| Summary: | The Askja volcanic system forms one of the 5-6 volcanic systems of the Northern Volcanic Zone, that divides the North-American and the Eurasian plates. Historical eruptions have occurred both within the central volcano and in its fissure swarm. As an example, repeated fissure eruptions occurred in the fissure swarm, and a Plinian eruption occurred within the volcano itself in 1875. This led to the formation of the youngest caldera in Iceland, which now houses the Lake Öskjuvatn. Six eruptions occurred in the 1920„s and one in 1961 in Askja. No historical accounts have, however, been found of eruptive activity of Askja before 1875, likely due to its remote location. To improve the knowledge of historic and prehistoric activity of Askja, we mapped volcanic fissures and tectonic fractures within and north of the Askja central volcano. The 1800 km2 area included as an example Mt. Herðubreið, Mt. Upptyppingar and the Kollóttadyngja lava shield, as well as Askja. The results indicate that the activity of different subswarms of the Askja central volcano alternates with time, as the NE subswarm ends suddenly at a 3500-4500 BP lava flow. This may possibly occur due to different locations of inflation centers in Askja. If, as an example, the inflation center is easterly in Askja, a dike might propagate from this inflation center into the eastern part of the fissure swarm. Volcanic fissures are most common close to Askja, but the number of tectonic fractures increases with distance from the volcano. This may indicate a higher magma pressure in dikes close to Askja, than farther away. The number of fractures decreases with altitude in Kollóttadyngja, which may indicate more depth to the top of the dikes under the center of Kollóttadyngja, than beneath its slopes, due to altitude. Shallow eartquakes are mostly originated at non-fractured areas, like the ones that occur near Mt. Herðubreið, where fault-plane solutions have indicated the formation of strike-slip faults. In only about 4 km distance from Herðubreið, dilatational ...
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