Jökull - 01.12.1984, Page 5

Further examination of available records
(Jóhannesson 1983) shows firm evidence for
eruptions in 1902-1904, 1922, 1933, 1934, 1938,
1945 and 1954 and possible eruptions in 1939,
1941 and 1948. The definite eruptions were
actually observed or conditions at the volcano
after a jökulhlaup show signs of an eruption. For
the otheT ones the indications for an eruption are
jökulhlaups at unusually short intervals which
suggests that the frequency may have been influ-
enced by eruptions. Jökulhlaups without accom-
panying eruptions took place in 1960, 1965, 1972,
1976, and 1982.
Little can be said about the volume of these
eruptions but the apparently short duration and
the thin or no tephra layers suggest that each
eruption had fairly small volume but this is com-
pensated by high eruption frequency. This high
eruption frequency makes the petrology of the
products interesting because they are likely to
interrupt the evolution process of the magma
that feed the volcano. To explain the 5000 MW
Fig. 2. The Grímsvötn caldera
showing the eruption site and
the ash layers formed in the
1983 eruption.
Mynd 2. Staðsetning gosstöðv-
anna í Grímsvötnum 1983 og
öskugeirarnar þrír sem þá
mynduðust.
geothej-mal activity 50 million m3 of mágma need
to be cooled down to 400°C (Björnsson et al.
1982). This large volume may either be steadily
supplied into high level magma reservoirs or
large magma volumes already present in the
upper crust may be the source. Both possibilities
may affect the magma about to be erupted and
since the products are instantly quenched at the
time of eruption the state of the magma at that
time is further reflected in the glass and minerals
of the tuffs.
THE 1983 ERUPTION
In the morning of May 28th 1983 several
seismometers of the Icelandic seismic network
recorded unusually intense seismic activity
originating in Grímsvötn. This lasted from early
in the morning until noon and was followed by a
continuous volcanic tremor which was most
intense on May 28th and 29th but lasted until just
JÖKULL 34. ÁR 3