Weathering classification, products & geological features

weathering classification, products and geological products

1. Weathering

2. Introduction Weathering term is applied to the combined action of all processes causing rocks to be disintegrated physically and decomposed chemically because of their exposure at or near the earth surface. It is an initial stage in the process of denudation and is considered as a static part of the general process of erosion.

3. An essential feature of weathering is that affect rocs insitu and doesn’t involve transport of the degraded rock materials. An weathered soft surface layer is called regolith. Regolith grades down from top to bed rock. Weathering helps the erosion in considerable extent.

4. Factors affecting weathering Major factors are as following, they are, a)Climate b)Topography c)Structure, texture & mineral composition d)Vegetation cover e)Time

5. a)Climate Climate is defined by meteorological elements lie temperature, moisture, humidity, rainfall, atmospheric pressure & wind , etc. It determines weather mechanical disintegration or chemical decomposition & speed of process. Physical disintegration predominates in arid or tropical climatic condition. i.e. in drier & colder region.

6. Rock decomposition is more active in humid regions. i.e. in moist, warm, & low-lying areas. The thermal expansion & contraction are expected to cause mechanical destruction of the rock. Besides, alternate freezing & thawing, due to temperature fluctuation also shatters the rock. Temperature also affects the rate of chemical reaction.

7. In polar & mountain region water traps on the pore spaces and intermittent freezing split the rocks. Water is an active chemical agent, lead to the chemical decomposition of rocs, thereby eliminating all the unstable minerals in rocks.

8. b)Topography Erosive power & weathering increases in high relief and decreases in low relief. Relief controls the degree of weathering on the region. Based on the topography the weathering areas can be easily identified to locate the weathering products lie soil & regolith.

9. c)Structure, The presence of joints, cracks and fissures in the igneous rocks, bedding in sedimentary as well as foliations of metamorphic rocks facilitates maximum weathering. Water when percolates in rock it will disintegrate or decompose the rock.

10. Texture, Texture of a roc determines hardness and strength of the rocks. Fine grained rocks most susceptible to weathering then the coarse grained rocks. Also that soft rocks weather more than hard rocks.

11. Mineral composition The stability in rocks on the silicate minerals are based on the Bowens reaction. Quartz is persists in all conditions of temperature and pressure, at last. Alkali feldspar stay longer than the plagioclase in weathering. Basic rocks are more weathered in chemical action than the acidic rocks.

12. d)Vegetation cover Surface covered by vegetation is more protected than the bare surfaces. Besides, plant roots grow in fissures & joints tend to disintegrate rocks Organic acids due to plant decay, which help in the decomposition of rocks.

13. e)Time Longer the period maximize the degree of weathering and greater the volume of rocks affected by it.

14. Agents of weathering The principle agents are as follows are, Water, wind, gases lie O2 & Co2, acids, gravity, variation in temperature, organisms etc. These are mainly responsible for the physical & chemical weathering.

15. Types of weathering Physical weathering or mechanical process of disintegration Chemical weathering or chemical process of rock decomposition Biological weathering or processes of rock decomposition associated with the activities of organisms.

16. Physical weathering Mineralogical composition of roc is not affected, Instead it is a process of reduction of material into smaller pieces. It is a process of fragmentation of roc due to some physical forces. They are as following,

17. a)Fluctuation in temperature Rock is a poor conductor of heat but the outer shell of the rock is subjected to the temperature fluctuation. The periodic expansion & contraction of rocks lead to the flaking off of the upper layer. The process of scaly peeling off of the rocks is known as exfoliation or desquamation.

18. The difference in their volumetric expansion may also lead to the development of cracks and gradual disintegration of rock. this process is also known as gradual disintegration. The weathering due to fluctuation in temperature is termed as thermal weathering.

19. b)Change in pressure on rocks Pressure exerted on the deeply buried rock by the column of overlying material are enormous. As a result of removal of the overlying rocks by denudation, the rock adjust to this unloading will result in the development of a closely spaced joint systems and fractures parallel to the surface. This phenomenon of response of rocks to release the confining pressure is called dilation.

20. c)Growth of crystals Water consists of minerals enter the rocks through fracture or cracks. With the evaporation of water, the salts it contains start crystallizing. As a crystal grow it exerts large expansive stresses that result in the rock fragmentation. it mostly occurs in dry region. Crystals of gypsum, magnesium, sulphate, phosphate, nitrates grow in this manner.

21. d)Freezing of water Water expands 9% in volume during freezing will exerts enormous pressure on the walls of the fissures or joints of rocks, when it is trapped in the pores of rocks. Alternate freezing & thawing is the most effective process of rock disintegration. This phenomenon is termed as Frost wedging

22. e)Frost action Water present in ground freezes form a ice layers. Thus the thickness of the ice forcing the soil above it upward. In this way the up heaved soil is disrupted by the expansion of freezing water. This is also known as frost heaving.

23. Chemical weathering It is aprocess of mineral alteration, which consists of a number of chemical reactions. Whereby the primary mineral is converted into new compound, the secondary minerals. Effectiveness of chemical weathering depends upon, 1)size of the particles, 2)composition of rocks 3) Favourable temperature and humidity

24. a)Hydration It refers to the chemical union of water with a mineral. Certain minerals takes up water, which lead to a change in mineral composition of rocs. Due to the minerals expand, causing more stresses within the rock and cause mechanical disintegration. i.e. gypsum, hematite, to limonite

25. b)hydrolysis Process of exchange reactions between the bases of the minerals and the hydrogen ions which is part of the water. The higher Ph value, the more dissociated is the water and the stronger its action. Water is more readily dissociated when it contains Co2. Feldspar reacts with water more, hydrolysis is more in granite.

26. c)oxidation The process of chemical union of o2 atoms of other metallic elements. Thus the minerals are altered with the production of oxides. Ferromagnesian minerals lie pyroxenes, hornblende & olivine etc. rapidly undergoes oxidation. By producing a brown crust consisting largely of oxides of iron. i.e. pyrite.

27. d)carbonation Process by which Co2 is added to minerals to form certain carbonates. Rain water passage through atmosphere, dissolves some Co2 present in air. It turns into a weak acid called carbonic acid. It dissolves carbonates easily. Alkali metals like sodium, potassium, calcium & magnesium. Feldspar to clay minerals by this process. Effective in limestone or chalk areas.

28. e)solution Some of the minerals get dissolved by water and thus removed in solution. Solution tae place at different rates for different rates for different rates for different rocks. Maximum in chlorides and minimum in sulphates and carbonates. Silica undergoes solution in alkaline fluid. The process of removal of soluble material from the rocks in solution is called leaching.

29. Biological weathering It mainly related to the biological activities of plants, animals and organisms lie bacteria etc. It involves the role of plants & animals in the breaking down of rocks through mechanical ways as well as in the decomposition of rocks.

30. Physical breaking down of rocks Plant roots growing between jointed blocks and along minute fractures of rocs exerts force on it & tend to widen the existing opening. Insects like earthworm, snail etc. and burrowing animals like rodents loosen the soil cover and creates suitable conditions for various geological agencies to have action on the underlying rocks.

31. Bio chemical actions Plant dissolves various constituents of the rocs with the acid evolved by the tips of their roots. Besides, certain bacteria algae and moose's breakup roc forming silicates directly. Bird droppings have been capable of weathering limestone's. Dead decays of plants & animals produce chemically active substances which capable of bringing about rock weathering.

32. Weathering products A mantle of broken & decomposed material of varying thickness & composition called the regolith lies over the derived rocks. Soluble salts are produced by the solutions which leach outs. Colloidal substances which are weathered products carried away by the ground water. Insoluble particles lie quartz, clay, undecomposed feldspar, zircon & tourmaline are deposited by weathering.

33. classification All the weathering products are grouped into two categories as: A)transported or mobile-Which transported over varying distance by mechanical, chemical & bio chemical means. B)residual or sedentary-remains of parent rocks after weathering like bauxite or laterite.

34. Geological features Differential weathering give rise to features like honeycombed rocks, hollows and niches in rock walls. Boulders by spheroidal weathering, tor or kopje- free standing outcrop capping hill or slope. Mass wasting on slope produce talus or scree.

35. Thank you