Jökull - 01.12.1979, Side 49

7 Geothermal activity in Iceland
INGVAR BIRGIR FRIDLEIFSSON
National Energy Authority, Reykjavík
INTRODUCTION
Like other constructive plate margins the Mid-
Atlantic Ridge is characterized by a high heat flow
in the crestal region, but with increasing distance
symmetrically away from the ridge crest the raean
heat flow falls until it reaches an average level for
the oceans. Iceland forms a 500 km broad segment
astride the ridge and falls entirely within the crestal
heat flow anomaly. The regional heat flow on the
island varies from about 80 mW/m2 furthest away
from the active volcanic zones crossing the country
to about 300 mW/m2 in some regions at the mar-
gins of the Reykjanes-Langjökull axial rift zone.
The geothermal gradient as measured in over 100
m deep drillholes outside known geothermal fields
and outside zones of active volcanism, ranges from
37°C/km to 165°C/km (Fig. 1).
Hot springs are very abundant in the country as
a result of the high heat flow (Fig. 2). To date there
have been recognized approximately 1000
geothermal localities in the country. Hot springs
have also been identified in a few places on the sea
floor surrounding the island. The thermal output
varies greatly from one locality to another. In the
Plio-Pleistocene and Tertiary regions the water
temperature varies from a few degrees above the
mean annual temperature to boiling springs, and
the flow rate varies from nil to a maximum flow of
about 180 1/s from a single spring. Steamfields are
confined to the active zones of rifting and volcan-
ism that run through the country. It has become
customary to divide the geothermal activity into
two types on basis of the base temperature
(maximum temperature) in the uppermost 1 km.
The base temperature is thus <.150°C in the low
temperature areas, but >:200oC in the high tem-
perature areas. The low temperature areas are in
Plio-Pleistocene and Tertiary volcanics. Due to the
oceanic climate there is heavy precipitation in the
island. Some of the precipitation percolates deep
into the bedrock in the highland areas and flows
laterally along faults and pervious horizons for
Fig. 1 . Geothermal
gradient measured in over
100 m deep drillholes out-
side known geothermal
fields and outside the active
volcanic zones. (Data from
Pálmason et al. 1978 and
files of the National Energy
Authority, Reykjavík).
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