Jökull - 01.12.1986, Síða 17

Fig. 5. Tectonic structure of the
Reykjanes peninsula. The dominant
orientations are those of the volcanic
zone (75° E) and of the volcanic sys-
tems (30—40° E) (Sigurðsson 1985).
Mynd 5. Höggun á Reykjanesskaga.
Ríkjandi eru stefnur gosbeltisins
(VSV-ANA) og gosfylkjanna
(SV-NA) (Freysteinn Sigurðsson
1985).
Explanations/SArýn'ngúT:
1- Fissure zones, trends of faults and fissures /
Sprungusvœði og sprungustefnur.
2. Seismically active zone / skjálftasvœði.
3. Central belt of The Reykjanes Volcanic Zone /
miðrœma gosbeltis.
4. Lava shield / Dyngjugígur.
5. Hyaloclastites / móberg.
1 ■ Scoriaceous lavas with vertical and horizontal joint-
ing.
2- Denser lavas with little scoria on the contacts
between the lava flows.
2- Interglacial basalts, clean pillow lavas and fresh
tuffs.
4. Dense interglacial basalts, pillow lavas with a
glassy matrix, breccias.
5- Altered breccias and tuffs. Perhaps some massive
basalts.
The different permeabilities are demonstrated in the
investigation area where the groundwater table drops
when crossing boundaries between lithological forma-
dons (e.g. Sigurðsson 1976) or where the relative ele-
vation of the groundwater table varies according to
lithology in the western part of the peninsula (Sig-
urðsson 1985). The regional permeability is affected
hy the tectonic pattern of the area, the open fissures
causing an increase in permeability and a horizontal
anisotropy.
The hydrological parameters of the freshwater body
are controlled by the permeability field caused by the
geological structure. Thus, the elevation of the
groundwater table and the direction and strength of
ihe groundwater currents are the results of geological-
hydrological interactions, where the mobile water
responds to the stable rock formations, resulting in the
groundwater flow. The quality of the groundwater , as
expressed in terms of temperature and chemistry, is
influenced by climatic factors and the impact of the
geothermal fields on the Reykjanes peninsula.
GEOLOGICAL STRUCTURE
The Reykjanes Volcanic Zone is narrowest in the
west, only 5 km wide, although its southern margin
might be out in the sea south of the peninsula. In the
east, where the volcanic zone merges with the Western
Volcanic Zone of southern Iceland, its width has in-
creased to 15 km. The volcanic zone is orientated
near to 70° E. The eruptive fissures are situated
oblique to the zone, commonly having an orientation
of 30—40° E. The hyaloclastite ridges have a similar
orientation, as do also the tectonic fissures and fissure
zones (Fig. 3). There seems to be a gradual change in
the median values of the orientation from 38° E in the
western part of the peninsula to 32° E in the eastern-
most part of it (fig. 5).
On the border of the volcanic zone the volcanic ele-
ments are frequently orientated 40—50° E. This
change in orientation coincides with some irregulari-
ties in the volcanic pattern and occurences of picrite
inside narrow belts, that delineate an inner volcanic
zone with a higher degree of regularity. The northern
part of this inner zone is seismically active, with a
great number of small earthquakes occurring every
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