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The 2011 unrest at Katla volcano
this activity. More likely, this pattern of radial fault-
ing and (rhyolitic) dyking is a feature of the Katla vol-
canic stress field (e.g. Burchardt et al., 2018), not of
purely tectonic activity. The presence of a number
of rhyolitic intrusions around the Gvendarfell ridge is
indicative of a main area of lateral magma supply and
flank eruption during the eruptive history of Katla. A
similar information can be derived from the general
radial dip around the Gvendarfell ridge of the hyalo-
clastic deposits observed along its middle-lower sides.
No radiometric ages are available for these prod-
ucts, which makes it not possible to define chrono-
logically the rhyolitic and tectonic activity identified
along the Katla’s southern flank around Gvendarfell.
The proposed interpretation of lavas erupted in sub-
glacial to subaerial environmental conditions suggests
that these lavas were most likely emplaced before (or
during) the Last Glacial Maximum (LGM), that oc-
curred in Iceland around 21 ka BP (Norðdahl, 1991).
This corresponds to a generic pre-Holocene age as
suggested by Lacasse et al. (2007) and Jóhannesson
and Saemundsson (2009). This is also coherent with
the presence of deep ravines that were most likely
eroded by retreating glacier streams after the LGM.
However, the possibility that volcanic and tectonic
processes have been active in the Gvendarfell area
during the Holocene cannot be ruled out.
The features of the deposits exposed in the area
of Gæsavatn are those of basaltic, palagonitized prod-
ucts of dilute pyroclastic density currents (PDC) and
ballistic fallout typical of Surtseyan eruptions devel-
oped in shallow-water to subaerial conditions. Their
radial outward bedding attitude describe a tuff cone
structure centred on the lake, which is also consistent
with the bomb sag direction and the route of PDCs in-
ferred from the bedform shapes (Figure 9c, d). Over-
all, the lithologies observed here are broadly similar
to those described at the Varda tuff cone in southwest
Öræfajökull (Smellie et al., 2016), but without com-
plex subsidence attributed to ice block melt-out. The
presence of the Gæsavatn tuff cone demonstrates that
the deposits of the southern flank of Katla are not en-
tirely of subglacial origin, as is commonly assumed
(Lacasse et al., 2007; Jóhannesson and Saemundsson,
2009).
DISCUSSION
The July 2011 unrest coincided with the beginning
of a period of elevated earthquake activity within the
caldera and the beginning of a new cluster of earth-
quakes on the southern flank of the volcano. More-
over, since 2011 the seismicity rate has become higher
inside the caldera than at Goðabunga, opposite to pre-
vious patterns. These noteworthy changes are likely
related to modifications in the volcanic system, as
also indicated by slight ground deformation consis-
tent with inflation observed with GPS between 2011
and 2012 (B.G. Ófeigsson and S. Hreinsdóttir, pers.
comm.). However, it is not simple to constrain the
origin of these changes, as this unrest, similarly to the
1955 and 1999 episodes, was not accompanied by di-
rect indications of volcanic activity or a clear defor-
mation field.
As Katla is partly covered by a glacier, it is possi-
ble that a part of its persistent seismicity is related to
glacial processes, since volcanic and glacial processes
can produce similar waveforms (Weaver and Malone,
1976; West et al., 2010). The controversial interpre-
tation of the Goðabunga seismic cluster is an example
(Soosalu et al., 2006; Jónsdóttir et al., 2009). Also the
ground deformation has controversial interpretations:
the 1999–2004 inflation episode has been interpreted
in association with either magma accumulation inside
the volcano (Sturkell et al., 2006, 2008) or glacial re-
bound (Spaans et al., 2015). In this respect, we note
that the ground deformation notably coincided with
a seismic crisis at Goðabunga that has had no ob-
served equals since and started after the 1999 unrest
episode, when jökulhlaup and ice cauldron formation
occurred. A similar trend has characterized the 2011
unrest, although with a less clear deformation pattern,
and is interpreted by Sgattoni et al. (2017) as originat-
ing from volcano-related (magmatic or hydrothermal)
rather than glacial processes. Therefore, we suggest
that volcanic processes generally dominate as a source
for seismic activity at Katla.
An influence of the glacial system is outlined by
the typical summer peaks of seismicity occurring in-
side the Katla caldera. They are frequently associated
with small jökulhlaups or increased water drainage
from the glacier, and with changes in the chemical
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