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Indian Journal of Geo-Marine Sciences Vol. 39(4), December 2010, pp. 557-561 Coastal geomorphic features around Indian Ocean P Nag National Atlas and Thematic Mapping Organisation Bidhan Nagar, Kolkata -700 064, India [E-mail: [email protected]] Received 20 August 2010; revised 21 December 2010 Coastal areas are important globally. These areas have also drawn attention of geomorphologists for in-depth studies. Several institutions have made contributions towards analysis, identifications of different geomorphic features and their mapping. Attempts have been made to map oceans and associated coastal geomorphic features. In this paper an effort has been made for the Indian Ocean. There are straits, channels, continental sub-marine margins and continental shelf which form different components of the coastal ocean. They are shaped by land erosion, sub aerial deposition, volcanic, diastrophic, marine erosion and deposition and organisms. The interactions of these features are so variable that the coastal features are almost infinitely complex. The coasts of Indian Ocean realm fall into more than one category. Therefore, a matrix reference can be used with definite colour band to depict each single or compound feature. [Keywords: Coastal features, Shepard's classification, Erosion, Marine deposition] Introduction According to United Nations estimate, 66 per cent of the world population lives within few kilometers of the coast. Consequently, food production, communication, settlements and even recreation facilities are concentrated along the coast. As such, coastal areas, all over the world draw attention of the geomorphologists for in-depth study of various coastal features of their interest. There are different considerations for demarcating the coastal area. As such, some schools of thought define that the coastal zone may extend landward and seaward keeping in view the influence of sea on land and land on sea respectively. The Indian Ocean Atlas1,2 depicts the physiographic profiles of the Indian Ocean and the adjoining land area. The contours indicate the asymmetrical distribution of the physical features in land part and the bathymetry backed by recent geophysical investigations indicate the most complex nature of the Indian Ocean with regard to its history of formation and thereby its physiography. In this context it is to be remembered that this ocean is nearly land-locked from three sides and open to the Southern Ocean. Again this Southern Ocean is itself considered a special subdivision of each adjoining major ocean of the world (Fig. 1). Materials and Methods Sources of Information and Techniques For the preparation of the map shown in Fig. 1, a scale of 1:25,000,000 is used. Isobaths and contours are given in kilometers. Bathymetric data are compiled from charts No.7070 to 7073 published by the National Hydrographic Office (NHO), Dehra Dun, and sheet No.1-9 and map of Indonesia, showing bottom topography of the Indian Ocean and also sheet 1-4 showing topography of the continental slope of Antarctica in Geological–Geophysical Atlas of the Indian Ocean3. The charts published by NHO are at a scale of 1:5,000,000. Land topography including that of Antarctica has been compiled from the maps and atlases published by Survey of India, National Atlas & Thematic Mapping Organisation and other agencies. Geophysical names for physical feature are more or less followed as generally used in English, though there are exceptions also viz. Massif Di Ivakoanv is the French term of Massif Ivakoanv, Bab-el–Mandeb is the Arabian term used for the strait of Mandeb, and Nafud is an Arabian term to designate desert and Dasht-e-Kebir is the Persian term of Kebir desert. The horizontal cross profile along 20˚S parallel has been drawn to show a representation of the bathy-orographical features of the study area. The inset map at a scale 1:100,000,000 show some selected geo-features viz. the epicentres of earthquakes, active volcanoes, sea mounts, guyots and canyons. In the map, the important physical features of land part are mountains and hills, plateaus and rift valleys, and river valleys and coastal plains. The important physical features of the ocean part are seas, bays, gulfs, straits, channels, banks, submarine ridges, plateaus, basins, abyssal plains, etc. Deep blue to light INDIAN J. MAR. SCI., VOL. 39, No. 4, DECEMBER 2010 558 Ocean with the South China Sea. It is approximately 805 km long and varies in width from 64 km in the south to 257 km in the north. The strait assumed its present configuration as a result of the post-glacial rise of sea level, which drowned the Sunda strait. Strait of Hormuz—The Strait of Hormuz, the entrance to the Persian Gulf from the Indian Ocean, is a narrow, strategically important stretch of ocean between the Gulf of Oman in the southeast and the Persian Gulf in the southwest. On the north coast of the strait is Iran (Persia) and on the south coast is the United Arab Emirates and Musandam, an exclave of Oman. The strait at its narrowest is 34 km having two 1.6 km wide channels for marine traffic separated by a 3.2 km wide buffer zone. This is the only sea passage to the open ocean for large areas of the petroleum exporting Persian Gulf states. Continental sub-marine margins This part of the ocean has direct links with the adjoining continents. The shelf regions, the continental slope and the continental foot are the three distinguished physiographic zones of continental sub-marine margins. Continental shelf Fig. 1―Indian Ocean – Physical. shades indicate variations in depth whereas the variation of land elevation from plains to mountains is shown by green to brown. The prominent features are written on the body of the map. Contours are drawn in area concerned. Results and Discussion Straits and Channels There are a large number of straits and channels in the Indian Ocean. The straits which are narrow stretch of sea connecting two extensive areas of sea have been formed by fracture across an isthmus or by the sea overflowing land, which had subsided or eroded. On the other hand, the channels as applicable for the ocean features are relatively narrow stretch of sea between two landmasses, connecting two or more extensive areas of sea. The straits and channels of Indian Ocean along with their strategic location have been enlisted. Malacca Strait—The Malacca Strait (also called the Straits of Malacca, and in Malay Salet Melaka), a shallow passage between the Malay peninsula and the Indonesian island of Sumatra, connects Indian This region directly adjoins the land and is made up of shallow water fringe zones with average depth of less then 0.2 km and a relatively gentle slope towards the ocean. The surface topography of the shelf includes old shorelines, flooded terraces, submarine extension of river valleys and other forms of the land relief. The width of the shelf varies from a few kilometers to several hundred kilometers. The Indian Ocean except for the relatively wide shelf zones in the eastern sector of the Arabian Sea, the eastern sector of the Bay of Bengal and the northwest coast of Australia, the continental shelf of much of the Indian Ocean and adjacent seas is narrow and in places precipitous. The shelf area of Indian Ocean is about 3,141,000 sq. km as against 11,150,000 sq. km for the Pacific and 14,180,000 sq. km for the Atlantic Ocean. Along the eastern coastline of Africa in general, the shelf is narrow, less than 4 km from shore, except southern tip of Africa adjoining Cape Agulhas and area adjoining Baria. From Mombassa to Cape Guardafui along the Somali coast, the extremely narrow shelf is suggestive of a fault coast. However, east of 51˚E off the African coast, the continental shelf is well developed and forms a platform. The continental shelf of Red Sea region widens in the central and southern sector. In some areas in south sector the shelf extends for more then 100 km off NAG: COASTAL GEOMORPHIC FEATURES AROUND INDIAN OCEAN shore. The shelf is approximately 37 km across the Gulf of Aden at the entrance of the Red Sea. East of Aden, the shelf becomes narrow. There are a couple of places along Yemen and Oman coasts where there is no shelf area at all. At the same time along the Arabian coast between 56˚E and 58˚E the shelf forms the platform supporting the Kuria-Muria Islands. The whole of Persian Gulf is under shelf region and less than 40 m in depth. The Gulf of Oman has a broad mud covered shelf on its head. Thereafter as one proceeds eastward, the shelf is narrow along the coast of southern Iran (Makran coast) and Baluchistan coast (west of Karachi). The continental shelf along the greater part of the coast of western Pakistan is about 40 km wide except across the Indus delta, where it becomes around 100 km wide. In some places east of Karachi, the shelf is 180 km or more in width. The continental shelf of western coast of India is comparatively wide between 20 and 160 km except near Bengal coast at the mouth of River Ganga, where it is as much as 160 km wide .The Andhra shelf has an average width of about 43 km. The continental shelf along the entire coastline of Bangladesh is broad varying between 125-160 km and the shelf area is approximately 170 sq. km. The continental shelf of Myanmar is in continuity with that of Bangladesh and is comparatively wide, ranging from 70 to 180 km. Thailand has two distinct parts of the coastlines. In Indian Ocean area, the coastline is small facing the Andaman Sea, where the continental self is rather narrow. The second part is Gulf of Thailand, which is actually a part of the South China Sea. Malaysia and Singapore have a wide continental shelf all around the peninsula. The continental shelf of the western part of the islands of Sumatra, Java, Bali, Lomba of Indonesia have a narrow continental shelf whereas the continental shelf of the eastern part of these islands including Borneo have very wide continental shelf known as Sunda shelf. In fact the Sunda shelf covers the area between the three large islands namely Sumatra, Borneo and Java and mainland Asia, including the southern part of South China Sea, the Gulf of Thailand and the Java Sea as well as the shallow part of the Strait of Malacca, previously called the Southeast Asiatic shelf. The northwestern Australia has a vast continental shelf and was previously known as Great Australian Bank of East (to match the Great Asiatic Bank–Sunda shelf). This Great Australian Bank has been divided into three parts viz. Rowley Shelf (115˚ to 122˚E), Sahul Shelf (122˚ to 131˚E) and Arafura shelf. The 559 first two have areas of about 3,00,000 km² while the third one is 9,30,000 km². In Western Australia, around Shark bay, the continental shelf is moderately wide but the other areas are comparatively narrow. The shelf area of Antarctica coast adjoining Indian Ocean is also narrow. Coastal land form features As an integral part of the earth system, the geomorphologic processes at work on coastal landforms are influenced by number of environmental factors such as geological, climatic, biotic, tidal and other oceanographic factors including salinity and sea level changes. These factors vary from one sector of the coast to another. The variation is zonal in terms of climatic regions and irregular in terms of geological outcrops. Thus, there are rocky coasts, beaches, river mouths, fiords, estuaries, lagoons, barrier islands and inter-tidal wetlands. In and around them, a number of specific biological communities have developed including inter-tidal and marsh communities, mangroves, sea grass, deep beds and coral reefs. The coastal area of the Indian Ocean is adorned with a large variety of features, which have been shaped by different agents in different environmental conditions. Broadly, the Indian Ocean coasts are shaped by the actions of seven features namely (Fig. 2): . Fig. 2―Indian Ocean - Coastal Features. 560 INDIAN J. MAR. SCI., VOL. 39, No. 4, DECEMBER 2010 (1) Land Erosion (2) Sub aerial deposition (3) Volcanic (4) Diastrophic (5) Marine erosion (6) Marine deposition (7) Organisms. The interactions of these features are so variable that the coastal features are almost infinitely complex. Many times the coasts of Indian Ocean realm fall into more than one category. Therefore, a matrix reference can be used with definite colour band for each single or compound feature (Fig. 3). Thereafter, where third feature comes into action, a colour pattern has to be superimposed as shown in the Indian Ocean Atlas prepared by the National Atlas & Thematic Mapping Organisation. For example, the coasts of extreme southern India are primarily formed by sub-aerial deposition as well as marine deposition. Simultaneously, marine erosion is also an action to that very area in shaping the coasts. Now, to depict the coastal features of that area, the first two actions are shown by a particular colour band, and for the third feature colour pattern (horizontal black lines) has been superimposed. Similarly, predominantly three features of which two features namely volcanic and marine erosion are shown by colour band while for the third feature i.e. ‘marine deposition’ by black dot pattern is superimposed on the colour band form the coast from Karachi to Gwadar in Pakistan. The Tanzanian coast is shaped by five predominant features namely marine erosion, marine deposition, organism, and diastrophic and sub-aerial deposition. This particular combination can be shown by a complete separate colour band and pattern. Another category, which has recently been receiving closer attention, is the ‘Frozen Coast’; it is well developed in Antarctica where ice shelves terminate in cliffs up to 35 m high. In summer, icebergs calves from these and in winter the adjacent sea freezes. Such condition in Antarctica is represented by ‘frozen coast’. Studies on coastal classification One of the early attempts at coastal classification was put forward by Suess4. He distinguished Atlantic coasts which run transverse to the general trend of geological structures from Pacific coasts, which run parallel to structural trends. The former is characterised by the Atlantic coast of Europe and the latter by the Pacific coast of North America. In another method of classification Gulliver5 distinguished the coasts formed by submergence from the coasts formed by emergence. The distinction made by him was developed into a genetic classification of shorelines by Johnson6, who proposed three categories, namely: (1) Shorelines of submergence (2) Shorelines of emergence and (3) Neutral shoreline formed neither due to submergence nor emergence but by deposit or tectonic movement e.g. delta shorelines, alluvial plain shorelines, outwash plain shorelines, volcano shorelines and fault shorelines. Cotton7 suggested a scheme, which is most straight forward. He made a fundamental distinction between coasts of stable and mobile regions. Tectonic movement during quaternary period has not affected stable regions, while mobile regions have been subject to quaternary period’s earth movement, which may still be continuing. Vatentin8 made a fundamental distinction between advancing and retreating coasts, noting that advance may be due to coastal emergence or progradation by deposition while retreat is due to coastal submergence and/or retrogation by erosion. The coastal features of Indian Ocean realm can however primarily be classified based on Shepard's method and with a combination of various processes. The following features came into force in shaping the coasts of Indian Ocean realm: a b c d e Fig. 3―Combination of Coastal Features. Sub aerial deposition+marine erosion. Sub aerial deposition+Marine deposition. Sub aerial deposition+Marine decomposition + marine erosion. Sub aerial deposition+Marine erosion +coast built by organisms. Sub aerial deposition+Marine deposition +land erosion. NAG: COASTAL GEOMORPHIC FEATURES AROUND INDIAN OCEAN f Sub aerial deposition+coast built by organisms. g Sub aerial deposition+land erosion. h Sub aerial deposition+volcanic. i Sub aerial deposition. j Diastrophic+marine erosion+marine deposition k Diastrophic+marine deposition. l Diastrophic+marine deposition+coasts built by organisms. m Diastrophic+coasts built by organism + volcanic. n Diastrophic. o Volcanic+marine erosion+marine deposition. p Volcanic+diastrophic. q Volcanic. r Marine erosion. s Marine deposition+marine erosion. t Marine deposition+marine erosion +land erosion. u Marine deposition v Coast built by organisms+marine erosion. w Coast built by organisms+marine erosion +marine deposition. x Coasts built by organisms. y Marine erosion+marine deposition +coast built by organisms+diastrophic+sub aerial deposition. On the whole, it is observed (rank—wise) that marine deposition, organism, sub aerial deposition, marine erosion, diastrophic, volcanic and land erosional factors are active in 43, 40, 34, 27, 7, 5 and 2 areas respectively in shaping the Indian Ocean coasts. It is also observed that organisms, marine 561 deposition, sub aerial deposition, marine erosion, volcanic and diastrophic factors have been predominantly responsible in shaping the coasts in 17, 14, 6, 4, 4 and 2 areas respectively in Indian Ocean realm. Conclusion The study of the coastal areas is exciting and also a challenging field. In such a study one might observe changes in the coastal features even within a single day. A point-to-point description of coastal features and its components have been given. But it should be noted that this picture is a generalized one. Exact cause of action and the coverage may be drawn by more detailed micro-level studies. However, this map would be a useful in the quest for the factors responsible in shaping the coasts of Indian Ocean realm. References 1 Nag P, (Ed.), Indian Ocean Atlas, (National Atlas & Thematic Mapping Organisation, Kolkata) 1998. 2 Nag P, (Ed.), Geography of the Indian Ocean, (National Atlas & Thematic Mapping Organisation, Kolkata) 2006. 3 Academy of Sciences of the USSR, International Indian Ocean Expedition (1960-1965), Geological-Geophysical Atlas of the Indian Ocean (Moscow) 1975. 4 Suess E, The Face of the Earth, (Carleton Press, Oxford) 1906. 5 Gulliver F P, Shoreline topography, (Proceedings of the American Academy of Arts & Science) 1899, Vol. 34, pp. 151258. 6 Johnson D W, Shore Process and Shoreline Development, (Wiley, New York) 1919. 7 Cotton C A, Criteria for the classification of coats, (Proceedings of the 17th International Geographical Congress, Washington) 1952, pp. 315-19. 8 Vatentin, Harmut, Die Kusten der Erde, (Justus Perthes Gotha, Berlin) 1952.