Thursday, February 9, 2017

Why Is There A "Lost Continent" Underneath Mauritius

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.

The "lost continent" underneath Mauritius is making news. 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 age of the zircon is however Archean in age, between 2.5 billion - 3 billion years. That means the zircon crystal did not form in the young lava, but belongs 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 the earth continents is primarily made up of granitic and andesitic rocks and sedimentary cover. This crust is light and thick (30km-40 km) and it sticks above sea level.  On the other hand, the crust making  up the ocean basins is made up of basalt and is denser and thinner (~10 km). 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 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.

Source: Lewis D. Ashwal, Michael Wiedenbeck, and Trond H. Torsvik 2017

Mauritius is part of a series of splinters that collectively are called Mauritia. These splinters were part of the Archean continental nucleus that made up Madagascar and the western Dharwar craton in south India.

Here is the interesting part that many 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/Somali plate to the west. Today, Seychelles and Mauritius is on the African/Somali 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 CIR or Central Indian Ridge is where sea floor spreading is forming the Mascarene basin. Seychelles and Mauritia lie to the east of this ridge on the Indian plate.

Source: Shankar Chatterjee et. al. 2013

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 mya) and between Mauritius and Chagos/Laccadives (42 mya).  These "ridge jumps", as they are called, formed the Carlsberg Ridge and  transferred Seychelles and Mauritius on to the African/Somali 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/ 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. They thus often get submerged under the sea.

Open Access


  1. I read about the discovery in the NY Times, so it was nice to find more detail here, thx.