Jökull - 01.12.1991, Page 26

Fig. 3. The large crater to the north of Svíahnúkur Vestri in April 1934. Note the fans of sand or tephra beneath
the ice walls. The western end of the island, formed in the eruption, can be seen in the lower left hand comer.
(photo Niels Nielsen, from Nielsen, 1937). Stóri gígurinn norður afSvíahnúk vestri í apríl 1934. Eyjan sem
myndaðist sést í neðra vinstra horninu. Aska þekur botninn undir ísveggjunum.
layer of tephra was spread over the area but the flat
interior parts of the ice shelf were apparently covered
with new snow, masking the tephra layer shortly after
the eruption (Askelsson, 1936; Nielsen, 1937).
Five earthquakes with epicentral distance fitting
with Grímsvötn, were recorded in Reykjavík at the
start of the eruption, ranging in magnitude from 3.5
to 4.5 (Tryggvason, 1960). Traces of smaller earth-
quakes with similar appearance can also be seen on
the seismic records (Brandsdóttir, 1984).
Expeditions to Grímsvötn in the summer of 1935
(Áskelsson, 1936; Nusser, 1948) reported that the
craters were still visible, as were the ice walls sur-
rounding the large crater. The island and the fans of
tephra below the ice walls were no longer visible as
the rise in the water level since April 1934 was of the
order of 40 m (Guðmundsson and Björnsson, in
prep.). In the spring of 1936 the craters were still vis-
ible but no water was detected (Nielsen, 1937). No
expeditions visited Grímsvötn in 1937. Air photos by
P. Hannesson from May 28, 1938 (Table A2 in
Appendix) indicate that no open water existed in the
craters at that time.
The eruption site of 1983 was at a similar location
as the large crater of 1934 (Figs. 2 and 3). A similar
opening in the ice shelf was formed (Fig. 4), about
500 m in diameter (Grönvold and Jóhannesson, 1984).
The eruption lasted a few days, started on May 28 and
was probably over on June 2. An island, semicircular
in form and with a diameter of about 80 m, was
formed in the lake. On the ice shelf, an ash and debris
fan radiated 0.5-1 km to the north from the crater. Ash
24 JÖKULL,No. 41, 1991