Jökull - 01.12.1986, Síða 3

A Model for the Reykholtsdalur and the Upper-
Árnessýsla Geothermal Systems with a Discussion on
some Geological and Geothermal Processes in
SW-Iceland
STEFÁN ARNÓRSSON
and
GUNNAR ÓLAFSSON
Science Institute, University of Iceland,
Dunhagi 3, 107 Reykjavík.
ABSTRACT
The heat source to the Reykholtsdalur and the
Upper-Ámessýsla geothermal systems, SW-Iceland, is
considered to be magmatic. The magma is assumed to
have originated under the nearby Thingvellir-Lang-
jökul] volcanic zone. Diminishing dilation in this
zone favours intrusion into the old crust flanking it as
witnessed by volcanic edifices of Upper-Quaternary
age resting unconformably upon considerably older
volcanics. Chemical geothermometry indicates sub-
surface temperatures of as much as 150°C in Reyk-
holtsdalur and in Upper-Árnessýsla they may be as
high as 200°C. Both of the geothermal systems are
considered to be convective systems. In Reykholts-
dalur the convection is probably Iargely confined to
relatively recent tectonic fractures. In Upper-Árnes-
sýsla convection is also envisaged to be concentrated
in tectonic fractures but downflow could occur ad-
ditionally through pores of primary permeability. The
ultimate source of the water in both fields is on ele-
vated ground inland. The water flows at relatively
shallow depth to the geothermal fields — part of it
even on the surface — where it enters the convecting
flow. Alteration mineralogy in a few geothermal
systems in SW-Iceland indicates that high-temperature
systems may develop into low-temperature ones con-
juncture with their drifting out of the volcanic zone.
The geothermal activity in Reykholtsdalur could
though hardly be of such origin due to its distance
from the volcanic zone. The high H2S content of the
Laugarvatn hot springs in Upper-Árnessýsla is taken
to indicate that magma is replenishing the heat source
at present.
INTRODUCTION
Einarsson (1937, 1942, 1966) developed a concep-
tual model for low-temperature geothermal activity in
Iceland. He considered this activity, which occurs in
Quaternary and Tertiary formations, to be entirely
non-volcanic in origin. Einarsson's model envisaged
deep circulation of ground water from the central
highlands of the country towards the lower coastal
regions. The water was assumed to be meteoric and its
dissolved constituents leached from the rock. The
low-temperature activity was regarded as being steady
state; during deep circulation the water became heated
by the terrestrial conduction current. Björnsson
(1980a) has concluded from investigation of hydrology
and the total energy balance of the low-temperature
activity that Einarsson's steady state model is a viable
possibility.
Bödvarsson (1950, 1961, 1982) expressed agreement
with the model of Einarsson regarding the non-vol-
canic nature of the low-temperature activity, but con-
cluded, on the basis of energy balance considerations,
that the conduction process involved in the heating
must be of transient nature and not steady state.
Bödvarsson (1982) considers that the effects of the gla-
ciations in Iceland must have had a profound effect on
ground water hydrology, stress in the crust that could
affect permeability by fracturing, and rapid erosion
that could lead to an increase in the near surface ther-
mal gradient. All these processes would create transi-
ent conditions in favour of developing relatively
intense low-temperature activity in postglacial times.
Studies of the stable hydrogen and oxygen content
of the low-temperature geothermal waters in Iceland
1