Jökull - 01.12.1979, Page 55

and epidote and in rare cases garnet and amphibole
in the central part.
The association of magmatic activity with a high
temperature hydrothermal system has been clearly
demonstrated during the current rifting episode of
the Krafla volcanic system in northern Iceland. A
magma chamber has been located below the center
of the 8 km broad Krafla caldera. A high tempera-
ture thermal area lies right above it. Magma that
flows steadily into the magma chamber causes in-
flation of the caldera and during sudden deflation
events magma is expelled laterally into the fissure
swarm that transects the caldera. Since 1975 small
basaltic fissure eruptions have occurred three times
in or just outside the caldera. The hydrothermal
activity inside the caldera has increased dramati-
cally along the eruptive fissure, and the most pow-
erful new springs have thrown mud and rocks and
formed craters that are about 15 m deep and up to
50 m in diameter. These look like explosion craters,
but have been formed by steam erosion rather than
single explosions. Surface hydrothermal activity
has increased even more in two other geothermal
areas on the fissure swarm, one lying about 7 km
north of the caldera and the other (Námafjall)
about 7 km south of the caldera. In a deflation
event in 1977 a small volcanic eruption occurred on
a fissure near the northern rim of the caldera. Seis-
mometers indicated that magma was also moving
southwards and nearly five hours later about 3 tons
of basaltic scoria were erupted up through a 1138 m
drillhole in the Námafjall steam field, about 12 km
south of the active crater in the north. The rifting
events and the magmatic activity have caused
pressure impulses in the water-dominated part of
the Krafla geothermal field. Magmatic gases have
similarly had pronounced effects on the chemistry
of the thermal fluid and caused serious deposition
in boreholes. In one of the magmatic pulses the pH
of the discharge from a well changed from about 9
to about 2 for a short while. Examples of such
injections of volcanic gases into geothermal fluids
can be seen in the secondary mineral assemblages
of some of the most deeply dissected cores of extinct
central volcanoes.
Deep drilling has been conducted in seven high
temperature fields in Iceland. Five of these have
been found to be entirely water-dominated, with
base temperatures of 200—300 °C, but two fields
have been found to be partly boiling. The reservoir
in the Krafla geothermal field has been found to be
rather complex consisting of two geothermal zones,
an upper water-dominated zone with temperatures
of about 200°C and a lower boiiing zone with
temperatures of 300—350°C.
Isotope studies indicate the hydrological cycle in
the high temperature areas to be much more
localized than that of the low temperature areas.
Local precipitation seeps deep into the bedrock; it
has an easy route along open fissures in the active
fault swarms that commonly extend through the
high temperature areas. The water is heated up by
contact with the hot rock, the ultimate heat source
being the general heat flux of the volcanic zone and
the shallow level intrusions in the core of the high
temperature system. The ascending hot water may
flash to steam at a depth of 1 km or less, and on
flashing the dissolved gases (carbon dioxide,
hydrogen sulphide, and hydrogen) are transferred
into the steam. When the steam mixes with local
groundwater the carbon dioxide may give rise to
carbonate springs but the hydrogen sulphide is
oxidized to sulphur or sulphate and the resulting
water is acid. This acid water leaches the rock and
is responsible for the intense alteration of the sur-
face rock in the high temperature areas. The grey
colour of the mud pools is due to tiny floating
specks of pyrite as well as clay (kaolinite and
montmorillonite). The white colours are due to
silica, calcite, aragonite and gypsum; the bright
yellow colours in hot patches are due to sulphur;
the yellowish, red, brown and green colours are
mostly due to iron oxides. Two high temperature
areas are known to be infiltrated by sea water
(Reykjanes and Svartsengi) and the resulting
geothermal fluids are brines. Chemical analyses of
selected waters from the high temperature areas are
shown in Table 3. Typical temperature profiles are
shown in Fig. 4.
The strata of the active high temperature areas
are like the Plio-Pleistocene strata composed of
layers of subaerial lavas intercalated by thick piles
of subglacially erupted pillow lavas and hyalo-
clastites. The proportion of intrusives normally in-
creases with depth. Most of the intrusives are
relatively fine grained basaltic dykes and sheets but
dolerites and granophyres have also been encount-
ered in some areas. The strata are generally highly
faulted. Measurements show the transmissivity to
be highly variable between areas and within in-
dividual fields (Table 1). No statistical analysis is
available on the occurrence of aquifers. The
maximum temperature measured is 346°C, the
maximum flow rate (total flow) from a single well is
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