Jökull - 01.01.2015, Blaðsíða 61

Reviewed research article
Mapping of the Eldgjá lava flow on Mýrdalssandur with
magnetic surveying
Sigrún Sif Sigurðardóttir1, Magnús Tumi Gudmundsson1 and Sigrún Hreinsdóttir2
1Nordvulk, Institute of Earth Sciences, University of Iceland, Sturlugata 7, 101 Reykjavík
2GNS Science, New Zealand
Corresponding author: sigrunsif23@gmail.com
Abstract — The 934–938 AD Eldgjá lava flow was formed in the largest flood basalt eruption in Iceland during
the last 1100 years. Since the eruption, sediments from Katla jökulhlaups have accumulated at Mýrdalssandur
outwash plain and partly buried the Álftaver lava field, the westernmost branch of the Eldgjá lava flow. Using
the results from a magnetic survey on Mýrdalssandur, the location of the buried edge of the Álftaver lava field
has been estimated. The magnetic measurements indicate that the edge of the Álftaver lava field lies 1–4 km
further to the west then previously suggested. The thickness of sediments on top of the lava edge is ∼10 m
with generally decreasing thickness towards the east and northeast. The depth to the lava flow suggests that
sediment accumulation on central and western Mýrdalssandur has amounted to 3–5 km3 since the Eldgjá lava
flow was emplaced. The buried part of the lava flow has an area of 64 km2 and volume of 1.4±0.5 km3. When
these values are added to older estimates, the total area of the Eldgjá lava flow increases to 844 km2 and the
volume to 20 km3.
INTRODUCTION
Eldgjá is a 75 km long discontinuous eruptive fissure
in the Katla volcanic system in S-Iceland. The fissure
extends from southwest to northeast from beneath the
Mýrdalsjökull glacier towards the Vatnajökull glacier
(Larsen, 2000). The exact date and duration of the
eruption is not known but the year 934 AD is often
used (Thorarinsson, 1955; Larsen, 1979; Hammer,
1984; Zielinski et al., 1995). A large acidity peak in
the Greenland ice core correlates well with an erup-
tion onset around 934 AD and indicates that it may
have lasted for up to 9 years (Hammer, 1984; Zielin-
ski et al., 1995). The activity during the eruption oc-
curred in episodes separated by intervals of low or no
volcanic activity (Larsen, 2000; 2010).
Huge amounts of tephra (6 km3), gas and lava
were produced in the eruption, which had major
impact on neighbouring areas (Thordarson et al.,
2001). Most of the tephra was produced in explosive,
predominantly phreatomagmatic activity on the sub-
glacial part of the fissure below Öldufellsjökull out-
let glacier (Thordarson et al., 2001; Larsen, 2010).
The bulk of the erupted magma, however, formed ex-
tensive lava fields in SE-Iceland. The lava followed
rivers and valleys down to the lowlands, forming the
lava fields of Álftaver, Meðalland and Landbrot (Fig-
ure 1) (Larsen, 2000). The tephra and lava fields have
the chemical characteristics of the Katla volcanic sys-
tem, being transitional alkali basalt with high concen-
tration of iron and titanium (Jakobsson, 1979). Es-
timates of the volume of the lava flow have ranged
from 14 km3 (Miller, 1989) up to ∼18.3 km3 (Thord-
arson et al., 2001). However, the westernmost part
of the Álftaver lava field is covered by the Mýrdals-
sandur outwash plain making definition of its western
margin difficult (Larsen, 2010). The most recent vol-
ume estimate of ∼18.3 km3 (Thordarson et al., 2001)
makes the Eldgjá lava flow one of the largest lava
flows on Earth in the last 11 centuries (Larsen, 2010).
JÖKULL No. 65, 2015 61