Jökull - 01.12.1987, Page 18

Fig. 4. Land elevation changes at Leir-
botnar during the present volcanic epi-
sode, adapted from Björnsson 1985. —
Mynd 4. Landhœðarbreytingar á Leir-
botnasvœðinu á yfirstandandi umbrota-
tímabili.
resulted in volcanic eruptions (Björnsson et. al., 1977,
1979; Einarsson, 1978; Einarsson & Brandsdóttir, 1980;
Tryggvason, 1980; Björnsson, 1985) (Fig 4). This activity
is centred in the center of the caldera near Leirhnjúkur.
A magma chamber has been located at a depth of 3-8 km
in the roots of the caldera (Einarsson 1978) (Fig. 5).
Lateral and vertical movements were observed due to
Fig. 5. The Krafla area and the underlying magma
chamber. — Mynd 5. Afstöðumynd af kvikuhólfinu í
Kröflu og helstu mannvirkjum og kennileitum.
flow of magma into the magma chamber and during
rifting episodes due to sudden discharges of magma
from the magma chamber. The inflation rate has been a
few millimeters per day for weeks, months or years until
the pressure in the magma chamber reached some crit-
ical state, sufficient to initiate rifting of the fissure
swarm.
The geothermal manifestations appear on the surface
as mudpots and fumaroles mostly connected to the tec-
tonic fractures and faults. Flot springs are absent.
EXPLORATION
The presence of numerous fumaroles and hot and
altered ground within the Krafla caldera indicate the
existence of a large geothermal system, about 15 km2 in
area. Systematic geothermal exploration of the area
started in 1970 with surface and later subsurface studies.
Early work included geological mapping, an aeromag-
netic survey, resistivity measurements, gravity map-
ping, and chemical analysis of fumarole fluids (Stefáns-
son 1980,1981). Later work has included more extensive
geological mapping along with more detailed resistivity
and ground magnetic surveys of specific drilling fields.
Gravity and geodetic measurements in conjunction with
levelling have been repeated frequently in order to mon-
itor the magmatic activity. Geochemical studies of fu-
marole fluids have been used extensively to monitor the
effect of magmatic gases on the chemistry of the reser-
voir fluids (Ármannsson & Hauksson 1980, Benjamíns-
son 1985, Ármannsson & Benjamínsson 1986). Stable
isotope surveys have been undertaken in an attempt to
estabish the origin of the fluids and the relation between
different drilling fields and neighbouring geothermal
fields (Darling and Ármannsson 1986).
DRILLING
Drilling in Krafla started in 1974 with two 1200 m deep
exploration wells. Three production wells were drilled
in 1975 and by 1978 twelve wells had been completed, all
in the Leirbotnar field. In 1976 the reservoir fluid at
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