Jökull - 01.01.2015, Side 4

Einarsson and Hjartardóttir
systems increases with distance from the EVZ-SISZ
junction (Óskarsson et al., 1982). Furthermore, FeTi
volcanism, characteristic for propagating rifts (e.g.
Sinton et al., 1983), is found within this area, be-
ginning 2–3 Myr ago (Jóhannesson et al., 1990). It
can thus be argued that the area between Torfajökull
and the south coast in Mýrdalur district (see Figure
2) has been created during the last 3 million years
by voluminous FeTi basalt placed unconformably on
top of older sea floor. This is supported by xenoliths
found in the hyaloclastites in Mýrdalur, on the S-flank
of Katla (Áskelsson, 1960; Einarsson, 1962, 1967).
These xenoliths are fragments of oceanic sediments
containing fossils. In spite of this copious volcan-
ism no measurable crustal spreading has begun yet,
as shown by GPS-geodesy in the last decade (LaFem-
ina et al., 2005; Geirsson et al., 2006; Árnadóttir et
al., 2009). Hence, Eyjafjallajökull is situated on sta-
ble Eurasia Plate and can be classified as an intraplate
volcano. Same applies to its nearest neighbour, Katla,
in spite of its occasional connection with rifting in the
EVZ, exemplified by the AD 934 Eldgjá eruption (e.g.
Larsen, 2000; Thordarson et al., 2001), (Figure 2).
The transition from the divergent EVZ to the
South Iceland flank zone is also clearly seen in
the structural architecture. Long, parallel extensional
structures such as normal faults and eruptive fissures
characterize the EVZ (Thórarinsson et al., 1973; Ein-
arsson, 2008). South of the EVZ-SISZ junction these
structures become short and their strike variable. The
southern fissure swarm of Torfajökull has a trend of
ENE, and the Hekla volcanic system similarly. The
Tindfjallajökull and Eyjafjallajökull volcanic systems
trend almost E-W (Figures 2 and 3).
STRUCTURE OF THE VOLCANIC
SYSTEM
The Eyjafjallajökull volcano is relatively old. The
oldest formations are found at the lowest stratigraphic
level on the south flank, near Þorvaldseyri farm (Fig-
ure 3). Reversely magnetised layers are found there
indicating an age older than the Matuyama-Brunhes
magnetic boundary (0.78 My), confirmed by K-Ar
dating (Kristjánsson et al., 1988). The bulk of the
volcano was, however, built up during the Brunhes
magnetic chron as shown by a large positive mag-
netic anomaly associated with the edifice (Jónsson
et al., 1991). The anomaly can be traced towards
the south and west, towards the Vestmannaeyjar vol-
canic system off shore. The construction of the ed-
ifice therefore occurred in the presence of water and
ice. Erosional scars, mainly in the southern flank area
of the volcano, show assemblages characteristic for
subglacial deposition and indicate that a good part of
the volcanic edifice was emplaced during deglaciation
(Loughlin, 2002).
Geothermal activity at Eyjafjallajökull is only lim-
ited. The only significant occurrence is near the Selja-
vellir farm on the south flank, where the oldest rocks
are exposed and the level of erosion is deepest. Jóns-
son (1998) reports a volume of highly altered rocks in
this area, cut by numerous dikes and veins, indicating
long-lasting geothermal activity.
Loughlin (1995) mapped 118 dikes on the south
side of Eyjafjallajökull. The area with the highest
density of eroded dikes is in the valleys and gorges
above Seljavellir and Þorvaldseyri farms (Figure 3).
The vast majority of the observed dikes are 0.1–0.7 m
wide. They dip steeply and about 50% of them have a
dip of 90◦. The distribution of dike strike is bimodal
with the bulk of the observations in a bell-shaped dis-
tribution with the center azimuth of 45◦, and the sec-
ond peak (small and narrow) at 180◦.
Jakobsson (1979) analysed 20 lava units at Eyja-
fjallajökull that he identified as being of Postglacial
age. Only two of them turned out to be of basaltic
composition, both located in the eastern fissure
swarm, at Fimmvörðuháls. Of the rest, 17 were of
intermediate composition and one quartz-trachytic.
Here it should be noted that the term Postglacial may
have a variable meaning because of the glacial cover
of the volcano and high elevation of most of the erup-
tion sites. The glacier attained its maximum areal cov-
erage around 1900 AD and has been shrinking ever
since at a high rate. Lavas erupted in the Holocene
may therefore be glacially eroded and show evidence
of close interaction with snow, ice and water. Even
the lava of March 2010 on the eastern flank at Fimm-
vörðuháls was influenced by such interaction.
4 JÖKULL No. 65, 2015