Three largest lava flows on Earth during the Holocene period are all originated from a single area in Iceland. First is Thjorsárhraun Lava Field (8,000 years ago), from the Bardarbunga Volcanic System. The magma did flow underground to the Veidivotn area, where the eruption took place. The second was the Eldgjá Lava Field. Then the magma originated from the Katla Volcanic System, forming about 60 km long row of craters. Now it has been found that the eruption took place in 938. Third is the Eldhraun Lava Field from 1783-84. The magma created a dyke from Grímsvötn Volcanic System over to Laki, and then the rift developed to the SW and NE from there.
All the eruptions seemingly started near a major upwelling division line of mantle convection rolls. The same division line takes part in forming the central area of Hveravellir.
The polygons, where the dykes and craters form, are subject to direct pulling effect from the large scale tectonic drift of the North American Tectonic Plate. Rifts open up perpendicularly to the drift direction, forming dykes and craters due to volcanic activity.
This indicates that the upwelling line encountered by the large scale dyke formation, somehow ´ignites´ the approaching magma, leading to eruption.