Jökull - 01.01.2004, Page 142

Freysteinn Sigmundsson og Magnús Tumi Guðmundsson
og um 0.45 km3 af kviku komu upp (Magnús Tumi
Guðmundsson o.fl., 1997). Eldgosið 2004 kom upp
rétt utan vatnsins í Grímsvatnaöskjunni, en bræðslu-
vatn frá gosstöðvunum rann þangað og varð hluti af
Grímsvatnahlaupinu sem hélt áfram á meðan gosinu
stóð. Grímsvatnahlaupið náði hámarki 2. nóvember
með yfir 3000 m3/s rennsli. Við goslok rann vatn í
djúpri rás við Grímsfjall til austurs frá Gríðarhorni (5.
mynd). Vatnið féll í fossi fram af bergþröskuldi við
Gríðarhorn og síðan í farvegi með bergbotni, þar sem
til sást. Ekki er vitað hvenær þessi farvegur myndaðist
enda er þetta í fyrsta sinn sem vatn hefur sést renna í
Grímsvötnum. Áður hefur allt vatnsrennsli verið und-
ir jökli.
Aukin virkni virðist nú vera að færast í eldstöðv-
arnar í Vatnajökli, með Grímsvatnagosum 1998 og
2004, eldgosinu í Gjálp 1996, og öru kvikuflæði milli
gosa. Meginhluti síðustu aldar einkenndist af lítilli
virkni, með engu staðfestu gosi í Grímsvötnum frá
1934 til 1983. Eldgosasaga Vatnajökuls sem greind
hefur verið út frá öskulögum í jöklinum (Guðrún Lar-
sen o.fl., 1998) bendir einnig til að þar skiptist á tíma-
bil lítillar og mikillar virkni. Vísbendingar eru því um
að nýtt virknitímabil sé hafið í Vatnajökli, og því má
búast við eldsumbrotum þar aftur innan fárra ára.
SUMMARY
Grímsvötn eruption, November 1–6, 2004
An eruption within the Grímsvötn caldera began
around 22 GMT on November 1, following an in-
tense swarm of earthquakes. Long-term precursors
to the eruption had been detected, including uplift
and expansion across the caldera measured by GPS-
geodesy, increased earthquake activity, and elevated
geothermal activity. The main eruptive site was near
the southwest rim of the caldera (Figure 1) where the
eruption melted a void in the ice, about 900 m long
and 700 m wide, where the ice had previously been
up to 200 m thick. The main tephra sector lies to-
wards the north from this crater. Furthermore, about
70 m deep ice cauldron formed immediately to the
east of the main void in the ice, marking the con-
tinuation of the eruptive fissure. A new ice cauldron
formed also east of Mt. Grímsfjall, and small amount
of tephra was issued from another previously exist-
ing cauldron in the same area. Amount of ice melted
is preliminary estimated as 0.1 km3, and amount of
eruptive material is less than 0.1 km3. At the main
eruptive site, periods of high and low explosive activ-
ity alternated. Increased tephra fallout followed after
periods of high activity, and on November 2 it was
seen how this lead to tephra clouds flowing along the
ice surface north of the eruptive site (Figure 2). Ex-
plosive activity occurred within the complete length
of the void formed in the ice at the main eruptive
site (Figures 3 and 4). Near the end of and after
the eruption, water was seen flowing along the north-
ern edge of Mt. Grímsfjall (Figure 5). Beginning of
a jökulhlaup from Grímsvötn preceded this eruption
by few days, the first time this has happened since
1934. A causal relationship is indicated, apparently
with release of overburden pressure associated with
the jökulhlaup triggering the eruption, as suggested
by Sigurður Þórarinsson (1953) for some of the ear-
lier eruptions at Grímsvötn.
HEIMILDIR
Gudmundsson, M. T., F. Sigmundsson og H. Bjornsson
1997. Ice-volcano interaction of the 1996 Gjalp sub-
glacial eruption, Vatnajokull, Iceland. Nature 389,
954–957.
Larsen, G., M. T. Gudmundsson og H. Bjornsson 1998.
Eight centuries of periodic volcanism at the center of
the Iceland hotspot revealed by glacier tephrostrati-
graphy. Geology 26, 943–946.
Sigmundsson, F., E. Sturkell, V. Pinel, P. Einarsson,
R. Pedersen, H. Geirsson, M. T. Gudmundsson, H.
Bjornsson og C. Pagli 2004. Deformation and eruption
forecasting at volcanoes under retreating ice caps: dis-
criminating signs of magma inflow and ice unloading
at Grimsvotn and Katla volcanoes, Iceland. Eos Trans.
AGU 85(47), Fall Meet. Suppl., 608.
Sturkell, E., P. Einarsson, F. Sigmundsson, S. Hreins-
dottir og H. Geirsson 2003. Deformation of
Grimsvotn volcano, Iceland: 1998 eruption and
subsequent inflation. Geophys. Res. Lett. 30, 1182,
doi:10.129/2002GL016460.
Þórarinsson, S. 1974. Vötnin Stríð: Saga Skeiðarárhlaupa
og Grímsvatnagosa. Bókaútgáfa Menningarsjóðs, 254
pp.
Thorarinsson, S. 1953. Some new aspects of the
Grímsvötn problem. J. Glaciology 4, 267–274.
142 JÖKULL No. 54, 2004