Náttúrufræðingurinn - 2016, Síða 15

87
Tímarit Hins íslenska náttúrufræðifélags
Summary
Volcanic episode in the Askja
volcanic system 11.000 years ago
The Askja volcanic system is located in
the North Volcanic Zone of Iceland. The
system is about 200 km long and up to
25 km wide. In its southern part a cen-
tral volcano is found with a large caldera,
Askja, about 45 km2. Numerous fissure
eruptions have occurred on the fissures
swarm during postglacial time. In gen-
eral, the eruption history of the Askja
volcanic system is not well known ex-
cept for the last c. 5.000 years. One of the
lavas has gained more attention than
others among geologists, i.e. the
Sveinahraun lava. The Sveinar-Randar-
hólar eruption fissure, where the lava
was erupted, is the longest one known
in the country, about 75 km long. During
this eruption the fissure transected the
Jökulsárgljúfur Canyon. In the walls of
this 100 m deep canyon cross-sections of
feeder dikes may be seen. Until recently
the age of the Sveinahraun lava has been
uncertain. Commonly it has been esti-
mated somewhere between 6–9 ka. In
the summers of 2011 and 2012 a geologi-
cal mapping of the Northern Volcanic
Zone was conducted by Iceland
GeoSurvey. During the fieldwork te-
phrochronology was applied in dating
individual lavas. One of the objectives
was to get a more accurate age estimate
for the Sveinahraun lava. At two loca-
tions soil profiles with the 11 ka old
Askja-S tephra overlying scoria of the
Sveinar-Randarhólar crater row were
found. The source craters of the Askja-S
tephra are not preserved but most prob-
ably were located within the Askja cal-
dera or its vicinity. The sections studied
are located some 20 km apart on the
Randarhólar-Sveinar crater row, i.e. at
Randarhólar and Vegasveinar, respec-
tively. Based on the profiles it is con-
cluded that the crater row and the whit-
ish Askja-S tephra are contemporary
formations, belonging to the same Fires
in Askja c. 11 ka ago. The volcanic activ-
ity seems to have started with basaltic
eruptions, a phreatomagmatic activity in
the Askja region and a fissure eruption
on the northern part of the Askja fissure
swarm, the Sveinahraun eruption. Then
a plinian eruption occurred in Askja pro-
ducing the Askja-S tephra (c. 1.5 km3 as
DRE), covering the Sveinar-Randarhólar
scoria and the Sveinahraun lava. In the
waning phase a part of the eruption fis-
sure, i.e. at Jökulsárgljúfur, erupted
again and also phreatomagmatic erup-
tions occurred in the Askja region. These
Fires resemble those occurring in the
Askja volcanic system in 1874–1875 AD.
Þakkir
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Heimildir
1. Árni Hjartarson & Kristján Sæmundsson 2014. Berggrunnskort af Íslandi.
1:600 000. Íslenskar orkurannsóknir, Reykjavík.
2. Guðrún Larsen & Jón Eiríksson 2008. Holocene tephra archives and
tephrochronology in Iceland – a brief overview. Jökull 58. 229–250.
3. Guðrún Larsen & Sigurður Þórarinsson 1977. H-4 and other acid Hekla
tephra layers. Jökull 27. 28–46.
4. Kristján Sæmundsson, Magnús Á. Sigurgeirsson, Árni Hjartarson,
Ingibjörg Kaldal, Sigurður Garðar Kristinsson & Skúli Víkingsson 2016.
Jarðfræðikort af Suðvesturlandi. 1:100 000. 2. útg. (frumútg. 2010).
Íslenskar orkurannsóknir, Reykjavík.
5. Kristján Sæmundsson, Árni Hjartarson, Ingibjörg Kaldal, Magnús Á.
Sigurgeirsson, Sigurður Garðar Kristinsson & Skúli Víkingsson 2012.
Jarðfræðikort af Norðurgosbelti. Nyrðri hluti. 1:100 000. Íslenskar
orkurannsóknir og Landsvirkjun, Reykjavík.
6. Magnús Á. Sigurgeirsson, Árni Hjartarson, Ingibjörg Kaldal, Kristján
Sæmundsson, Sigurður Garðar Kristinsson & Skúli Víkingsson 2015.
Jarðfræðikort af Norðurgosbelti. Syðri hluti – Ódáðahraun. 1:100 000.
Íslenskar orkurannsóknir, Reykjavík.
7. Haukur Jóhannesson & Kristján Sæmundsson 1998. Jarðfræðikort af
Íslandi. 1:500 000. Höggun. Náttúrufræðistofnun Íslands, Reykjavík.
8. Kristján Sæmundsson 1982. Öskjur á virkum eldfjallasvæðum Íslands.
Bls. 221–239 í: Eldur er í norðri (ritstj. Helga Þórarinsdóttir, Ólafur H.
Óskarsson, Sigurður Steinþórsson & Þorleifur Einarsson). Sögufélag,
Reykjavík.
9. Kristján Sæmundsson & Freysteinn Sigmundsson 2013. Öskjukerfið. Bls.
318–357 í: Náttúruvá á Íslandi – eldgos og jarðskjálftar (ritstj. Júlíus
Sólnes, Freysteinn Sigmundsson & Bjarni Bessason). Viðlagatrygging
Íslands og Háskólaútgáfan, Reykjavík.
10. Magnús T. Guðmundsson, Kristín Jónsdóttir, Hooper, A., Holohan, E.P.,
Sæmundur A. Halldórsson, Benedikt G. Ófeigsson, Cesca, S., Kristín S.
Vogfjörð, Freysteinn Sigmundsson, Þórdís Högnadóttir, Páll Einarsson,
Olgeir Sigmarsson, Jarosch, A.H., Kristján Jónasson, Eyjólfur Magnússon,
Sigrún Hreinsdóttir, Bagnardi, M., Parks, M.M., Vala Hjörleifsdóttir,
Finnur Pálsson, Walter, T.R., Schöpfer, M.P.J., Heimann, S., Reynolds, H.I.,
Dumont, S., Bali, E., Guðmundur H. Guðfinnsson, Dahm, T., Roberts,
M.J., Hench, M., Belart, J.M.C., Spaans, K., Sigurður Jakobsson, Gunnar
B. Guðmundsson, Hildur M. Friðriksdóttir, Drouin, V., Dürig, T.,
Guðfinna Aðalgeirsdóttir, Riishuus, M.S., Pedersen, G.B.M., van Boeckel,
T., Björn Oddsson, Pfeffer, M.A., Barsotti, S., Baldur Bergsson, Donovan,
A., Burton, M.R., Aiuppa, A. 2016. Gradual caldera collapse at Bárdar-
bunga volcano, Iceland, regulated by lateral magma outflow. Science 353,
6296. DOI: 10.1126/science.aaf8988.
11. Ólafur Jónsson 1945. Ódáðahraun. I–III. Norðri, Akureyri. 425+447+405
bls.
12. Sigurður Þórarinsson 1963. Eldur í Öskju. Almenna bókafélagið, Reykja–
vík. 48 bls.
13. Haraldur Sigurðsson & Sparks, R.S.J. 1978. Rifting episode in North Ice-
land in 1874–1875 and the eruptions of Askja and Sveinagjá. Bulletin of
Volcanology 41. 149–167.
14. Hartley, M.E. & Þorvaldur Þórðarson 2012. Formation of Öskjuvatn cal-
dera at Askja, North Iceland: Mechanism of caldera collapse and implica-
tions for the lateral flow hypothesis. Journal of Volcanology and Geother-
mal Research 227–228. 85–101.
15. Annertz, K., Nilsson, M. & Guðmundur E. Sigvaldason 1985. The post-
glacial history of Dyngjufjöll. Norræna eldfjallastöðin 8502, Háskóli
Íslands, Reykjavík. 22+7 bls.
16. Guðmundur E. Sigvaldason, Annertz, K. & Nilsson, M. 1992. Effect of
glacier loading/deloading on volcanism: postglacial volcanic produc-
tion rate of the Dyngjufjöll area, central Iceland. Bulletin of Volcanology
54. 385–392.
17. Dugmore, A.J., Cook, G.T., Shore, J.S., Newton, A.J., Edwards, K.J. &
Guðrún Larsen 1995. Radiocarbon dating tephra layers in Britain and
Iceland. Radiocarbon 37. 379–388.
18. Kristján Sæmundsson, Magnús Á. Sigurgeirsson & Karl Grönvold 2012.
Þeistareykir. Jarðfræðirannsóknir 2011. ÍSOR-2012/024. Íslenskar
orkurannsóknir, Reykjavík. 61 bls.
19. Esther Ruth Guðmundsdóttir, Guðrún Larsen & Jón Eiríksson 2011. Two
new Icelandic tephra markers: The Hekla Ö tephra layer, 6060 cal. yr BP,
and Hekla DH tephra layer, 6650 cal. yr BP. Land-sea correlation of mid-
Holocene markers. The Holocene 21. 629–639.
20. Mangerud, J., Furnes, H. & Jóhansen, J. 1986. A 9000-years old ash bed
on the Faroe Islands. Quaternary Research 26. 262–265.
21. Andrews, J.T.,Áslaug Geirsdóttir, Jórunn Harðardóttir, Principato, S.,
Karl Grönvold, Gréta Björk Kristjánsdóttir, Guðrún Helgadóttir, Drexler
J. & Árný Erla Sveinbjörnsdóttir 2002. Distribution, sediment magnetism
and geochemistry of the Saksunarvatn (10,180 +/-60 cal. yr BP) tephra in
marine, lake and terrestrial sediments, northwest Iceland. Journal of
Quaternary Science 17. 731–745.
22. Lohne, Ø.S., Mangerud, J. & Birks, H.H. 2013. Precise 14C ages of the
Vedde and Saksunarvatn ashes and the Younger Dryas boundaries from
western Norway and their comparison with the Greenland Ice Core
(GICC05) chronology. Journal of Quaternary Science 28. 490–500.
23. Rasmussen, S.O., Andersen, K.K., Svensson, A.M., Steffensen, J.P.,
Vinther, B.M., Clausen, H.B., Siggaard-Andersen, M.-L., Andersen, S.J.,
Johnsen, S.J., Larsen, L.B., Dahl-Jensen, D., Bigler, M., Röthlisberger, R.,
Fisher, H., Goto-Azuma, K., Hansson, M.E. & Ruth, U. 2006. A new