron oxides and clay minerals that take up more volume and expand when hydrated. Expansion causes the outer weathered layers to peel off.  On the right, is an exfoliation in the Adirondacks dome formed by the formation of curved joints.
aqueduct
Figure 8.  Granular disintegration of  the Moxie Gabbro, central Maine.  The knobby weathering is related to the gabbro's unique texture. Figure 9.  Spheroidal weathering of the Roman aqueduct in Segovia, Spain.  Rounding of the corners represents approximately 2000 years of weathering.

Examples of weathering: slides from Duke University

Soils

Soil is the by-product of weathering that forms as a thin cover over the surface of the earth.  Soil provides the nutrients required for living organism, controls the fate of precipitation, and can reflect the maturity of a landscape.  It is a non-renewable resource that can affect the fate of an economy. In a general sense, soil is the loose mixture of mineral and organic material that overlies bedrock. It can be formed in situ (residual soil) or dumped by some geologic agent (transported soil) such as wind, water  or ice.  Nearly all soils in New England and the upper Midwest are transported, deposited directly by glacial ice (till), meltwater, or strong winds relocating sediment on barren post-glacial landscapes.  Geomorphologists study soils to prevent their destabilization by natural and man-induced processes, and to learn about landscapes and surface processes.

Chemical and physical processes of weathering, eluviation and illuviation, and mineral precipitation create a series of distinct zones in the sediment called soil horizons. Each horizon can be distinguished   by its color, mineral and organic composition and particle size. Many would argue that without at least two or three of these horizons a sediment would not be considered a true soil--but just sediment.  The degree of soil development and character of its layers depends on the parent material (regolith or rock), climate, topography and time.

Go to these sites to learn more:

Questions
  1. Review 28 in Bloom. Explain why water is the most important agent of weathering on Earth. Discuss the role of water in both chemical and mechanical weathering. Give examples.
  2. How does mechanical weathering prepare the rock for chemical weathering?
  3. What is the effect of temperature on weathering?
  4. Explain why limestone forms cliffs in arid climates and valleys in humid climates?
  5. Weathering processes are typically described as either physical or chemical, however the distinction between these two categories is often vague.  List and describe two weathering phenomena that involve both physical and chemical processes.
  6. Explain why the chemical weathering of metamorphic and igneous minerals aids in the disintegration of rocks?
  7. Identify and explain the reactions that turns ferromagnesium minerals orange and feldspars chalky white.
  8. Explain why water flowing over and through rock is a more effective agent of chemical weathering than water that is trapped in rock. 
  9. In a single brief statement define exfoliation. Explain how exfoliation is accomplished by both chemical and mechanical processes.
  10. Explain how solid rock becomes weathered into spheroidal masses.  How does the mechanism of spheroidal weathering of a basalt differ from that of a granite?
  11. Reorder the following minerals from most stable to least stable: biotite, olivine, pyroxene, quartz, calcium-plagioclase, orthoclase, mica.
  12. Explain why rugged mountains weather and erode faster than low hills. List and discuss all the variables that you think apply.
  13. Explain why limestone and marble buildings in urbanized areas weather faster than their rural counterparts.
  14. List and give examples of the three products of weathering. How does each participate in the rock cycle?

 

Bibliography

Bloom, Arthur. 2004, Geomorphology, A systematic analysis of Late Cenozoic Landforms, (4th edition): Waveland Press Inc., Longe Grove , IL 482 p.

Chorley, R. J., Schumm, S. A., Sugden, D. E., 1984, Geomorphology: Methuen and Co. Ltd., London, 605 p.

Easterbrook, Donald J., 1993, Surface Processes and Landforms: Macmillan Pub. Co., 520 p.

*Goldich, S., 1938, A study of rock weathering. Jour. Geology 46:17-58.

Hart, M. G., 1986, Geomorphology pure and applied: George Allen And Unwin, Boston MA, 227 p.

Jones, N. W., 1998, Laboratory manual for physical geology: WCB McGraw-Hill, Boston MA, 303 p.

*Ollier, C. D., 1975, Weathering: Longman, London, 304 p.

Prick, Angelique, 2004, Frost and frost weathering: in Encyclopedia of Geomorphology, A. S. Goulde (ed), Roultledge, New York, NY, p. 412-414.

Ritter, D. F., Kochel, C. R., and Miller, J. R., Process Geomorphology (3rd Edition): Wm. C. Brown Publishers, Dubuque, IA, 544 p.

Summerfield, M. A., 1991, Global Geomorphology. John Wiley and Sons, New York, NY, 536 p.

*Wampler, J. M., 1997, Mythical influences of crystallization temperature and pressure on the susceptibility of minerals to weathering: Journ. Geol. Edu., v. 45, p. 74-76.

Winkler, Erhard, M., 1998, The Complexity of urban stone decay, Geotimes, September, p. 25-29.
 

Lindley Hanson/Department of Geological Sciences/Salem State College/Geomorphology/GeoIndex/QkRef