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Weathering: Importance and Types
Weathering is the breakdown and alteration of rocks and minerals at or near the Earth's
surface into products that are more in equilibrium with the conditions found in this
environment. Most rocks and minerals are formed deep within the Earth's crust where
temperatures and pressures differ greatly from the surface. Because the physical and chemical
nature of materials formed in the Earth's interior are characteristically in disequilibrium with
conditions occurring on the surface. Because of this disequilibrium, these materials are easily
attacked, decomposed, and eroded by various chemical and physical surface processes.
Weathering is the first step for a number of other geomorphic and biogeochemical processes.
The products of weathering are a major source of sediments for erosion and deposition. Many
types of sedimentary rocks are composed of particles that have been weathered, eroded,
transported, and terminally deposited in basins. Weathering also contributes to the formation
of soil by providing mineral particles like sand, silt, and clay. Elements and compounds
extracted from the rocks and minerals by weathering processes supply nutrients for plant
uptake. The fact that the oceans are saline in the result of the release of ion salts from rock
and minerals on the continents. Leaching and runoff transport these ions from land to the
ocean basins where they accumulate in seawater. In conclusion, weathering is a process that
is fundamental to many other aspects of the hydrosphere, lithosphere, and biosphere. There
are three broad categories of mechanisms for weathering: chemical, physical and biological.
Products of Weathering: The process of weathering can result in the following three
outcomes on rocks and minerals:
(1) The complete loss of particular atoms or compounds from the weathered surface.
(2) The addition of specific atoms or compounds to the weathered surface.
(3) A breakdown of one mass into two or more masses, with no chemical change in the
mineral or rock.
1. Chemical Weathering:
involves the alteration of the chemical and mineralogical
composition of the weathered material. A number of different processes can result in
chemical weathering. The most common chemical weathering processes are hydrolysis,
oxidation, reduction, hydration, carbonation, and solution.
a. Hydrolysis: is the weathering reaction that occurs when the two surfaces of water and
compound meet. It involves the reaction between mineral ions and the ions of water (OHand H+), and results in the decomposition of the rock surface by forming new
compounds, and by increasing the pH of the solution involved through the release of the
hydroxide ions. Hydrolysis is especially effective in the weathering of common silicate
and alumino-silicate minerals because of their electrically charged crystal surfaces.
Lecture Delivered By: Dr. Tanveer Ahmad Malik
Weathering: Importance and Types
b. Oxidation: is the reaction that occurs between compounds and oxygen. The net result of
this reaction is the removal of one or more electrons from a compound, which causes the
structure to be less rigid and increasingly unstable. The most common oxides are those of
iron and aluminium, and their respective red and yellow staining of soils is quite common
in tropical regions which have high temperatures and precipitation. Reduction is simply
the reverse of oxidation, and is thus caused by the addition of one or more electrons
producing a more stable compound.
c. Hydration: involves the rigid attachment of H+ and OH- ions to a reacted compound. In
many situations the H and OH ions become a structural part of the crystal lattice of the
mineral. Hydration also allows for the acceleration of other decompositional reactions by
expanding the crystal lattice offering more surface area for reaction.
d. Carbonation: is the reaction of carbonate and bicarbonate ions with minerals. The
formation of carbonates usually takes place as a result of other chemical processes.
Carbonation is especially active when the reaction environment is abundant with carbon
dioxide. The formation of carbonic acid, a product of carbon dioxide and water, is
important in the solution of carbonates and the decomposition of mineral surfaces because
of its acidic nature.
e. Solution: Water and the ions it carries as it moves through and around rocks and minerals
can further the weathering process. Geomorphologists call this phenomena solution. The
effects of dissolved carbon dioxide and hydrogen ions in water have already been
mentioned, but solution also entails the effects of a number of other dissolved compounds
on a mineral or rock surface. Molecules can mix in solution to form a great variety of
basic and acidic decompositional compounds. The extent, however, of rock being
subjected to solution is determined primarily by climatic conditions. Solution tends to be
most effective in areas that have humid and hot climates.
The most important factor affecting all of the above mentioned chemical weathering
processes is climate. Climatic conditions control the rate of weathering that takes place by
regulating the catalysts of moisture and temperature. Experimentation has discovered that
tropical weathering rates, where temperature and moisture are at their maximum, are three
and a half times higher than rates in temperate environments.
2. Physical Weathering: is the breakdown of mineral or rock material by entirely
mechanical methods brought about by a variety of causes. Some of the forces originate within
the rock or mineral, while others are applied externally. Both of these stresses lead to strain
Lecture Delivered By: Dr. Tanveer Ahmad Malik
Weathering: Importance and Types
and the rupture of the rock. The processes that may cause mechanical rupture are abrasion,
crystallization, thermal insolation, wetting and drying, and pressure release.
Types of Physical Weathering: The various types of Physical Weathering are:
a. Thermal stress: Thermal stress weathering (sometimes called insolation
weathering) results from the expansion and contraction of rock, caused by
temperature changes. For example, heating of rocks by sunlight or fires can cause
expansion of their constituent minerals. As some minerals expand more than others,
temperature changes set up differential stresses that eventually cause the rock to
crack apart. Because the outer surface of a rock is often warmer or colder than the
more protected inner portions, some rocks may weather by exfoliation – the peeling
away of outer layers. This process may be sharply accelerated if ice forms in the
surface cracks. When water freezes, it expands with a force of about 1465 Mg/m^2,
disintegrating huge rock masses and dislodging mineral grains from smaller
fragments.
b. Frost Action: Frost weathering, frost wedging, ice wedging or cryofracturing is the
collective name for several processes where ice is present. These processes include frost
shattering, frost-wedging and freeze-thaw weathering. Severe frost shattering produces
huge piles of rock fragments called scree which may be located at the foot of mountain
areas or along slopes. Frost weathering is common in mountain areas where the
temperature is around the freezing point of water. Certain frost-susceptible soils expand
or heave upon freezing as a result of water migrating via capillary action to grow ice
lenses near the freezing front. This same phenomenon occurs within pore spaces of rocks.
The ice accumulations grow larger as they attract liquid water from the surrounding
pores. The ice crystal growth weakens the rocks which, in time, break up. It is caused by
the approximately 10% (9.87) expansion of ice when water freezes, which can place
considerable stress on anything containing the water as it freezes.
c. Pressure Release: pressure release, also known as unloading, overlying materials (not
necessarily rocks) are removed (by erosion, or other processes), which causes underlying
rocks to expand and fracture parallel to the surface. Intrusive igneous rocks (e.g. granite)
are formed deep beneath the Earth's surface. They are under tremendous pressure because
of the overlying rock material. When erosion removes the overlying rock material, these
intrusive rocks are exposed and the pressure on them is released. The outer parts of the
rocks then tend to expand. The expansion sets up stresses which cause fractures parallel
Lecture Delivered By: Dr. Tanveer Ahmad Malik
Weathering: Importance and Types
to the rock surface to form. Over time, sheets of rock break away from the exposed rocks
along the fractures, a process known as exfoliation. Exfoliation due to pressure release is
also known as "sheeting".
d. Salt-crystal growth: Salt crystallization, otherwise known as haloclasty, causes
disintegration of rocks when saline solutions seep into cracks and joints in the rocks and
evaporate, leaving salt crystals behind. These salt crystals expand as they are heated up,
exerting pressure on the confining rock. Salt crystallization may also take place when
solutions decompose rocks (for example, limestone and chalk) to form salt solutions of
sodium sulfate orsodium carbonate, of which the moisture evaporates to form their
respective salt crystals.
The salts which have proved most effective in disintegrating rocks are sodium
sulfate, magnesium sulfate, and calcium chloride. Some of these salts can expand up
to three times or even more. It is normally associated with arid climates where strong
heating causes strong evaporation and therefore salt crystallization. It is also common
along coasts. An example of salt weathering can be seen in the honeycombed stones
in sea wall. Honeycomb is a type of tafoni, a class of cavernous rock weathering
structures, which likely develop in large part by chemical and physical salt weathering
processes.
e. Biological effects on mechanical weathering: Living organisms may contribute to
mechanical weathering (as well as chemical weathering, see 'biological' weathering
below). Lichens and mossesgrow on essentially bare rock surfaces and create a more
humid chemical microenvironment. The attachment of these organisms to the rock
surface enhances physical as well as chemical breakdown of the surface microlayer of the
rock. On a larger scale, seedlings sprouting in a crevice and plant roots exert physical
pressure as well as providing a pathway for water and chemical infiltration
3. Biological Weathering: involves the disintegration of rock and mineral due to the
chemical and/or physical agents of an organism. The types of organisms that can cause
weathering range from bacteria to plants to animals. Biological weathering involves
processes that can be either chemical or physical in character. Some of the more important
processes are:
i. Simple breaking of particles, by the consumption of soils particles by animals.
Particles can also fracture because of animal burrowing or by the pressure put forth by
growing roots.
Lecture Delivered By: Dr. Tanveer Ahmad Malik
Weathering: Importance and Types
ii.
Movement and mixing of materials. Many large soil organisms cause the
movement of soil particles. This movement can introduce the materials to different
weathering processes found at distinct locations in the soil profile.
iii. Simple chemical processes like solution can be enhanced by the carbon dioxide
produced by respiration. Carbon dioxide mixing with water forms carbonic acid.
iv. The complex chemical effects that occur as a result of chelation. Chelation is a
biological process where organisms produce organic substances, known as chelates,
that have the ability to decompose minerals and rocks by the removal of metallic
cations.
v. Organisms can influence the moisture regime in soils and therefore enhance weathering.
Shade from aerial leaves and stems, the presence of roots masses, and humus all act to
increase the availability of water in the soil profile. Water is a necessary component in
several physical and chemical weathering processes.
vi. Organisms can influence the pH of the soil solution. Respiration from plant roots releases
carbon dioxide. If the carbon dioxide mixes with water carbonic acid is formed which lowers
soil pH. Cation exchange reactions by which plants absorb nutrients from the soil can also
cause pH changes. The absorption processes often involves the exchange of basic cations for
hydrogen ions. Generally, the higher the concentration of hydrogen ions the more acidic a
soil becomes.
Importance of Weathering:
All the processes of weathering, physical, chemical or organic, besides preparing the surface
material for removal by various agencies of denudation, perform other functions also.
Rock disintegration by weathering does not result only in fragmental materials to be
transported and deposited by rivers, wind or glaciers; or their mass movement by the
gravitational pull, but it also provides raw material in the form of regolith for the
development of soil.
1. It is a visible proof that geologic cycle is operating.
2. It shows how geologic material and certain kinds of energy react to bring the material into
adjustment with conditions prevailing- environment, atmosphere, moisture, temperature
fluctuation, organism etc.
3. It converts bed rock into regolith and soil. Regolith is also notable for being the basic
source for the inorganic part of the soil. The importance of soil which indirectly supports all
animal forms of life in the form of vegetation cannot be overemphasized.
Lecture Delivered By: Dr. Tanveer Ahmad Malik
Weathering: Importance and Types
4. Weathering creates raw material that become sediments and sedimentary rocks- loosens to
individual minerals, attack and alter individual minerals and solution.
5. It forms economic deposits/ importance in engineering projects.
Weathering breaks rocks into their mineral components. It also creates new compounds
through chemical changes.
6. During the prolonged period of time, the weathering processes produce concentrations of
valuable mineral ores of iron, manganese, tin, aluminium, and uranium etc. For example, by
chemical weathering soluble bases and even silica are removed, leaving behind increasingly
rich residual concentrations of metallic oxides. This occurs under humid tropical climatic
conditions as part of the laterization process.
Landforms of Weathering
I.
Regolith and Soil: Most landforms to some extent show the effects of weathering. On
the bedrock surface of these landscapes are the accumulations of the products of
weathering. Within these accumulations are materials displaying various degrees of
physical, chemical, and biological alteration. These materials range in size from large
boulders to clay sized particles less than 0.004 millimeters in diameter.
Geomorphologists refer to these accumulations as regolith. Regolith can be further
altered by climate, organisms, and topography over time to create soil. Soil is the
most obvious landform of weathering.
II.
Limestone Landforms: Among the most interesting and most beautiful landforms of
weathering are those which develop in regions of limestone bedrock. These
landscapes are commonly called karst. In karst landscapes weathering is concentrated
along joints and bedding planes of the limestone producing a number of different
sculptured features from the effects of solution. Depressions of all sizes and shapes pit
the landscape surface and are the most obvious features associated with karst. Beneath
the surface, solution results in the formation of caves, springs, underground water
channels, and deposits from evaporation.
III.
Periglacial Landforms: Unique weathering landforms are also found in polar and
sub-polar regions. In these regions, physical weathering processes are dominant, with
active freeze-thaw and frost-shattering being the most active. Associated with these
weathering processes are a number of unique surface features that develop only in
periglacial environments. Collectively known as patterned ground, these surface
features resemble circles, polygons, nets, steps, and stripes. The outlines of all of
Lecture Delivered By: Dr. Tanveer Ahmad Malik
Weathering: Importance and Types
these features consist of elevated accumulations of coarse regolith fragments.
Scientists believe that these outlines result from the systematic sorting of particles of a
wide range of texture sizes by freeze-thaw action. The sorting causes larger fragments
to move vertically upward and horizontally outward. Horizontal movement stops
when one feature encounters another, linking the perimeter of two or more features.
The linking of many adjacent features creates net-like patterns.
Lecture Delivered By: Dr. Tanveer Ahmad Malik