Jökull - 01.12.1978, Blaðsíða 106

Association of Hydrogeologists (IAH). In spite
of the uncommon hydrogeological conditions
of the country these methods proved to be very
useful in representing the general
hydrogeological features of Iceland. The map
is partly based on the Internat. Geol. Map of
Europe and the Mediterr. Region 1:1 500 000.
The type of bedrock and unconsolidated
rocks is indicated by dim signature in the base
along with fissure swarms, central volcanoes
etc. The areal colours of the map indicate the
hydrogeological character of the rocks.
Among the novelties in Icelandic
hydrogeology shown on the map, is the sub-
division of the bedrock and the separation of
the unconsolidated rocks into groups of dif-
ferent permeabilities. The most productive
springs and spring areas in Iceland are indi-
cated and their relation to fissure swarms and
lavafields shown. The catchment areas and
the mean runoff of the main rivers are shown
as well as the high temperature areas, the
main low temperature areas, carbonate
springs, salt water intrusions, brackish water,
and the permafrost moors. This map is the
Icelandic contribution to The International
Hydrogeological Map of Europe, published in
the scale of 1:1 500 000 by IAH and
UNESCO.
THE OXIDES OF THE JAN MAYEN
ROCKS
Páll Imsland,
Nordic Volcanological Institute
The primary oxides of the alkaline rocks of
Jan Mayen are chromian spinel, titanomag-
netite and magnesian ilmenite. The spinel
occurs primarily in ankaramites. The iron-
titanium oxides are most abundant in the
basalts but also occur in intermediate rocks.
The oxide compositions show that the iron-
titanium oxides have crystallized at 1040—
1080°C with f02 close to the FMQ-buffer.
MgO and V203 show covariation with tem-
perature. This covariation is believed to result
from decreasing amount of these components
with increased crystallization and decreasing
temperature. Magnesian ilmenite has been
believed to occur in kimberlites only but a
continuous increase of Mg in ilmenites from
Mg-free ilmenites in oceanic tholeiites to
highly magnesian ilmenites in kimberlites is
revealed.
THE AGE OF TWO RECENT LAVA
FLOWS IN HNAPPADALUR,
W-ICELAND
Haukwr Jóhannesson,
National Energy Authority
The Eldborg and Raudhálsar lava flows in
Hnappadalur W-Iceland are described. They
are both basaltic in composition. The Eidborg
lava flow was formed in a single eruption but
not in two as has been suggested by a
previous worker. The eruption took
place when sea-level was at least 2 m lower
than at present. The country covered by
the lava flow was barren at the time of the erup-
tion. This suggests an age of 5000 to 9000
years and the higher age is more likely. The
Raudhálsar lava flow is the freshest-looking
flow in Hnappadalur and it was erupted from
the Raudhálsar cinder cone. Unlike the Eld-
borg eruption, the Raudhálsar eruption in-
cluded an early phreatic phase. The ash pro-
duced can be traced in the wetlands south of
Raudhálsar. In soil profiles the dark peat be-
low the ashlayer changes to brown above it.
This change in colour has been attributed to
soil erosion soon after the settlement of Ice-
land. The Raudhálsar eruption thus appears
to have taken place during, or shortly after,
the settlement period (874—930 A.D.) Land-
náma (the Book of Settlement), written in
the eleventh century, refers to a volcanic
eruption in Eldborg in the ninth century but
the present study has shown that the Eldborg
lava flow is much older and may therefore
1 04 JÖKULL 28. ÁR