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JOURNAL GEOLOGICAL SOCIETY OF INDIA Vol.78, September 2011, pp.249-257

Weathering and Landslide Occurrences in Parts of Western Ghats, Kerala K. S. SAJINKUMAR1, S. ANBAZHAGAN2, A. P. PRADEEPKUMAR3 and V. R. RANI4 1

Geological Survey of India, Kerala Unit Manikanteswaram P O, Trivandrum - 695 013 2 Department of Geology, Periyar University, Salem - 636 011 3 Department of Geology, University College, Trivandrum - 695 034 4 Central Groundwater Board, Trivandrum – 695 004 Email: [email protected]

Abstract: The climatic condition of Western Ghats has influenced the process of weathering and landslides in this mountainous tract along the southwest coast of India. During the monsoon period, landslides are a common in the Western Ghats, and its intensity depends upon the thickness of the loose unconsolidated soil formed by the process of weathering. Debris landslides with a combination of saprock, saprolite and soil, indicate the role of weathering in landslide occurrences. This paper reports on how the weathering in the windward slope of Western Ghats influences the occurrence of landslides and the factors which accelerate the weathering process. Rock and soil samples were collected from the weathering profile of hornblende gniess and granite gneiss. The chemical analysis and the calculated Chemical Index of Alteration (CIA) indicate the significant weathering and its possible influence on landslide occurrences in the study area. Mainly, the CIA value of lateritic soil and forest loam indicated the extent of high chemical weathering in this region. Rainfall is the dominant parameter influencing the chemical weathering process. In addition, deforestation, land use practices and soil erosion are some of the other important factors accelerating the weathering process and landslide occurrences in the region. The locations of the previous landslides superimposed on geology and soil show that most of the landslide occurrences are associated with the highly weathered zone, particularly lateritic soil and the ‘severe’ (rock outcrop) erodability zone. Keywords: Weathering, Landslides, CIA, Western Ghats, Kerala. INTRODUCTION

Monsoon climate is prevalent throughout the length of the Western Ghats, which is an elevated passive continental margin along the southwestern coast of India (Subrahmanya, 2001). The climate in this region is interlinked with two common natural phenomena; predominance of chemical weathering and the occurrence of devastating landslides. Rainfall and availability of water, apart from temperature, are the important factors that influence the intensity of chemical weathering (Deepthi and Balakrishanan 2005). At first glance, these geological processes look unconnected, however a reversible relation exists between the two processes (Sajinkumar, 2005). Weathering, a process operating over millions of years, bear direct relationship with the spontaneous occurrence of landslides. Weathering processes facilitate landslide incidence and is considered as a conditioning mechanism for such occurrences (Terlien, 1998) and landslide itself is a part of the weathering process. Many physio-chemical variables like rocks types, climate,

topography and exposure age affect the weathering environment. Several studies were conducted worldwide to show the relation between weathering and landslides. Pasuto and Silvano (1998) showed that the effect of weathering on rocks resulted in the formation of vast scree slopes at the foot of the large landslide scarp. Studies by Zhou et al. (2002) in Hong Kong showed prominent weathering in the volcanic rocks. The volcanic terrain is characterized by colluvium at its foot slopes. Terlien (1998) identified the progressive weathering of slope materials by slow natural processes as the real causative factor for landslides. Buma and Duhn (1998) studied the role of climate and weathering in landslide occurrences and concluded that changing weathering pattern occurs due to climate change which results in landslides. In the present study, the extent of weathering, factors which influence the weathering process and landslides in parts of the Western Ghats in Kerala are discussed.

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K. S. SAJINKUMAR AND OTHERS

STUDY AREA

The hill range of Western Ghats, a prominent feature in the geography of India (Oldham, 1894) has played a major role in shaping the climate of peninsular India, by creating conditions for the orographic precipitation during southwest monsoon (Pascoe, 2001). The selected area of study also experiences similar climate. It lies in the windward slope of

Western Ghats and characterized by rugged hills and deep valleys. Geographically it is located between north latitudes of 9°53'00" and 10°05'00" and east longitudes 76°45'00" and 76°57'30" covering an area of 420 sq km in the Ernakulam and Idukki districts of Kerala (Fig.1). The average annual rainfall is between 380 and 520 cm and the temperature varies between 13° and 29°C. The study area is effectively interconnected and accessible by roads.

Fig.1. The study area located in parts of Ernakulam and Idduki districts, Kerala. JOUR.GEOL.SOC.INDIA, VOL.78, SEPT. 2011

WEATHERING AND LANDSLIDE OCCURRENCES IN PARTS OF WESTERN GHATS, KERALA

GEOLOGY

The Western Ghats encompasses different geologic and tectonic regimes. The study area forms a part of Madurai Granulite Block (MGB) which is perhaps the single largest crustal block among the collage of terrains on the southern high grade metamorphic terrain of South India (Koshimoto et al. 2004) and is sandwiched between Palghat-Cauvery shear zone in the north and Achankovil shear zone in the south. The study area consists of hornblende gneiss (hornblende-biotite and quartz-mica gneiss composite) and pink granite gneiss (Munnar Granite), traversed by bands of pyroxene granulites, calc-granulites, quartzites and quartz magnetites of different dimensions (Fig.2). Hornblende gneiss is formed by the retrogressive metamorphism of charnockites due to the emplacement of Munnar granite (Rajan et al. 1984). The general mineral assemblage of this gneiss comprises of quartz, K-feldspar, oligoclase, biotite and hornblende. Munnar granite, on the other hand, is emplaced within the Precambrian Gneisses and spatially related to the intersection zone of the NE-SW Attur lineament (Santosh et al. 1987). This flesh-coloured rock is characterized by granulitic texture with quartz, orthoclase and biotite as major minerals. GEOCHEMISTRY AND MINERALOGY

The major part of the study area is characterized by the outcrops of hornblende gneiss and granite gneiss. To study the extent of weathering, two weathering profiles were examined; one developed over hornblende gneiss and the other over granite gneiss. The weathering profile at Kaliyar is developed over hornblende gneiss and overlain by lateritic soil. The other profile studied at Adimali is developed over pink granite gneiss and overlain by forest loam. Chemical analysis was carried out to understand the chemical composition of two rock types (Table 1). Geochemically, the hornblende gneiss and granite gneiss have significant difference in major and trace element content. Though the rock types have shown almost identical major elements of geochemistry, variations observed in the percentage of elements. The trace elements show significant differences, particularly in the content of Zn is higher by 5 ppm and Cu by 10 ppm in granite gneiss over hornblende gneiss. Rocks represent minor variations in the major elements, however noticeable variation observed in the soils developed over the rocks. The forest loam developed over granite gneiss show variation in the percentage of Al 2O3, Fe2O3 and K2O, when compared to lateritic soil associated with hornblende gneiss. XRD analysis was carried out to JOUR.GEOL.SOC.INDIA, VOL.78, SEPT. 2011

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Table 1. Geochemical analysis of rocks and soils of two different weathering profiles Weathering Profile in KaliyarHornblende Gneiss-Lateritic Soil Major Element

Weathering Profile in AdimaliGranite GneissForest Loam

Hornblende Gneiss

Lateritic Soil

Granite Gneiss

Forest Loam

SiO2 Al2O3 FeO Fe2O3 K2O Na2O CaO MgO MnO P2O5 TiO2 LOI* Total (%)

68.908 12.452 5.460 2.707 3.435 1.956 2.402 0.678 0.038 0.303 0.432 0.740 99.511

28.161 29.079 2.350 11.435 1.900 0.145 0.256 1.295 0.106 0.158 1.920 22.700 99.505

67.760 13.971 4.730 2.950 3.869 1.680 2.197 0.626 0.014 0.150 0.682 0.520 99.149

28.696 26.688 1.150 14.156 3.274 0.002 0.207 0.927 0.147 0.163 1.715 22.640 99.765

Trace Element Sr Pb Zn V Hg Cr Cu Total (ppm) CIA**

2.800 0 23.640 0 0 0 51.800 78.240 61.507

0.002 0.002 0.016 0.027 0 0.016 0.009 0.072 92.667

2.670 0 28.950 0 0 0 60.990 92.610 64.331

0 0.002 0.013 0.028 0 0.022 0.009 0.074 88.456

* Loss on Ignition ** Chemical Index of Alteration

understand the clay mineralogy of the two rock types. The technique has successfully been adopted in studying the vermiculitization of chlorite through weathering in landslide areas (Yoneda et al. 1995) and in the study of clay mineralogy of the Western Ghats (Deepthi and Balakrishanan, 2005). Examination of the XRD pattern of the hornblende gneiss (Fig.3b) reveals the presence of phlogopite, plagioclase and quartz. In granite gneiss (Fig.4b), additional to these phases, orthoclase is also present. The major difference in mineralogy is noted in comparing the XRD of lateritic soil (Fig.3a) developed over the hornblende gneiss and the XRD of forest loam (Fig.4a) developed over the granite gneiss. Mineralogy was determined from XRD by comparison with standard spectra available in published literature. The presence of kaolinite and montmorllinite in the forest loam is noticed. This can be directly correlated with the higher percentage of feldspar in granite gneiss relative to hornblende gneiss. Gibbsite is present in both soil types, indicating that the aluminum content was high, and that the weathering was persistent enough to convert higher silicates to aluminum hydroxides.

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Fig.2. Geological map of parts of Ernakulam and Idukki districts, Kerala JOUR.GEOL.SOC.INDIA, VOL.78, SEPT. 2011


Fig.3. XRD pattern of weathering profile over hornblende gneiss.

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indices change systematically with depth. However, the weathering of heterogeneous metamorphic rocks compounds the weathering index with depth (Price and Velbel, 2003). In the present context, CIA is calculated to know the intensity of weathering of rocks and soil profiles in the study and to compare the samples to understand the role of weathering in landslide occurrences. The calculated CIA values are shown in Table 1. Hornblende gneiss and granite gneiss show CIA values of 61.50 and 64.53 respectively, indicating the significant weathering condition in the area. The CIA value of soils show higher index than the rocks. The CIA value of lateritic soil and forest loam vales of 92.66 and 88.45 respectively, indicating the high extent of chemical weathering. Moreover, these values show that the profiles in the study area are in the final stage of weathering process. The leaf litter on the forest floor too contributes to accelerated weathering, but its influence over the total extend of weathering has not been studied. Previous studies in forested areas have documented the proactive role of plants and their roots in accelerating weathering, leading to clay mineral formation (Ranger, 2002). FACTORS FACILITATING WEATHERING AND LANDSLIDES

Fig.4. XRD pattern of weathering profile over granite gneiss.

Gibbsite is the dominant weathering residue in both the lithologies, probably due to the fact that the iron content is below 8% in both cases, while Al2O3 is around 13%. CHEMICAL INDEX OF ALTERATION

Chemical Index of Alteration (CIA) is a parameter commonly used as a measure of the intensity of chemical weathering of soils and sediments (Nesbitt and Young 1984). It is expressed as CIA = Al2O3/(Al2O3+CaO*+Na2O+K2O) x 100

(1)

Where, all components are expressed in molecular proportions and only the silicate part of total CaO* is considered. The chemical weathering indices is also used for characterizing the weathering profiles. Chemical Index of Alteration has been effectively utilized for palaeoclimatic studies (Soreghan et al. 2002), slope stability probability classification (Lindsay et al. 2001) and erosion due to tectonic activities (Dokuz and Tanyolu, 2004). Generally, on homogenous parent rocks, weathering JOUR.GEOL.SOC.INDIA, VOL.78, SEPT. 2011

The factors mentioned under this section facilitate both weathering and landslides and act in tandem, not as individual phenomenon. Rainfall, anthropogenic activities and soils are the factors which simultaneously influence the weathering phenomena and also the occurrence of landslides in the study area. Rainfall

Rainfall is an important triggering mechanism in landslide occurrences. It induces shallow landslides, mostly soil slip and debris flows, initiated by a transient loss of shear strength resulting from the increase in pore-water pressure, caused by intense rainfall on loose surface soil underlying finer less permeable bedrock (Wilson and Wieczorek, 1995). Temporal occurrence of landslides and movement activities is controlled by rainfall pattern with different types of resolution (van Asch et al. 1999). The study area experiences heavy showers than the state average annual rainfall (300 cm). The geographical location of the study area in the windward side of the Anamudi and Munnar massif, which are effective barriers for cloud movement, results in pockets of unusually high orographic precipitation. Most of the locations in the study area receive more than 380 cm rainfall per annum. The amount of precipitation increases from southeast to northwest.

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Neriyamangalam, a rain gauge station located in the study area receive the highest rainfall in Kerala with more than 500 cm of annual rainfall. Rainfall is a paramount factor influences chemical weathering. It controls the supply of moisture for chemical reactions and for the removal of soluble constituents of the minerals (Jenny, 1941). Due to this factor, rocks are well leached through the continual downward movement of percolating water. Because of the ample supply of moisture, the water table is often at shallow depth and much of the altered zone permanently saturated and in a reduced state. Under these conditions, the chemical weathering tends to proceed rapidly through the loss of soluble constituents and the residue becomes progressively enriched in minerals containing high proportion of alumina. Anthropogenic Activities

The surface of the earth is continuously altered by human beings for their sustenance. Improper land use practices like agricultural development on steep slopes, mining and quarry without considering the terrain conditions, construction of roads and buildings on unfavourable slopes can contribute landslide occurrences. Deforestation is another significant factor to be considered in occurrence of landslides. In Idukki district, the area covered by natural vegetation was 2049 sq km in 1998 compared to 4352 sq. km in 1966-67 (Jha et al. 2000). This indicates that the deforestation has occurred at a fast rate in the study area, due to the spread of human settlement. A quantification of degradation of natural forest cover done through topographic map, satellite data and field verifications, shows that 34% of deforestation has taken place within a span of 25 years (1978-2002). The reduction of forest cover has adversely impacted the equilibrium of the environment, results in weathering, accelerating soil erosion and landslides. Many studies have proven that the mass removal of forest cover leads to a drastic increase in the probability of landslides occurrence (Derose et al. 2006; Vasanthakumar and Bhagavannulu, 2008). Because, deforestation reduces soil cohesion, increases soil erosion and runoff, which induces landslides. The consequential defacing of such deforested area is extreme during a landslide, as the volume of debris liberated is high due to thick regolith cover. The sediment delivery rate is always higher in the deforested landslides than landslides in forest cover area (Fuente et al. 2002).

soil are the different types of soil, which covers the massive hornblende gneiss and granite gneiss in the study area. The knowledge of the spatial distribution and variability of soil thickness and its properties, coupled with detailed measurements of rainfall, would allow for the temporal and spatial prediction of slope instability during a storm. The physico-chemical properties of the soils are mostly consistent with the lithological diversities of the rocks as well as the physiographic and vegetation distribution pattern. Increase in moisture content makes a soil to lose its strength. Hence, soils when saturated with water, always have the tendency to flow because, the loss of shear strength resulting from increase in pore-water pressure. The geotechnical studies of soil could provide the more reliable information about the area prone for landslide occurrences (Anbazhagan et al. 2011). Kerala State Landuse Board has developed a soil erosion map, classified on the basis of physiography, soil association, soil depth, soil temperature regime, structure (surface and sub-surface), texture (weighted average), slope percentage

Fig.5. Soil erosion map of parts of Ernakulam and Idukki districts, Western Ghats.

and land use. The soil erosion map shows four different categories (Fig.5). The category ‘Severe with rock outcrop’ is characterized by rock outcrop with unfavourable conditions of slope, vegetation cover and soil temperature; whereas in ‘moderate to severe’ category, erosion is heavy without rock outcrop. The ‘moderate category with rock outcrop’ erosion is also characterized by the presence of rock outcrop but with certain limitations viz., wet soil structure, slope and soil depth. The fourth class called as ‘moderate’ type is prevalent in the study area and the quantity of erosion is less due to thick forest cover.

Soil Erosion

Soil is the final product of weathering and an important conditioning factor in the occurrence of landslides. Lateritic soil, forest loam, riverine alluvium and brown hydromorphic

DISCUSSION

The study area, in parts of Ernakulam and Iddukki JOUR.GEOL.SOC.INDIA, VOL.78, SEPT. 2011


districts in Kerala state, located in Western Ghats, bears testimony of frequent landslides, particularly during monsoon. The major type of landslides are debris flow, debris slide and rock fall. The factors which influence the occurrence of landslides in the study area include geographical location, topography, rainfall, geology, geomorphology, structure, slope, land use, land cover, drainage, soil etc. Rainfall and anthropogenic activities are the triggering mechanisms for landslides in the study area. The real cause of landslides involves the progressive weakening of slope materials by slow natural process such as continuous weathering and tectonic forces. Considering the importance of climate in the windward side of the Western Ghats and its influence on weathering and landslide occurrences, due attention has been given to study the weathering in this region. Weathering is responsible for changes in the mechanical properties of the rocks, in particular leading to increases in bulk density, changes in permeability, reduction in strength and increasing deformability. The prevalence of tropical climate facilitates the chemical weathering and aids in landslides in the study area. The essential element of tropical climate is the monsoon, which is the most important triggering mechanism for chemical weathering and landslides. Chemical weathering in this region has influenced the formation of thick columns of soil over the Precambrian rocks and the process continues to depth. The overburden is removed and brought downhill due to landslides that occur during the monsoon period. The magnitude and intensity of damage caused by landslides solely depend upon the quantity of material brought downhill which in turn depends upon the quantity of precipitation. This process produces new surfaces for nature to continue the cycle of weathering with more intensity. In addition to rainfall, the anthropogenic activities such as improper land use practices along steep slopes and deforestation are the major causes accelerating the process of weathering in the study area. Mixed cultivation with settlements is confined to the plains, gentle slope and communication corridors. Rubber and cardamom plantations cover the midland and highland regions respectively and occupy areas cleared of natural vegetation. Open scrub, the response product of deforestation, is noticed at number of places. Pineapple cultivation in the hillocks is seasonal as well as a summer crop, usually harvested before the onset of monsoon. During monsoon, the hillocks remain barren and the debris gets saturated, which leads to debris flow in this region. In the densely forested cover region, large trees always provide strong root structures that penetrate fragile cracks JOUR.GEOL.SOC.INDIA, VOL.78, SEPT. 2011

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in the underlying bedrock and anchor the soil. However, in the deforested area, the root system eventually dies off which leaves the soil vulnerable to over saturation. The vegetation and forest clearance for road construction and residential purposes, often allows rapid soil erosion and weathering to occur, increasing the potential for landslides (Smyth and Royle 2000). Earthquake-induced landslides cannot be ruled out in the study area. Periyar and Kerala are the two major faults in the study area, considered as active site of earthquakes. Rastogi et al (1989) deciphered reactivation along a NW-SE plane along the NW-SE flowing Kallar River, which is continuation of Periyar River in the area. The CIA value of hornblende gneiss, granite gneiss, lateritic soil and forest loam have indicated that the significant level of weathering process in the study area. Since the majority of the area is covered by these rocks and soil, the intensity of weathering process play major role in landslide occurrences. The nature of clay minerals formed due to weathering process differs in tropical regions according to high and low rainfall conditions. The weathering profile of west coast of India, which receives high rainfall (300 cm) have clays, aluminium hydroxides and Fe-Al oxides. However, the inland rain shadow zones (