Weathering Processes

Assignments

1. 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
2. Text: Bloom, Arthur, 2004, Geomorphology: Geomorphology: Chapter 7 - Rock 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.

Online Resources

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

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

Introduction

Weathering is simply the physical breakup and chemical breakdown of a rock. The two processes work in concert to weaken and tear apart rock so it can be more easily eroded. Each processes increases the effectiveness of the other. Mechanical weathering processes, such as unloading, tectonic fracturing and ice wedging breakup bedrock into smaller blocks thereby increasing the surface area along which chemical reactions can occur. Chemical weathering alters hard unstable minerals into softer minerals 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 that has been thoroughly altered by weathering is called saprolite. In New England and in other glaciated regions saprolite is quite rare, having been removed by glaciation.

 Mechanical/Physical weathering (disintegration)

Fracturing Joints Fracturing, or jointing, is often the first step in weathering. Rock fracturing prepares the rock for chemical weathering by offering conduits through which 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 caused by thermal changes or  removal of surrounding rock.   Commonly jointing occurs along some inherent weakness in a rock (primary 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. 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 surrenders to weathering and rock fall. In 2001 the profile collapsed in response to frost wedging and chemical weathering along horizontal and vertical dilation joints that weakened the face. Modified from Russel 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.

Organic activity (growth of roots and rootlets, bioturbation, hominidation, etc) Burrowing and drilling organisms, plants, and people are also responsible for mechanical weathering.  Man is probably the most destructive considering the millions of tons/year blasted for mining 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. Ice Wedging (frost wedging, ice riving, frost shattering) 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 could be forced into the rock hydrofracture the rock at joint tips and grain boundaries.

Figure 3 . Plant wedging of macadam driveway.

Thermal expansion and contraction

Observations of shattered rock in deserts has lead to the 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).

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 sea spray (fig. 4).  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 flack.   Salt weathering by F. J. P. M. Kwaad

Figure 4. Salt weathering along the shores of Misery Island, MA.

Hydration: A chemical reaction entailing the adsorption of water that 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 disintegration of granite.   Shale slacking is facilitated 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.

Abrasion by geologic agents:

Agents of abrasion are also responsible for erosion which exposes fresh material for abrasion and chemical weathering. Abrasion is accomplished by the debris carried by each agent:

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

 Relative rates of abrasion of minerals depend on:

• hardness, tenacity, and cleavage of mineral
• abrasion environment (e.g. wind vs. water)

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 solution.  Incongruent 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.

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 )
  • Some minerals (e.g. zircon ZrSiO₄, an orthosilicate) have unique metal ion-oxygen bonds that are particularly strong

•  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.

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.

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:

• Flash animation of the development of soil horizons (NC State Soil Science)
• Nasa Soil Science Education Site > Soil Forming Factors
• USDS NRSC soil site > the Twelve Orders  (brief synopsis)
• University of Idaho > Soil Taxonomy > Twelve Soil Orders (detailed with maps of distribution)

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 Geomorpphology, 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 suceptibility 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.