Jökull - 01.12.1979, Síða 50

distances of tens of km before it appears on the
surface along dykes or faults on the lowlands. The
water withdraws heat from the regional heat flow
during its passage through the strata. The high
temperature areas are confined to or on the mar-
gins of the active zones of rifting and volcanism,
and are thought to draw heat both from the
regional heat flow and from local accumulations of
igneous intrusions cooling at a shallow level in the
crust.
LOW TEMPERATURE AREAS
The crustal thickness of Iceland varies from 8 to
15 km, and the crustal structure is known in a
considerable detail from geological and seismic
surveys. The crust is formed almost entirely of ig-
neous rocks. The uppermost 4 km or so are com-
posed of subaerial lavas and much subordinate
airborne tuffs in the Tertiary areas, but of subaerial
lavas intercalated with morainic horizons and thick
piles of subglacially erupted pillow lavas and
hyaloclastites in the Plio-Pleistocene provinces,
which flank the active volcanic zones (Fig. 1). Each
eruptive unit is fed by a dyke, and consequently the
dyke intensity increases with depth in the crust.
Below 5 km or so the crust probably consists mostly
of very low porosity impermeable intrusions. This
layer (the oceanic layer, Vp = 6.5 km/s) may form
the base to water circulation in the low tempera-
ture areas. In the high temperature areas and other
parts of the active rift zone the water may circulate
down into the intrusive layer during its formation.
A comparison of the deuterium content of the
thermal water and the local precipitation in the
individual areas has shown that the thermal water
is of meteoric origin. In most cases it is precipitation
which has fallen in the highlands. There the water
manages to percolate deep into the bedrock and
then, driven by the hydrostatic gradient, flows
laterally for distances of tens and up to 150 km
before it appears on the surface along dykes or
faults on the lowlands. This model was originally
proposed by Trausti Einarsson in 1942. The age of
the thermal waters varies from a few decades to
thousands or even tens of thousands of years, dep-
ending on the distance between the hot spring areas
and the recharge areas.
Fig. 3 shows the general flow pattern of thermal
groundwater systems according to deuterium
measurements, with arrows joining the individual
hot spring areas with possible recharge localities,
superimposed on a geological map of Iceland. On
basis of a comparison of the flow directions with
hydrostatic pressure isolines of the country, the hot
water appears to flow equally in every direction
away from the highlands. A close comparison of the
□
TERTIARY FLOOO 8ASALTS
PLIO-PLEISTOCENE BASALTS
ANO HYALOCLASTITES
ACTIVE Z0NESOFRIFTING O THERMAL SPRINGS 45-IOO°C
ANO VOLCANISM • THERMAL SPRINGS 20-45°C
STRIKE / OIP --- DIRECTION OF MAJOR VALLEYS
HIGH TEMPERATURE AREA ANDFJORDS
Fig. 2. Geological map of
Iceland showing the dis-
tribution of natural geother-
mal activity, and the direc-
tion of major erosional
features (valleys and fjords).
The volcanic strata dip
towards but age away from
the active volcanic zones.
High temperature areas
(with temperatures above
200 °C in the uppermost 1
km) are confined to the
active zones of rifting and
volcanism. The low temper-
ature activity is most intense
in areas where the major
erosional directions are
approximately parallel with
the geological strike.
(Slightly modified from
Fridleifsson, 1978).
48 JÖKULL 29. ÁR