Jökull - 01.12.1986, Side 10

permeability thus produced in rocks of otherwise very
low primary permeability was sufficient to initiate
water convection in the fractures. The presumed em-
placement of magma under the geothermal system
may have contributed to the building up of the strain
in the rock that led to the fracturing. The geothermal
system is probably recharged from the northeast along
young fractures. As overall permeability is expected to
decrease with depth recharge at a relatively shallow
level is favoured. Infiltration of ground water stored in
recent unconsolidated sediments covering the bedrock
in the westernmost part of the area may also contri-
bute to the recharge to the geothermal system especi-
ally, if the sediments overlie permeable fractures.
In Upper-Árnessýsla the recharge water is ulti-
mately derived from the area around the southern
margin of Langjökull 30 km away. The water flows as
cold ground water to the geothermal field where it
becomes involved in a deep convection cycle. The
convection is considered to be concentrated in tec-
tonic fractures but downflow may occur additionally
along pores of primary permeability.
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