Jökull - 01.12.2003, Blaðsíða 19

The 1783–1785 Laki-Grímsvötn eruptions
plain of the Skaftá River in front of Skaftárjökull be-
fore entering the upper reaches of the Skaftá River
gorge. The total magma volume produced by the
Laki eruption is 15.1 km3±1 km3, thereof 14.7 km3
was erupted as lava and 0.4 km3 as tephra (Thordar-
son and Self, 1993).
CONTEMPORARY CHRONICLES
The contemporary chronicles discussed by Thordar-
son (this issue) provide valuable information about
the Laki eruption and are used here to reconstruct the
course of events during the 1783–1785 Laki-Gríms-
vötn eruptions. This information has been catalogued
according to the processes it describes and the cat-
egories are seismicity, explosive activity, lava flows,
and sense of directions in the Fire districts (Thord-
arson, 1990, 1991). Tables A to D in the appendix
contain quotations containing information relevant for
each of the four categories. Each table is organised
such that it relates to the course of events in chrono-
logical order.
For the sake of simplicity, the references in the
following sections are not to the original accounts, but
to the translated quotations tabulated in the appendix.
For example, reference (A1) refers to the quotation
with index number A1 in Table A. Also note that text
enclosed by [ ] are inserted as explanatory notes by
the present authors.
GEOGRAPHIC DIRECTIONS AND
LOCATION OF ACTIVE VENTS
The geographic features and place names are com-
monly used in the contemporary accounts as reference
points for positioning the phenomena being described.
The orientation of certain topographic features af-
fected the inhabitants’ perception of geographic direc-
tions, and resulting in a 15◦–25◦ counter-clockwise
offset of directions in the Síða and Landbrot districts
(Figure 4). It is important to consider these offsets to
fully comprehend the importance of the descriptions
in reconstructing of the course of events (Thordarson
and Self, 1988; 1993). Therefore, location of all rel-
evant landmarks referred to in the texts are given on
the maps in Figures 1 and 2.
Steingrímsson’s references to particular localities
give valuable information about sense of direction
in the Síða, Landbrot and Fljótshverfi districts (D1–
D16) at the time of the Laki eruption. The north-
ernmost boundary of these districts is a 200m high
scarp, trending NE (065–075◦) in the Síða and Land-
brot districts and E (090◦) in the Fljótshverfi district
(Figures 1 and 2). The trend of this scarp was the con-
trolling factor on perceived directions and at the time
of the eruption, as the scarp was believed to run due
east-west along its entire length. This is confirmed
by contemporary maps made by Hólm (1771), Step-
hensen (1785) and Pálsson (1794), where the scarp
in the Síða district is shown trending east-west (see
Figure 1 in Thordarson, this issue). This results in
a counter-clockwise offset of directions given in the
Síða and Landbrot districts of the order of 15◦–25◦
(Table 1 and Figure 4). In the Fljótshverfi district, the
offset is minimal if any. Steingrímsson lived in the
Síða district and most of his directional information is
based on local conditions and landmarks in that dis-
trict. Therefore, the counter-clockwise offset in his
descriptions must be accounted for when the direc-
tions of reported features are evaluated. This offset in
the perception of geographic directions has prompted
re-evaluation of some major conclusions reached by
earlier studies (e.g. Thorarinsson, 1974, 1984; see
also next section).
The offset of geographic directions is consistent in
southerly and easterly directions, but is more variable
in northerly and westerly directions, with the great-
est degree of variation in westerly directions (Table
1). Many of Steingrímsson’s observations were made
from his home, the farm Prestbakki, but some were
also made from the farm Kirkjubæjarklaustur, where
the church was located (Figure 2). The relative po-
sition of these farms as perceived by the inhabitants
at the time differs considerably from their actual geo-
graphic relationship. A likely explanation for this dis-
crepancy is the change in the landscape that occurs
between these two farms. A large valley, trending
NW (320◦) cuts into the main scarp between Prest-
bakki and Kirkjubæjarklaustur. On the old maps this
valley is shown trending due north and Prestbakki
is shown NE to ENE (055–080◦) from Kirkjubæjar-
JÖKULL No. 53, 2003 17