Jökull - 01.01.2014, Side 105

Umbrot tengd Bárðarbungu og Grímsvötnum á 19. öld.
ar hafa staðfest tilvist grunnstæðra kvikuhólfa undir
Grímsvötnum og Öskju (Magnús T. Guðmundsson og
John Milsom, 1997; Erik Sturkell og fl., 2006; Rai-
mon Alfaro og fl., 2007). Frekari rannsókna er þörf til
þess að skera úr um hvort þau eru fleiri.
SUMMARY
Bárðarbunga was not recognized as an active volcano
until the first cloud-free ERTS-1 images of Vatnajök-
ull appeared in January 1973.
In June 1839, the mathematician and cartographer
Björn Gunnlaugsson and his assistant Sigurður Gunn-
arsson hiked up to the northern slope of Bárðarbunga
in order to gain view of the surrounding areas (Fig-
ures 1–4). Sigurður, later a priest at Hallormsstaður,
E-Iceland, published several descriptions of the then
unknown interior of Iceland in the 18th century news-
papers. His accounts of changes along the NW mar-
gin of Bárðarbunga suggest eruptive activity there be-
tween his trips in 1852 and 1871.
Historical accounts of eruptions within the Bárð-
arbunga volcanic center are limited to tephra produc-
ing subglacial events and fissure eruptions along the
NW margin of the Vatnajökull ice cap. Several 18th
century eruption accounts associated with jökulhlaups
(glacial lake outburst floods, GLOFs) from Grímsvötn
onto the Skeiðarársandur plain have been attributed
to Grímsvötn. In order to produce larger than aver-
age jökulhlaups onto Skeiðarársandur, glacier melt-
ing outside the subglacial caldera lake, i.e. within the
water divide north of the caldera, is required (Fig-
ure 1). Large jökulhlaups in 1838, 1852 and 1861
were thus associated with subglacial volcanism north
of Grímsvötn. As during the 1996 Gjálp eruption,
large 18th century jökulhlaups may have been caused
by magmatic activity within Bárðarbunga rather than
Grímsvötn. An intermittent fissure eruption in Trölla-
gígar, along the SW flank of Bárðarbunga occurred
in 1862–1864. It is likely that the 1861 subglacial
event and Tröllagígar eruptions were phases in an
eruptive/rifting episode within the Bárðarbunga cen-
tral volcano, fed by lateral diking in a similar way
as the Gjálp eruption and the 2014 fissure eruption
in Holuhraun, northeast of the volcano (Sigmundsson
et al., 2015). Although unconfirmed, this sequence is
likely to have been followed by subglacial eruptions
draining south into Grímsvötn and north into Skjálf-
andafljót and Jökulsá á Fjöllum, and fissure eruptions
along the glacier margin at Dyngjuháls and Tröllagíg-
ar (Table 1).
Earthquake monitoring indicates a causal relation-
ship in seismicity between the large subglacial vol-
canoes at the center of the Iceland hotspot (Figure
5). A regional increase in seismicity has been ob-
served since 2002–2003 and an increase seismicity
within the Bárðarbunga volcano is generally accom-
panied by enhanced seismicity within the neighboring
regions, Kistufell, Kverkfjöll and Tungnafellsjökull,
Lokahryggur and N-Grímsvötn. Clear strain relation-
ship is also observed between Grímsvötn and Loka-
hryggur although Grímsvötn seemingly have an inde-
pendent source of strain, reflected in increased seis-
micity prior to the 1998, 2004 and 2011 eruptions fol-
lowed by a seismic quiescence.
HEIMILDIR
Alfaro, Raimon, Bryndís Brandsdóttir, Daniel P. Row-
lands, Robert S. White og Magnús T. Gudmundsson
2007. Structure of the Grímsvötn volcano under the
Vatnajökull icecap. Geophys. J. Int. 168, 863–876,
doi:10.1111/j.1365-246X.2006.03238.x.
Gwenn E. Flowers, Helgi Björnsson, Finnur Pálsson og
Gary K.C. Clarke 2004. A coupled sheet-conduit
mechanism for jökulhlaup propagation. Geophys. Res.
Lett., 31, L05401, doi:10.1029/2003GL019088.
Haukur Jóhannesson 1983. Gossaga Grímsvatna 1900–
1983 í stuttu máli. (A brief review of the volcanic ac-
tivity of the Grímsvötn volcanic system 1900–1983).
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Helgi Björnsson 2009. Jöklar á Íslandi. Bókaútgáfan
Opna, 479 bls.
Helgi Björnsson og Páll Einarsson 1990. Volcanoes bene-
ath Vatnajökull, Iceland: Evidence from radio echo-
sounding, earthquakes and jökulhlaups. Jökull 40,
147–168.
Jónas Jónasson 1884. Frjettir frá Íslandi 1883. Hið Ís-
lenska Bókmenntafjelag, Reykjavík, 62 bls.
Hjörleifur Guttormsson 2014. Upprifjun um Grímsvatna-
jökul, Vatnajökulsveg og Holuhraun. Jökull 64, 105–
122.
Magnús T. Gudmundsson og John Milsom 1997. Gra-
vity and magnetic studies of the subglacial Grímsvötn
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