Jökull - 01.01.2015, Síða 11

Structure and tectonic position of the Eyjafjallajökull volcano, S-Iceland
The idea of gravitational stresses being responsi-
ble for the Eyjafjallajökull fissure swarm, its evolu-
tion and presently anomalous E-W orientation is sup-
ported by, or consistent with, the following lines of
argumentation:
1. Eyjafjallajökull and Katla were constructed in a
propagating rift (Óskarsson et al., 1982). They ex-
tended the land mass of Iceland beyond the insular
shelf. This happened before the youngest volcanic
system, the Vestmannaeyjar system, was formed.
2. The observations and model calculations show-
ing sill intrusions in 1994 and 1999 (Pedersen and
Sigmundsson, 2004, 2006), accompanied by earth-
quake focal mechanisms with vertical T-axis (Dahm
and Brandsdóttir, 1997), and sill intrusion followed
by a vertical E-W dike preceding the 2010 flank erup-
tion within the fissure swarm (Sigmundsson et al.,
2010) are all consistent with an idea of an E-W fissure
swarm primarily affected by gravitational stresses.
3. The oldest dikes exposed at the base of the volcano
have a dominating strike of NE (Loughlin, 1995),
whereas the most recent eruptive fissures show a clear
E-W strike. This indicates that the original stress field
was modified as the volcanic edifice was built up.
4. Topography of the volcano correlates with the ori-
entation of eruptive fissure in the western branch of
the fissure swarm, demonstrating the effect of gravity
on stresses in the volcano.
In this paper we point out the rather unique posi-
tion of the Eyjafjallajökull and Katla volcanoes that
they are placed unconformably on top of old seafloor,
at least partly. In this respect they resemble oceanic
island volcanoes more than any of the other volcanic
systems in Iceland do. The anomalous orientation
of the E-W rift zone or fissure swarm that connects
the two central volcanoes may be a result of vol-
cano spreading that often characterizes oceanic vol-
canoes (e.g. McGuire, 1996; Van Wyk de Vries and
Borgia, 1996). This structural relationship may in-
duce hazards that are associated with flank instabil-
ities and sudden failure of the edifice (e.g. Acocella
and Puglisi, 2010). The question must be considered
under what conditions the flank may become unsta-
ble. Intrusion of dikes or sills at shallow level or even
within the edifice is an obvious source of weakness.
Measurements of the deformation of the volcanoes
during the present unrest period since 1994 have given
no indications of flank failure so far. The deforma-
tion field prior to the 2010 eruptions was successfully
modelled as the result of Mogi-type point sources
(Sturkell et al., 2003, 2008) or rather deep sill intru-
sions (Pedersen and Sigmundsson, 2004, 2006). The
deformation during the eruptions and immediately be-
fore them also indicated sills, dikes and rather deep
source of deflation (Sigmundsson et al., 2010) with
no hint of flank instability. This possibility should be
kept in mind, however, during future unrest periods.
The anomalous E-W orientation of the volcanic
systems of Tindfjallajökull and Eyjafjallajökull has
attracted attention and is mentioned in several places
in the literature. The opinion has been expressed that
the E-W orientation of the rift of Eyjafjallajökull is
yet another expression of horizontal shear in the flank
zone of S-Iceland (e.g. Saemundsson, 1978) or even
the South Iceland Seismic Zone (e.g. Jónsson, 1998).
Our investigation does not support these views. The
structures we find do not resemble any of the struc-
tures associated with strike-slip faulting (e.g. Einars-
son, 2010). In spite of a clear E-W orientation of the
SISZ no major faults with that orientation have been
identified there (see Figure 2).
Other processes have been invoked as possible
causes of anomalous fracture orientations in Iceland,
such as loading and unloading of the crust by glacia-
tion and deglaciation. E-W oriented fractures in the
rift zone of Central Iceland may be related to recent
changes in the load of the Vatnajökull glacier (Hjart-
ardóttir et al., 2015), and the anomalous orientation
of the Upptyppingar dike intrusion in 2007-2008 has
been attributed to the present reduction in glacier load
due to global warming (Hooper et al., 2011). The
Eyjafjallajökull edifice was built up mostly in the
Pleistocene during which Iceland was repeatedly cov-
ered by thick glacier ice. During glacial periods the
ice load north of the volcano was large, whereas the
ice south of the volcano would have been floating on
the ocean. Changes in the ice load would therefore
have resulted in differential stress changes in the crust
at the location of the volcano that may have influenced
the orientation of its fissure swarms.
JÖKULL No. 65, 2015 11