Jökull - 01.12.1978, Side 107

have been confused with the Raudhálsar
eruption. A study of the Landnáma indi-
cates that the eruption may have taken place
in 950—970 A.D.
SIGALDA HYDRO-POWER SITE:
GROUNDWATER AND LEAKAGE
STUDIES
Bj'órn Jónasson,
National Energy Authority
In connection with the impounding of the
Sigalda reservoir intensive studies of the be-
haviour of the local groundwater have been
carried out together with observations of pos-
sible leakage paths from the reservoir and
headrace canal. Measuring the temperature of
groundwater in boreholes and springs in the
Sigalda area was an important part of the
study as well as the chemical analysis of water
samples. The temperature measurements
have revealed the presence of two ground-
water systems. On the one hand there is a
“warm system” (4.5—5°C) in the moberg
(headrace canal and vicinity), in the northern
part of the lavas in the reservoir basin and on
the damsite, and on the other hand a “cool
system” (3—4°C) which occurs in the lavas to
the south. Springs issuing into the Sigalda
canyon downstream of the training dyke are
derived from the cool system but springs
further upstream from the warm system. The
temperature measurements also show that
leakage out of the reservoir basin occurs
mainly in a westerly direction along the
scoriaceous top layer of the Tungnaá lava
THf, along the bottom layer of THh as well as
along the interbed between them, primarily
around the southern dam abutment and from
there to the NW into the canyon downstream
of the dam, while leakage underneath and
through the dam is negligible. Leakage from
the headrace canal, i.e. in the moberg, seems
to be associated with contacts between
moberg facies and/or along tectonic
lineations. Evidently, the leakage paths follow
an east-west striking trend which probably is
the dominating fault direction in the Sigalda
moberg. The chemical analysis supports the
established pattern quite well.
VOLCANIC ERUPTIONS ON THE
REYKJANES PENINSULA IN
HISTORICAL TIME
Jón Jónsson,
National Energy Authority
Investigations during the last few years
have revealed that several volcanic eruptions
have occurred in the above mentioned area
after the arrival of the first Scandinavian set-
tlers to Iceland in about 874. Relatively reli-
able historical records exist for only 3 erup-
tions, but the existence of at least 9 can now be
proved, partly with the aid of tephrocronology
and partly by C14 age determinations. There
seems to have been relatively intense volcanic
activity shortly before and after the year 900.
Accordingly there is no evidence for a decrease
in the volcanic activity in the Reykjanes area.
PALEOMAGNETISM OF THE ESJA
AREA, SOUTHWESTERN ICELAND
Leó Kristjánsson, Science Institute, Ingvar Birgir
Fridleifsson, National Energy Authonty, N. D.
Watkins (deceased, 2 Nov 1977), University of
Rhode Island
Since 1973, we have sampled 286 lava flows
of Lower Quaternary age in the Esja area for
plaeomagnetic work. K-Ar dating has con-
firmed that the youngest of these units most
probably correspond to the Gilsá geomagnetic
polarity event. A short sequence of normally
magnetized flows in Kistufell and elsewhere
might hence be of Olduvai or possibly
Reunion age, and the oldest exposed lavas
JÖKULL 28. ÁR 105