The unprecedented connection between India and its neighbouring island nation of Sri Lanka can be regarded in many ways such as geographical, historical, mythological, etc. It is well encrypted in Ramayana about Ram Setu as the bridge constructed by Lord Rama and his army from Dhanushkodi (Rameswaram Island) of India to Jaffna district of Sri Lanka across the intervening sea. But, historically, this narrow crossway between Tamil Nadu and the northern province of Sri Lanka had been documented as ‘Palk Strait’, named after Robert Palk who was a governor of Madras during the British India. One of the most debated questions that probably carried through past millennium and still remaining to be a scientifically unresolved problem would be ‘whether the Ram Setu or the so-called Palk Strait is a man-made or rather a naturally occurred feature? In fact, India and Sri Lanka share several hundred million years of inheritance since both of these continental masses where once existed together as a single land and later India gave birth to Sri Lanka with the formation of a sea intervene between them. A group of geologists, led by Dr. Ratheesh Kumar R.T. (Assistant Professor, Department of Marine geology and Geophysics, Cochin University of Science and Technology) have made this important discovery.
Scientifically, the most ambiguous, but fundamental question relating India and Sri Lanka is that how a sea evolved between the two land masses. The answer to this query comes from the theory of plate tectonics. This theory states that Earth’s rigid outer shell called ‘lithosphere’ exists as several major and minor broken pieces or ‘plates’ that are floating over a denser magma ocean called the ‘asthenosphere’ very much like a wood floating on water. These plates, which include continental (e.g., Indian continent, African continent, etc.) and oceanic (e.g., Indian ocean, Atlantic Ocean, etc.) lithospheres, that are constantly in motion either away from or towards or slide past each other. When two continental plates move towards and collide each other, great mountains like Himalaya will form. In contrast, when a continental plate break-up into two masses and rift away from each other, ocean forms in between them. Here the oceanic lithosphere generates through the fracture zone left by the broken continents through which hot-molten rock materials called ‘magma’ upwells from the Asthenosphere and cools down to form denser oceanic plate, and this long-linear belt of intense and continuous igneous activities is known as ‘Mid-Ocean Ridge’. The ocean grows large as the continental masses rift away each other from the mid-ocean ridge. Today’s world’s oceans which exist between the continental plates were evolved in this manner. The best example for this phenomenon is the Mid-Atlantic ridge located along the floor of Atlantic Ocean, which is formed by the break-up and subsequent rifting between North America and Eurasia in the north, and between South America and Africa in the south. However, this general understanding of evolution of the oceanic lithosphere is not applicable in the case of the sea intervened between India and Sri Lanka as a mid-ocean ridge system is absent in this sea floor, and this imposed great mysteries and debates in its evolutionary ideas.
To this end, Dr. Ratheesh Kumar R.T. and his team of scientists conducted detailed investigations by using different geophysical techniques based on the data sets including bathymetry, gravity and magnetic anomalies, and generated models of deep structure and mechanical behaviour of the lithosphere in and around India and Sri Lanka. Their results reveal that the south-eastern margin of India and western margin of Sri Lanka were once-conjugated margins as they share similar deformational signatures, and that their rifting brought the sea in. The major findings of this study are as follows. At about 240 million years ago all the continental plates were existed as a single giant continental mass named as Gondwana supercontinent’ or often known as ‘Pangea’, in which the position of Sri Lanka was aligned in between India and Antarctica (See Figure). This supercontinent began to break up at about 140 million years ago, during which the break-up between India and Antarctica imposed great uncertainty in the position of the intervened Sri Lanka. Initially Antarctica was holding Sri Lanka on its side, which imposed Sri Lanka to rift away from the Indian side. The rift movement of Sri Lanka was rather like drawing a semi-circle using a compass such that Sri Lanka’s northern end (Palk strait area) stayed stand still as a pivot while its southern side rotated in an anticlockwise direction away from India. This movement caused a progressively southward stretching and thinning of the lithosphere in between India and Sri Lanka and that resulted in the formation of the Mannar gulf and opening of the sea at about 132 million years ago. However, the lithosphere beneath the Palk Strait or Rama Setu area, which was least affected by this rift movements, was still thicker and mechanically stronger enough to abort the mid-ocean ridge and break-up between India and Sri Lanka. Therefore, it became a failed rift, and the strong lithospheric bond across the Palk Strait acted as a continental ‘Umbilical Cord’ between India and Sri Lanka. These developments caused a migration of the mid-ocean ridge between Sri Lanka and Antarctica, and that ended up in their break up at about 128 million years ago. Thus, Sri Lanka and India became integral parts of a single plate, after which they were able to drift together away from Antarctica and still continue their voyage. Palk Strait or Ram Setu, therefore, represents a remnant of the naturally formed lithospheric bond between India and Sri Lanka. Nonetheless, if the story of Ram Setu portrayed in Ramayan is true, Rama and his army were indeed smart as they chose this naturally bonded shallowest Palk Bay area as an ideal location to construct their bridge to Sri Lanka! However, further detailed studies and solid scientific evidences are yet to come out in order to prove the existence of an ancient man-made bridge in this area.
This study was recently published in ‘Journal of Geophysical Research’, which is one of the world’s top renowned scientific journals. The researchers who were part of this work as Dr. Ratheesh Kumar’s team include Prof. Brian F. Windley (University of Leicester, UK), Dr. P.L. Dharmapriya (University of Peradeniya, Sri Lanka), Prof. Xiao Wenjiao (University of Chinese Academy of Sciences, China), and U. Jeevan (Research Scholar, CUSAT). Dr. Ratheesh Kumar R.T, who hails from Pothencode at Trivandrum, is currently working as a visiting scholar at University of Kentucky for a NASA project on Mars.
Reference
Ratheesh-Kumar, R.T., Dharmapriya, P.L., Windley, W.F., Xiao, W.J., Jeevan, U. The tectonic 'Umbilical Cord' linking India and Sri Lanka, and the tale of their failed rift. Journal of Geophysical Research: Solid Earth, In press article: doi:10.1029/2019JB0182250
