african plate ammonite basalt carbonatite continental drift cretaceous deccan plateau dinosaur emeishan traps erosion extinction flood basalt fossil geographic coordinate system geology of venus india indian ocean lamprophyre large igneous province list of volcanoes in india mantle plume mascarene plateau matheran meteoroid mumbai nephelinite pune seafloor spreading shiva crater siberian traps tertiary tholeiitic basalt volcanic gas western ghats xenolith