Jökull - 01.01.2015, Page 1

Reviewed research article
Structure and tectonic position of the Eyjafjallajökull volcano,
S-Iceland
Páll Einarsson and Ásta Rut Hjartardóttir
Institute of Earth Sciences, University of Iceland, Sturlugata 7, 101 Reykjavík, Iceland
palli@hi.is, astahj@hi.is
Abstract — The Eyjafjallajökull volcano, one of the oldest active volcanoes in Iceland, is located in the vol-
canic flank zone of South Iceland, a few tens of kilometers south of the nearest branch of the mid-Atlantic plate
boundary. It is an elongated, broad cone of about 1650 m height. A 100-200 m thick glacier covers the upper
part of the volcano and its elliptical 2.5–km-wide summit crater or caldera. An E-W trending rift zone transects
the volcano, but a few radial fissures are observed around the summit area. Eruptive fissures on the west flank
are curved and tend to be aligned along the maximum gradient of the topography. The E-W orientation of the
rift zone and the apparent correlation with the topography suggests strong influence of gravity. Dikes in the
older parts of the volcano strike north-easterly and indicate a change in the stress orientation during the last
0.78 My. This change may be related to a southward propagation of the Eastern Volcanic Rift Zone of Iceland
and the transfer of spreading from the Western to the Eastern Volcanic Rift Zone. We suggest that the anoma-
lous orientation of the Eyjafjallajökull volcanic system is the result of preexisting topography and gravitational
stresses when the volcanic edifice was built up unconformably on old oceanic crust. All known episodes of
activity in Eyjafjallajökull have been accompanied by activity in the neighbouring volcano Katla. The most
recent examples are the two thermal events, possibly subglacial eruptions, of 1999 and 2011 at Katla following
the 1999 sill intrusion and 2010 eruption of Eyjafjallajökull. The coupling mechanism between the volcanoes
remains enigmatic. One volcano may be triggered by the other by direct dike or sill injection. Furthermore,
pressure perturbation in the mantle may affect the magma sources of both volcanoes.
INTRODUCTION
The 2010 eruptive activity of the Eyjafjallajökull vol-
cano in Iceland demonstrated the vulnerability of
modern society. A relatively modest eruption of a
moderately active volcano caused widespread disrup-
tion of everyday life in Europe for several weeks.
The Eyjafjallajökull volcanic system is one of over 30
presently active volcanic systems in Iceland (Figure
1). The activity of these systems builds the Icelandic
crust, partly to compensate for the divergence along
the N-America – Eurasia plate boundary, and partly
to contribute to the thickening of the crust above the
Iceland hotspot. The plate boundary crosses the coun-
try from SW to NE and calculated from global mod-
els of plate motions (e.g. DeMets et al. 1994) the full
spreading rate in the area is about 19 mm/year and the
direction of spreading about 104◦. The majority of
the volcanic systems are located at the plate boundary
and their activity is heavily influenced by the tectonic
movements. Others are located off the boundary, in
the so called flank zones (e.g. Saemundsson, 1978),
and their activity is only affected slightly by the plate
movements. Examples of these are Eyjafjallajökull
and Vestmannaeyjar, including Surtsey and Heimaey
eruptions (Figures 1 and 2). As a consequence of
this varied tectonic environment Icelandic volcanoes
display an unusually wide range of behaviour, both
during eruptions and the preparation time of erup-
tions, i.e. the time from the initiation of recharge
until eruptions break out (e.g. Thordarson and Hösk-
JÖKULL No. 65, 2015 1