Jökull - 01.01.2015, Page 6

Einarsson and Hjartardóttir
summit region, about 15 km to the west and 10 km to
the east where it merges with the edifice of the neigh-
bouring Katla volcano. There is a notable difference
between the eastern and western fissure swarms. The
E-W structural grain is strong in the eastern swarm.
Eruptive fissures in the col between the two volca-
noes (Fimmvörðuháls) are mostly E-W striking, and
so are the normal faults that are prominent in the
area. Hyaloclastite ridges formed by subglacial fis-
sure eruptions on the SE-flank also have a strong E-W
strike. The western fissure swarm, on the other hand,
shows a divergent pattern towards the west. Fissures
along the main ridge of the edifice trend E-W, but fis-
sures on the side of the ridge have an arcuate shape.
This is most prominent in the Skerin fissure on the
NW-flank, thought to be the source of the eruption
around the year 920 AD (Óskarsson, 2009), but is
also seen in older fissures on the SW-flank. On the
NW-flank the fissures are concave to the north, on the
SW-flank they are concave to the south.
It is noteworthy that the two short eruptive fissures
active in the Fimmvörðuháls flank eruption of March-
April 2010 had strikes that are inconsistent with the
general E-W structural grain.
Although the fissure swarms, or rift zones, of
Eyjafjallajökull are distinct and well defined, their
characteristics are different from those of the typical
fissure swarms of the divergent plate boundary defor-
mation zones. At Eyjafjallajökull they are defined by
the orientation of eruptive fissures and the preferred
location of eruption sites on the eastern and western
flank of the volcano. The rift zones or the fissure
swarms do not extend beyond the volcano’s edifice.
A typical fissure swarm at a divergent branch of the
plate boundary, such as the Askja and Kverkfjöll fis-
sure swarms in the Northern Volcanic Zone (Figure 1)
contain long normal faults and open fissures, in ad-
dition to eruptive fissures (Hjartardóttir et al., 2015;
Hjartardóttir et al., 2009; Hjartardóttir and Einarsson,
2012; Hjartardóttir et al., 2012). They also extend for
tens of kilometers beyond the edifice of the central
volcano of the respective system. The rift zones of
Eyjafjallajökull thus show evidence of weak tectonic
control, as opposed to the strong tectonic influence on
the volcanic systems at the divergent plate boundary.
MORPHOLOGY
The Eyjafjallajökull edifice has the shape of an elon-
gated, relatively flat cone that rises about 1,650 m
above the coastal plain. The volcano is capped by a
glacier, from which the volcano takes its name (jökull
in Icelandic is glacier). The glacier is up to 200 m
thick (Guðmundsson and Högnadóttir, 2005). The
volcano has an elliptical 2.5-km-wide summit crater
or caldera. The outlet glacier Gígjökull originates in
the summit crater and flows towards the north through
an opening in the crater rim. In the classification
scheme of Icelandic volcanoes Eyjafjallajökull was
frequently called a stratovolcano, mainly with refer-
ence to its mixed eruptions of lavas and tephra (e.g.
Thórarinsson, 1960; Saemundsson, 1978, Gudmunds-
son, 2000). Jakobsson (1979) pointed out, however,
that morphologically Eyjafjallajökull is comparable to
Hekla, which is classified as a stratified ridge due to its
elongated shape. Indeed, the aspect ratios of the two
volcanoes (long axis divided by the short axis) is quite
similar. For Eyjafjallajökull it is 2.5 measured at 1100
m elevation contour, for Hekla it is 2.4 at 1000 m. So,
if anything, Eyjafjallajökull is a bit more elongated
than Hekla. Slope has been used to classify volca-
noes, particularly to distinguish between stratovolca-
noes and shield volcanoes. Because of the elongated
shape of Eyjafjallajökull the slopes are quite different
along the long or short axes of the volcano. We find an
average slope of 0.20 along the short axis (N-S) and
0.09 along the long axis (E-W). These slopes are com-
parable to that of some of the Hawaiian shield vol-
canoes. Thordarson and Larsen (2007) point out the
close morphological resemblance to the polygenetic
Hawaiian shield volcanoes and call Eyjafjallajökull
"a shield-like volcano". Then, Thordarson and Hösk-
uldsson (2008) classify Eyjafjallajökull as a shield
volcano. We concur with this idea and emphasize the
resemblance of Eyjafjallajökull to the Kohala volcano
in Hawaii. The likeness is striking with respect to di-
mensions, shape and structure. Both volcanoes are
about 1600 m a.s.l. and have an uncertain extension
below sea level. The elongation is about the same,
similarly the slopes. The rift zones have similar ap-
pearance and structure.
6 JÖKULL No. 65, 2015