Jökull - 01.12.1984, Síða 6

after one o’clock in the morning of June 2nd (P.
Einarsson and B. Brandsdóttir pers. comm.).
In the evening of May 28th pilots on board
Icelandair aircraft TF-FLN on a scheduled flight
from Egilsstadir in east Iceland noted a white
column rising about 100 m above the cloud
blanket covering the volcano. This was at about
21.35. The aircraft had left Reykjavík at 19.30
and nothing unusual had been seen on the out-
ward journey. This observation was not reported
until later.
The first direct observation of the eruption site
was at 10.30 in the morning of May 29th and only
after the scheduled flight to Egilsstadir had been
diverted to Grímsvötn because of the nature and
intensity of the seismic activity. Weather condi-
tions were very favourable that day and the erup-
tion was closely watched from the air.
The eruption site was within the caldera close
to the southern wall, just north of Vestri-Svíahn-
júkur peak (Figure 2). This site is the same as the
main crater of the 1934 eruption (see Nielsen
1937). An oval shaped opening had formed in the
ice cover of the caldera lake, about 300 m across
in the beginning but increasing to 500 m at the
end of the eruption. The lake was covered with
floating ice blocks continuously supplied from the
overhanging cliffs of the caldera wall. No volca-
nic activity was observed during the first sighting
but during the day explosions took place in the
lake at varying time intervals sending black ash
jets about 50 m into the air while the accompany-
ing steam column rose to one to two km height
above the surroundings.
When the eruption was first observed two ash
fans had formed, one to the south outside the
caldera and another to the north on the ice cover
inside the caldera (Figure 2).
The tephra fan to the south was extremely thin,
about five kilometres long and one km wide next
to the caldera. This tephra fan was most likely
formed in the afternoon of May 28th (B.
Brandsdóttir pers. comm.) as northerly trending
wind was blowing between 12.00 and 18.00 and
volcanic tremors were recorded just after 15.00
indicating eruption activity.
The ash fan to the north, deposited on the ice
cover of the caldera lake was about one km long
and 0.5 km wide. It seems to have formed by a
water wave since large blocks of ice and volcanic
bombs intermingled with the ash that formed a
radiating pattern extending from the lake. The
distal boundary was relatively sharp and no
apparent thinning was observed. This suggests
that the fan was formed by a flood wave either
created by an explosion or a snow avalanche into
the lake from the overhanging cliffs or combina-
tion of both.
The eruption was observed from aircrafts on
and off from May 29th until the last sighting in
the afternoon of May 31st, between 15.00 and
16.00. During this period activity continued in a
similar way with occasional explosions sending
ash jets 50—100 m into the air and the accom-
panying steam column rising one to two km.
During the last observation on May 31st no new
ash fan had formed (Kristján Saemundsson pers.
comrn.).
From the afternoon of May 31st and until June
5th the volcano was covered by a thick cloud
blanket and the only observations available are of
the steam column rising through the clouds. On
June lst a steam column was seen all day and at
about 10 in the morning aircraft from Eagle Air
on its way to Fáskrúdsfjördur in the east passed
over Grímsvötn. The cloud blanket reached
about 3000 m height a. s. 1. but the steam column
about 5000 m height. The pilots observed pulses
of steam at 2Vi to 3 minute intervals and each
pulse was accompanied by a black ash bearing
cloud that rose to about 4300-4500 m height. On
the return flight at about 11.00-11.30 pulses
were observed at 4—5 minute intervals. On a
series of photographs taken by one of the passen-
gers the column is swept towards east by a gentle
WSW breeze. When the volcano was visited after
the eruption a very thin north-easterly tephra fan
was observed on the caldera rim (Figure 2) about
one km wide but of uncertain length. It is most
likely that this fan formed during these pulses.
When the eruption site was next observed in
the afternoon of June 5th no eruption activity was
observed but a semicircular island, about 80 m in
diameter, was seen steaming in the middle of the
ice free lake.
It is worth noting that no changes were
observed in the river Skeiðará which drains the
area and where jökulhlaups seem to occur accom-
panying the majority of known eruptions.
CHEMICAL ANALYSES
OF THE TEPHRA
The ash and scoria is made up of glass and
4 JÖKULL 34. ÁR