Geology

The Western Indian Ocean alone contains every type of tectonic plate boundary, both active and fossil, some of the deepest fracture zones, the most complex mid-ocean ridge configurations and some of the thickest sedimentary sequences of the world’s ocean basins. It is one of the most geologically diverse ocean basins on the planet. The continental land mass of Africa, the micro-continent Madagascar, and the North Seychelles Bank, are remnants of the supercontinent Gondwana, dating from pre-Cambrian times, over 650 mya, and that started to break up 180 mya.

Ocean basins – The Indian Ocean spreading ridge rises to just under 2000 m depth, isolating the WIO from deep waters to the east. The four deep basins in the WIO include the Madagascar (5500 m, southeast of Madagascar), Mascarene (4900 m, west of the Mascarene Plateau), Mozambique (5000 m, south of the Mozambique channel) and Somali basins (5100 m, between Somalia and the Seychelles). Little has been done on the abyssal plains and soft sediments of these basins, though drilling on and near the Mascarene Plateau shows thick accumulations of marine sediments, and in the Mozambique channel of terrestrial sediments.

Plate tectonics - The ocean floor is composed of three major plates, the African, Indian and Australian plates, and the Arabian plate in the north, with an active rift along the Central Indian Ocean ridge. The Indian Ocean started to form 180 million years ago (mya), when the land mass containing the future Madagascar, Australia, India and Antarctica split from the African coast. From 120 mya Australia started to separate from Madagascar-India, and from 80 mya, India started to separate from Madagascar and move northwards. From about 70-50 mya the Indian Plate exhibited among the highest recorded speeds, up to 16 cm/yr, potentially as a result of superplume activity of the Reunion hotspot. Currently the Indian and Australian plates are rifting away from the African plate, but with apparently little relative motion relative to one another. The Arabian plate is rifting from the African plate, forming the Red Sea, starting in the Eocene and accelerated during the Oligocene.

Hotspots and magmatic provinces – the western Indian Ocean contains a number of these, including the Mascarene-Reunion hotpot, the Comoros hotspot and the remnants of massive magmatic extrusions including features in India (Deccan Traps), Madagascar and the underwater plateau extending southwars (Cretaceous activity) and others.

Mascarene-Reunion hotspot - at the Cretaceous-Tertiary (K-T) boundary, about 67-64 mya, a massive eruption of magma that formed the Deccan Traps in India (classified as a Large Igneous Province, or LIP) occurred as India was already moving away from Madagascar, and is implicated in the stranding of part of the Indian continent as part of the northern Seychelles bank in the middle of the ocean. The ‘superplume event’ that produced the Deccan Traps may have played a role in the last of five mass extinctions that have occurred in the Earth’s history, at the K-T boundary, in which the all the dinosaurs and up to 30 % of genera of many marine taxa (including corals), went extinct.
Following this, the Mascarene-Reunion hotspot remained active, and combined with the rapid tectonic movements of India, has produced a unique mid-ocean feature – the volcanic islands and carbonate-topped banks of the western and central Indian Ocean, the Mascarene Plateau. Starting with the Lakshadweep – Maldives chains (57-60 mya), the series of banks and islands strung southwards down the Indian Ocean were produced: the Chagos archipelago (48 mya), Saya de Malha (45 mya), Nazareth and Cargados Carajos banks (34 mya), Mauritius (7-8 mya) and finally , Reunion (0-2 mya), which is still volcanically active today. Two complications add further interest to this system: the northern Seychelles bank and its granitic islands are remnants of continental rock, stranded in mid-ocean as India passed over the spreading ridge 65 mya. Second, the spreading ridge itself moved over the hotspot about 45 mya, splitting the Chagos and Saya de Malha banks, and forming a kink in the otherwise continuous chain from India to Reunion. These islands and banks form a classic example of the progression from volcanic high islands to sunken coralline banks, from Reunion to India, charting the span of the entire Cenozoic era.

Marion hotspot - Madagascar Plateau – the continental fragment of Madagascar lay over the Marion hotspot during the Cretaceous, with significant evidence of activity during this period in south of the island. The submarine Madagascar Plateau was formed by basaltic lava from the Marion hotspot, as Madagascar and Antarctica separated during the Cretaceous. Called the Rowley shoals in maritime charts and known for highly turbulent conditions induced by the Plateau, this geological feature is a southward extension of Madagascar at depths of 1,000 – 2,000 m, rising above the deeper basins to the east and west up to 5,000 m deep. Activity of the Marion magmatic plume is also implicated in the breakup of Madagascar and India, and India’s rapid migration northwards preceding the activity of the Mascarene-Reunion hotspot.

Mozambique channel - within the Mozambique channel, the Davies ridge runs north-south between about 13 and 18°S, rising to nearly 300 m below the surface, with origins reminiscent of old continental or lithospheric fractures left over by the initial separation of Madagascar from the African continent. The formation of the Comoro archipelago is poorly known, with one theory based on magmatic extrusion associated with these lithospheric fractures. An alternative explanation emphasizes recent volcanic activity during the Miocene from about 10 mya, such as of the Montagne d’Ambre, a large massif at the northern tip of Madagascar. This preceded activity of the Comoros hotspot which has produced the Comoros archipelago, ranging from the oldest island, Mayotte (5.4 mya), to the youngest island, Ngazidja (Grande Comore, ≈ 130,000 ya).

Seychelles-Mascarene activity - A complex geological history is also suggested by the multiple islands and groups in the Seychelles, such as the Aldabra group and others such as Farquhar/Providence and the Amirantes. The island of Rodrigues is associated with the Soudan bank that stretches between it and Mauritius, and are a result of the complex fractures extending at right angles from the Central Indian Ocean ridge. Capping these diverse geological island/bank features, are a diverse array of shallow-water carbonate-producing systems, from classic island-arc subduction series of fringing – barrier – atoll – submerged reefs (Mascarene Plateau and Comoros-Glorieuses islands) to isolated carbonate caps (Farquhar, Amirantes).

Seamounts in the WIO are concentrated on the mid-ocean ridges, particularly the South West Indian Ridge to the south of the WIO, and scattered around the Mascarene Plateau, most likely a result of the same features that formed the individual island and bank systems mentioned above. These latter seamounts are the only ones within the EEZs of the regional countries.

Key References - Ali & Huber (2010); Cande & Stegman (2011); Duncan (1990); Dyment (2004); Parson & Evans (2004); Rajan et al. (undated). --> References