Weathering of Rocks.pptx

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: Thermal stress Spheroidal weathering and block disintegration Frost action Pressure release Slacking and haloclasty Hydraulic action 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 H 2 O + CO 2 (in soil) --------> H 2 CO 3 (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 (CaSO 4 ) 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: Oxidation Carbonation Hydration 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

7. Mechanical Weathering – Different Types 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.

8. 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

9. 3. Frost Action The 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 Found mainly in the cold mountainous regions such as the Himalayan terrains in India The end product of frost action may be cone shaped deposits of slope materials called scree or talus, seen at the foothills A 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 Landforms associated with these deposited materials may be very thick and are known as talus piles

10. 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

11. 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.

12. 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.

13. Haloclasty Slaking

14. 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

15. 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

16. 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: Oxidation Carbonation Hydration Hydrolysis

17. 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

18. 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 (H 2 CO 3 ) 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

19. 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 Calcium Sulphate Gypsum Hydrolysis – The exchange of water molecules with mineral ions. Common process of weathering of silicate minerals Orthoclase H ion from water 3. Oxidation and Reduction Iron bearing minerals are especially prone to weathering through oxidation and hydration  

20. 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 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  

21. 4. Carbonation Process of weathering of rocks under the combined action of atmospheric CO 2 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 (2KAlSi 3 O 8 ) results in kaolinite (a clay mineral) silica 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  

22. 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

23. Factors Affecting Weathering 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

24. 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

25. 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

26. 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.