Jökull - 01.01.2015, Page 10

Einarsson and Hjartardóttir
is inherited from, and reflects the original coastline or
the original shelf edge. The N-S extension across the
Eyjafjallajökull rifts is thus the result of minor slump-
ing of the south flank towards the oceanic side. The
north flanks are buttressed against the pre-existing
landmass (Figure 5). Similar explanations were pro-
posed by Fiske and Jackson (1972) for the location
and orientation of the rifts of the Hawaiian volca-
noes. According to their ideas each new volcano of
the Hawaiian chain was built up on the flank of the
preexisting volcanoes and the orientation of the new
rifts was governed by the gravitational effect and the
buttressing effect of the preexisting edifice. Similar
conditions may arise where a new volcano is formed
at the tip of a propagating rift. Today the Eyjafjalla-
jökull edifice is buttressed on the SW-side by the ad-
dition to the insular shelf area produced by the young
Vestmannaeyjar volcanic system, that did not exist
during the main building phase of Eyjafjallajökull.
The westwards divergence of the faults and erup-
tive fissures of the western fissure swarm is a remark-
able feature of the volcano. Since dikes and eruptive
fissures generally tend to form at right angle to the
least compressive principal stress, the fissure swarm
pattern should reflect the stress field. Nakamura et
al. (1980) showed how the local stress field of a vol-
cano dominates the fissure pattern in the central part
of a volcano, but farther away the regional stress field
takes over and the fissure pattern begins to reflect the
regional stress. Under these conditions the fissure
pattern is radial close to the volcano and parallel far-
ther away, opposite to what we observe in the western
branch of the Eyjafjallajökull fissure swarm. The fis-
sures become more radial with increasing distance.
This is an argument for a weak influence of the re-
gional stress field. We suggest that the fissure pattern
is the result of topographic influence through gravity.
The eruptive fissures tend to form perpendicular to the
elevation contours on the lower flanks of the volcano.
This is in line with recent papers that show a strong
topographic control on the propagation of dikes be-
neath volcanic edifices. The horizontally propagating
dike from Bárðarbunga volcano in August 2014 was
shown to follow a path of maximum gradient in po-
tential energy along its way to feed the Holuhraun
eruption at a distance of 45 km from the center of
the volcano (Sigmundsson et al., 2015; Heimisson
et al., 2015). Even gently sloping flanks of mono-
genetic lava shields have been shown to influence the
intensity of fissure swarms that pass through them
(Hjartardóttir and Einarsson, 2015).
Figure 5. A North-South topographic profile through Eyjafjallajökull and surrounding area. The buttressing
effect of the Tindfjallajökull volcano is evident, also the lack of support on the south side. – Þversnið með
N-S stefnu í gegnum Eyjafjallajökul og nærliggjandi svæði. Sjá má hvernig Tindfjallajökull styður við norðan
megin, en stuðning vantar við suðurhlíðar fjallsins.
10 JÖKULL No. 65, 2015