Jökull - 01.12.1991, Page 82

Hellnahraun lava flow (other names: Hvaleyrarhraun,
Flatahraun), as well as the flows from the Obrin-
nishólar craters (Fig. 1), were formed during the
Krísuvík Fires. Later the present authors discovered
that the Hellnahraun lava flow was formed in two
different eruptions. The Younger Hellnahraun lava
flow overlies the Obrinnishólar lava flow which in
turn overlies the Older Hellnahraun lava flow. In
1986 the present authors succeeded in finding a soil
section which shows that the Younger Hellnahraun
lava flow overlies the Settlement tephra layer, but in
turn underlies the Medieval tephra layer (Fig. 8) and
consequently is of historic age. The Hellnahraun
lava flow had up to then been thought to be a single
flow and of prehistoric age. Jónsson (1974) dated the
Obrinnishólar lava flow as 2100 years old, but he
also stated that it is younger than the Hellnahraun
lava flow, which now has been partly disproved.
Further mapping of the Younger Hellnahraun lava
flow has enabled us to trace it to its origin in the
Brennisteinsfjöll fissure swarm which is east of the
Trölladyngja fissure swarm. According to this new
evidence the Younger Hellnahraun lava flow is a sea-
ward extension of the Tvíbollahraun lava flow earlier
described by Jónsson (1977). We also suggest that
the Breiðdalshraun lava flow described by Jónsson
(1978a) was formed during the same volcanic event
as the Yonger Hellnahraun lava flow.
Calibration curves for two new radiocarbon dates
of the Younger Hellnahraun and two earlier dates
from Jónsson (1977, 1978a) from Tvíbollahraun and
Breiðdalshraun are shown in Fig. 10. The weighted
mean of these four dates (Fig. 10(e)) suggests that the
following time intervals are within the 68.3% proba-
bility limits: 894-923 and 938-983 AD. Thus the
Younger Hellnahraun flow is older than the Krísuvík
Fires and was formed during the lOth century.
A historic account exists of a volcanic eruption in
1000 AD in the Hellisheiði region farther east.
Jónsson (1979) suggested that the Svínabruni lava
flow was then formed. This flow belongs to the
Brennisteinsfjöll fissure swarm. This brings up the
question whether the eruption of the Younger
Hellnahraun lava flow and the Svínabruni lava flow
could be contemporaneous volcanic events. It has
been noticed that 14C dates from the settlement peri-
od and later in Iceland tend to give 30-50 years too
high ages compared with the chronicles. It is worth
noticing that although the suggested age of the
Krísuvík Fires in 1151 AD is within the probability
limits of the five available 14C dates, the mean value
of the dates lies close to the year 1100 AD, or 50
years earlier than the suggested age, According to I.
Olsson (pers. comm.), who has studied the activity of
atmospheric 14C in Iceland, a deviation of about 30
years is to be expected for 14C dates on Icelandic
material.
80 JÖKULL, No. 41, 1991