Revealed: Weathering movie. Courtesy of Anneberg
Media, URL <http://www.learner.org/resources/series78.html>. Requires
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- 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
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
||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,
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,
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
Rock thoroughly altered by weathering is called saprolite
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,
Mechanical/Physical weathering (disintegration)
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.
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,
- 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
Organic activity (growth
of roots and rootlets, bioturbation, hominidation, etc)
Burrowing and drilling organisms, plants
(flg. 3), and people are also responsible for mechanical
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
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.
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).
||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.
by chemical processes
Precipitation of salts (CaSO4.2H2O,
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.
weathering by F. J. P. M. Kwaad
Figure 5. Salt weathering along the shores
of Misery Island, MA.
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
- 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
- the agent carrying the abrasive (e.g. wind, water, glacier,
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
2. Hydrolysis: decomposition caused by reaction with H+ and
OH- in water
- 2KAlSi3O8 (orthoclase)+ 2H+ +
9 H2O >
+ 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
- CO2 + H2O > H2CO3 carbonic
- H2CO3 + CaCO3 =
Ca++ + 2HCO3- (calcium and bicarbonate
ions in solution)
- Factors that would drive the reaction to the right (dissolve
- 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.
(controlled by mineral stability, temperature, water, removal
Characteristics of the parent material
- 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
- 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
Olivine and Ca plagioclase feldspar
Na plagioclase feldspar
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
- In a sediment texture controls the porosity and permeability--solution
- 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,
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
Additional notes and comments:
Why is water an effective agent of
- 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:
- 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
- Eh (redox potential, oxidation-reduction) and pH (acidity
, H+ concentration)
- presence of organic acids and chelating compounds increases
Such reactions continue until
- free H+ are no longer available
- replacement cations are no longer present
- the solvent has reach saturation with respect to ions
- 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.
- 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
- 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
|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
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:
- 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.
- How does mechanical weathering prepare the rock for chemical
- What is the effect of temperature on weathering?
- Explain why limestone forms cliffs in arid climates and
valleys in humid climates?
- 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
- Explain why the chemical weathering
of metamorphic and igneous minerals aids in the disintegration
- Identify and explain the reactions
that turns ferromagnesium minerals orange and feldspars
- Explain why water flowing over and through
rock is a more effective agent of chemical weathering than
water that is trapped in rock.
- In a single brief statement define exfoliation. Explain
how exfoliation is accomplished by both chemical
and mechanical processes.
- 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?
- Reorder the following minerals from
most stable to least stable: biotite, olivine, pyroxene,
quartz, calcium-plagioclase, orthoclase, mica.
- Explain why rugged mountains weather
and erode faster than low hills. List and discuss all the
variables that you think apply.
- Explain why limestone and marble buildings
in urbanized areas weather faster than their rural counterparts.
- List and give examples of the three
products of weathering. How does each participate in the
of Geological Sciences/Salem
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
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,
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.