Jökull - 01.12.1978, Blaðsíða 108

south of Hvalfjördur of Mammoth age. This
interpretation is consistent with aeromagnetic
survey results and stratigraphic correlation
with dated formations in Borgarfjördur. The
rate of lava extrusion in the Esja area, about
2.4 km thickness/M.y., is unusually high, and
more details of geomagnetic and glacial events
might be recorded here than in other contem-
poraneous formations in Iceland.
CHEMICAL EXCHANGE BETWEEN
HYDROTHERMAL FLUID AND
BASALTIC ROCKS IN THE KRAFLA
GEOTHERMAL AREA
Hrefna Kristmannsdóttir and Jan Swantesson,
National Energy Authority
The chemical exchange during hydrother-
mal alteration in the Krafla geothermal area is
evaluated on the basis of analyses of the major
and some of the trace elements in drill cuttings
from the main rock formations. Similar
studies have been carried out previosly in a
few other Icelandic geothermal areas. A rather
slight change could be demonstrated there in
the relative contents of major elements. The
rocks were highly hydrated and locally en-
riched or depleted in silica and the most
mobile elements of the basalts. A more sig-
nificant change in composition of rocks was
observed in areas with saline geothermal
fluids than in the others. The effects of
chemical exchanges could become somewhat
modified due to the method of sampling the
rock chips during drilling. A slow total out-
wash from the rocks during alteration is indi-
cated by the low content of dissolved solids in
the non-saline geothermal fluid. Mineralogi-
cal studies suggest a mobilization of most
elements in the basalts. The effects of chemical
exchanges during hydrothermal alteration
appear to be considerable in the Krafla area.
Mg and the alkalies show the most significant
changes. Mg increases in the upper levels of
the section, whereas the alkalies are depleted.
For Si02, Fe and Ca a strong mobility is
demonstrated during tjne hydrothermal alter-
ation. No general pattern of enrichment or
depletion could be shown for these elements.
THREE LARGE FISSURE ERUPTIONS
ON THE SE VOLCANIC ZONE,
S-ICELAND
Gudrún Larsi.i,
Nordic Volcanological Institute
Two large fissure swarms on the SE volcanic
zone seem to erupt large amounts of magma at
fairly regular interval of 600—800 years each.
A major eruption can be expected within the
next 150 years.
In the period between 850 and 1600 A.D.
three large eruptions took place in this area.
Two of them, dating back to the 9th and 16th
centuries respectively, erupted on parallel
NE—SE trending fissures, the Vatnaöldur
and Veidivötn eruptive fissures, the SW part
of which lie within the caldera of the Torfa-
jökull volcanic complex. Both eruptions pro-
duced voluminous tephra layers and small
lavas. A rather abrupt change in composition
of the magma is observed where the fissures
cut the Torfajökull volcanic complex.
In the lOth century there was a major
eruption in Eldgjá producing both tephra and
lava. The lava fields Medallandshraun,
Landbrotshraun and, in part, Alftavershraun
seem to be of the same age as the Eldgjá
tephralayer. If that proves correct all the Eld-
gjá fissure may have erupted simultaneously
and the material produced is of the same order
of magnitude as that of the Lakagígar erup-
tion in 1783.
106 JÖKULL 28. ÁR