Náttúrufræðingurinn

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Náttúrufræðingurinn - 1995, Side 104

Náttúrufræðingurinn - 1995, Side 104
and the fissure system associated with the Grímsvötn central volcano may also extend into the mapped area as Holuhraun lava. Figure 2 shows a general geological map of' the research area where the lavas from each fissure swarm are as a rule grouped together as a unit. No attempt was made in separating the different lava flows originating in the same fissure swarm, with the exception of some lavas from craters or crater rows situated somewhat outside the main fissure swarms. Still the lavas from the Dyngjufjöll fissure swarm crossing Gígöldur area are devided into two groups, Kverktjallahraun and Kreppu- tunguhraun, based on the great difference in appearance and texture of the lavas and their craters and also due to their different age. The main lava fields introduced on the map are Kverkfjallahraun, Trölladyngjuhraun, Kreppu- tunguhraun, Dyngjufjallahraun, and some lavas surrounding Herðubreið and Holuhraun. Kverkfjallahraun lavas originate in at least five volcanic fissures in the western part of Kverkfjallarani ridge, but on its eastern slope there are two crater rows that have produced the two lavafields, Kreppuhraun and Linda- hraun. No attempt was made to differentiate lavafields originating in the Trölladyngja shield volcano from lavas from the Bárðar- bunga fissure swarm on Dyngjuháls which at least six eruptive fissures. Krepputunguhraun originates in two or more old volcanic fissures in Gígöldur, which in both texture and flow structure are very similar to the Þjórsárhraun lavas in southern Iceland. They can also be es- timated to be of the same age, about 6000- 8000 years old, since they are everywhere the oldest unit in the lava sequence. The lavafield ot Krepputunguhraun can be traced toward the east as far as the end of Krepputunga at Herðubreið, and its western branch can even be traced as Bárðardalshraun almost all the way down to the coast. Dyngjufjallahraun lavas originate in Dyngjufjöll central volcano and its fissure swarms. They have flowed from a great number of craters on the southern slopes of Dyngjufjöll and in Gígöldur as well as from craters in the Askja caldera. Mount Herðubreið is surrounded by lavas from two small shield volcanoes, Flatadyngja and Svartadyngja, as well as lava streams from NE trending fissure swarm of Dyngjufjöll central volcano. Holuhraun at the northern edge of Dyngjujökull is most likely a historical lava and can possibly be related to the fissure swarm of Grímsvötn central volcano. The tectonic features of the research area are very distinct and obvious in the field and are closely connected with the evolution of the central volcanoes and their associated fissure swarms. The main SW-NE tectonic trend of the rift-zone of southern Iceland here gradu- ally shifts to the N-S direction of northern Ice- land and it is commonly characterized by transverse volcano-tectonic features such as eruptive fissures and faulting. The most prominent faulting is that trending to the NE from Kverkfjallarani, where single faults in a postglacial lava have been measured up to 20 m. Krepputunguhraun lava was erupted 6000- 8000 years ago and by studying its flow struc- ture between Gígöldur and the river Kreppa, it is clear that the downthrow of the block be- tween the fissure swarms of Dyngjufjöll and Kverktjöll can be estimated to be at least 60- 70 m, whereof at ieast 20 m date from the last prehistoric, catastrophic flooding of river Jökulsá occurring about 2000 years ago. That is in good agreement with the average subsid- ence of the main rift-zone. In the next article of this series the cata- strophic floods of the river Jökulsá and related volcanism will be discussed. PÓSTFANG HÖFUNDAR/AUTHOR'S ADDRESS Guttormur Sigbjarnarson Skaftahlíð 8 IS-105 Reykjavík Iceland 212
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