Jökull - 01.01.2015, Side 8

Einarsson and Hjartardóttir
of the SE flank and increase in seismicity beginning
in April and ending in August (Sigmundsson et al.,
2010). Towards the end of the year, however, inflation
resumed at a higher rate. A sill and then a dike were
intruded, ending with an outbreak of a lava eruption
on March 20, 2010, in the eastern fissure swarm, at
Fimmvörðuháls, the col between Eyjafjallajökull and
Katla. The eruption issued from two short fissures
and produced alkali basalt. The lava flow field was
1.3 km2 in area and the volume only 0.020 km3 (Gud-
mundsson et al., 2012). This small eruption ended on
April 12 and did not lead to any deflation of the pre-
viously inflated volcano. A new eruption then broke
out in the summit region of the volcano on April 14
(e.g. Gudmundsson et al., 2012). The product was tra-
chyandesite, mostly in the form of fine ash, but also a
lava flow was issued down the Gígjökull outlet glacier
of the caldera. The eruption came to an end towards
the end of May and was accompanied by deflation of
the volcano, not in the same area as the pre-eruption
inflation, however (Sigmundsson et al., 2010).
The 2010 activity at Eyjafjallajökull did not seem
to affect the seismicity at Katla much. But the follow-
ing year, on July 8, a burst of tremor was recorded at
Katla that lasted 23 hours (Sgattoni et al., 2015). Sev-
eral cauldrons formed in the glacier at the SE caldera
rim and a jökulhlaup issued from the Kötlujökull out-
let glacier. The bridge on the Múlakvísl river was
washed away. A new seismic area became active on
the south flank of Katla (Sgattoni et al., 2014) and the
seismicity increased in the caldera as well. To account
for these observations it is considered possible that a
small eruption occurred under the glacier, accompa-
nied by an intrusion into the south flank, but direct
proof is missing.
INTERNAL STRUCTURE OF
EYJAFJALLAJÖKULL AND KATLA
In order to come up with sensible suggestions for a
mechanism of the interaction between the volcanoes
it is necessary to review all evidence that may have
bearing on the problem. The available indications are
compiled in a conceptual model shown in a W-E cross
section through the volcanoes in Figure 4.
The shallow-level magma chamber that is shown
in Figure 4 beneath the Katla caldera is based on seis-
mic undershooting by Guðmundsson et al. (1994) that
revealed an area of P-wave delay and S-wave attenu-
ation in the caldera. Its existence is further supported
by the inflation measured by Sturkell et al. (2003,
2008) during 1999-2004. The topographic expression
of the caldera is derived from the radio-echo sound-
ing of Björnsson et al. (2000). Topography and the
thermal effects of a magma chamber are also con-
sidered to be responsible for a pronounced negative
magnetic anomaly over the caldera region (Jónsson
and Kristjánsson, 2000). The small dike extending
towards the surface near the western extreme of the
chamber (Figure 4) is meant to represent the feeding
channel of the presumed small eruption that occurred
on July 18, 1999 (Guðmundsson et al., 2007).
The Katla caldera is surrounded by several sili-
cic domes (Jóhannesson et al., 1990), one of which,
Kötlukollur, is shown in the cross section. Some other
central volcanoes in Iceland have similar halo of sili-
cic domes, notably Krafla in the Northern Volcanic
Zone. These silicic formations have been suggested
to be the result of remelting of hydrated basaltic
crust by frequent feeding of the volcano by primary
basaltic melt from the mantle (Jónasson, 1994). The
cryptodome shown on the west side of the Katla
caldera is based on the interpretation of Soosalu et al.
(2006) and Einarsson et al. (2005), that the persistent
seismicity cluster at Goðabunga is caused by a slowly
rising body of semi-molten rock of low density. An
alternate explanation of the seismicity at Goðabunga
is offered by Jónsdóttir et al. (2007), who suggest that
the low-frequency seismic events are caused by falling
ice blocks.
The complex of sills and dikes shown beneath
Eyjafjallajökull represents the intrusive bodies re-
vealed by the repeated inflation episodes of 1994-
2010. Shown are the three sills of 1994, 1999, and
2009, respectively, also the sill that immediately pre-
ceded the eruptive activity in 2010 (Pedersen and Sig-
mundsson, 2004, 2006; Sigmundsson et al., 2010).
The final dike that fed the flank eruption at Fimm-
vörðuháls appears as a large red blob because it lies
in the plane of the cross section. The summit eruption
8 JÖKULL No. 65, 2015