Jökull - 01.12.1965, Blaðsíða 22

much higher or more similar to high tempera-
ture areas. Assuming chemical equilibrium be-
tween water and wall rock in the caldera sub-
strata, the circulating water would have similar
composition to the high temperature water of
Table 2. The amount of water, which in this
case would actually have circulated to depth in
the thermal system is approximately 14 of the
total amount or 1.75 km3 in a 10 year period.
This amounts to an average rate of circulation
of 6000 liters per second.
Thorarinsson (personal communication) esti-
mates the total surface melting in the Gríms-
vötn area to approximately 25% of the precipi-
tion. The geothermal heat produces the energy
for the melting of the remaining 75% or ap-
proximately 0.5 km3 water yearly or 1300*10?
cal/sec. Using the very rough estimate of
6000 liters/sec. for the deep circulating water
the heat content after circulation would be
217 cal/gram. This is similar to the heat con-
tent of deep water in high tempreature thermal
areas. Actually this figure merely shows an
order of magnitude, but it indicates that the
processes causing melting and the buildup of
dissolved elements in the meltwater can be
explained on the basis of the suggested model.
ABSTRACT.
Analysis of glacier burst water from Skeidará
in Sept. 1965 shoW high amounts of dissolved
solids during the period of peak discharges.
The water accumulates as snow and ice in the
caldera Grímsvötn over a period from 4 to 10
years. The total amount of precipitation in the
caldera in the period between glacier bursts
equals the amount of water discharged during
the burst. Strong thermal activitv in the caldera,
observed especially after emptying, indicates
that geothermal heat is the main source of
energy to melt the ice. A model is suggested
based on a system with closed water circulation.
The water circulating to depth comes into solu-
tion equilibrium with the wall rock at tempera-
tures probably exceeding 200° C. Approximately
14 of the total amount of water contained in
the caldera would have to círculate in thís
way in order to raise the amount of dissolved
solids to the observed value.
REFERENCES.
Böðvarsson, G. and Pálmason, G. 1961: F.xplora-
tion of Subsurface Temperature in Iceland.
Jökull 11: 39-48.
EIlis, A. J. 1957: Chemical equilibrium in mag-
matic gases. Am. J. Sci. Vol. 255, p. 416.
Nielsen, N. 1937: Vatnajökull. Kampen mellem
Ild og Is. Köbenhavn.
Rist, S. 1955: The hlaup of Skeidará 1954.
Jökull 5: 30-36.
Sigvaldason, G. and Elísson, G. 1966: Report
on Collection ancl Analysis of Volcanic
Gases from Surtsey. Surtsey Research Pro-
gress Report II. Reykjavík.
Thorarinsson, S. 1954: The jökulhlaup (glacier
burst) from Grímsvötn in July 1954. Jökull
4: 34-37.
— 1953: The Grímsvötn Expedition June—
July 1953. Jökull 3: 6-23.
ÁGRIP
Efnagremingar á vatni úr Skeiðará í jökul-
hlaupi í september 1965 sýna óeðli.lega hátt.
magn uppleystra efna í hápunkti hlaupsins.
Vatnið safnast sem snjór og is í Grimsvatnaöskj-
unni i 4—10 ára fyrningum. Urkoma á Gríms-
vatnasvÆÖinu á undan jökulhlaupi er jöfn vatns-
magni hlaupsins. Mikill jarðhiti, sem einkum
er áberandi efti.r tœmingu öskjunnar, er að öll-
um likindum orsök þeirrar bráðnunar, sem á
sér stað milli hlaupa. Hugsanleg skýring er
byggð á kerfi með loliaðri hringrás vatns. Vatn-
ið streymir niður á allmikið dýpi og kemst í
upplausnarjafnvœgi við berg i veggjum vatns-
rásanna við hitastig, sem að öllum líkindum er
yfir 200° C. Efnainnihald þessa vatns verður
svipað og hveravatns af háhitasvœðunum og um
það bil 14 hluti þess vatns sem. fyrir er í öskj-
unni þarf að streyma um dýpri berglög til þess
að heildarefnasamsetning alls vaUisins verði
sviþuð því, sem fannst í Skeiðarárhlaupinu.
128 JÖKULL