(Gulf of Kachchh, Gulf of Mannar, Lakshadweep, and
Andaman and Nicobar Islands) is mainly governed by depositional and erosional processes, which occur during glacioeustatic sea level fluctuations. Other factors involved,
directly or indirectly, are structure, lithology, sedimentation,
tectonic movements, and climatic changes.
Seismic data from the west coast of India revealed the
occurrence of buried reefs off Saurashtra and Ratnagiri,
indicating pinnacles and protuberances of different
heights. The cliffy coast of southern Saurashtra has preserved a record of the sea level changes during the late
Quaternary (Rao and Wagle, 1997). Based on the geomorphic positions of the notches, the older sea strand lies at an
elevation ranging from 12 to 15 m above the present
BMSL and has been attributed to the last interglacial
(MIS-5). Following this, a major tectonic uplift of about
6–9 m was experienced by the southern cliffed coast.
The Holocene sea level was recorded at 4–5 m which is
about 2 m higher than the general MIS-1 sea level. Thus,
the submarine terraces on the shelf must have also been
formed during the late Pleistocene to mid-Holocene still
stands of the sea level. Radiocarbon dates of surficial samples of algal, oolitic, and coralline limestone (Nair and
Hashimi, 1980) collected from the terraces and/or in their
vicinity suggest that the innermost terrace along the shelf
(55–60 m) is ca 7,845 yrs B.P., and the outermost terrace
on the shelf is ca 11,150 yrs B.P.
The distribution of reefs on the west coast shelf is random which perhaps may imply that conditions for reef
growth have not been uniform. It is interesting to note that
between Mumbai and Mormugao, where the shelf is relatively broader, reefs occur abundantly on the middle and
outer shelf. On the other hand, south of Mormugao, where
the shelf is narrow, they are sparsely present and only on
the outer shelf. Submerged coral reefs may be defined as
reefs that were established in relation to a pre-existing
sea-level lower than present and occur at depths greater
than that commonly associated with vigorous growth of
reef building, hermatypic corals. Further, the radiocarbon
dating of coral and associated fauna indicates that they
are of early Holocene age.
The Lakshadweep Islands rise from a submarine platform with coral reefs in the form of an atoll. It is crescent-shaped having a north-south orientation. The
western margin of the lagoon is a submarine bank with
a narrow reef below. The lithology is composed of an
upper 1–2 m-thick layer of coral rubble, below which is
a compact porous crust of conglomerate and soil composed mostly of coral sand. Topography is a flat 1–2 m
above sea level. Erosion occurs mostly along the shore
towards the northeast and northwest (Anon, 1990).
The Gulf of Mannar extends from Tuticorin to
Rameswaram Island in a SW-NE direction and lies
between 78
5’ & 79
30’E longitudes and 8
47’ &
9
15’N latitudes, a distance of about 140 km. There are
21 islands between Tuticorin and Rameswaram. All
islands are composed of a calcareous framework of dead
reef and sand and have a low and narrow sandy coast.
Around all offshore islands, well-developed coral reefs
occur. Geomorphologically, coral reefs in this area are of
fringing type, though some patchy corals are also
observed in between Appa Island and Pilliyarmuni Island,
and in some areas such as the Bharathinagar coast and the
southeast coast of Kariya Shuli Island (Loveson, 1993).
Wave cut platforms are common on the coast of
Mandapam,
Ramaswami
Madam,
Pudumatam,
Valinokkam, etc. Along the coast of the Gulf of Mannar,
cliffs occur though at some places these features have been
destroyed due to slumping. In the Gulf of Mannar, the
slope and width of the continental shelf is approximately
the same as the average for the eastern coast of India
(Ahmed, 1972). The total width of the shelf is around
30 km having a slope of about 21’. The slope near the
shore is about 4’. Recent depth contour maps indicate that
the seafloor level decreases along the coast and around the
islands due to emergence of the land or lowering of sea
level (due to tectonism) and sediment deposition. The
beach is gently sloping and marked with altered crests
and troughs that are formed due to wave action. As the
Gulf of Mannar is on the lee side of the northeast monsoon, there is no long-shore drift from the northeast that
might affect the morphology of the spit, (Ahmed, 1972).
The southwestern shore of Rameswaram has a tongue
shaped spit.
The Andaman and Nicobar Islands are parallel to
the arcuate plate boundary separating the Indian and
Sunda Plates east of the 9
E ridge (Dasgupta and
Mukhopadhyaya, 1993; Rajendran and Gupta, 1989).
The ridge is believed to have been formed from sediments
scraped off the descending Indian Plate interleaved with
ophiolites from the ocean floor beneath the Bengal Fan.
In that, the islands are bounded to the east by strike-slip
faults and spreading centers, and to the west by
a subduction zone, the Andaman and Nicobar ridge acts
as a small tectonic plate that has been referred to as the
Burma Plate by Curray et al. (1982) and as the Andaman
Plate by Dasgupta (1993). The geology of the Andamans
is described by Oldham (1884) and Tipper (1911).
A series of earthquakes ranging up to a magnitude of
7.5 under the Andaman and Nicobar Islands on 26 December 2004 shook the entire Andaman Sea region. The end
result was that the Burma Plate, which contains the
Islands, tilted during an earthquake “swarm” lasting for
about 8 min after the major quake off Sumatra. Thus, these
islands were a source of the tsunamis that spread out
across the Indian Ocean and themselves impacted by tsunamis from the whole series of plate movements on that
morning. The effects were very different on the mainland
of India to the offshore Andaman and Nicobar Islands.
The primary slip offshore of Sumatra was followed by
two other slips towards the north. Though some of these
findings need to be further substantiated, the fact remains
that the Sumatra earthquake caused significant changes to
the earth’s tectonic processes (Ortiz and Bilham, 2003).
The Andaman and Nicobar area, including Car Nicobar,
Katchel, and Campbell Islands were not only affected by
270
CORAL REEFS OF INDIA
Andaman and Nicobar Islands) is mainly governed by depositional and erosional processes, which occur during glacioeustatic sea level fluctuations. Other factors involved,
directly or indirectly, are structure, lithology, sedimentation,
tectonic movements, and climatic changes.
Seismic data from the west coast of India revealed the
occurrence of buried reefs off Saurashtra and Ratnagiri,
indicating pinnacles and protuberances of different
heights. The cliffy coast of southern Saurashtra has preserved a record of the sea level changes during the late
Quaternary (Rao and Wagle, 1997). Based on the geomorphic positions of the notches, the older sea strand lies at an
elevation ranging from 12 to 15 m above the present
BMSL and has been attributed to the last interglacial
(MIS-5). Following this, a major tectonic uplift of about
6–9 m was experienced by the southern cliffed coast.
The Holocene sea level was recorded at 4–5 m which is
about 2 m higher than the general MIS-1 sea level. Thus,
the submarine terraces on the shelf must have also been
formed during the late Pleistocene to mid-Holocene still
stands of the sea level. Radiocarbon dates of surficial samples of algal, oolitic, and coralline limestone (Nair and
Hashimi, 1980) collected from the terraces and/or in their
vicinity suggest that the innermost terrace along the shelf
(55–60 m) is ca 7,845 yrs B.P., and the outermost terrace
on the shelf is ca 11,150 yrs B.P.
The distribution of reefs on the west coast shelf is random which perhaps may imply that conditions for reef
growth have not been uniform. It is interesting to note that
between Mumbai and Mormugao, where the shelf is relatively broader, reefs occur abundantly on the middle and
outer shelf. On the other hand, south of Mormugao, where
the shelf is narrow, they are sparsely present and only on
the outer shelf. Submerged coral reefs may be defined as
reefs that were established in relation to a pre-existing
sea-level lower than present and occur at depths greater
than that commonly associated with vigorous growth of
reef building, hermatypic corals. Further, the radiocarbon
dating of coral and associated fauna indicates that they
are of early Holocene age.
The Lakshadweep Islands rise from a submarine platform with coral reefs in the form of an atoll. It is crescent-shaped having a north-south orientation. The
western margin of the lagoon is a submarine bank with
a narrow reef below. The lithology is composed of an
upper 1–2 m-thick layer of coral rubble, below which is
a compact porous crust of conglomerate and soil composed mostly of coral sand. Topography is a flat 1–2 m
above sea level. Erosion occurs mostly along the shore
towards the northeast and northwest (Anon, 1990).
The Gulf of Mannar extends from Tuticorin to
Rameswaram Island in a SW-NE direction and lies
between 78
5’ & 79
30’E longitudes and 8
47’ &
9
15’N latitudes, a distance of about 140 km. There are
21 islands between Tuticorin and Rameswaram. All
islands are composed of a calcareous framework of dead
reef and sand and have a low and narrow sandy coast.
Around all offshore islands, well-developed coral reefs
occur. Geomorphologically, coral reefs in this area are of
fringing type, though some patchy corals are also
observed in between Appa Island and Pilliyarmuni Island,
and in some areas such as the Bharathinagar coast and the
southeast coast of Kariya Shuli Island (Loveson, 1993).
Wave cut platforms are common on the coast of
Mandapam,
Ramaswami
Madam,
Pudumatam,
Valinokkam, etc. Along the coast of the Gulf of Mannar,
cliffs occur though at some places these features have been
destroyed due to slumping. In the Gulf of Mannar, the
slope and width of the continental shelf is approximately
the same as the average for the eastern coast of India
(Ahmed, 1972). The total width of the shelf is around
30 km having a slope of about 21’. The slope near the
shore is about 4’. Recent depth contour maps indicate that
the seafloor level decreases along the coast and around the
islands due to emergence of the land or lowering of sea
level (due to tectonism) and sediment deposition. The
beach is gently sloping and marked with altered crests
and troughs that are formed due to wave action. As the
Gulf of Mannar is on the lee side of the northeast monsoon, there is no long-shore drift from the northeast that
might affect the morphology of the spit, (Ahmed, 1972).
The southwestern shore of Rameswaram has a tongue
shaped spit.
The Andaman and Nicobar Islands are parallel to
the arcuate plate boundary separating the Indian and
Sunda Plates east of the 9
E ridge (Dasgupta and
Mukhopadhyaya, 1993; Rajendran and Gupta, 1989).
The ridge is believed to have been formed from sediments
scraped off the descending Indian Plate interleaved with
ophiolites from the ocean floor beneath the Bengal Fan.
In that, the islands are bounded to the east by strike-slip
faults and spreading centers, and to the west by
a subduction zone, the Andaman and Nicobar ridge acts
as a small tectonic plate that has been referred to as the
Burma Plate by Curray et al. (1982) and as the Andaman
Plate by Dasgupta (1993). The geology of the Andamans
is described by Oldham (1884) and Tipper (1911).
A series of earthquakes ranging up to a magnitude of
7.5 under the Andaman and Nicobar Islands on 26 December 2004 shook the entire Andaman Sea region. The end
result was that the Burma Plate, which contains the
Islands, tilted during an earthquake “swarm” lasting for
about 8 min after the major quake off Sumatra. Thus, these
islands were a source of the tsunamis that spread out
across the Indian Ocean and themselves impacted by tsunamis from the whole series of plate movements on that
morning. The effects were very different on the mainland
of India to the offshore Andaman and Nicobar Islands.
The primary slip offshore of Sumatra was followed by
two other slips towards the north. Though some of these
findings need to be further substantiated, the fact remains
that the Sumatra earthquake caused significant changes to
the earth’s tectonic processes (Ortiz and Bilham, 2003).
The Andaman and Nicobar area, including Car Nicobar,
Katchel, and Campbell Islands were not only affected by
270
CORAL REEFS OF INDIA