The fissure swarm of the Askja central volcano

Abstract

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 fractures, aged 4500-10.000 BP can be found. This may indicate that the maximum stress axis may have rotated since the formation of the dilatational fractures took place. The latest dike intrusions into either the Askja or the Kverkfjöll fissure swarms may have caused this rotation. Volcanic fissures are either oriented away from, or circle around the calderas in Askja. The volcanic fissures that are oriented away from the calderas may have formed after an inflation in a caldera, and the ones that circle around the calderas may have formed shortly after an inflation started in a previously deflating caldera. The first four eruptions that took place in the 1920s, and occurred around the newly formed caldera, may therefore indicate that an inflation had started in the caldera. The irregular orientation of fissures and fractures close to Askja suggests that it has a local stress field. The 1.7 km long pit crater chain in the Kollóttadyngja lava shield lies in and parallel with the Askja fissure swarm. We suggest that the topography of Kollóttadyngja caused a horizontal component of magma flow in an underlying dike. Increased magma flow in the upper part of the dike caused lower magma pressure, causing even more magma to flow into this part of the dike. A pipe was eventually formed which later collapsed, and formed the pit crater chain.