Adam's Bridge - Geological Evolution

Geological Evolution

Considerable diversity of opinion and confusion exists about the nature and origin of this structure. In the 19th century, there were two prevalent theories explaining the structure. One considered it to be formed by a process of accretion and rising of the land, while the other surmised that it was formed by the breaking away of Sri Lanka from the Indian mainland. The friable calcerous ridges are broken into large rectangular blocks, which perhaps gave rise to the belief that the causeway is an artificial construction.

According to V. Ram Mohan of the Centre of Natural Hazards and Disaster Studies of the University of Madras "reconstruction of the geological evolution of the island chain is a challenging task and has to be carried out based on circumstantial evidence". The lack of comprehensive field studies explains many of the uncertainties regarding the nature and origin of Adam's Bridge, which essentially consists of a series of parallel ledges of sandstone and conglomerates that are hard at the surface and grows coarse and soft as it descends to sandy banks.

Studies have variously described the structure as a chain of shoals, coral reefs, a ridge formed in the region owing to thinning of the earth's crust, a double tombolo, a sand spit, or barrier islands. It has been reported that this bridge was formerly the world's largest tombolo before it was split into a chain of shoals by the rise in mean sea level few thousand years ago.

Based on satellite remote sensing data, but without actual field verification, Marine and Water Resources Group of Space Application Centre (SAC) of Indian Space Research Organisation (ISRO) states that Adam's Bridge comprises 103 small patch reefs lying in a linear pattern with reef crest (flattened, emergent – especially during low tides – or nearly emergent segment of a reef), sand cays (accumulations of loose coral sands and beach rock) and intermittent deep channels. The coral reefs are designated by the different studies variously as ribbon and atoll reefs.

The geological process that gave rise to this structure has also been attributed to crustal downwarping, block faulting, and mantle plume activity by one study while another theory attributes it to continuous sand deposition and the natural process of sedimentation leading to the formation of a chain of barrier islands related to rising sea levels. Another theory affirms that the origin and linearity of the Adam's bridge may be due to the old shoreline – implying that the two landmasses of India and Sri Lanka were once connected – from where coral reefs evolved.

Another study explains the origin the structure due to longshore drifting currents which moved in an anticlockwise direction in the north and clockwise direction in the south of Rameswaram and Talaimannar. The sand was supposedly dumped in a linear pattern along the current shadow zone between Dhanushkodi and Talaimannar with later accumulation of corals over these linear sand bodies. In a diametrically opposing view, another group of geologists propose crustal thinning theory, block faulting and a ridge formed in the region owing to thinning and asserts that development of this ridge augmented the coral growth in the region and in turn coral cover acted as a `sand trapper'.

The tombolo model affirms a constant sediment source and a strong unidirectional or bi-directional (monsoonal) longshore current. One study tentatively concludes that there is insufficient evidence to indicate eustatic emergence and that the raised reef in south India probably results from a local uplift. Other studies also conclude that during periods of lowered sea level over the last 100,000 years, Adam's Bridge has provided an intermittent land connection between India and Sri Lanka, which according to famous ornithologists Sidney Dillon Ripley and Bruce Beehler supports the vicariance model for speciation in some birds of the Indian Subcontinent.