Jökull - 01.01.2015, Side 7

Structure and tectonic position of the Eyjafjallajökull volcano, S-Iceland
HISTORICAL ACTIVITY AND
SYMPATHETIC BEHAVIOUR OF KATLA
Eyjafjallajökull is only a moderately active volcano.
Only four episodes of activity are known since Iceland
became inhabited in the ninth century, one of which is
only known from geological evidence. It is remark-
able that all the episodes have been accompanied by
some activity or unrest at the neighbouring, and much
more active, volcano Katla. In most cases this sym-
pathetic reaction has been small, even insignificant.
But the possibility that Eyjafjallajökull could trigger a
large Katla eruption is intriguing in light of the gener-
ally dangerous behaviour of Katla (e.g. Larsen, 2000;
Óladóttir et al., 2005, 2008). The known episodes are
the following:
Around 920: No written documents exist of these
eruptions. Stratigraphy of flood deposits and tephra
on the NW-flank of Eyjafjallajökull indicate that an
eruption of Eyjafjallajökull occurred synchronously
with an eruption of Katla about the year 920 AD
(Óskarsson, 2009). The most likely eruption site is
a 4.5 km long radial fissure, Skerin, that extends from
the summit down the flank to the WNW (Figure 3).
The eruption was a mixed eruption, the lower part
erupted alkali basalt and the upper part produced tra-
chyte. Intermediate compositions also were produced.
1612–1613: The interpretation of historical doc-
uments on eruptive activity in 1612–13 has changed
considerably with time. In his compilation of written
documents on eruptions of Katla, Thorarinsson (1975)
found two contradictory entries. One mentions erup-
tion in Eyjafjallajökull, the other eruption and jökul-
hlaup from Katla. He dismissed the Eyjafjallajökull
entry as erroneous and only lists eruption of Katla
that year. Thoroddsen (1925) only lists an eruption of
Eyjafjallajökull in 1612, beginning on October 12. In
her tephrochronological studies Larsen (1978) finds
evidence for tephra from both volcanic systems con-
sistent with eruptive activity in 1612-1613. Contem-
porary description by a Czech traveler, Daniel Vetter
(Vetter, 1983, Jónsson and Sigurðsson, 2010) strongly
suggest an eruption of Eyjafjallajökull. So apparently
there were small eruptions in both volcanoes. There
still is no tephrochronological evidence to indicate
which volcano erupted first, Eyjafjallajökull or Katla.
1821–1823: An eruption began on December 19,
1821, apparently in the western part of the sum-
mit caldera of Eyjafjallajökull (Thoroddsen, 1925;
Larsen, 1999). It continued with varying intensity
for several months, accompanied by moderate ash-
fall and jökulhlaups. Increased intensity was noted in
June 1822 and eruptive activity was documented well
into the year 1823. On June 26, 1823, Katla began
erupting after a 68-year repose (Larsen, 2000). The
eruption was explosive and originated in the SE part
of the caldera (Figure 3). It lasted 28 days and pro-
duced jökulhlaups for most of that time. It ranks as
a moderately small Katla eruption, if compared to the
eruptions of 1721, 1755, and 1918.
1994–2011: No activity is known in Eyjafjalla-
jökull following the end of the 1823 activity. Seismic
network installed in South Iceland after 1973 shows
that no significant activity took place in Eyjafjalla-
jökull (e.g. Einarsson and Brandsdóttir, 2000) until
1992 when a flurry of microearthquakes was recorded.
The seismicity increased significantly in 1994 and
was associated with upheaval of the SE-flank of the
volcano (Sturkell et al., 2003; Dahm and Brandsdóttir,
1997; Jakobsdóttir, 2008; Pedersen et al., 2007). Ped-
ersen and Sigmundsson (2004) interpreted the infla-
tion bulge as the result of a sill intrusion at the depth of
about 4.6 km. A slightly larger sill intrusion occurred
in 1999 (Pedersen and Sigmundsson, 2006; Sturkell et
al., 2003). This sill was horizontally offset from the
1994 sill and at a greater depth, about 6 km. The intru-
sion began in July and lasted several months. At the
same time unrest was detected at Katla. A small jök-
ulhlaup issued from the Sólheimajökull outlet glacier
on July 18, presumably caused by a small subglacial
eruption at the SW rim of the Katla caldera (Guð-
mundsson et al., 2007). Inflation of the caldera re-
gion followed, accompanied by large increase in seis-
micity, both in the caldera and the Goðabunga seis-
micity cluster on the W-flank of Katla (Sturkell et al.,
2003, 2008, 2009). Increase in geothermal activity
in the caldera and along its rims was also noted and
documented (Guðmundsson et al., 2007). This unrest
came to an end in 2004 or 2005. The next chapter
in the development was the intrusion of the third sill
beneath Eyjafjallajökull in 2009, indicated by uplift
JÖKULL No. 65, 2015 7