Jökull - 01.12.1991, Side 81

Summary
AGE OF THE KAPELLUHRAUN AND HELLNAHRAUN LAVA FLOWS,
REYKJANES PENINSULA, SOUTHWEST ICELAND.
THE KRÍSUVÍK FIRES
It was suggested in an earlier paper of Jóhannesson
and Einarsson (1988) that the Ögmundarhraun lava
flow on the Reykjanes peninsula was formed in an
eruption in 1151 AD. This was based on 14C dates,
tephrochronological studies and historical records.
The eruption took place within the Trölladyngja
fissure swarm. The total length of the eruptive fissure
is about 25 km, but there is a gap in its central part.
The southern part of the fissure produced the
Ögmundarhraun lava flow while the northern part
produced the Kapelluhraun lava flow. At the central
part of the fissure two minor flows were produced, i.e.
Lækjarvellir and Mávahlíðar lava flows (Fig. 1). It is
suggested that the four lava flows were produced dur-
ing one volcanic episode which we have named the
Krísuvík Fires.
According to a petrological study by Gunn-
laugsson (1973) the lavas are olivine tholeiites in
composition. Close to the craters the flows are thin
pahoehoe flows, but as they flowed farther away they
became more viscous and formed aa flows.
The lava flows of the Krísuvík Fires cover an area
of 36.5 km2 and the estimated volume is 0.22 km3.
The production of tephra was negligible.
The general trend of the Krísuvík Fires eruptive
fissure is N45°E, but west of Krísuvík the fissure dis-
plays a more easterly trend which farther north
breaks up into short segments and shifts en echelon
northwards until it resumes a northeasterly trend.
This N-S trending anomaly coincides with the narrow
seismic zone which extends from the South Iceland
Seismic Zone west along the center part of the
Reykjanes peninsula (Jóhannesson 1989).
It is mentioned in the Flateyjarbók chronicle
(Flateyjarbók 1945) that earthquakes also occurred in
the year 1151 AD.
RADIOCARBON DATES OF
THE KRÍSUVÍK FIRES
Jónsson (1978a, 1978b, 1982) published four 14C
dates of charcoal from the Kapelluhraun lava flow and
three from the Ögmundarhraun lava flow. Calibration
curves for these dates and their weighted mean are
shown in Fig. 7, indicating that within the 68.3% (1
standard deviation) probability limits the volcanic
event occurred during one of the four following time
intervals: 1026-1045, 1052-1065, 1089-1125 or
1138-1153 AD. The proposed year of eruption, 1151
AD, is within one of these limits and supports our sug-
gestion which was based on the old chronicles (Storm
1888, Flateyjarbók 1945) where it says that in the year
1151 AD there was “fire in Mt. Trölladyngja”.
TEPHROCHRONOLOGY
Two tephra layers in the Reykjanes peninsula are of
great importance for the present study, i.e. the
Settlement tephra layer (Landnámslag in Icelandic),
which was formed around the year 900 AD (Larsen
1984, Róbertsdóttir and Jóhannesson 1986), and the
Medieval tephra layer (Miðaldalagið in Icelandic),
which formed in 1226 AD (Jóhannesson and
Einarsson 1988). Selected soil profiles are shown in
Figs. 2, 3, 6 and 8, where the relations of all the indi-
vidual lava flows (or their craters) of the Krísuvík
Fires to known tephra layers are shown. The lavas or
the cinder from the craters always falls in between
the two above-mentioned tephra layers.
THE AGE OF THE
HELLNAHRAUN LAVA FLOW
In the earlier paper on the Krísuvík Fires (Jóhannes-
son and Einarsson 1988) it was suggested that the
1
JÖKULL, No. 41, 1991 79