Int. j. econ. env. geol. Vol:3(2), 1-4, 2012
Introduction
The processes of weathering and erosion change the physical condition and chemical composition of fresh rocks. The broken materials from fresh rocks are carried away as the sediments by the natural transporting agents. Sediments which are transported by rivers are either from chemical weathering or physical weathering of fresh rocks. These sediments are deposited in the lower basins or finally join with the sea sediments as a part of the same geological process. Part of this is being used as a construction material.
The process of weathering depends on the various local factors within the tropical zone. Thus the rate of weathering cannot be estimated within a short period. But the material loss due to weathering (and erosion) can be estimated by basic experiments because the remaining residual materials have strong relationship with their parent rocks. The ions separated after the chemical reactions in rocks join with the surface water or ground water. The total dissolved ions and total suspended solids in surface water and ground water are the result of the chemical weathering of rocks in the entire region or the river basin. Change of the chemical composition of the parent material due to chemical weathering forms secondary minerals as the residual materials. It also reduces the bulk density of parent rocks due to loss of some ions. This is a part of the total process of destruction of land. This material loss can be estimated by simple basic methods because the separation of weathered products from the parent materials changes their original properties.
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Estimation of Material Loss due to Chemical Weathering of Some
Rocks in Sri Lanka
U. de S. JAYAWARDENA
Department of Civil Engineering, Faculty of Engineering, University of Peradeniya,
Peradeniya, Sri Lanka
E-mail: udsj@pdn.ac.lk, udesja@gmail.com
The river sedimentation may be high in tropical countries because rock weathering is most intense in the humid tropical region. Sri lanka is a country in the tropical region with high humidity. Therefore study on the rock weathering, river sedimentation and landscape reduction in Sri lanka may provide useful information to the scientists and engineers in the country. There are very few publications available on the study of rock weathering in Sri lanka (Vitanage etal., 1983; Jayawardena and Izawa, 1994 a&b; Jayawardena, 2000 and 2003). It is the continuation of the same research which highlight the physical condition of rocks during chemical weathering (Jayawardena and Izawa, 1994a&b; Jayawardena, 2000 and 2003). The aim of this study is to find out the weight loss of some major parent rocks with the increase of chemical weathering in Sri lanka.
General geography and geology of Sri Lanka. Physiographically Sri lanka consists of a central mountainous mass or central highland surrounded by a low, flat plain on all sides and extending to the sea (Vitanage, 1970). Sri lanka is considered to have a humid tropical climate. On the basis of rainfall, the dryness and topography, Sri lanka can be divided into three climatic zones namely the Dry Zone having rainfall less than 2000mm, the Intermediate Zone having rainfall between 2000mm to 3000mm and the Wet Zone having rainfall above 3000m (Walker, 1962). Weathering features in Wet and Dry Zones are different due to this climatic difference.
Geologically 90% of Sri lanka is made up of high grade metamorphic rocks of Precambrian age (Fig.1). The remaining rocks are sedimentary rocks of predominantly Abstract. The processes of weathering reduce the materials from the parent rocks. The material loss due to weathering can be estimated by a fundamental method because the remaining residual materials have strong relationship with their parent rock. The objective of this basic research is to find out the weight loss with the increase of chemical weathering in Sri lankan rocks. This may be an important data to calculate sedimentation rates in the reservoirs. Irregular samples from fresh rocks and differently weathered grades of each rock were collected from different localities without disturbing the structures. Bulk density and the porosity of each sample were measured. The total number of samples was about 300 from 73 locations. The results show the decrease of bulk density and the increase of porosity and weathering. It indicates that some materials moving away and separate from the original place create voids within the same volume during the processes of chemical weathering. The total loss of material is about 18 -29 % by weight from the fresh rock stage to highly weathered stage. Increase of porosity goes up to 38 % in highly weathered rocks.
ÓSEGMITE
Miocene age in the north-west (and very few places of south east) with some Jurassic sediments preserved in small faulted basins. There are recent sedimentary formations, identified as Pleistocene deposits in a few locations. Intruding the metamorphic rocks of Sri lanka are some granites, dolerites, pegmatites, quartz veins and a carbonatite (Cooray, 1967).
The land surface of Sri lanka has been subjected to a prolonged period of weathering and erosion under different climatic conditions. The secondary formations arising from weathering such as laterite, ferricrete, calcrete are found throughout the Island belonging to the younger Pleistocene formations. Recent deposits include both residual, alluvial and colluvial deposits. Residual deposits include the deep weathered zones or soils to be found in the central hill country and in the intermediate slopes. These deposits are not uniform in character and contain fragments of un-decomposed rocks (Herath, 1963a and 1963b). In some areas of the Wet zone, the thickness of the weathered profile may go up to 50 meters. In general the thickness may be between 10-15 meters. Residual shallow weathered zones are mainly confined to the Dry zone areas and the general thickness is less than 10 meters (KDWSSP, 1981), The weathering is not uniform in any place in the country and the thickness change drastically from place to place.
Materials and Methods
Engineering classification of weathered rocks(Fookes, and Horswill, 1969), was used to recognize the different weathering grades within the weathering profile of the fresh rock. Different samples from fresh rock and its each weathering grade were selected future study. Irregular samples, similar to the same size, were collected from the different grades of insitu weathered materials overlying the parent rocks. The samples represented their grade of weathering according to the engineering classification of weathered rocks. Except a few locations, all samples were collected within the Wet zone of Sri lanka where the rainfall is generally high. In Dry zone, the weathering profiles are much thinner and therefore, differentiation and recognition of different weathering groups are difficult. About 300 samples were collected from 73 locations in the country and all are from different metamorphic rock types (Fig.1).
Irregular fresh and weathered samples were cut from six sides, using a laboratory rock-cutting machine and prepared rectangular block specimens in different sizes. The opposite faces of the blocks are parallel to each other. The block sizes of different weathered grades were not equal but the dimensions (length, width and height) are nearly 10 cm each. For weathered rocks, slightly weathered,
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Fig. 1. General geological map of Sri lanka and the location of sampling points
Simplified Geological Map of Sri lanka
Quaternary Miocene Jurassic Precambrian
moderately weathered and highly weathered grade samples were chosen for sample preparation. Here the samples were cut very carefully and slowly, keeping parallel faces without damaging to the softer materials. The porosity and bulk density for all samples were determined according to the standard method ISRM Committee on Laboratory Test, Document No.2. (ISRM, 1972).
Results and Discussion
The reduction of bulk density of the fresh rocks with the increase of weathering and the increase of porosity with the increase of weathering of the same rock is given in Table 1. Table 2 indicates the reduction of density from 18 to 29% and the increase of porosity from 15 to 38 times when a fresh rock becomes highly weathered due to the reactions of chemical weathering. This may increase further leading to completely weathered rocks and residual soils. Fresh rocks occur at the bottom of the weathered profile and other grades occur above it. Close to the ground surface level the loose residual soil layer can be seen. The removal of materials from these loose soils is generally higher than the other layers. The materials separated from the fresh rocks due to the weathering and erosion join with water either as suspension materials or dissolved materials. Materials separating from the original rocks occur as bed loads.
The actions of chemical weathering change the mineralogy of fresh rock and create new secondary minerals. As a result some elements of the fresh rock may disappear and then increase the porosity while absorbing water or moisture. Due to the reduction of some ions the bulk density also decreases in the weathered rocks. This process is continues while the degree of weathering increases. The ions separated from the fresh rock may join with water as dissolved ions or suspended ions. Some minerals like quartz and heavy minerals such as garnets, rutile, ilmenite, corundum, spinel are strongly resistant minerals to weathering. These minerals separate from the weathered rocks and join with the large rock pieces (gravels, and boulders) and moving along the river flow as the bed load. In addition clay like suspension material also separate from weathered rocks and join with water.
Conclusion
The bulk density decreases and the porosity increases with the increase of the degree of weathering. The bulk density reduction may be about 18-29% at highly weathered stage. Increase of porosity goes up to 38 times than the fresh rocks in Highly Weathered rocks. However rate of chemical weathering per year cannot be calculated by this method.
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SW = Slightly weathered; MW = Moderately weathered; HW = Highly weathered; ** = Weathering grade Rock type Weathering
grade **
Bulk density Kg/m3
Porosity %
Maximum Minimum Maximum Minimum Charnockite
(38 locations)
Fresh rock 2981 2627 0.98 0.23
SW 2905 2605 2.30 0.65
MW 2510 2231 7.90 2.32
HW 2435 2080 11.94 5.34
Garnet sillimanite gneiss (7 locations)
Fresh rock 3031 2718 0.86 0.34
SW 2875 2655 4.20 1.40
MW 2450 2420 10.70 5.70
HW 2280 1600 26.40 11.80
Hornblende biotite gneiss (9 locations)
Fresh rock 2950 2740 0.45 0.32
SW 2820 2596 1.10 0.63
MW 2525 2245 6.20 3.50
HW 2280 2135 12.80 10.20
Biotite gneiss (10 locations)
Fresh rock 2801 2620 0.54 0.23
SW 2695 2520 3.30 0.95
MW 2470 2332 7.50 3.50
HW 2290 2160 16.15 9.60
Migtmatite (2 locations)
Fresh rock 2810 2795 0.63 0.42
SW 2750 2710 1.50 1.20
MW 2520 2495 3.80 4.30
HW 2045 2035 12.50 13.80
Quartzite ( 3 locations)
Fresh rock 2632 2538 0.49 0.35
SW 2596 2502 2.45 2.38
Marble ( 4 locations)
Fresh rock 2832 2795 0.42 0.3
SW 2752 2579 4.6 3.5
MW 2370 2313 8.4 8.13
Table 1. The variation of bulk density and porosity of rocks with the increase of chemical weathering.
References
Cooray, P.G. (1967) An Introduction to the Geology of Ceylon, 1st
Edition, p340, Dept. of National Museums, Govt. Press, Colombo, Ceylon.
Fookes, P.G; Horswill, P. (1969) Discussion on the load deformation behaviour of the middle chalk at mundford, norfolk, in in-situ investigations in soils and rocks. British Geotechnical Soc., 53-57. Herath, J.W. (1963a) The mineralogical composition of some laterites and lateritic soils of Ceylon. Industrial Minerals, 4, 36-42.
Herath, J.W. (1963b) Kaoline in Ceylon. Economic Geology, 58, 769-773.
I.S.R.M. Committee on Laboratory Tests (1972) Suggested Methods for Determining Water Content, Porosity, Density, Absorption and Related Properties and Swelling and Slake Durability Index Properties, I.S.R.M. Committee on Laboratory Test, Document No.2.
Jayawardena, U.de S; Izawa, E. (1994a) A New Chemical Index of Weathering for Metamorphic Rocks in Tropical Region - A Case Study from Sri lanka, Engineering Geology, 36, 303-310.
Jayawardena, U.de S; Izawa, E. (1994b) Applications of present indices of chemical weathering for metamorphic rocks in Sri lanka. Bulletin of the
International Association of Engineering Geology, No. 49, (April), p56-61.
Jayawardena, U.de S. (2000) Geochemistry and engineering properties of weathered metamorphic rocks in Sri lanka, DEng Thesis, p231, Kyushu University, Japan,
Jayawardena, U.de S. (2003) Use of the correlation between ultrasonic wave velocity and point load strength of weathered rocks - a new method for site investigation. Journal of The Geological Society of Sri Lanka, 11, 107-114.
Kandy District Water Supply and Sanitation Project (KDWSSP) (1981) Kandy District Water Resources Potential Study, Interim Report, p90, Plancenter Ltd Publication, Kandy, Sri Lanka.
Vitanage, P.W. (1970) A study of the geomorphology and morphotectonics of Ceylon, In: Proceedings of the Second Seminar on Geological Prospecting Methods and Techniques, United Nations, p.391-405, New York, USA.
Vitanage, P.W; Hatva, T; Lumiaho, K. (1983) Some aspects of weathering in Sri lanka, Rapautuminen Kallioperassa Symposium, IX/ (1) 23, Nov.9, p1-23.
Walker, R.C. (1962) The Hydrometeorology of Ceylon, (Part 1), Canada Colombo Plan Project, Government Press, Ceylon.
4 Rock type Weathering
Grade **
Bulk density
Kg/m3 Porosity %
Average Reduction Loss % Average Increase Charnockite
(38 locations)
FR 2804 00 00 0.55 0
SW 2722 82 2.9 1.81 3 times
MW 2404 400 14.2 4.72 8 times
HW 2227 577 20.4 8.25 15 times
Garnet sillimanite gneiss (7 locations)
FR 2858 00 00 0.53 0
SW 2755 103 3.6 2.65 5 times
MW 2516 342 11.97 7.09 13 times
HW 2027 831 29.07 20.30 38 times
Hornblende biotite gneiss (9 locations)
FR 2819 00 00 0.37 0
SW 2703 116 4.11 1.12 3 times
MW 2304 515 18.26 4.76 12 times
HW 2156 663 23.52 10.94 29 times
Biotite gneiss (10
locations)
FR 2705 00 00 0.39 0
SW 2632 73 2.7 1.50 3 times
MW 2396 309 11.42 5.25 13 times
HW 2203 502 18.56 12.15 31 times
Migtmatite (2 locations)
FR 2802 00 00 0.52 0
SW 2730 72 2.6 1.35 2 times
MW 2507 295 10.53 4.05 7 times
HW 2040 762 27.2 13.15 25 times
Quartzite (3 locations)
FR 2590 00 00 0.43 0
SW 2561 29 1.12 2.40 5 times
Marble (4 locations)
FR 2822 00 00 0.33 0
SW 2690 132 4.67 3.92 11 times
MW 2354 468 16.58 8.11 24 times
Table 2. Loss of bulk density as percentage and increase of porosity of rocks with the increase of chemical weathering
