Jökull - 01.12.1988, Blaðsíða 88

Summary
AGE OF THE ÖGMUNDARHRAUN
LAVA FLOW AND
THE MEDIEVAL TEPHRA LAYER,
REYKJANES PENINSULA,
SOUTHWESTICELAND
Haukur Jóhannesson
lcelandic Museum ofNatural History
Laugavegur 115, P.O. Box 5320
125 Reykjavík, Iceland
Sigmundur Einarsson
Birtingakvísl 19
110 Reykjavík, Iceland
The age of the Ögmundarhraun lava flow on the
Reykjanes peninsula, southwest Iceland (Fig. 1) has
been a matter of discussion. Jónsson (1982, 1983)
suggested on the basis of 14C datings that the lava
flow dated from the early llth century. Rafnsson
(1982) argued on the bases of evidence from
historical records that the flow had formed in an er-
uption between 1558 and 1563.
The flow is certainly of historic age (i.e. younger
than 900 AD) because it has overrun the old Krísu-
vík farm (Fig. 2) ruins of which have long been
known on the outskirts of the lava field.
The Ögmundarhraun lava flow was erupted within
the Trölladyngja fissure swarm. The crater row is
about 25 km long and has produced lava flows
which have reached both the south and north coast
of the Reykjanes peninsula (Fig. 1). The Ögmundar-
hraun flow is on the south of the peninsula but
Kapelluhraun and Hvaleyrarhraun are contemporary
flows on its north side.
The age of the Ögmundarhraun lava flow was det-
ermined by considering tephrochronology, 14C
datings and by evidence from historical records.
TEPHROCHRONOLOGY
Three historic tephra layers are found in soil sect-
ions in the region (Fig. 3). Firstly the Landnám
(Settlement) layer which formed around the year
900 AD (Larsen, 1984; Róbertsdóttir and
Jóhannesson, 1986; Hallsdóttir, 1987). Secondly the
Medieval tephra layer whose age is determined in
this paper. Thirdly the Katla 1485 tephra layer (Lar-
sen, 1978).
AGE OF THE MEDIEVAL TEPHRA LAYER
Olafsson (1983) has mapped the distribution of
the Medieval tephra. It is found all over the Reykja-
nes peninsula and was formed by a submarine er-
uption off the tip of Reykjanes. Ruins of the crater
are still to be seen at the coast. Olafsson (1983) tried
to date the layer. According to him the relative
position of the layer to the Landnám (Settlement)
and the Katla 1485 layers in soil sections suggested
the year 1340 but the historical records 1226/27 or
1231.
In the old Icelandic annals (Storm, 1888) many
references are made to volcanic eruptions in the sea
off the Reykjanes peninsula. They mention eruptions
or phenomena related to such eruptions in the foll-
owing years: 1210 and/or 1211, 1223, 1225, 1226,
1227, 1231, 1238 and 1240. A heavy ashfall is
reported in the summer of 1226 and it is also stated
that darkness occured in the middle of the day (no
eclipse was in that year). The following winter, i.e.
1226/27 is called "sandwinter". In 1231 contem-
porary sources mention eruption and ashfall ("sand-
summer"). Some sources mention eruptions in the
years 1340 and 1389 and/or 1390 but in a fictional
way.
Our conclusion from the review of the historical
records is that a volcanic episode took place in the
westem part of the Reykjanes peninsula and in the
sea off the tip of the peninsula in the years 1210
and/or 1211 to 1240. The ashfall in 1226 seems to
have been considerable and in fact is the only histor-
ic evidence for an eruption producing enough am-
ount of tephra to account for the Medieval tephra
layer.
The position of the Medieval tephra relative to
the Landnám (Settlement) and Katla 1485 tephra
layers in soil sections is not reliable due to a vari-
able soil thickening rate caused by soil erosion. On
86 JÖKULL, No. 38, 1988