Náttúrufræðingurinn - 1981, Síða 45

stratigraphically controlled. In high
temperature areas the volcanic and tectonic
risk may be of concern, and hence it is
important to study the volcanic history of
the area and analyse the fault pattern with
regard to identifying faults showing the
most recent movement.
Geothermal gradients across Iceland vary
from about 50°C/km for the oldest rocks in
West and East Iceland to about 100°C/km
inferred for the axial rift zones (Fig. 3).
Higher gradients are due to movement of
hot water in the bedrock. Isothermal sur-
faces, inferred from secondary minerali-
zation in the eroded lava pile, indicate
thermal gradients of about 70°C/km for the
peak heat flow before erosion set in (Fig. 5).
The permeability of the bedrock is reduced
immensely as a result of zeolitization and
alteration (Fig. 4). The consequence is that
primary permeability is reduced to almost
zero and secondary permeability becomes
prevailing. The secondary permeability
relates (a) to fractures, faults and dykes that
formed under extension within axial rift
zones during growth of the lava pile and (b)
to fractures and faults that formed later,
sometimes under different stress conditions,
outside the zones of crustal growth. A case is
made for secondary porosity due to dis-
solution from the deeper parts of geothermal
systems.
The high temperature areas are evident
from the occurrence of steaming ground,
fumaroles and mud pools. They are
localized within active geological structures
such as central volcanoes or the most active
parts of fissure swarms. The central vol-
canoes have acid rocks associated with them
and some have calderas. Two high tem-
perature areas lie on the border of the axial
rift zone. They lack active volcanism but
faulting is active (Hveragerdi and Geysir).
The size of the high temperature fields varies
between 1 km- and over 100 km'- (Table
1). Effluent seepage of the high temperature
fluid may emerge as C02-springs on their
outskirts, as warm ground water in open fis-
sures or as HgS-contaminated subglacial
streams. The roots of eroded extinct high
temperature hydrothermal systems reveal
cupolas of high temperature alteration cen-
tered on intrusive complexes or sheet and
dyke swarms. The intrusives may attain up
to 50% of the total rock involved in the
deepest parts of the exposed hydrothermal
aureoles. High temperature areas are situ-
ated in zones of active faulting which oc-
curs intermittently at intervals of tens of
years (western part of Reykjanes Peninsula)
and up to hundreds of years (e.g. northern
volcanic zone). New fractures may allow
upflow of fluid near boiling temperature. If
boiling occurs, hydrothermal activity signi-
ficantly increases, and hydrothermal ex-
plosion craters, which are common in many
high temperature fields, may form. Most of
the high temperature areas have been af-
fected by volcanism during postglacial time,
however, to a varying degree as regards type
of eruption, frequency of eruptions and
distribution of eruptions with time. Assess-
ment of volcanic risk is an important aspect
in the study of high temperature areas. Even
though constructions may be sited relatively
safely, gas pulses (CO., and SO,2) may cause
severe problems in utilizing the geothermal
system.
The low temperature geothermal activity
is less clearly defined as individual areas.
There may be, however, over 250 separate
areas with over 600 main hot springs. The
total natural flow, excluding springs of less
than 20°C, amounts to about 1800 1/s.
Utilization and successful prospecting for
hot water has hitherto been more or less
limited to known hot spring areas.
By drilling and pumping the natural flow
has been increased 10—20 times without
signs of overexploitation. On the basis of
geological and structural studies several dif-
ferent types of low-temperature areas have
been identified. (a) Dykes syngenetic with
the growth of the lava pile are probably the
most common type of aquifer. These pre-
dominate throughout the Tertiary plateau
basalt areas (Fig. 10). Sometimes the dykes
are associated with faults of similar strike
and age (Fig. 12). (b) Geologically young
faults or fault zones, cutting at an angle
across older structures syngenetic with the
growth of the lava pile (Fig. 11). (c) Zones of
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