Jökull - 01.12.1979, Síða 53

strata (Table 1). The most intensely drilled thermal
area in Plio-Pleistocene strata is at Reykir in Mos-
fellssveit, SW-Iceland. The production area is in a
heavily tilted and blockfaulted zone just outside a
two million year old caldera. Basaltic lavas form
40—70% of the strata and these are intercalated by
thick and thin beds of subglacially erupted pillow
lavas and hyaloclastites as well as detrital beds.
Table 2 shows the occurrence of aquifers in the
different rock types in drillholes in the area. It is
apparent that large aquifers are by far more likely
to occur at the contacts of lithological units than in
lavas alone or in subaquatic volcanics alone.
Several individual 2 km wells in the area can give
100 1/s with pumping and a drawdown of a few
tens of m. .—
The largest natural hot spring in Iceland is at
Deildartunga in Reykholtsdalur, W-Iceland. The
strata is of Tertiary age and mostly of basaltic
lavas, but extrapolation from surface outcrops in-
dicates that at 0.5—1 km depth under the hot
spring area sediments may amount to 30% of the
strata. There is a 1.4 km long line of hot springs
spread along a NW-trending relatively young frac-
ture. The Deildartunga hot spring, with a dis-
charge of about 180 1/s of boiling water, is at the
northern end of the line where the fracture cuts a
NE-trending dyke and an associated fault. The
Kleppjárnsreykir hot spring, with a discharge of
about 70 1/s of boiling water, is at the southern end
of the line and close to this spring the fracture also
cuts a NE-trending dyke. The total discharge along
the line is about 253 1/s, which shows that the
numerous hot springs along the line between the
big end members have insignificant discharge.
There are several other tectonically controlled lines
of hot springs in the Reykholtsdalur area, some of
which are associated with historically active faults.
There was movement on one of these faults
(Helgavatn fault) during an earthquake episode in
1974 and a hot spring disappeared for about 3
weeks and then reappeared with hotter water and
greater discharge than before.
The thermal waters of the low temperature areas
are characteristically alkaline with dissolved solids
of the order of 200—400 ppm. The composition is
dominantly governed by the temperature of the
water and the rock type which it flows through and
leaches. There has been noted a general although
subtle increase of the chlorine content of the ther-
mal waters towards the coast indicating some in-
filtration of seawater into the strata. Table 3 shows
the chemical composition of typical waters from
the low temperature areas. Carbonate springs
(both cold and warm) are found in a few localities
in Tertiary and Plio-Pleistocene strata and most
commonly in the vicinity of deeply eroded central
volcanoes. The carbon dioxide is considered to be
mostly of juvenile origin.
Geysers (erupting springs) can at present be in-
spected in two low temperature areas; Arhver in
the Tertiary strata of Reykholtsdalur, W-Iceland,
erupts irregularly a meter or two, whereas Ystihver
in the Plio-Pleistocene strata of Reykjadalur, N-
Iceland, erupts at a few minutes interval up to
about five meters. A few erupting springs in the low
temperature areas have been captured for utili-
zation.
TABLE 2.
Occurrence of aquifers in the different rock tyþes of 29 dnll holes (800— 2043 m deep) in the Reykir thermal area
(from Tómasson et'al, 1975).
Rock type:
Lavas .............................................
Hyaloclastites1 ...................................
Dolerites .........................................
Lavas + hyaloclastites1 ...........................
Lavas + dolerites .................................
Hyaloclastites1 + dolerites........................
1) The term hyaloclastite is here used in a collec-
tive sense for all subaquatic volcanic products,
thus comprising pillow lavas, pillow breccias, and
Aquifers Total
< 2 l/s 2—20 1/s > 20 1/s number
44 27 2 73
29 12 4 45
1 1 2
53 38 20 111
13 1 3 17
5 2 1 8
tuffs. Included in this group are also reworked
hyaloclastites and detrital beds.
JÖKULL 29. ÁR 51