Gulf of Mannar

A shallow, reef-fringed gulf between India and Sri Lanka, and India's first marine biosphere reserve.

The Gulf of Mannar is a warm, shallow arm of the Indian Ocean lying between the southeast coast of Tamil Nadu and the northwest coast of Sri Lanka.

Along its Indian side runs a chain of 21 low coral islands with fringing reefs, set in clear, shallow water among meadows of seagrass and old pearl-oyster beds. It is one of the richest stretches of sea in South Asia, and became India’s first Marine Biosphere Reserve.

The gulf sits in a far older feature: a deep rift in the Earth’s crust. India and Sri Lanka were once fused together inside the supercontinent Gondwana. As Gondwana pulled apart, the crust between them was stretched and thinned, and Sri Lanka rotated slightly away from India, opening the Mannar basin. The stretching came in pulses, with the main phase running from about 118 million years ago, in the Cretaceous, to the end of that period.

The split never finished. It slowed and stopped before it could grow into a new ocean, leaving what geologists call a failed rift, so the two lands stayed joined by shallow, thinned crust instead of drifting apart. Crustal thinning peaked around 70 to 65 million years ago, when basalt lava spread across the basin. Over the tens of millions of years that followed, the basin slowly sank and filled with sediment.

All of that left a broad, gently sloping, shallow shelf. Much more recently, during the last ice age, so much water was locked up in ice sheets that the sea stood roughly 120 metres lower, and much of this shelf was dry land. When the ice melted, sea level rose and then steadied a few thousand years ago, and corals colonised the shallow, sunlit platform, building the reefs and low islands that ring the gulf today.

Now the gulf is a protected sea, home to dugongs that graze its seagrass, sea turtles, hundreds of reef species, and pearl beds that people have worked for over two thousand years. Its two timescales connect. The ancient rift is the reason the young reef can thrive: stretching and sagging left a broad, shallow, clear shelf with no large muddy river draining into it, which is exactly the warm, well-lit, sediment-poor water corals need. The tectonics built the platform, and the corals grew on it.

A short reference note: this summary draws on third-party sources such as the Geological Survey of India and Wikipedia. In time I plan to give every geological feature its own learning-oriented write-up — with custom diagrams, graphs, and study pointers — as I’ve already done for the 11 locations that have an Instagram reel.

Sources