Jökull - 01.12.1986, Síða 4

indicate that these waters are precipitation that has
fallen in the interior highlands of the country (Árna-
son, 1976, 1977). Árnason (1976, 1977), therefore, con-
cluded that the isotopic studies supported the model
of Einarsson (1942, 1966). However, Bödvarsson
(1982) correctly pointed out that the isotopic data did
not provide any evidence of the mode of heating of the
water.
The temperature distribution in some drilled low-
temperature systems in Iceland shows that they are
convective systems and not upflow systems as would
be expected from Einarsson's (1942, 1966) model
(Björnsson 1980a). Thus, temperatures in deep drill-
holes are lower than would be expected from knowl-
edge of thermal gradient. For example, the regional
gradient in Eyjafjörður is about 60°C (Sœmundsson
and Fridleifsson 1980) but temperature in a deep drill-
hole in the area (Laugaland) only reaches 100°C at
2800 m depth (Björnsson 1980b). In Reykjavík where
the regional gradient is over 100°C/km, a temperature
of only 160°C occurs at the bottom of a 3000 m deep
drillhole (Gunnlaugsson, pers. comm).
It has been established in numerous low-tempera-
ture fields that faults and dykes control upflow zones
(e.g. Einarsson 1937, Sœmundsson and Fridleifsson
1980, Georgsson et. al. 1984, 1985). Fridleifsson (1979)
considered that various structural features, together
with hydrostatic head, determined subsurface flow of
water from the central highlands towards the coast.
These structural features constitute faults, dykes and
ridges of hyaloclastite formed by subglacial fissure
eruptions.
The model for the low-temperature activity in Ice-
land initially proposed by Einarsson (1942) fails to
explain the overall distribution and the intensity of
this activity. The model rests on basic hydrological
principles, but does not attempt to link the origin of
the low-temperature activity with specific geological
processes or events. This is to be contrasted with the
hypothesis of Bödvarsson (1982) regarding the transi-
ent nature of this activity. In the model generally ac-
cepted for high-temperature geothermal activity in
Iceland there is a strong genetic link between hydro-
thermal and geological processes. The origin of this
activity is related to intrusion of magma into the roots
of central volcanic complexes (Pálmason and Sce-
mundsson 1974, Arnórsson et. al. 1978, Sœmundsson
and Fridleifsson 1980).
Since Einarsson (1942) first proposed his model for
the Iow-temperature activity in Iceland very extensive
geological data have accumulated pertinent to the
understanding of the low-temperature activity in the
country. These data have been reviewed briefly and
using them a new conceputal model has been devel-
oped for the two largest low-temperature systems in
the country, in Reykholtsdalur and in Upper-Árnes-
sýsla. The new model favours that the heat source is
magmatic and presumably located directly below the
fields. The magma is assumed to have originated in
the adjacent volcanic zone but intruded, probably as
sheets, into the older rocks on both sides.
GEOTHERMAL ACTIVITY IN
REYKHOLTSDALUR AND
UPPER-ÁRNESSÝ SLA
The integrated natural flow from hot springs in the
low-temperature fields of Iceland is estimated to be
1800 1/s (Gudmundsson and Pálmason 1982). Two-
thirds of the total heat output is limited to the three
largest low-temperature areas, BorgarQörður, Árnes-
sýsla and Mosfellssveit (Sœmundsson and Fridleifsson
1980). These areas are all located in SW-Iceland, in
Quaternary and Late-Tertiary rocks on both sides of
the Reykjanes-Langjökull volcanic zone. In Borgar-
fjörður natural flow from hot springs is close to 400
1/s (Georgsson et al. 1984) and far the larger part of it
is confined to the Reykholtsdalur field which is of the
order of 50 km2 in area. Total flow from low-tempera-
ture springs in Árnessýsla is reported as 350 1/s
(Arnórsson 1970) of which 250 1/s is confined to
100/km2 in Upper-Árnessýsla (Laugardalur and
Biskupstungur). In both Reykholtsdalur and Upper-
Árnessýsla most of the water emerges at boiling tem-
perature. Chemical geothermometry indicates subsur-
face temperatures of up to 150°C in Reykholtsdalur
and as high as 200°C in Upper-Árnessýsla. Therefore,
natural heat output from these fields can be expected
to be considerably higher than indicated by water dis-
charged from hot springs. The natural heat output in
Reykholtsdalur and Upper-Árnessýsla is similar to
that estimated by Bödvarsson (1961) for many high-
temperature fields.
Elaborate studies of the geothermal resource in
Reykholtsdalur, described by Georgsson et al. (1984),
revealed elongated anomalies of low bedrock resistiv-
ity coinciding with faults striking approximately
northeast-southwest. They concluded that the re-
charge area was to the northeast and that the water
flowed along the faults into the discharge area. Árna-
son (1976) indicates a recharge area more to the east.
The heat discharged by the hot spring waters is equi-
valent to terrestrial heat conduction over 2000 km2
(Georgsson et al. 1984).
Arnórsson (1970) and Stefánsson and Arnórsson
2