Jökull - 01.12.2003, Page 23

The 1783–1785 Laki-Grímsvötn eruptions
the Síða highlands (C13–14, Figure 5b). Activity at
the fissures continued with vigour (B30–32). On the
17th of June the wind was from the north and it was
on this day that Steingrímsson saw from his home a
reddish blaze and sparks of fire (i.e., glowing magma
blobs) rising above Geirlandsheiði (C15). This activ-
ity has been shown to have resulted from rootless vent
eruptions in Laki lava in the vicinity of Mt. Leiðólfs-
fell, demonstrating that by this time the lava in the
Varmárdalur valley had begun its advance down the
Hellisá river channel (Figure 5b, Thordarson et al.,
1998).
On 18 June, the third lava surge emerged from
the Skaftá River gorge (C16–17) and was accom-
panied by earthquakes (A12–13). This lava surge
was enormous and the lava advanced rapidly in two
branches. One branch progressed to the east follow-
ing the channel of the Skaftá River, while the other
advanced rapidly down the channel of the river Mela-
kvísl reaching the Steinsmýri farms on 22 June, trav-
elling 15-16km in about 5 days (C18–20, C22–23;
Figure 5b). On 20 June, the farmers took another trip
to the mountains north of the Síða district to observe
how the eruption was proceeding. Three lava streams
were seen emerging southwards from the vents into
the Varmárdalur Valley. As these lava streams merged
they advanced southwards over the boggy pasture and
the fluvial plain of the Hellisá River (C21). The flow
continued down the riverbed of Hellisá until it joined
the earlier lava flow in the Skaftá River gorge just
west of Leiðólfsfell (Figure 2). Sources from Skaft-
ártunga indicate that lava in the Hellisá River channel
was flowing into the gorge by 24 June (C25, Figure
5c).
On 21 June, the wind was blowing from the east
along with sleet and snow. A considerable fall of
sandy tephra was reported from Síða while greyish
coloured sand was observed on thin boards and white
paper in Skagafjörður, Northern Iceland (B34–35;
Figure 1, inset). Easterly winds continued through
22–25 June (B36–40). Earthquakes were felt again
on 23–26 June (A14) and at the same time the lava
advanced at a steady rate from the gorge (C24). Ex-
plosive activity increased between 23–25 June, but
the plume was directed westwards by the wind and
may have caused tephra to fall north of Mýrdalsjökull
(B39–40, Figure 5c).
On 27 and 28 June, with a strong westerly wind,
a considerable tephra-fall occurred in the eastern part
of the Fire districts (i.e., in Síða and Fljótshverfi dis-
tricts; B41–43, Figure 5c). Steingrímsson’s descrip-
tion of the tephra fall on the 27th is as follows: “We
here in the Síða district saw a huge black sandy cloud rise up
from Vesturgjá, which twisted and spun over the mountains
forming bolsters [i.e., cumulus-shape plume]. This cloud
dispersed sandy ash containing some pumice-like lapilli
scoria over Fljótshverfi and the eastern part of Síða and pro-
duced by far the most voluminous tephra-fall in these dis-
tricts.” (B41)
This tephra-fall continued through the 28 th, forc-
ing farmers in Fljótshverfi away from their homes and
to seek refuge in the settlements west of the Fire dis-
tricts (B43). The wind was easterly on 29 June and the
fourth lava surge emerged from the Skaftá River gorge
that day (C26). From that time until 12 July, lava
flowed continuously out from the gorge, although at
pulsating rates, further enlarging the flow field (C27,
Figure 5c).
A considerable amount of tephra fell in Síða on 9
and 10 July, but it did not reach the Meðalland district
(Figure 5, B48–50). Increased activity was noted on
13 July, with thunder, lightning and earthquakes for
the next four days. The flow rate of the lava increased
and the last big lava surge emerged from the gorge on
13 or 14 July and the subsequent advance of the lava
fronts completed the construction of the western flow
field in front of the Síða scarp (C28–29, Figure 5d).
However, lava was seen flowing in the gorge for the
rest of the summer (C32, C36). The intensity of the
eruption appears to have increased between 18 and 20
July, such that livestock gathered in groups because of
seismic activity and earthquakes were reported on the
20th (A18–19, B52–53, C30–31). On 18 July, ash fell
in the Síða and Fljótshverfi districts and Steingríms-
son describes it as follows: “When these above described
phenomena had been going on the whole night and a large
part of the day, a sandy and muddy ash was dispersed over
the ground here so it blackened. This tephra-fall came from
east-northeast and differed from previous ones in colour and
touch. It came from another eldgjá [i.e., Austur-Landnorð-
JÖKULL No. 53, 2003 21