Jökull - 01.12.1985, Page 97

SUMMARY
ON THE A GES OF THE TWO RECENT LA VA
FLOWS IN EYJAFJÖLL AND THE LATE
GLACIAL TERMINAL MORAINES
IN SOUTH ICELAND
Haukur Jóhannesson
lcelandic Museum of Natural History
P. O. Box 5320
Reykjavík, Iceland.
The Eyjafjöll central volcano is one of many vol-
canoes in the Eastern Volcanic Zone (Fig. 1). The vol-
canic activity in the Eyjafjöll region is limited to a 5 km
wide and 30 km long fissure zone which stretches E-W
from the Markarfljót river in the west to the Mýrdals-
jökull in the east. The zone corresponds roughly to the
fissure swarms of the volcanic systems which characte-
rize the axial rift zones of Iceland. Only minor rifting is
assumed to take place in the zones accompanying the
volcanoes of the Eastern Volcanic Zone. The Eyjafjöll
volcano itself is located on the central part of the fissure
zone. It reaches 1666 m height and the top of the
volcano is occupied by a 2.5—3.0 km wide summit crater
or small caldera subsidence. The volcano is still active
and the last eruption took place in 1821-1823. The
intensity of volcanic activity is, however, very low in
comparison to other Icelandic volcanoes. The Eyjafjöll
volcano has been active for at least 700.000 years. The
oldest exposed formations were erupted during the final
stages of the Matuyama paleomagnetic epoch. The vol-
cano is built of a pile of lava flows erupted during
interglacial periods and hyaloclastites formed by subgla-
cial eruptions during glacial periods. Evidence for 9
glacial and 9 interglacial periods has been found.
Only two lava flows are of Holocene age. They are in
the western part of the Eyjafjöll volcano, named
Kambagil and Hamragardar lava flows, respectively
(Fig. 2). Kjartansson (1958a, 1970) believed that their
age was about 11.000—10.000 years, erupted during the
Allerod stage. In this paper the geology of the two flows
is reviewed.
The Kambagil lava flow was erupted from a crater
named Raudahraun (Figs. 2 and 3) on the north side of
the mountain range. Kjartansson (1958a) suggested that
the flow terminated at about 325 m above the present
sealevel. Kjartansson concluded that a glacier termed as
Markarfljót glacier flowed along the valley between the
Eyjafjöll and Tindfjöll massifs and that the lava flowed
down to the glacier and spread out on top of it. The
lower part of the flow was then subsequently removed
by the glacier. Detailed mapping of the lava flow has
revealed that it does not terminate at 325 m height but
has reached the lowlands west of the Eyjafjöll moun-
tain. But below the 325 m mark the flow is covered by
ground and terminal moraines indicating that the
Markarfljót glacier has overrun the lower slopes of the
Eyjafjöll mountain while the upper slopes were icefree.
The lava rests on conglomerate or tillite and no traces of
organic material were found thus preventing C-14 dat-
ing of the flow.
The margin of the former Markarfljót glacier was
traced by mapping the glacial striae (Fig. 2). Haraldsson
and Palm (1980) carried out seismic studies of the areas
south of the research area and suggested that a subsur-
face terminal moraine occurred in the outwash plain in
Austur-Landeyjar. A comparison of the seismic data
and the height of the former Markarfljót glacier on the
slopes of the Eyjafjöll mountain with the present
Skeidarárjökull indicate that the snout of the Markar-
fljót glacier might have been at the terminal moraine of
Haraldsson and Palm (1980) (Figs. 7 and 8). A further
three minor terminal moraines were observed on the
top of the Kambagil lava flow (Fig. 3).
The Hamragardar flow was erupted from a nameless
crater southwest of the Raudahraun crater (Fig. 2 and
6). The lava flowed along a small valley towards the
southwest but the main stream stopped at the cliff edge
above the Hamragardar farm and only two rivulets
seem to have flowed off the edge. The Hamragardar
flow has not been overrun by a glacier and is thus
undoubtedly younger than the Kambagil lava. The flow
rests also on conglomerate or tillite and no material for
C-14 datings is to be found.
The major terminal moraines of South Iceland are
shown on Fig. 9. They can be divided into three groups,
Outer (or Y-), Middle (or M-) and Inner (I-) terminal
moraines. C-14 datings of shells from raised terraces in
South and West Iceland suggest that the Y-moraine is at
least from the Older Dryas stage or about 12.000 years
old but may be older. The M-moraines are at least from
the Younger Dryas stage or 10.000 years old. The
Kambagil flow is thus older than 12.000 years, and the
Hamragardar flow is at most 11.000 years old as it has
not been overrun by the glacier which formed the Y-
moraines but might be younger.
JÖKULL 35. ÁR 95