Náttúrufræðingurinn - 1974, Page 76

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N ÁTT Ú RU F RÆÐINGURINN
— J 967: The Eruptions of Hekla in Historical times. Vísindafél. ísl., Reykja-
vík.
— 1968: Skaftáreldar og Lakagígar. Náttúrufr. 37: 27—57.
— 1971: Aldur ljósu gjóskulaganna úr Heklu samkvæmt leiðréttu geislakols-
tímatali. Náttúrufr. 41: 99—105.
S U M M A R Y
Postglacial volcanic activity in the Jökulsárgljúfur Canyon, NE-lceland
by
Sigurvin Eliasson, Skinnastadur
The author describes two crater rows and the remnants of postglacial lava-
flows in the 30 km long canyon of the glacier river Jökulsá á Fjöllum. The
older craters are the 6 km long H1 jódaklettar and Raudliólar row some 10 km
upstream from the mouth of the canyon at Áshöfdi, and the younger craters
the Randarhólar and Sveinar row about 25 km south of the canyon’s mouth.
At least what seems to be the older lava flowed a short distance out to the
sandur plain north of the canyon. Tlte lavas are greatly eroded ancl the remn-
ants mostly cap the canyon walls or are perched upon them. The bigger and
younger flow seems to have come from the Randarhólar and Sveinar craters.
Dated by tephrochronological methods (Sigurdur Þórarinsson, 1944 and 1958)
both the lava flows seern to be a little more than 8500 years old. The river
was most probably at least two times temporarily dammed by the fissure erup-
tions.
Superposed on the lavaflows are coarse gravel terraces, in a few places with
great superficial blocks, covered by tliin soil which, judging by asli layers, is
lesser than 2900 years old. The possibility of a great catastrophic flood (jökul-
hlaup) some 2000 years ago is shortly discussed.
Tlie existence of a big ice-dammed lake in the Mosar—Svínadalur area,
probably in the late Older Dryas periocl or perhaps as late as the Younger
Dryas period is pointed out. This lake must have had an outlet west of Lamba-
fell and could have contributed greatly to the carving out of the famous Ás-
byrgi Canyon, notably if the morainic dam had lturst sucklenly.