Hólmatungur is an erosional basin formed by catastrophic flooding of the Jökulsá river shortly after the end of the last ice age. Sveinasprunga fissure erupted some 11,000 years ago, cutting across the river by Hafragilsfoss, and causing lava to flow into the Jökulsá river at Hólmatungur. The river surged over the lava while it was still unconsolidated except at the base. That part is preserved as normal columnar basalt. The river quenched the upper part forming the cube-jointed lava which may be observed widely in Hólmatungur, for example at Vígaberg. Numerous springs, associated with cracks, faults and lithological boundaries, burst forth in the Hólmatungur area. Some originate from underneath the basaltic interglacial Grjótháls lava shield while others emerge from the lavas of the Rauðhólar and Randarhólar craters which belong to the Sveinar volcanic fissure. The total flow from the Hólmatungur springs is about 4000 l/s.