Náttúrufræðingurinn - 1990, Blaðsíða 64

Walker, G. P. L. (1966) Acid volcanic
rocks in Iceland. Bulletin Volcanolog-
ique, 29. 375-406.
SUMMARY
On the origin of low-temperature
geothermal activity in Iceland
by
Stefán Arnórsson
and
Sigurður R. Gíslason
Science Institute
University of lceland
Dunhagi 3
IS-107, REYKJAVÍK
Iceland.
Low-temperature geothermal activity in
Iceland is confined to Early Quaternary
and Tertiary formations. High-temper-
ature activity is, on the other hand, associ-
ated with the presently active belts of vol-
canism. Drillhole data indicate subsurface
temperatures of <150° and >200°C at
depths of less than 1000 metres in the low-
and high-temperature fields, respectively.
It is generally considered that the low-
temperature activity is non-volcanic but
that the high-temperature systems have a
magmatic heat source.
Einarsson (1937, 1942, 1966) proposed a
hydrogeological model for the low-tem-
perature fields in Iceland. He envisaged
that the water was of meteoric origin, that
it seeped deep into the bedrock in the in-
terior highlands and ascended in lowland
areas. During its gravity driven flow the
water picked up some of the terrestrial
conduction heat. Einarsson’s model im-
plied that the low-temperature activity
was steady state and that the low-temper-
ature areas represented upflow zones.
Extensive studies of deuterium in nat-
ural waters in Iceland led Árnason (1976)
to the conclusion that the deuterium con-
tent of waters from the low-temperature
fields generally represented precipitation
that had fallen on higher ground farther
inland. The deuterium data were, there-
fore, compatible with Einarsson’s model.
Bödvarsson (1982) considered that the
model of Einarsson (1966) was not accept-
able on the basis of energy balance consid-
erations, at least for the strongest low-
temperatures fields. The heat output from
these fields was too high for them to be
represent steady state systems.
On the basis of drillhole data on per-
meability and temperature Björnsson
(1980) showed that, at least, some of the
low-temperature systems were convection
systems. At great depths (2.5-3.2 km) in
two fields temperatures were lower than
anticipated from the regional thermal gra-
dient (Fig. 6), thus indicating that cooling
of the rock had occurred at great depths
and in all likelihood by the convecting wa-
ter. The above described results indicate
that these low-temperature systems are
neither steady state nor upflow systems as
proposed by the general model of Einars-
son (1966).
The authors of the present contribution
propose that one or more of the following
four processes contribute to the develop-
ment of low-temperature geothermal sys-
tems:
1) Flow of water deep in the bedrock
from highland areas to lowland areas.
2) Convection in young permeable frac-
tures which have formed by tectonic
movements in otherwise impermeable
bedrock.
3) Gradual cooling of high-temperature
geothermal systems as they drift out of
the active volcanic belts and their mag-
matic heat source becomes extinct.
4) Magmatic intrusion into fractures or
permeable bedrock flanking the vol-
canic belts.
If it is possible to define a characteristic
common to all low-temperature activity,
then it would be the recent formation of
tectonic fractures enhancing permeability
and subsequent water convection in these
fractures. It is evident that the natural
heat output is highest and subsurface tem-
peratures are highest for those low-tem-
perature fields which lie in areas with the
highest regional thermal gradient.
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