Náttúrufræðingurinn - 1990, Blaðsíða 46

SUMMARY
The Nature of Hot Spring Systems
in Iceland
by
Axel Björnsson, Guðni Axelsson and
Ólafur G. Flóvenz
Orkustofnun
National Energy Authority
of Iceland
Grensásvegur 9
IS-108 REYKJAVÍK
Iceland
During the first decades of this century
most scientists believed that the main en-
ergy source of both high temperature and
low temperature geothermal fields in Ice-
land was magmatic intrusions in the crust.
The water was thought to be meteoric wa-
ter, which circulated in the crust and was
heated by the cooling intrusions.
Trausti Einarsson (1942) presented a
model of the low temperature fields in N
Iceland where he demonstrated that the
energy source was not of magmatic origin.
Later he extended his theory to all low
temperature fields as well as the high tem-
perature fields in Iceland (Trausti Einars-
son, 1966). According to his model the
geothermal fields are fed by a general cir-
culation of meteoric water, which perco-
lates deep into the crust in the central
highlands and flows towards the coast.
The water is heated by the continuous
conductive terrestrial heat flow from be-
low, and the temperature in hot springs is
controlled by the depth of the flowing wa-
ter. This steady state model of low tem-
perature systems has enjoyed general rec-
ognition among geoscientists for some
decades.
Gunnar Bodvarsson (1950), on the oth-
er hand, concluded on the basis of energy
balance calculations that a steady state
model could not explain the power of the
larger low temperature systems in Iceland.
He concluded that these systems must be
of a transient nature, and that the heat
source is the heat stored in the crustal
rocks in which the water circulates. Later
Gunnar Bodvarsson (1982, 1983) present-
ed a conceptual model of the low temper-
ature fields. According to his model the
circulating ground water is heated in
narrow fissures, which continuously mi-
grate downward during the heat mining
process.
Great amounts of new data on the low
temperature areas have been collected
during the last two decades. These data
are not compatible with the steady state
model but support strongly the heat
mining model of Gunnar Bodvarsson. De-
tailed geological mapping within and
around low temperature areas demon-
strates that they are closely related to fiss-
ures, which have recently been tectonical-
ly active. Temperature profiles in deep ge-
othermal drillholes clearly demonstrate
local circulation of geothermal water and
transport of heat from the roots of the
geothermal systems in the lower crust up
to the surface. New estimates of the per-
meability of the Tertiary and Quaternary
formations around low temperature areas
indicate that the general deep ground wa-
ter flow from the central highland to the
coast is far too slow to be able to feed the
larger geothermal systems.
Theoretical considerations of the heat
mining process suggested by Gunnar Bod-
varsson indicates that the process appears
to be able to explain the power of even
the largest low temperature areas in Ice-
land. In addition, these considerations
show that the power of such low temper-
ature systems is controlled by the temper-
ature conditions in the crust and in partic-
ular the local stress field. Given the ther-
mal conditions in the crust of Iceland it
appears therefore that the regional tecton-
ics and the resulting local stress field con-
trol the low temperature activity.
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