Jökull - 01.01.2015, Page 12

Einarsson and Hjartardóttir
We have shown that activity of Eyjafjallajökull is
often accompanied by activity of Katla. The Katla
events are not all large, as shown best by the recent
events accompanying the intrusion of 1999 and erup-
tions of 2010. Mechanisms of possible interaction of
the two volcanoes include alteration of stress around
Katla magma chambers caused by inflation or defla-
tion of an Eyjafjallajökull magma chamber, and di-
rect injection of Eyjafjallajökull magma into a pas-
sive magma chamber at Katla. Other mechanisms are
possible, even if less likely, such as a deep pressure
connection through the mantle magma sources, or the
triggering effects of increased melting rate of the over-
lying glaciers induced by ash fall. Albino and Sig-
mundsson (2014) showed that stress interactions be-
tween magma bodies are strongly dependent on the
distance between the bodies and their shape. This in-
teraction is probably insignificant in the case of Eyja-
fjallajökull and Katla.
CONCLUSIONS
1. The Eyjafjallajökull volcano belongs to a group
of volcanoes that are located off the plate boundary,
south of the rift-transform junction where the EVZ
and the SISZ meet. Rifting in the area is insignifi-
cant but the volcanoes represent activity at the the tip
of a propagating rift. The volcanoes are likely to be
unconformably placed on top of ocean sediments.
2. The volcanic edifice has the shape of an elongated
shield volcano with a summit elevation of 1650 m,
long-axis of about 30 km and short-axis 15 km.
3. The volcanic system has an immature rift zone, or
a fissure swarm, with an E-W orientation. It does not
extend beyond the edifice. The western branch is 15
km long and the eastern branch extends at least 10 km
where it merges with the neighbouring Katla volcanic
system. Both volcanoes may inject magma into this
eastern branch of the rift zone.
4. The anomalous orientation of the rift may be the
result of gravitational stresses generated by the edi-
fice and the preexisting topography when the volcano
started growing at the propagating rift tip.
5. On the W flank the pattern of lateral eruptive fis-
sures becomes more radial with increasing distance
from the volcanic center. This is inconsistent with
tectonic control of the dike orientations and suggests
that gravitational stresses of the edifice are more im-
portant.
6. Geological and historical evidence show that ac-
tivity of Eyjafjallajökull is often accompanied by ac-
tivity of Katla. The most likely mechanisms of inter-
action of the two volcanoes is considered to be di-
rect injection of Eyjafjallajökull magma into a passive
magma chamber at Katla. Other mechanisms are pos-
sible.
Acknowledgements
This paper benefits from long discussions with numer-
ous scientists and laymen through several decades.
In particular we like to mention the farmer Einar H.
Einarsson (1912–1992), who meticulously operated
a seismograph at his farm Skammadalshóll for more
than two decades and had a special relationship to his
volcanoes, Katla and Eyjafjallajökull. He also was
tireless pointing out the importance of the fossilifer-
ous xenoliths found at his farm in the fifties. Rósa
Ólafsdóttir helped draft Figures 1 and 4, and Finnur
Pálsson provided ice thickness data for Figure 4. Con-
structive criticism by two reviewers led to significant
improvements of the paper.
ÁGRIP
Eyjafjallajökull er ein af elstu virku megineldstöðv-
um Íslands. Hann stendur í jaðargosbelti Suðurlands,
í nokkurra tuga kílómetra fjarlægð frá flekaskilum
Atlantshafsins sem liggja í gegnum Ísland. Fjallið er
ílöng, flöt keila, sem stendur um 1650 m yfir sjáv-
arborði. Lítill jökull hylur efsta hluta fjallsins, þar
með talda litla toppöskju, sem er um 2,5 km að þver-
máli. Fremur vanþroskaður sprungusveimur liggur
í gegnum fjallið. Hann hefur A-V stefnu og fylgja
gossprungur og gígaraðir oftast þeirri stefnu. Næst
toppnum hafa þó gossprungur tilhneigingu til að vera
geislalægar. Í vesturhluta sprungusveimsins sveigja
gossprungur frá meginstefnunni og liggja þá hornrétt
á hæðarlínur fjallsins. Meginstefna sprungusveimsins
gefur til kynna spennusvið með tognun í N-S, sem er
ekki í samræmi við þekktar flekahreyfingar. Stefna
sprungna bendir til þess að lögun fjallsins og spenna
sem af henni stafar stjórni miklu um ferðir kviku á
12 JÖKULL No. 65, 2015