St. Mary's Islands (Karnataka) - Geology

Geology

The columnar basaltic lava found in these Islands, which is very well developed in the basalts of Deccan Traps, exhibit an imposing range of hexagonal shaped or multi-faced (polygonal) columns split into a horizontal mosaic. In geological terms these are called "columnar joints". The lava rocks form regular five, six or seven-sided pillars, called "laminar lava", and are found in varying heights in all the islands; the tallest of the columns is about 6 m (20 ft). Considering the importance and rarity of such an occurrence, these islands were classified as a National Geological Monument in 2001 by the Geological Survey of India.

The Deccan Traps, that formed during Cretaceous–Eocene time about 60 million years ago. emerged from the vast deluge of hot molten basaltic lava in the western part of India which is now seen as flat topped hills and step like terraces. Scientific studies carried out at the Indian Institute of Technology, Bombay on the petrology, palaeomagnetism and volcanics of the rocks of the island has brought out the following facts.

• Islands comprise fully of igneous rocks. They have acid composition that consist of dacites, rhyodacites, rhyolites and granophyres and carry basic patches.
• The columnar jointing pattern is well developed on Coconut Island.
• Mineralogically, plagioclase, K-feldspar, quartz, ortho- and clinopyroxenes, olivine, magnetite, and ilmenite are recorded in the ground mass phases
• Magnetic granulometric studies (susceptibility and hysteresis at different temperatures) of island rock samples indicate the presence of a multi domain (MD) state of magnetite. It is inferred that: "the formation of MD could have affected the stability and consistency of magnetic directions in these rocks" and that "this igneous body has been either annealed or could be an intrusive."

An analysis of palaeomagnetic data from India and Madagascar hypothesizes a new India–Madagascar fit related to the Late Cretaceous, directly prior to and during the early phase of Madagascar–India separation. A scientific study paper on Late Cretaceous India–Madagascar fit and timing of break–up related magmatism by several scholars reported in the Wiley Inter Science Journal states: "St. Mary magmatism is linked to the initial break–up between India and Madagascar, and magmatism probably resulted from rift related extensional processes initially induced by the Marion hotspot underlying southern Madagascar during the Late Cretaceous."