Jökull - 01.12.2003, Side 20

Thordarson et al.
klaustur, where the true direction is N15◦E (Figure 2
and Figure in Thordarson this issue). Because of the
mistaken direction of the valley between the farms,
references to the geographic west in the vicinity of
Prestbakki are slightly north of west (or 280–300◦).
This effect can be seen in some of Steingrímsson’s de-
scriptions of tephra fall that he reported to have been
dispersed by westerly wind (D10). On the other hand,
Kirkjubæjarklaustur is situated right under the scarp
and at that location geographic west was inferred to
be parallel to the scarp (245–255◦) and therefore re-
sulting in a southward displacement of the reported
westerly directions at this location.
Table 1. Direction (in degrees) of geographic direc-
tions as they were sensed in Síða district at the time of
the Laki eruption and their offset compared to true ge-
ographic directions. – Frávik átta á Síðunni á 18. öld.
Cardinal directions True position1 Offset2
rétt stefna frávik
Northwest 320◦ to 325◦ +5◦ to +10◦
North — —
Northeast 345◦ to 360◦ - 45◦ to -60◦
East northeast 20◦ to 30◦ -37.5◦ to -47.5◦
East 65◦ to 70◦ -20◦ to -25◦
Southeast 120◦ to 135◦ 0◦ to -15◦
South — —
South-southwest 181◦ -21.5◦
Southwest 215◦ to 225◦ 0◦ to -10◦
West-southwest 245◦ to 255◦ -2.5◦ to +7.5◦
West 250◦ to 300◦ -20◦ to +30◦
1) Geographic directions in degrees (east of north) as esti-
mated from position of landmarks used as a point of refer-
ence by Steingrímsson in his accounts.
2) Shows deviation of the values in column 2 from the true
geographic directions. Clockwise offset is indicated by plus
sign, counter-clockwise by minus sign.
A considerable offset is also in reported northwest
and northeast directions, which after correction be-
come; NW = 325◦, NE = 345–360◦ and ENE = 20–
30◦ (Table 1). It is vital to keep this discrepancy in
mind when locating the active vents during the Laki
eruption using the contemporary accounts. In the fol-
lowing compilation of the eruption, geographic direc-
tions are given as stated in the contemporary accounts
(i.e., not corrected), unless otherwise noted. Degrees,
when given, refer to the actual geographic directions.
LOCATION OF ERUPTION VENTS
Reverend Steingrímsson (1783; 1788) identifies three
distinct eldgjás1 in his accounts of the Laki eruption.
These eldgjás indicate the sites where he saw fire or
ash columns rising from the active vents from his
home, the farm Prestbakki. Two of these gjá’s clearly
refer to segments of the Laki vent system, whereas the
third gjá indicates vents that were situated a good dis-
tance to the east of the Laki f issures (e.g. Thordarson
and Self, 1988; 1993).
During the first days of the Laki eruption, the
fires were seen rising above the moors to the west of
Mt. Kaldbakur, in the direction of the Mörtunga farm
(320–325◦) when viewed fromSteingrímsson’s home,
Prestbakki (Figure 2). These sightings indicate that
the fire fountains at the very beginning of the erup-
tion emerged from the southwestern most vents on the
Laki fissures near the Hnúta Mountain. As demon-
strated by Thordarson (this issue), these vents are sit-
uated right in front of the now lava-filled Úlfarsdalur
valley, as is stated in Steingrímsson’s accounts (B26).
He refers to these vents as either Útnorðursgjá (the
northwest vents) or Vesturgjá (the west vents), terms
that he later applied to all vent activity visible to the
west of Mt. Kaldbakur. On 29 July, eruption columns
were seen rising for the first time above the eastern
shoulder of Mt. Kaldbakur, in the direction of Mt.
Blængur (Figure 2). The line of sight to these vents
(i.e., fissure 6 on Figure 3) is 10◦ to the west of north
but not to the northeast as indicated in the original
accounts. In his descriptions Steingrímsson refers to
this and other vent activity that revealed itself to the
1Eldgjá is a term that is often used in old Icelandic literature to describe erupting vents or sites. Literal translation of the term is “the
f issure of fire” and is clearly derived from the most common experience Icelanders have had with volcanic activity, a fiery display of
fountains emerging from a linear vent system or a f issure. Here, Steingrímsson uses the term more loosely or simply to indicate erupting
vents.
18 JÖKULL No. 53, 2003