Jökull - 01.12.2003, Síða 8

Thorvaldur Thordarson
north of Mt. Blængur is not internally consistent. In
Eldrit 2 the description is as follows: “Between 22
July and 2 August the weather was calm with occa-
sional heavy rainfall. During this period columns of
fire and steam were constantly seen, along with oc-
casional thunders and rumble in the pasture. On 3
August people noticed that the water in Hverfisfljót
River was getting warmer. Its temperature increased
steadily, until the river finally dried up.
On 9 August intensive thunder and lightning were
seen around the fissure north of Fljótshverfi. The same
day a lava surge emerged out from Hverfisfljót River
gorge and advanced like running water out onto the
sandur plain, one mile [i.e. Danish mile = 7532 m]
beyond so called Orustuhóll hillock, and from there
due south until it stopped. — along with occasional
thunders, claps and fountain activity which were seen
off and on emerging from the fissures up in the pasture
as well as lava emerging from the gorges out over the
settlement. It was not foreseen what these fires and
the rivers that were dammed by the lava might do in
the following fall and winter.”
In Eldrit 3 the same events are described as fol-
lows: “On 29 July a rumble and boiling sound was
first heard northeast of Mt. Kaldbakur, on the strike
with [directly behind] a high mountain called Blæng-
ur. The rumble and cracking were not any less than
those heard from Vesturgjá at the time when activity
was at the greatest vigour, but had dwindled consid-
erably at this stage. Later that day a dreadful vol-
canic cloud emerged from the site with sandy tephra
fall that was dispersed over Fljótshverfi and the east-
ern part of Síða and caused almost complete dark-
ness indoors. On 30 July there was calm and nice
weather, thunders, rumbles and cracking sounds were
heard almost continuously from all sides. On 31 July
a steam cloud was seen advancing down the Hverf-
isfljót River gorge, which was almost as deep and
wide as the Skaftá River gorge and the river carried
same amount of water which now effervesced/boiled
in many channels due to the heat. — 1, 2, and 3 Au-
gust the same whizzing continued to be heard from
this fissure [Austurgjá], along with tremor, rumble,
thunder and lightning, along with flowing lava north
of the mountains, which dried up Hverfisfljót River.
— The first lava surge came out from the Hverfis-
fljót River gorge on 7 August. On 8 and 9 August
it advanced down one of channels of Hverfisfljót, the
channel closest to the eastern margin of Síða district
trending south-southwest. The lava flowed far out
onto the sandur plain, beyond the hill Orustuhóll. —
on the eastern side [of the gorge] it advanced a short
distance beyond the promontory Dalshöfði.”
In the quotation from Eldrit 2 the heating of the
Hverfisfljót River is said to have begun on 3 August,
or 4 days after activity was first noticed north of Mt.
Blængur. In the Eldrit 3 this is said to have occurred
on 31 July and that the river dried up on 4 August.
Furthermore, the day when the river dried up is not
specified in former quotation, but it is evident that it
occurred after 3 August and most likely on the 4th. It
is known from the accounts that the Skaftá and Hverf-
isfljót rivers were of similar size. As stated in Stein-
grímsson’s descriptions, the flow of lava at the begin-
ning of the eruption dried up the Skaftá River in one
day. Then why should it take 4 days for the lava flow
to dam and dry up the Hverfisfljót River?
The volume or the flow rate of the lava was not
much less in the latter case (Thordarson and Self,
1993). Therefore, it is logical to assume that it would
take a roughly equal amount of time for the lava to
dry up both rivers. This indicates that the lava did
not reach the Hverfisfljót River gorge until 3 August
as stated in Eldrit 2 and that it took the lava 4 to 5
days to flow from the vents and down into the gorge.
The location of the vents straight north of Mt. Blæng-
ur explains the time it took the lava flow to reach the
gorge. The lava had to flow down a narrow valley be-
tween Blængur and Innri Eyrar, a distance of 13 km,
before entering the gorge near Mt. Miklafell (see Fig-
ure 2 in Thordarson et al., this issue). This implies
that the lava advanced at rates of ∼3 km per day. This
estimate is consistent with the calculated average flow
rate for the lava as it continued its advance down the
gorge. The lava travelled from Miklafell to the mouth
of the gorge (∼15 km) in about 4 days, giving average
flow rates of 3–4 km per day (Thordarson, 1990).
According to Eldrit 3 the lava surge emerged out
from the Hverfisfljót River gorge on 7 August, but in
Eldrit 2 this is said to occur on 9 August. In both ac-
6 JÖKULL No. 53, 2003