 The stratigraphic subdivision is supported by geochemical and mineralogical analyses, which indicate that the basaltic rocks are mantle-derived and have, unusually, relatively intraplate (or hotspot) type chemical affinities.The multiple andesite flows were probably derived from basaltic magma by the crystallization of the magma material.The relatively rare, more felsic flow units, are thought to have originated from high-end crystallization inside the magma chamber.