There are four large islands in the St.Marys Islands group, namely Coconut Island, North Island, Darya Bahadurgari Island and South Island. Out of the four Islands the one from St.Marys Islands the northern most Island has a basaltic rock formation in a hexagonal form, the only one of its type in India. The rocks have very well-developed columnar joints and hence the island has been gifted by the Karnataka state Government to the Geological Survey of India which has declared the rocks as a national Geological Monument.

Geological Background of the region

Many plate reconstructions propose a close link between Madagascar and Greater India from late Precambrian to Cretaceous times, and the separation is estimated to have started during or shortly after a period of Late Cretaceous basic and felsic magmatism that is well-known from Madagascar.

The St. Mary’s islands was sub aerial in nature  as  that of  in  Madagascar was still attached to India. Rifting of Madagascar from India took place around 105 M.Y. The volcanic activity which gave rise to the St.Marys Islands was sub aerial in nature as that of in Madagascar. The St.Marys Island consists entirely of acid composition and includes Dacites, Rhyodacities, Rhyolites and Granophyres and carries basic patches. These rock types are present in all the large islands  of the St.Marys Islands  group and of well-developed columnar joint is seen on Coconut  Island.

How these Columnar joints are formed?

 

When a mud puddle dries, it loses water, and the mud cracks in symmetric shapes, forming mud polygons. This is because the lose of water causes the mud to shrink, hence contraction is relieved by breaking- as the dry mud becomes brittle. Basalt and other lavas behave similarly. As lava cools, it doesn’t dry out like mud, but it does shrink. As it becomes cold and brittle, the lava contracts and relieves the stress by cracking. The cracking produces a polygonal pattern that extends through the lava flow. As weathering cuts into lava, the rock breaks along the joints, exposing this geometric regularity. Although many of the polygons are six- sided, four, five, seven or eight sides columns are also relatively common. The degree and perfection to which this is developed depends on the thickness and composition of the lava and how fast it cools.

Really thick lava flows have several layers of joints. Ideally, there is a well jointed lower zone, called the lower colonnade. The middle section in the flow can a more chaotic set of thinner columns and is called the entablature. A thinner, less well-developed upper colonnade is also possible. The upper and lower colonnades reflect normal heat loss to the surface and the ground. In the middle of the flow, the entablature’s more chaotic pattern suggests that the heat loss direction was less well-defined and varied locally.

 

Examples of similar Columnar Jointing features-

 

Acknowledgements

The author is thankful to Department of Geology, V.O.Chidambaram College, Thoothukudi, Tamilnadu, India, Engineering Geophysics, NGRI, Hyderabad, India, for making the data available through scholarsresearchlibrary.