Jökull - 01.11.1998, Side 5

New data on the age and origin of
the Leiðólfsfell Cone Group in south Iceland
Þorvaldur Þórðarson1
Institute of Geological and Nuclear Science, Wairakei Research Centre,
Private Bag 2000, Taupo, New Zealand
D. Jay Miller
Ocean Drilling Program, Texas A&M University Research Park
1000 Discovery Drive, College Station, Texas 77845-9547
Guðrún Larsen
Science Institute, University of Iceland,
Dunhaga 3, IS-107 Reykjavík, Iceland
Abstract - TheLeiðólfsfell cone group is an association ofscoria cones and ramparts in the 1783-
84 Laki lavaflow in Southern Iceland. The related scoriafall deposit has a local dispersal within
a 1.5 km radius ofthe cones. The age and the origin ofthis cone group have been debated in re-
cent years. New data based on tephra stratigraphy in nearby soil profiles show that it wasformed
in the year 1783 by rootless vent eruptions when the Laki lava flow entered the channel of the
Hellisá River. These results arefurther supported by radiometric dating of carbonized mossfound
immediately below the Leiðólfsfell scoria deposit, giving uncorrected C14-age of250 ± 60 years
and also by the chemical composition ofthe Leiðólfsfell scoria which is identical to the products
of the 1783-84 AD Laki eruption. The contemporary accounts which detail the course of events
during the Laki eruption, indicate that the rootless vent eruptions at Leiðólfsfell tookplace around
17June1783.
INTRODUCTION
The Leiðólfsfell cone group is an association of ir-
regularly spaced and partly overlapping scoria cones
interbedded with the 1783-84 Laki lava flow to the
northwest of the Leiðólfsfell mountain in the Síða high-
lands (Fig. 1). The explosive activity that built the
cones also produced a tephra layer which blanketed
the surrounding landscape up to 1.5 km from source.
In the last decade the published data on the origin and
the age of the Leiðólfsfell cone group have differed
considerably. Jónsson (1985a, 1985b, 1990) postulat-
ed that the cone group was the product of an unknown
eruption in the 12th Century. Þórðarson (1990, 1991),
however, used tephrochronological evidence to sug-
gest that the cone group was produced by rootless
eruptions, as the 1783 Laki lava first flowed over the
water saturated beds of the Hellisá River, which at the
time ran north of Mt. Leiðólfsfell before flowing into
the Skaftá River gorge (see below). Þórðarson's results
were preliminary because they were based on recon-
'Current address: CSIRO Magmatic Ore, Deposit Group, Division of Exploration and Mining, Private Bag P. O. Box Wembley 6022 W.A., Australia.
JOKULL, No. 46, 1998
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