Jökull - 01.01.2005, Blaðsíða 53

Reviewed research article
The Katla volcano S-Iceland: Holocene tephra stratigraphy and
eruption frequency
Bergrún Arna Óladóttir1, Guðrún Larsen2, Þorvaldur Þórðarson2,3 and Olgeir Sigmarsson1,2
1 Université Blaise Pascal, OPGC and CNRS, 63038 Clemont-Ferrand, France
2Institute of Earth Sciences, University of Iceland
3SOEST, University of Hawaii at Manoa, Honolulu, HI 96822, Hawaii, USA
b.oladottir@opgc.univ-bpclermont.fr
Abstract — The Katla volcano of the Eastern Volcanic Zone (EVZ) is characterised by subglacial explosive
eruptions of Fe-Ti basalts. The Holocene eruption history and magma evolution at Katla volcano is assessed
based on the stratigraphy and major-element composition of tephra layers in a composite soil section east of
the volcano. A total of 208 tephra layers were found, 18 layers are of historical age and 190 are prehistoric,
deposited during the last ∼8400 years. Of the 208 layers, 126 were analyzed for major-element composition.
Of those 126 tephra layers, 102 are typical Katla Fe-Ti basalts, another 7 are Fe-Ti basalt layers containing
rare grains of silicic composition, and 2 layers are basaltic icelandites with high Fe-Ti content. Ten silicic
Katla (SILK) tephra layers were also identified in the composite section. An age model for the section was
constructed using soil accumulation rate (SAR) between tephra layers previously dated by the 14C method.
According to the tephrochronology on the east side of the volcano, the prehistoric eruption frequency is higher
than that of historical time by a factor of 2. In view of the knowledge that prehistoric Katla tephra layers were
also dispersed to the south, west and north of the volcano, the true prehistoric eruption frequency is likely to
be as high as 4 events/100 years. Moreover, if prevailing wind directions have not changed much during the
Holocene, over 300 explosive basalt eruptions are likely to have occurred at the Katla volcano in the last 8400
years. The highest eruption frequency of basalt is observed between 2.5 to 4.5 ka and between 7.0 to 8.5 ka.
Both these periods correspond to main peaks in eruption frequency of the silicic SILK layers. Finally, abrupt
shifts in magma compositions divide the ∼8400 years record into eight intervals of 500-1700 year duration,
designated with either a constant composition, irregular or systematic variations of major-element composition
with time.
INTRODUCTION
The majority of Holocene and Quaternary tephra in
North Europe originate in Iceland despite the domi-
nance of basaltic volcanism. Many of the most ac-
tive volcanoes are capped by or covered by glaciers
and, consequently, subglacial eruptions driven by ex-
plosive water-magma interactions are common in Ice-
land. Such eruptions are the characteristic style of ac-
tivity at the three most active basalt-dominated cen-
tral volcanoes in Iceland; the Grímsvötn, Bárðarbunga
and Katla volcanoes of the Eastern Volcanic Zone
(e.g. Thorarinsson, 1974, 1975; Larsen; 2000, Larsen
et al., 1998; Hafliðason et al., 2000). Phreatomag-
matic eruptions at these volcanoes and on their asso-
ciated fissure swarms have produced some of the most
widespread basaltic tephra layers of the Holocene
era, which include historical eruptions of the Katla
volcano 1755 AD and 1625 AD (e.g. Thorarinsson,
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