Jökull - 01.12.2003, Side 26

Thordarson et al.
ursgjá] as later became evident.” (B52) [This tephra fall
originated at Grímsvötn volcano, see also Figure 5d].
On 19 and 20 July, this ash cloud was directed to-
wards the west by easterly winds, but to the northwest
on the 21st (B54–56). From that day: “no loud rum-
bling sounds were heard from the Vesturgjá, despite the fact
that fires were seen and lava continued to flow from it into
September.” (B56)
Fires were seen at the fissure to the end of July
and an intense fire-coloured glow was seen in the
sky on 24 and 25 July (B58, B60). Fine ash, de-
scribed to look like finely ground tobacco, fell at
the Kjölur mountain-route some time before 24 June,
about 250km north-northwest of the Laki fissures
(B59; Figure 1, inset). On the 28th rain containing
ash fell in Síða district and was accompanied by a bad
stench (B61).
In summary: The chronicles report seven occurrences
of tephra fall in the Fire district during the first 45
days of the eruption (Figure 5a-d). Five of these
tephra falls can be traced back to the Laki fissures,
whereas two (i.e., 21 June and 18–21 July) clearly
originated from a more easterly source, namely at the
Austur-Landnorðursgjá (= Grímsvötn volcano). The
chronicles also show that five of the tephra falls from
the Laki fissures were preceded by an increase in
earthquake activity and that each was followed by
a major lava surge emerging from the Skaftá River
gorge three to five days later (Table 2). The end of
this period also associated with marked change in the
eruption, as the focus of activity was shifted to sites
the northeast of Mt. Laki (see below and Figure 3).
COURSE OF EVENTS IN LATE SUMMER AND
FALL 1783
On 29 July, rumbles and a boiling sound were first
heard north of Mt. Blængur, accompanied by tremors
of similar intensity to those generated by the activ-
ity on Vesturgjá. Shortly thereafter, a “huge and
dreadful” volcanic cloud emerged from this site, dis-
persing sandy ash and pumice-like scoria over Fljóts-
hverfi and the eastern part of Síða (B62, Figure 5e).
Earthquakes continued and were felt intermittently
throughout August (A20–21). The water in the Hverf-
isfljót River began to warm up on 3 August when a
steam cloud was seen advancing down its gorge. The
river dried up the following day. The first lava surge
emerged from the gorge on 7 August and two days
later the lava had advanced 4 km beyond the hill Orus-
tuhóll (C37–40, Figure 5e).
Intense rumbling and cracking was heard from
the new fissure between 30 July and 7 August and a
continuous boiling sound was heard until 23 August
(B63, B66, B68–70). Southwesterly winds kept the
eruption column away from the Fire districts for the
first two weeks of August, except for 6 August when
fine-grained tephra was dispersed over Fljótshverfi
(B67). Showers containing ash fell in the Síða dis-
trict between 17 and 23 August (B72). Some time be-
tween June and August the Laki eruption column was
seen from the farm Húsafell in western Iceland rising
above the eastern part of the Kaldidalur Valley (B71),
indicating column heights in excess of 9 km (Thordar-
son and Self, 2003). The lava emerged at a steady rate
from the gorge until 14 August, but then the outflow of
lava dwindled considerably and remained so through
August (C42).
Earthquakes started again on 23 August and con-
tinued to the end of the month (A21). On 1 Septem-
ber a second lava surge emerged from the Hverfis-
fljót River gorge and in the following days a branch
of the lava advanced rapidly to the east towards the
farm Núpar (Figure 5f). The flow of lava was so in-
tense on 7 September, when Steingrímsson crossed
the Hverfisfljót sandur plain on that day all the rivers
and streams were dry (C43–44). In the Fire dis-
tricts the first week of September was characterised
by sandy rain, mist, thunder and lightning (B76).
Ash-fall was reported in Vestmannaeyjar on the
7th, lasting for 2–3 hours (B77, Figures 1 and 5f). The
third lava surge emerged on 10 September and the lava
flowed in between earlier lava (C45). September 14
came with an easterly wind and heavy ash-fall which
only reached as far west as the Geirlandsá River (B78,
Figure 5f). From this day until the 26th an intermis-
sion occurred in the flow of lava from the Hverfis-
fljót River gorge and the flow of water resumed (C46).
According to Steingrímsson, the Skaftá and Hverfis-
fljót rivers reappeared in their gorges at this time. The
glow of fire was seen at night above themountains and
24 JÖKULL No. 53, 2003