Jökull - 01.12.1976, Page 59

noted in mid-February. This gas consisted most-
ly of CO2 and was a few to several degrees
warmer than the outside air. The speed of the
gas coming out of basement floor drains in
town was measured with a Wallac thermo-ane-
mometer at various points through March and
April (Fig. 5) and intermittently later. The gas
speed maximum corresponds in time approxi-
mately to the discharge minimum at the drill-
hole. Gas outflow in town persisted into Aug-
ust, its CO2 content in early August being about
5% as compared to 30% in late June and 90%
in March (B. Arnason, pers. comm. 1974).
Changes in ground water levels and discharge
of CO2 from the ground were noted by Kjart-
ansson (1957) during and after the 1947—1948
eruption of Hekla. Pálmason (1967) has dis-
cussed convection of ground water in Iceland,
showing that in the liighly permeable neovol-
canic zone, convection may be responsible for
much of the vertically transported heat. Our
results and those of Kjartansson demonstrate
lateral movement of groundwater up to kilo-
meters away from an eruptive fissure. Such
convection, not generally included in theoretical
studies on the cooling of intrusive bodies, may
well be an effective mechanism of spreading
the heat away from the immediate vicinity of
intrusives over much larger volumes of rock
and thus reduce the intensity of tlieir meta-
morphic effects.
NOTE ADDED IN PROOF
According to Dr. Páll Einarsson, Science In-
stitute, University of Iceland, there was a signi-
ficant change in the seismic activity in the
vicinity of Heimaey arouncl March 15, i.e. at
the same time as a minimum was noted in the
temperature and discharge from the well. Be-
fore March 15 most of the eartliquakes were
located within a small volume at a depth of
approximately 20 km, but around that time the
activity increased and the active volume was
extended to the north.
Tlie authors are grateful to Dr. Páll Einars-
son for drawing their attention to this correla-
tion between the seismic activity and the gas
pressure. 1
ACKNOWLEDGEMENTS
We thank all the people wlio helped us in
measurements at the Heimaey drillhole and in
monitoring the gas outflow. We are particularly
grateful to Dr. G. Pálmason for his construc-
tive comments on this manuscript.
REFEREN CES
Bödvarsson, G. 1950: Geofysiske metoder ved
varmtvandsprospektering i Island. Tímarit
Verkfrædingafélags íslands, 35, 49—59.
Carslaw, H. S., and Jaeger, J. C. 1959: Con-
duction of lieat in solids, Oxford University
Press, 510 pp.
Kjartansson, G. 1957: Some secondary effects of
the Hekla eruption. Soc. Sci. Isl. The Erup-
tion of Hekla 1947—48, III, 1—34.
Pálmason, G. 1967: On heat flow in Iceland in
relation to the Mid-Atlantic Ridge. In:
Iceland and Mid-Ocean Ridges, ed. by S.
Björnsson, Soc. Sci. Isl. Rit, 38, 111—127.
Pálm.ason, G., Tómasson, /., Jónsson, /., and
Jónsson, í. 1965: Djúpborun í Vestmanna-
eyjum (Deep drilling in Vestman Islands).
National Energy Authority, Reykjavík. 43
pp.
Tómasson, ]. 1967: On tlie origin of sediment-
ary water beneath the Vestmann Islands.
Jökull, 17, 300-311.
Á G R I P
Árið 1964 var boruð á vegum Orkustofnunar
1565 m djúp borhola undir Klifinu á Heimaey
í leit að fersku vatni. Vatnsæðar voru í hol-
unni á 800—1000 m dýpi, en vatnið var óhæft
til drykkjar vegna seltu. Nokkrir metrar voru
niður á vatnsborð í holunni þegar borun lauk.
Aðfaranótt 23. janúar 1973 hófst eldgos í
sprungu á Heimaey í um það bil 1700 m fjar-
lægð frá holunni, og 24. janúar tóku menn
eftir því að vatnsborð holunnar hafði hækkað
og runnu um 0,4 sekúndulítrar af 33 stiga heitu
vatni úr henni.
Hitastig vatnsins og rennslið eða vatnsborðið
var síðan mælt nær daglega til áramóta 1974.
Bæði rennsli og hitastig náðu hámarki um 6.
febrúar 1973, féllu síðan niður í lágmark um
miðjan mars, en jukust síðan aftur fram til 25.
maí. Úr því minnkaði rennslið jafnt og þétt
JÖKULL 26. ÁR 57