3. Results and Discussion
In the shore between Tuticorin and Vaippar River, the seafloor is sloping gently down to 5 m depth, while in Sippikkulam coast where the seafloor is very steep down to 2 m depth, it extends up to 0.129 km from the coast. In between 4 m and 5 m depths, some elevated rises and islands are observed. At a distance of 4.30 km (8°49′ N–78°12′16″ E) from Tuticorin coast, a 3.9 m elevated rise was observed. Just north-east of this rise a 6 m elevated island (8°49′35″ N–78°12′28″ E) called Van Island, which is situated 5 km away from Tuticorin coast. Around this island, the seafloor is sloping very gently down to 2 m depth in southeast, east, north-east and north directions. Whereas in the west, southwest and northwestern parts of the island, the seafloor slope is very steep. On the north-eastern, eastern and southern sides of the island, the seafloor is covered by fringing coral reef, extending down to 2 m depth with an approximate distance of 1.50 km from the coast of the island. Between the depths of 3 m and 4 m, there is another island called Koswari Island, which is located at 8°52′2″ N–78°13′22″ E, with a distance of 6.09 km from Tharuvaikulam coast. In between 5 m and 10 m depths, the seafloor slopes moderately. From the depth of 10 m to 20 m, the seafloor slopes gently, having a distance of 16.65 km between them. At a depth of 20 m, the seafloor falls suddenly with a very steep slope (continental slope) extending till 30 m depth. The width of this slope has been calculated as 7.27 km. The total width of continental shelf in Tuticorin region has been calculated as 26.75 km. In the coast from Veppalodai to Sippikkulam, between 4 m and 6 m depths, elevated islands, namely, Kariya Shuli and Vilangu Shuli Islands, are situated at a distance of 4.77 km and 6.56 km from Sippikulam coast respectively. The seafloor is found to be sloping gently towards north, east and south from the shore of Vilangu Shuli Island, whereas west of the seafloor tends to slope very steeply. An extensive well-developed fringing reef has been identified around Vilangu Shuli and Karia Shuli Islands extending to 3 m depth.
In the shore between Vaippar and Muttiapuram, the seafloor is at a depth of 3 m, which extends to an approximate distance of 3.51 km from the shore. In the area near the mouth of Vembar, the seafloor topography is plain with a depth of 2 m. Between 2 m and 20 m, the seafloor slopes very gently and extends to a distance of 19.75 km. A sudden steep slope is encountered at 20 m depth; this area is the starting point of the continental slope and it extends to the depth of 30 m. The width of the slope has been calculated as 5.45 km, and the width of the continental shelf has been calculated as 19.75 km. In the area from Terkku Narippeyur to Gundar River, the shelf is found at a depth of 3 m, extending to 0.36 km south and southeast of Narippeyur coast and 0.88 km south of Gundar River-mouth. Between the depths of 7 and 8 km, the seafloor is considerably broad and the slope is very gentle. An elevated island is found exposed above sea level, viz., Uppu Tanni Island situated at 6.72 km from Gundar River-mouth. The seafloor is plain up to 1 m depth in all directions around the island except in the north and north-eastern directions where the slope is very steep. In the shore between Gundar and Palar River, the seafloor is found at a depth of 3 m, and this deep extends to 1.19 km from the coast. At the south of Mel Mendal coast, the seafloor slopes very steeply to 7 m depth and extends to 1.44 km from the coast. Between the depths of 10 and 20 m, the seafloor slopes moderately with a width of 7.52 km. At an average depth of 20–30 m, there is a continental slope with 16.56 km width. In between 7 and 8 m depths, the seafloor is broad and has a very gentle slope. In this region particularly at latitude of 9°6′5″ N and longitude of 78°32′10″ E, an island, namely, Shalli Island, having 9 m elevation from seafloor is encountered. On the northern and southern sides of this island, the seafloor slopes very steep, while on the eastern and western sides, the slope is gentle. Towards the east from Shalli Island, another island, namely, Nalla Tanni Island, is encountered (9°6′11″ N–78°34′29″ E). Around this island, the seafloor slopes very gently, particularly in the north-east, east and southeastern sides of the island, which extends to a depth of 4 m from the shore of the island. Whereas in the northern and southwestern sides of this island, the seafloor have a very steep slope, extending to a depth of 4 m at a distance of about 0.36 m from the coast of the island. In the region between the Palar River-mouth and Dhanushkodi shelf, the topography has some irregularities. In the area opposite to the Palar River-mouth, there is a gentle depressed channel, having an approximate length of 4.41 km towards south. In the coast between the Palar River-mouth and Kalachimundal, the seafloor is found at a depth of 1 m, and it extends to a distance of 0.28 km from the shore. Between these areas, the seafloor gradient is very steep to a depth of 7 m. The width of this area is 1.60 km. After reaching 7 m depth, the seafloor rises up to 3 m depth; this depressed channel runs to a distance of 21.27 km towards the north-east and south and lies between 9°9′15″ N–78°40′4′ E and −9°13′8″ N–78°47′37″ E. The average width of this channel has been measured as 0.75 km. After reaching 3 m depth, the seafloor has a sudden fall to 10 m depth. In between the two depressions, a flat-topped continental rise has been observed. The average width of the continental rise is 1.58 km and is located 5.7 km from the shore. On this rise, there are two elevated islands rising 4 m from the seafloor and are located between 9°9′3″ N–78°41′28″ E and 9°9′4″ N–78°43′32″ E. These islands are called as Anaipar and Pilliyarmunai Islands. Around these islands, the seafloor slopes very gently, and extensively developed fringing corals are found within 2 m depth. Between 4 and 5 m depth, at a lat long of 9°9′12″ N–78°45′8′ E, an island, namely, Puvarasanpatti Island, having an elevation of 4.50 m from the seafloor, is encountered. At a depth between 10 m and 20, the seafloor slopes gently. The width of this continental shelf is 18.17 m. In the area between Keelakarai and Pudumadam, the seafloor is almost plain and has a depth of 3–4 m. This plain extends to a distance of 9.72 km from the coast. In this plain, some low elevated rise and Islands are observed. Located at a lat long of 78°49′10″ E–9°9′31″ N, an island, namely, Appa Island, having an elevation of 5 m from seafloor, has been noticed. Around this island, the seafloor slopes gently to 2 m depth. It extends to an average distance of 1.50 km. At 9°14′2″N–78°51′25″ E and 9°14′32″ N–78°52′31″ E, two continental rise have been observed having an elevation of 1 m from the seafloor. In between 4 and 5 m depths, there are some low elevated islands (5.50 m from seafloor), namely, Talairi, Valai and Muli Islands. They are located at an average distance of 8.45 km from Kaplar River-mouth. In this region, the seafloor gradient is very steep extending from 5 m depth to 10 m depth and then it slopes gently up to 30 m depth.
In the coast between Pudumadam and Thoniturai, the seafloor is almost plain having a depth of 2-3 m. This plain extends 6.95 km from the coast. Near Seeniappa Dargah and Thoniturai, this plain is encountered at 2 m depth, and it extends to an average distance of 2.17 km from the coast of Seeniappa Dargah and 0.50 km from Thoniturai coast. Along this plain, some low elevated continental rise (9°12′14″ N–79°5′19″ E) and chain of islands have been observed. From Seeniappa Dargah to 6.65 km towards south, an island, namely, Musal Island, having an elevation of 3.5 m from seafloor is encountered. Around this island, fringing corals have developed very extensively to a depth of 2 m; they extend 1.45 km towards the north, 1.75 km towards the northwest, 1.69 km towards the west and 1.42 km towards the south from the coast of the island. Around this island, the seafloor slopes gently up to a depth of 2 m. On the seaward side of this island, the seafloor slope is very steep to 10 m depth and to an average distance of 2 m. About 6.18 km away from Maraikayar Pattinam, there are two islands, namely, Manalli and Manalliputti Islands (9°12′23″ N–79°7′26″ E and 9°12′23″ N–79°8′16″E). Around these islands, the seafloor is encountered at 1 m depth, and the topography of the seafloor around this island has a moderate gradient. On the seaward side of this island, the seafloor slope is very steep, between 3 m and 6 m depths. The width of this slope is 0.73 km. At a depth of 6–7 m, a plain having a width of 3.52 km is encountered. The seafloor slopes gently between 7 and 30 m depths. In the area between Thoniturai and Pamban canal, the seafloor is encountered at a depth of 1 m and extends to an average distance of 1.07 km from the coast of Thoniturai and Velupilliyarkovil. In this area, the seafloor slopes very gently up to 2 m depth. In between 2 and 3 m depths, there is a vast plain seafloor extending 3.80 km from north to south and 23.3 km from east to west. Along this plain, there are low elevated chains of islands (9°14′28″ N–79°10′28″ E and 9°13′28″ N–79°14′16″ E), namely, Pumurichan, Kovi, Kursadi and Shingle Islands. Around these islands, the seafloor is encountered at a depth of 0.5 m and extends to an average distance of 0.49 km towards north, 1 km towards south, 0.19 km towards east and 0.27 km towards west. In the north of these islands, the seafloor slope is steep (till a depth of 1 m), whereas it is very gentle in the south (till a depth of 3 m). In between 3 m and 10 m, the seafloor slope is steep having a width of 1.83 km. From 10 to 30 m, the seafloor slopes moderately. In the area between Dhanushkodi and Kundugal, the seafloor is at a depth of 6 m and slopes moderately up to 30 m depth. The width of the continental shelf in this area has been measured approximately as 26.25 km.
A comparative study was conducted using the contour map of 1999 and bathymetry map of 1975. The study exhibited a decrease in the seafloor level in the study area. This may be attributed to emerging of land due to tectonic activity. Several marine scientists have also confirmed that the coast of Gulf of Mannar is on an emerging phase due to tectonic movement (Foot 1888; Ahmad 1982; Stoddart and Pillai 1972; Loveson and Rajamanickam 1988; Ramasamy 1989). Ramasamy (1997) has build up a post collision tectonic model for the southern part of Indian in which he has observed a series of geo-environmental problems being caused due to such ongoing tectonic movement. In some areas (particularly at river-mouths and some islands), the seafloor level increased as a result of erosional activities. In and around the Tuticorin group of islands, the illegal removal of corals resulted in an increase in depth (Thanikackalam and Ramachandran 2002). The average depth reduction of seafloor along the coast of the study area has been estimated as 0.51 m over a period of 24 years, which is equivalent to 0.021 m/year along the coast and 0.023 m/year around the island. This estimation is based on the assumption that the rate of change of seafloor is uniform throughout the study period. Thus, we conclude from this result and discussion that tectonic movement coupled with anthropogenic activities (like coral mining) play a crucial role in regulating the anatomy, physiology and morphology of seafloor.