Both formed as a result of the hotspot, now slowly tracking beneath the Indian Ocean, which formed the massive basalt flows of the Deccan Traps, when it was beneath India in the Late Cretaceous.
The Old Series includes olivine basalt, agglomerate, trachyte intrusions and trachyandesite plugs.
The Early Volcanic, also known as the Intermediate Series continued from 3.5 to 1.7 million years ago, into the Quaternary.
The central plateau has high permeability in intermediate volcanics and also receives the highest rainfall.
In the past, the island had had basalt and coral sand quarrying for building material.