Description of different type of weathering processes

1. Weathering of Rocks
Dr. K. Pavan Kumar
Associate Professor
Department of Civil Engineering
Vasavi College of Engineering

2. Learning Objective
Different processes associated with rock weathering
Agents that exert stress to break the rocks in mechanical weathering
Chemical weathering and the agents that help to bring about changes in
rocks
Living organisms that contribute to biological weathering of rocks
Impact of weathering on engineering geology

3. Introduction
Weathering is the breaking down of rocks
No rock is immune to weathering
Weathering includes two process: physical or mechanical weathering and chemical
weathering
Thermal stress is the main reason for mechanical weathering
Chemical weathering involves the direct effect of atmospheric chemicals or
biologically produced chemicals in the break down of rocks
Plant roots that develop along cracks and joints in rocks expand them eventually
resulting in their crumbling

4. Types of Weathering
Mechanical Weathering
- Includes processes that involves the breakdown of rocks into fragments or their
disintegration into smaller pieces without altering their mineral composition
- Mechanical weathering destroys a rock but leaves its chemical composition
unchanged
- Types of mechanical weathering:
a) Thermal stress
b) Spheroidal weathering and block disintegration
c) Frost action
d) Pressure release
e) Slacking and haloclasty
f) Hydraulic action
g) Tree root action

5. Chemical Weathering
- Changes the composition of rocks, often transforming them when water interacts with their minerals
forming various chemical reactions
- Water plays a very important role in chemical weathering
H2O + CO2 (in soil) --------> H2CO3 (carbonic acid, a weak acid)
(infiltrated rain water) (microbial respiration) (dissolves the minerals in rocks, especially the
carbonate minerals that make up limestone and marble)
Hydration – Water absorbed onto the mineral lattice. Ex: Conversion of anhydride into gypsum (CaSO4)
Hydrolysis – Breaking up of minerals by water which can take place at both surface and at shallow depths.
Most common group of minerals, the silicates, is derived from hydrolysis
- In humid climate rocks are aggressively by chemical weathering
- Different types of chemical weathering are:
a) Oxidation
b) Carbonation
c) Hydration
d) Hydrolysis

6. Biological Weathering
 Main agent in the biological weathering is the organic acids released by
organisms such as bacteria, lichens, mosses, and decaying plants of many
types
 The acid attacks the rock forming minerals
 Mineral composition can also be initiated and accelerated by soil organisms
 The most common forms of biological weathering are the release of chelating
compounds (i.e., organic acids) and of acidifying molecules (i.e., protons,
organic acids) by plants so as to break down aluminium- and iron-containing
compounds in the soils beneath them
 The decaying remains of dead plants in soil may form organic acids, which
when dissolved in water cause chemical weathering

8. Mechanical Weathering – Different Types
1. Thermal Stress
- Thermal stress results from the expansion or contraction of rocks, caused by
temperature changes
- It comprises two main types, namely thermal shock and thermal fatigue
- There is a continuous alternation of compression and tensile stresses because
of the temperature changes
- If the pores of a rock contain water, then when the water freezes, it expands
and the rock fails in tension
- Forest fires also cause weathering of rocks and boulders to a significant extent
because the sudden intense heat can rapidly expand a boulder.

10. 2. Spheroidal Weathering and Block Disintegration
• Spheroidal weathering is the flaking of highly heated, exposed rock as it
expands more than the cooler rock underneath it
• This process produces rounded rock mass structures and sometimes
exfoliation domes
• It is less common in sedimentary rocks than in igneous rocks
• This process is predominant in granitic rocks where the process of
disintegration happens through a layer-by-layer removal to evolve towards
rounded forms
• The process of block disintegration results from sharp temperature changes
causing expansion and contraction of rocks, especially in very dry climatic
conditions

12. 3. Frost Action
oThe process wherein snow or ice inside cracks cause their expansion and the
ultimate fragmentation of the rock is known as frost weathering, frost
wedging, or ice wedging
oFound mainly in the cold mountainous regions such as the Himalayan terrains
in India
oThe end product of frost action may be cone shaped deposits of slope
materials called scree or talus, seen at the foothills
oA talus cone is developed by the accumulation of broken rock pieces in various
shapes and sizes at the base of a mountain cliff or steep hill slopes
oLandforms associated with these deposited materials may be very thick and
are known as talus piles

14. 4. Pressure Release
In the pressure release (also known as ‘unloading’) phenomenon, the overlying rock by
erosion or other processes causes the underlying rocks to expand and develop fractures
parallel to the surface
It also promotes sheeting or peeling of rock from the inner mass into a series of concentric
shells
Retreat of an overlying glacier which is a mechanical weathering process of rocks, can also
lead to disintegration of rock mass due to pressure release phenomenon

15. Exfoliation
- Recognizing the presence of exfoliation joints can have important
implications in geological engineering
- Most notable may be their influence on slope stability
- Exfoliation joints following the topography of inclined valley walls, bedrock
hill slopes, and cliffs can create rock blocks that are particularly prone to
sliding
- Foundation work may also be affected by the presence of exfoliation joints,
for example in the case of dams
- Exfoliation joints underlying a dam foundation can create a significant leakage
hazard, while increased water pressure in joints may result in lifting or sliding
of the dam
- Exfoliation joints can exert strong directional control on groundwater flow
and contaminant transport.

17. 5. Slacking and Haloclasty
 Slacking is the process that causes the crumbling of rocks when exposed to
air or moisture
 It is more apparent in clay-rich sedimentary rocks as they dry out during
drought
 Crystallization of salts, which is also known as haloclasty, causes the
disintegration of rocks when water (acidic solution) seeps into cracks and
joints in rocks and evaporates, leaving the salt crystals behind
 When heated up, the salt crystals expand and exert pressure on the confining
rock
 This process splits the rock and honeycomb structures develop on its surface
 The salts that are most effective in disintegrating a rock are sodium sulphate,
magnesium sulphate, and calcium chlorite.

18. Haloclasty
Slaking

19. 6. Hydraulic Action
• In coastal areas, when water from
powerful waves rushes rapidly into
the cracks on the rock face, hydraulic
action takes place
• This causes the trapping of a layer of
air at the bottom of the cracks,
which compresses them and
weakens the rock.
• When the waves retreat, the trapped
air is suddenly released with an
explosive force.
• This causes widening of the cracks or
crumbling of the rock, thereby
hastening the process of weathering

20. 7. Tree Root Action
• Tree roots can widen the joints and
fractures in rocks as they grow up,
causing weakness and ultimately the
crumbling of the rock mass
• This is a frequently observed process
of physical weathering.
• The disintegration process is
activated by the exposure of the
rock, especially by the removal of
soil cover

21. Chemical Weathering
 Chemical weathering changes the composition of rocks, often transforming
them when water interacts with their minerals forming various chemical
reactions
 Water plays a very important role in chemical weathering
 Different forms of chemical weathering are:
a) Oxidation
b) Carbonation
c) Hydration
d) Hydrolysis

22. Oxidation
Oxidation happens when atmospheric oxygen combines with the minerals in
some rocks
Most commonly observed oxidation process is:
Fe2+ in minerals combines with O2 in water and gives rise to oxides such as:
hematite, limonite, and geothite
This gives the affected rock a reddish brown colour on surface, which
crumbles easily, weakening the rock
This process is known as rusting
Most commonly seen in limestones

23. 1. Solution
Some rocks contain minerals that are soluble in water to some extent
Example: Rock salt, gypsum, and calcite
Pure water is not a good solvent of minerals
However, carbonated water (H2CO3) enhances the solvent property for many
minerals
Ex: Limestone is not easily soluble in pure water but the carbonated water
dissolves the rock effectively
Limestone gets pitted and porous due to chemical weathering
2. Hydration and Hydrolysis
Direct attack of atmospheric moisture on the individual minerals

24. The surface of many rocks contain partially unsatisfied valences
When polarized water molecules come in contact with them, it gives rise to
any two of the following:
Hydration – Process of addition of the water molecule to mineral ions
𝐶𝑎𝑆𝑂4 + 2𝐻2𝑂 −−−−→ 𝐶𝑎𝑆𝑂42𝐻20
Calcium Sulphate Gypsum
Hydrolysis – The exchange of water molecules with mineral ions. Common
process of weathering of silicate minerals
𝐾𝐴𝑙𝑆𝑖3𝑂8 + 𝐻+ −−−−→ 𝐻𝐴𝑙𝑆𝑖3𝑂8 + 𝐾+
Orthoclase H ion from water
3. Oxidation and Reduction
Iron bearing minerals are especially prone to weathering through oxidation
and hydration

25. Oxidation – Ferrous ion (Fe2+) of the minerals is oxidized to ferric ion (Fe3+) on
exposure to air rich in moisture, which further oxidizes to ferric hydroxide
4𝐹𝑒 + 3𝑂2 −−−−−→ 2𝐹𝑒2𝑂3
𝐹𝑒2𝑂3 + 𝐻2𝑂 −−−−−→ 𝐹𝑒2𝑂3𝐻2𝑂
Reduction – Minerals and rocks containing iron oxide may undergo reduction of
the oxides to elemental iron
- Seen mostly in the environment where soil is rich in decaying vegetation
(swamps)
- Effects of oxidation weathering can be easily observed from the colour
changes produced in iron bearing rocks
- Rocks in which iron is oxidized to ferric state – show a marked brown colour
- Oxidation has reached only ferrous state – typical colours developed in rocks
are, green, blue and grey

26. 4. Carbonation
 Process of weathering of rocks under the combined action of atmospheric
CO2 and moisture, which results in carbonic acid
 Carbonation leads to corrosive action over a number of silicate bearing rocks
 Silicates of K, Na, and Ca are particularly vulnerable to carbonation
 Example: Carbonation of orthoclase (2KAlSi3O8) results in kaolinite (a clay
mineral) silica
2𝐾𝐴𝑙𝑆𝑖3𝑂8 + 2𝐻2𝐶𝑂3 −−−→ 𝐴𝑙2𝑆𝑖2𝑂5 𝑂𝐻4 + 𝐾2𝐶𝑂3 + 4𝑆𝑖𝑂2
Orthoclase carbonic acid Kaolinite Silica
 This process results in alteration of physical constituent of a rock
 A soft clay mineral (kaolinite with H = 1) is formed in place of a hard mineral
(orthoclase, H = 6)
 Carbonation is very common in igneous rocks due to presence of feldspar

27. 5. Spheroidal Weathering
A complex type of weathering observed in jointed rocks and characterized
with breaking of original rock mass into spheroidal blocks
Both mechanical and chemical weathering actively cooperate in causing
spheroidal weathering
The original solid rock mass is split into small blocks by development of
parallel joints due to thermal effects
Simultaneously, the chemical weathering processes corrode the border and
surfaces of the blocks causing their shapes roughly into spheroidal contours

28. Factors Affecting Weathering
a) Nature of the rock
- Granite and sandstone exposed to hot and humid climate will show different resistance to
weathering
- Sandstone will be highly resistant to weathering due to presence of quartz mineral
- Granite are likely to undergo chemical decay due to carbonation, hydration or hydrolysis
- Chemical composition of rock is an important factor in determining the stability of a rock
in a given environment
b) Climate
- Cold and humid conditions favour chemical and mechanical weathering
- Totally dry and cold climates do not favour chemical or mechanical weathering
- In hot and humid climates chemical weathering predominates
- In hot and dry climates mechanical weathering predominates

29. Resistance to Weathering
 In igneous rocks, the resistance to weathering is broadly related to the stage
of their formation from a composite igneous melt
 Feldspar (allumino-silicates) – are easily weathered as they are formed at
initial stages
 Quartz (silica) – less susceptible to weathering due to forming at later stage
 For most common rock forming minerals, resistance to weathering increases
in the following order:
Dark coloured minerals – Olivine<Augite<Hornblende<Biotite
Light coloured minerals – Calcic feldspar<Sodic feldspar<potash
feldspar<mica<quartz

30. Engineering Considerations of Weathering
When the foundations are to be carried to the bed rock:
- The depth of weathered cover
- Degree of weathering
- Trend of weathering in that area
Have very important bearing on the safety of the project
For construction engineer it is always necessary to find out:
- To what extent the area for the proposed project has already been physically deteriorated
due to weathering
- What would be the likely effect of weathering on the construction material proposed to be
used in the project
Any process of slope stability must ensure protection of slope rocks from
attacks by weathering agencies in and around the zone of slope failure

31. How to Protect Buildings From Weathering
 The attack of weathering starts from the surface of a structure. We can prevent the
structure from weathering attack, by painting it from the outside. The good quality of paint
does not allow external factors to affect the concrete structure.
 We can protect the structure by applying a stone sealer or sheathing to the surface of the
structure. The sheathing or stone sealer will not allow water and chemicals to enter the
surface of the concrete structure.
 Air-leaks of the structures should be filled with caulk. Doing so, water, chemicals will not be
absorbed by the surface of the structure.
 Use detergents and antiseptic agents to prevent molds, algae, lichen, and moss to appear
on the sides of the wall of the building. Building sides should be cleaned periodically to
prevent moisture and salts accumulate on the surface.