chemical and physical weathering of rocks

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Nauki ścisłe priorytetem społeczeństwa opartego na wiedzy
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Marta Miedźwiedziew
CHEMICAL AND PHYSICAL WEATHERING OF ROCKS AND MINERALS
Introduction
The article is devoted to students of secondary schools participating in chemistry classes both at the
basic and extended levels. The issues presented in the article may be treated as extension of knowledge
concerning materials of natural origins – rocks and minerals – and their properties.
All the rocks form earth crust; they are subject to
external influence of the atmosphere and hydrosphere.
The whole of the changes which rocks and their
ingredients (minerals) undergo, exposed to the
atmosphere, hydrosphere and biosphere is called
weathering. It occurs on the surface of the crust and
in the so called weathering sphere. Depending on
geological structure, climate, the depth of groundwater
1. Alabama Hills [www.geotripperimages.com]
and the presence of living organisms, the depth of
weathering sphere ranges from a few to several meters.
Changes which are experienced by rocks can have the character of physical processes
(physical or mechanical weathering) whose effects include spallation and disintegration of rocks
without the change of their chemical composition or the character of chemical processes (chemical
weathering) which result in disintegration of rocks, the chemical composition of mineral
substances changes.
Biosphere plays an important role both in physical weathering, by bursting slots through roots
and the activity of burrowing animals, as well as in chemical weathering, thanks to produced by
plants juice and combinations created as a result of dead organic matter disintegration.
Physical weathering
The crucial factor of physical weathering is insolation (heating by solar rays) and temperature
fluctuations, especially those connected with daily cycle, often quite rapid. Depending on the
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structure of a rock and minerals’ properties, weathering caused by temperature can proceed in three
different ways.
In the case of minerals of homogenous composition,
conducting heat poorly, the surface layer is heated quickly
and as a result of varied extension of the sunlit part and the
inner part of the rock, cracking and peeling occur
(exfoliation) [fig. 2]. If a rock consists of various minerals
(e.g. granite) with different physical properties, the
disintegration occurs as a result of uneven expansion and
contraction of adjacent grains (crystals) and loosening of
2. Efekty eksfoliacji [www.geology.about.com]
connection between them. Then, we deal with granular
disintegration.
Due to rocks expansion and contraction it is possible for invisible before multidirectional
cracking to become visible, dividing blocks of rocks into parts (so called block disintegration). An
important factor, especially in the mountains and polar areas, is frost causing the increase of ice
crystals in cracks and pores of rocks (similar effects are caused also by the increase of other
crystals, e.g. of salt). Disintegration action of freezing water in the rock is called frost action.
An important role in physical weathering is played by biosphere. Plants roots may penetrate
crevices and while growing they can expend them, or even split rock solids, therefore, facilitating
the access of water and air. The similar influence have burrowing animals (such as earthworms,
moles), drilling canals in the top layers of the earth and revealing deeper lying rocks.
A certain contribution in the processes of physical weathering has also varied expansion of
rocks caused by lowering pressure after uncovering the surface.
Chemical weathering
In the process of chemical weathering the crucial role is played by water. Regardless of
whether the rock is in contact with surface waters, humidity in atmosphere, or soil water (soil
solution), it is in the water and with the contribution of dissolved in water organic and inorganic
substances chemical reactions occur.
The action of water and contained in it ions is based on dissolution, oxidation and reduction.
Oxidation reaches in the earth crust so deeply as deeply can reach oxygen, so up to the
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groundwater levels. Due to this fact, the area above the groundwater levels is called oxidation
sphere.
During the oxidation of silicates containing divalent iron oxides (FeO) trivalent iron oxides are
created (Fe2O3), usually hydrated. As a result, the colour of these minerals changes from green or
black into red or yellow-brown. A magnetite (Fe2O3  FeO) of metal black colour which changes
into a red hematite (Fe2O3) may serve as an example. Iron salts oxidize very slowly in acidic
solutions, whereas in solutions of a weak acid or alkaline they oxidize very quickly.
Sulphides also easily undergo oxidation, such as pyrite (FeS2) in oxidation process transforms
into furrous sulphate:
FeS2 + H2O + 7/2 O2  FeSO4 + H2SO4
and gallium (PbS) into anglesite (PbSO4).
Hydration is based on attaching water molecules. An example of a simple hydration is the
transformation of anhydrite (CaSO4) into anhydrite (CaSO4  2H2O)
CaSO4 + 2 H2O  CaSO4  2 H2O
hematite (Fe2O3) into yellow-brown limonite (2Fe2O3  3H2O) or braunite (Mn2O3) into manganite
(Mn2O3  H2O).
Hydration usually is links with hydrolysis and the action of oxide and carbonic acid. For
instance, from siderite through hydrolysis and oxidation a limonite is created:
FeCO3 + H2O = Fe(OH)2 + CO2 4Fe(OH)2 + O2 = 2Fe2O3  3H2O + H2O
Another example of processes accompanying hydration is silicate alteration by water
(hydrolitic disintegration) and carbon dioxide. Then, silty minerals are created, among them
kaolinite (H2Al2Si5O8); the process of its creation is called kaolinization. The kaolinization is
experienced first and foremost by feldspars, e.g. orthoclases:
K2Al2Si6O16 + 2 H2O + CO2 = H2Al2Si5O8  H2O + K2CO3 + 4 SiO2
Reduction processes are caused mainly by two factors: distribution of organic matter and
bacteria activity. Disintegrating organic matter is linked not only with free oxygen, but also with
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oxygen contained in chemical solutions. Some bacteria have ability to split off the oxygen which
they need to live from organic and inorganic connections.
Reduction of iron compounds by organic matter may be demonstrated by following equations:
2Fe2O3 • 3H2O + C = 4FeO + CO2 + 3H2O
FeO + CO2 = FeCO3
Sulfates are reduced by organic matter or bacteria and, in some cases, by hydrocarbons. An
example of sulfates distribution is gypsum distribution leading to limestone and and sulfur creation:
CaSO4 + 2C = CaS + 2CO2 2CaS + 2H2O = Ca(OH)2 + Ca(SH)2 Ca(OH)2 + Ca(SH)2 + 2CO2 =
= 2CaCO3 + 2H2S H2S + O = H2O + S
The next important for chemical weathering process is carbonatization, i.e. changes occurring
due to dissolved in water carbon dioxide. Limestone, comprised mainly of calcium carbonate,
dissolve poorly in pure water. However, their dissolution increases ten times if the water is
saturated with carbon dioxide. Calcium carbonate changes then into more soluble calcium
bicarbonate:
CaCO3 + H2O + CO2  Ca2+ + 2 HCO3–
This reaction is reversible.
Limestone dissolves also easily under the influence of organic acids, e.g. acetic:
2CaCO3 + 2C2H4O2 = Ca(C2H3O2)2 + CaH2(CO3)2
The weathering products and characteristic weathering forms
The product of physical and chemical weathering is eluvium cover created on rocks.
Weathering products might remain in one place if a horizontal surface is the subject of weathering
or the products might also be transported by gravity along the slope or by water, wind or glacier.
If a rock undergoes chemical weathering which results in part of material being dissolved and
insoluble part remains in one place, an eluvium cover is created on the surface (e.g. eluvium clay).
If the subject of weathering is a rock containing insoluble minerals, they can be gathered in one
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place creating eluvium deposits.
There where factors intensively removing weathering products do not work, eluvium cover
appears on the rock which is directly exposed to external factors and is transformed under the
influence of organic environment. This cover is called soil, and processes leading to soil creation–
the process of soil formation.
In moderate climates – and therefore also in Poland – both physical and chemical weathering
occurs. Block disintegration of rocks as a result of physical weathering leads to creation of rubble
and sharp edge blocks of various sizes called boulder field [fig. 3]. If weathering products roll
down along the slope and are gathered at its foot, screes are created. In Poland boulder fields and
screes occur in Góry Świętokrzyskie, Sudety and Karpaty.
Other typical of this sphere weathering products are rubbles and eluvium clays.
In Poland, there are also forms which were created in
earlier
geological
epochs
when completely different
conditions were present. For instance, boulder fields from
Góry Świętokrzyskie and Karkonosze come from glaciation
periods or terra rosa (red ground), the chemical weathering
product of calcareous rocks of characteristic red or rust
colour, originated in Tertiary or Triassic period when climate
was much warmer and more humid than nowadays.
In cold polar climate the weathering of rocks and soil
3. Boulder field in Sudety [www.nationalgeographic.pl]
occurs not only due to temperature but also to freezing and
thawing water, present in top surface layers of soil. Thus,
deformations are formed, called cryoturbation which
includes involution, traverse and streaked soils. Involution
means deformation in top surface layer of the ground being
the result of crushing and folding of rock material.
Polygonal soils [fig. 4] are stone structures in the shape of
polygons of diameter 1-7m. They are formed by pushing the
4. Polygonal soils at Hudson Bay, Canada
[www.physicalgeography.net]
soil upwards during freezing. Thus, distensions appear from which pushed to the surface bigger
rock fragments roll to the sides. Streaked soils develop from polygonal soils when ground rolls
down along slopes.
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In the hot dry climate zones where temperature differences during
a day reach sometimes dozens degrees, and precipitation practically
does not occur, we deal mainly with mechanical weathering due to
temperature. A characteristic form for dry and hot climate are also
colourful covers or efflorescence (e.g. so called desert roses [fig. 5])
on the ground surface, formed as a result of soaking water upwards and
precipitation and crystallization of dissolved in water compounds (e.g.
5. Desert rose
[www.en.wikipedia.org]
silicas, oxides Fe and Mg).
6. Laterite soils [www.en.wikipedia.org]
7. The loaf of sugar, Rio de Janeiro [www.nationalgeographic.pl]
In hot and humid climate chemical weathering is crucial. Characteristic products of weathering
are laterites [fig. 6], loose cherry-coloured eluvium, comprised mainly of aluminum and iron
hydroxide and bauxites, mineral laterites and a valuable resource for aluminum production.
Other forms typical of hot are so called loaves of sugar [fig. 7], rounded hills comprised most
often of granites, created as a result of physical and chemical weathering.
Summary
Weathering means the whole of changes which are experienced by rocks and their components
(minerals) exposed to the activity of atmosphere, hydrosphere and biosphere. It takes place on the
surface of the earth crust and in top surface layers called weathering sphere.
Changes which are undergone by rocks may be of physical processes character (physical or
mechanical weathering) whose effects include crashing and disintegration of rocks without the
change of their chemical composition or of chemical processes character (chemical weathering),
which result in rocks disintegration, the change of chemical composition of mineral substances.
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Physical weathering occurs mainly as a result of uneven expansion and contraction of minerals due
to temperature and the increase of crystals in cracks and pores of rocks. Chemical weathering
happens under the influence of water and dissolved in it substances and is based mainly on
dissolution, oxidation and reduction.
Depending on rocks properties and environmental conditions weathering processes may create
various morphological forms. In Poland these are mainly boulder fields, scree cones, rubble and
eluvium clay.
Experiment
Collect two not very big pieces of limestone, marl, clay slate, granite, and sandstone. In five
transparent glasses (e.g. beakers) prepare water with a high content of CO2, whereas, to five
remaining beakers pour distilled water. One fragment of each pair of rocks put into distilled water,
the second one to water with CO2. Observe what will happen with rocks and water at the moment of
beginning the experiment, after 20 minutes and after 24 hours.
Bibliography
1. M. Książkiewicz, Geologia dynamiczna, Wydawnictwo Geologiczne, Warszawa 1979.
2. W. Mizerski, Geologia dynamiczna dla geografów, Wydawnictwo Naukowe PWN, Warszawa
2002.
3. http://www.physicalgeography.net/
4. http://geologicalintroduction.baffl.co.uk/
5. http://www.w3.salemstate.edu/
6. http://www.geology.about.com/

chemical and physical weathering of rocks

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