Jökull - 01.12.1987, Side 17

considerable work has been added, several wells have
been drilled and new well fields put into production. The
purpose of this paper is revision and extension of previ-
ous work in the light of new information.
geological framework
The boundary between the European and the Amer-
ican plates runs along the axial rift zone in northern
Iceland. It is characterized by five sub-parallel volcanic
systems, which are a part of the Neovolcanic zone. The
Krafla volcanic centre, situated NE of Lake Mývatn is
inside one of a NNE-SSW directed 100 km long fissure
swarm, which bisects the volcanic centre in the form of a
graben (Fig. 1).
Although tectonic features of this centre are made
rather complex by the presence of a caldera, they can be
divided into three groups:
L Features due to the main moyements within the area,
which are oblique to the crustal plate boundaries and
perpendicular to the minimum compressive stress
(Björnsson 1985).
2. WNW-ESE fractures and faults due to the shearing
stress (Sæmundsson 1974).
3. Fractures and faults associated with the caldera.
The geological features of the Mývatn region are
mostly connected to the Krafla fissure swarm. The Kra-
fla volcano features a caldera which is considered to
have formed during the early part of the last interglacial
period about 100 thousand years ago. Since then the
volcano has been very active and virtually filled the
caldera with eruptive products. Acidic volcanism was
initiated during its formation as is evident from an ash-
flow surrounding it and the rhyolite ridges in and near
the eastern and western parts of the caldera rim (Jörun-
dur, Hrafntinnuhryggur and Hlíðarfjall). In the Neovol-
canic zone outside the caldera the basalts are mostly of
olivine normative composition but dominantly quartz
normative inside the caldera. The dominant geological
features inside it are hyaloclastite ridges parallel to the
fissure swarm and postglacial lava flows. Glacial allu-
vials are close to the southern rim of the caldera and
large areas of the surface are covered with pyroclastics,
clay and mud from explosive craters.
It has proved possible to determine the age of lavas or
eruptives from late postglacial time by tephrachronolog-
ical methods (Thórarinsson, 1960; Saemundsson, 1984).
The main episodes are listed in Table 1. In December
1975 a major rifting episode started in the Krafla fissure
swarm comprising 21 rifting events nine of which have
TABLE 1 AGE OF VOLCANIC ERUPTIONS IN THE NÁMAFJALL-KRAFLA AREA BASED ON
TEPHRACRONOLOGCIAL STUDIES.*
Time Tephra Episodes Eruptions
scale layers Hiatus
KRAFLA-ERUPTIONS
250 years
1717 AD M 1 cm black MÝVATN-ERUPTIONS
1477 AD a 4 cm black
1104 AD H, narrow white band 800 years
800-AD h
u twin layer each 1 cm black
600 AD C
1000 years
THRENGSLABORGIR-
ERUPTIONS
1200- H3 4 cm white
1300 BC
HÓLL-ERUPTIONS
HVERFJALL-ERUPTIONS
300 years
400 years
>4000 years
*(Sæmundsson 1984)
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