Jökull - 01.01.2015, Síða 9

Structure and tectonic position of the Eyjafjallajökull volcano, S-Iceland
Figure 4. A schematic E-W cross section through Eyjafjallajökull and Katla showing the positions of inferred
magmatic bodies in the roots of the volcanoes. Earlier versions of this section, or part of it, were published
by Sturkell et al. (2009) and Sigmundsson et al. (2010). The ice thickness is from Björnsson et al. (2000).
The inferred basaltic magma bodies are red, intermediate bodies are orange and the more silicic bodies yellow.
Sill 1 and 2 denotes the sills that formed in 2009 and 2010 immediately before the eruptions 2010. – Hugsað
þversnið með A-V stefnu í gegnum Eyjafjallajökul og Kötlu. Á þversniðið eru teiknuð helstu kvikuhlot sem þekkt
eru eða vísbendingar eru um í rótum þessara eldstöðva. Eldri útgáfur af þessu þversniði eru í greinum eftir
Sturkell o.fl. (2009) og Sigmundsson o.fl. (2010). Ísþykkt er samkvæmt Björnsson o.fl. (2000). Basaltkvika er
merkt með rauðu, súr kvika með gulu og ísúr með rauðgulum lit. Sill 1 og 2 tákna laggangana sem mynduðust
2009 og 2010 í aðdraganda gosanna 2010.
was fed from a body of evolved magma near the sill
complex but separate from it.
DISCUSSION
The strong E-W alignment of the fissure swarm of
Eyjafjallajökull shows that the regional minimum
compressive principal stress is oriented N-S. This is
not consistent with the stress field set up at a plate
boundary as a result of plate separation. Fault plane
solutions of earthquakes and the strike of faults and
fissures at the plate boundary are broadly consis-
tent with the spreading orientation of azimuth around
104◦ (e.g. Einarsson, 1991, 2008). The stress orien-
tation indicated by the Eyjafjallajökull rift is there-
fore anomalous and requires an explanation. The ef-
fect of stress interaction between neighbouring volca-
noes may lead to fissure swarms connecting the vol-
canoes (e.g. Gudmundsson and Andrew, 2007), which
is consistent with the orientation of the eastern branch
of the Eyjafjallajökull rift connecting Eyjafjallajökull
and Katla. This would not, however, explain all as-
pects of the western branch.
We suggest that the E-W orientation of the Eyja-
fjallajökull volcanic system is the result of pre-
existing structural control. The Eyjafjallajökull and
Katla volcanic systems were formed at a tip of a prop-
agating rift and were emplaced unconformably on top
of crust generated at the plate boundary to the west.
They extended the Iceland landmass southward, into
the oceanic area. It still remains to be determined
whether the south flank of Eyjafjallajökull rests on
oceanic sediments, but sedimentary xenoliths found
in the hyaloclastite formation on the south flank of
Katla (e.g. Áskelsson, 1960; Einarsson, 1962, 1967)
show that Katla was at least partly built on oceanic
sediments. We propose that the E-W structural grain
JÖKULL No. 65, 2015 9