The Neovolcanic zones in Iceland are of threefold origin: the Mid-Atlantic rift-zone, the Snæfellsnes “leaky-transform fault”, and South-Iceland transgressive volcanic zone or propagating rift segment. Basalt compositions range from alkali basalts through transitional basalts to tholeiites along the off-rift zones towards the centre of Iceland. Diminishing proportion of garnet-pyroxenite melts towards the centre of the mantle plume is the preferred explanation. The origin of silicic magmas reflects their tectonic settings and appears to be controlled by the crustal geothermal gradient.