By Suvrat Kher
Yes, the term “lost continent” brings up visions of a lost world full of fantastic creatures that once existed deep in the Earth’s past. Or, of a civilization that once was, but was swallowed up by rising seas and which now only remains on the margins of human memory.
“Scientists have confirmed the existence of a “lost continent” under the Indian Ocean island of Mauritius that was left-over by the break-up of the supercontinent, Gondwana, which started about 200 million years ago.”
It is more accurate to say that there is continental crust underneath the oceanic lavas of Mauritius. And that continental crust is very old. Geologists found crystals of zircon in young lava that erupted on Mauritius about 5.7 million years ago. The zircon is, however, Archean in age – between 2.5 billion and 3 billion years old. Meaning these zircon crystals did not form in the young lava, but belong to the older foundation of the island. They were extracted from this Archean crust by rising molten material and brought to the surface about 5. 7 million years ago.
The crust making up Earth’s continents is primarily granitic and andesitic rocks, and sedimentary cover. This crust is light and thick (30 km – 40 km) and sticks out above sea level. On the other hand, crust at the bottom of the ocean is denser and thinner (~10 km) and mostly basalt. So, what is Archean continental crust doing in the middle of the Indian ocean, surrounded by Cretaceous-Cenozoic oceanic basaltic crust?
The answer lies in the way Gondwanaland broke up, or rather the way India broke away from Madagascar, about 88 million years ago. This was a continuation of the progressive breakup of Gondwanaland that began in the late Jurassic period, about 150 million years ago.
A large, rigid continent need not break into two clean pieces. Very often, the edges splinter. Several smaller fragments of continental crust are left isolated near the edges of the two continents. The map below shows these continental splinters scattered in the Indian ocean, as Madagascar and India broke apart and drifted away from each other.
Mauritius is part of a series of splinters collectively called Mauritia. These splinters were originally part of the Archean continental nucleus that made up Madagascar and the western Dharwar craton in south India.
But here is the interesting part news reports haven’t touched on. Look closely at the map above. Trace the Carlsberg Ridge southwards. The Indian plate, which is drifting northwards, lies to the east of this ridge, and the African plate to the west. Today, Seychelles and Mauritius are on the African plate, and Chagos and the Laccadives on the Indian plate. But when the initial separation happened about 84 million years ago, Seychelles and most of Mauritia were on the northerly drifting Indian plate. This is because 84 million years ago, the plate boundary between the Indian and African plates was formed by sea-floor spreading in the Mascarene basin.
This is depicted in the paleo-geographic reconstruction below. At 65 million years, the Central Indian Ridge (CIR) is where the sea-floor spreads, forming the Mascarene basin. Seychelles and Mauritia lie to the east of this ridge on the Indian plate.
Later, beginning around 62 million years ago and continuing up to about 41 million years ago, the loci of sea-floor spreading jumped eastwards. The result was the formation of new plate-boundaries between the Seychelles and Laxmi Ridge (62 million years ago) and between Mauritius and Chagos/Laccadives (42 million years ago).
These “ridge jumps”, as they are called, formed the Carlsberg Ridge and transferred Seychelles and Mauritius on to the African plate. Continued northward drift of India coupled with sea-floor spreading, and the formation of new oceanic crust along the Carlsberg Ridge, has formed the broad oceanic basin of the Arabian Sea and Indian ocean.
The process of continental breakup involves extensional forces that stretch and thin the crust. Fault movements cause a subsidence of crustal blocks. Many of the splinters at the edge of major continental margins are such thinned, downfaulted blocks, and thus often get submerged under the sea.
The author, Suvrat Kher, is a Sedimentary Geologist and Science Writer. His work focuses on the geosciences, biological evolution and environmental issues.
This article was originally published on Suvrat’s personal blog, Rapid Uplift.
Tweet at Suvrat: @RapidUplift
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