How do the volcanic zones really work? A tension between the large scale tectonic drift and local opposite mantle flow create a pulling effect on the area, so rifts and faults appear.
An upwelling line markes the western boundary of the east volcanic zone, and a downwelling line mars the eastern side. The flow under is from west to east, opposite to the drift of the tectonic plate. The local and global trends are opposite to each other.
The western volcanic zone is shaped in the same way. The eastern side is found along the next downwelling line, but the effect of pulling is only found in half the polygon. Snæfellsnes volcanic zone is also facing upwelling lines in the north. The Öræfajökull zone is facing the eastern trend of the tectonic drift, therefore having an upwelling line to the east.