The Tinna Central Volcano in the Skagafjordur Valleys belongs to the Neogene succession of N-Iceland. It was active during the period 6-5 Ma. The total volume of the volcano is 210 km (super 3) . Four rhyolite lava domes are described. The largest one, the Skati Dome, was formed during an immense explosive eruption accompanied by a heavy tephra fall 5.5 Ma. It is the most voluminous monogenetic rhyolite formation known in Iceland both with respect to lava and tephra. The area of the dome is about 80 km (super 2) and its volume is 8 km (super 3) . The tephra layer might additionally account for 10 km (super 3) of dense rock, or 18 km (super 3) altogether. The tephra layer is correlated with an acid ash layer found at ODP-site 907, 500 km NNE off Iceland's coast and can serve as an important marker horizon for the late Miocene in the deep-sea sediments. A collapse caldera was formed during the final stage of the volcano. After its extinction it was buried by the younger lava pile while it drifted away from the active rift zone.