Jökull - 01.12.2003, Side 15

The 1783–1785 Laki-Grímsvötn eruptions
to evaluate the mechanics of the Laki eruption (e.g.
Thoroddsen, 1879, 1925; Helland, 1886; Thorarins-
son, 1967, 1969, 1984; Thordarson and Self, 1993).
The accuracy and reliability of these accounts vary
greatly, but when evaluated critically and corrected
for inconsistencies (see Thordarson, this issue), they
are a rich source of information about the course of
events. They also provide information about eruption
processes that otherwise would be unobtainable.
Here we present a comprehensive compilation and
a critical analysis of the information on the course of
eruptive events in 1783 to 1785 as revealed in the
contemporary chronicles, which are published here
for the first time in English. We also present new
evaluations of key eruption parameters derived from
this analysis. This includes a re-examination of the
source, timing and nature of seismic swarms, ex-
plosive activity, and lava surges and their inference
for number of eruption episodes during this volcano-
tectonic episode on the Grímsvötn volcanic system.
We have also evaluated the characteristic mode of lava
flow emplacement using information from the eyewit-
ness accounts in conjunction with new field observa-
tions. An extended list of references is included and
English translations of the original text used in the re-
construction presented here are given as quotations in
the appendix.
GEOGRAPHICAL AND GEOLOGICAL
SETTING
GEOGRAPHY
Geographic features and place names are commonly
used in the contemporary accounts as reference points
for positioning the phenomenon that the authors are
describing. Thus to fully comprehend the importance
of the descriptions it is useful to have some knowledge
of the local geography.
The Laki fissures and lava flow are located
in South Central-Iceland in theshire of Western-
Skaftafell (Figure 1). A distinct topographical feature
in this area is a scarp, an old sea cliff, which trends
across the central part of the area fromWSW to ENE.
Cultivated areas are all located along this scarp or on
the Holocene outwash plain to the south and consist of
six districts: Álftaver, Meðalland, Skaftártunga, Síða,
Landbrot and Fljótshverfi (Figure 1). These districts
were most severely affected by the Laki eruption and
thus often referred to as the Fire districts. North of the
scarp is the Síða highlands, bounded on three sides by
the Skaftá and Hverfisfljót rivers. The highlands fea-
ture SW-NE trending hyaloclastite ridges separated by
low-lying wetlands. A few gorges and smaller val-
leys trend northward from the main scarp into the Síða
highlands, including those of the Skaftá and Hverfis-
fljót rivers which now are largely filled by the Laki
lava (Figure 2). The area around the Laki fissures was
boggy before the eruption and the remains of these
bogs are preserved along the edges of the lava-filled
Úlfarsdalur and Varmárdalur valleys. Northeast of the
Síða highlands is the Vatnajökull ice cap where the
subglacial Grímsvötn central volcano is located (Fig-
ure 1).
GEOLOGY
The Laki cone-row is located within the Grímsvötn
volcanic system which consists of the Grímsvötn cen-
tral volcano and an ∼100 km long and ∼15 km-wide
volcanic fissure swarm (Figure 1). The Grímsvötn
volcanic system is tholeiitic in character and is esti-
mated to have erupted between 50–55km3 of magma
during the Holocene (Jakobsson, 1979). The volume
of magma erupted in the ice-free region of the sys-
tem is 21.5 km3, of which 15.1 km3 (or 65%) were
extruded by the Laki fissures in 1783–1784. The
Grímsvötn volcano has erupted at least 40 times since
1598AD with an average repose period of 10–15
years. The most recent eruptions occurred in 1983,
1996 and 1998 (Thorarinsson, 1974; Jóhannesson,
1984; Guðmundsson et al., 1997; Larsen et al., 1998;
Sigmarsson et al., 2000). A noticeable pause in the ac-
tivity at Grímsvötn volcano occurs from 1785 to 1823,
the 38 years following the Laki eruption (Thordarson
and Self, 1993).
LAKI FISSURES AND LAVA FLOW
The Laki vent system is 27 km long, extending from
Úlfarsdalur Valley in the west towards the tip of Síðu-
jökull glacier in the east (Figure 2). It consists of 10
northeasterly trending en echelon volcanic fissures,
which together host more than 140 vents (Figure 3).
JÖKULL No. 53, 2003 13