Náttúrufræðingurinn - 2016, Blaðsíða 15

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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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