Náttúrufræðingurinn - 1962, Blaðsíða 27

NÁTTÚRUFRÆÐINGURINN
17
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der Askja und Knebelsee, sowie den Rudloffkrater in Zentralisland. —
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SUMMARY
The Askja-Caldera and the Askja-Eruption 1961
by Þorleifur Einarsson
University Research Institute, lleykjavik.
This article is a short narrative about the Dyngjufjöll-Massiv and the Askja-
caldera in Middle-Iceland and its volcanism, especially the Askja-eruption 1961.
In the SE-part of the great lava-desert Ódáðahraun (ca 4000 km2) lies the
Dyngjufjöll-Massiv, which consists of pleistocene, subaerial and/or subglacial,
volcanic breccias and tuffs (”palagonite-tuff“). In the southern part of Dyngju-
fjöll lies the recent subsidence-caldera Askja (50 km2). ln the SE-part of Askja
lies the Öskjuvatn-caldera, that collapsed after a violent, rhyolithic eruption
in 1875.
Dyngjufjöll are 1200—1500 m high (all heights over sea-level), the highest
peak is Þorvaldstindur 1510 m. The floor of Askja lies between 1100 and 1200
nr. The Ódáðahraun-lavafields around Dyngjufjöll lie in 600—800 m.
The lavas flown before the subsidence of the Askja-caldera are called
Dyngjufjallahraun (pretectonic lavas). (Icelandic hraun = postglacial lava.)
The lavas flown during and aíter the caldera subsidence are called Öskju-
hraun (posttectonic lavas), and they liave generally poured out from the
ring-faults around the Askja-caldera and spread out over the floor of Askja
and then flown through the graben-pass Öskjuop. The Dyngjufjöll- and
Askja-lavas are all basaltic and most of them are prehistoric. (Iceland was sett-
led in the 9th century A. D.). Very little is known about the volcanism in Askja
in historic times and the clironology of the older lavas is unknown.
In the night 28/29th March 1875 a violent, explosive eruption took place
in the maar Víti. After the eruption a 11 km2 area subsided in the SE of Askja.
This subsidence was over 230 m deep in 1876. In 1907 the waterlevel of the
caldera lake Öskjuvatn had reached its present level (1053 m).
In the years 1921-30 several small eruptions occurred in Askja, which
produced 6 different lavaflows. All ol' the vents lie on the ring-faults but one, a
7 km long Nso°E-striking fissure soutli of Dyngjufjöll, that produced the
lava Thorvaldshraun, 16 km2.