Jökull - 01.12.1984, Side 24

orifice approached the lake surface. With this
physical concept in mind one may want to look
back at observations of earlier eruptions and
jökulhlaups from Grímsvötn. During these
events two processes are in progress, i.e. building
of a cone and lowering of the lake level because
of the jökulhlaup. The eruptions have usually
been seen from the lowland about the time when
the jökulhlaup is near its maximum. In light of
the 1983 experience it seems Iikely that the erupt-
ions did, in fact, begin a few days before they
were seen, even possibly in some cases before the
jökulhlaup began.
The 1983 eruption was a small eruption, and
yet it had a very distinct seismic signature, detec-
table by many of the more sensitive seismographs
in eastern and southern parts of Iceland.
Seismographs of similar design have been in
operation in this area since 1971, when a proto-
type instrument was installed at Skammadalshóll
(SK in Fig. 1). During this period of comparable
seismographic coverage no activity resembling
that of 1983 was detected in the Grímsvötn area.
In particular we note that no earthquakes origin-
ating in Grímsvötn were recorded by the perman-
ent seismographs during the three jökulhlaups
from Grímsvötn in 1972, 1976, and 1982. From
the seismic evidence and the 1983 experience it
therefore seems very unlikely that any volcanic
eruptions occurred in connection with these
jökulhlaups. This contrasts with the suggestion of
Tómasson (1974), who found a peak in sediment
load and dissolved solids during the 1972 jökul-
hlaup that could not be explained by the maximum
in water flow. The sediment consisted of volcanic
glass from Grímsvötn, and Tómasson suggested
that it could be explained as a result of a small
subglacial eruption. In light of the seismic evi-
dence one must find another mechanism to exp-
lain a temporary increase in the sediment load,
e.g. stirring up of the bottom sediment in Gríms-
vötn, either by slumping, current erosion or sett-
ling of the ice cover on the bottom as the water
drains away.
The seismicity pattern of Grímsvötn in 1983, with
a long period of premonitory seismicity and qui-
escence during and following the eruption, bears
a close resemblance to the seismicity pattern of
the Krafla Volcano in the rift zone in NE-Ice-
land. This pattern is different from that of the
Hekla Volcano in South Iceland, both associated
with the 1970 eruption (Einarsson and Björnsson
1976) and the 1980-81 eruptions (Grönvold et al.
1983). Seismicity associated with the Heimaey
eruption in 1973 also showed a different pattern
(Einarsson and Björnsson 1979). Perhaps we are
seeing a common feature of typical basaltic rift
volcanoes, such as Krafla and Grímsvötn, where-
as andesitic volcanoes such as Hekla and off-rift
volcanoes such as Heimaey behave in a more
individualistic way.
AKN O WLED GEMENTS
The seismic network used in this study is operated
jointly by the Meteorological Office, the National
Energy Authority and the Science Institute, Uni-
versity of Iceland. Station attendants are thanked
for their cooperation and interest. Special thanks
are due to the station attendant at Skamma-
dalshóll, Einar H. Einarsson, whose keen eye and
persistence led to the discovery ofthe 1983 Gríms-
vötn eruption.
REFERENCES
Björnsson, H. 1974: Explanation of jökulhlaups
from Grímsvötn, Vatnajökull, Iceland. Jökull
24: 1-26.
Björnsson, H. 1975: Subglacial water reservoirs,
jökulhlaups and volcanic eruptions. Jökull 25:
1-14.
Björnsson, H. 1977: The cause of jökulhlaups in
the Skaftá River, Vatnajökull. Jökull 27: 71 —
78.
Björnsson, H., S. Björnsson and Th. Sigurgeirs-
son 1982: Penetration of water into hot rock
boundaries of magma at Grímsvötn. Nature
295: 580-581.
Björnsson, H. and H. Kristmannsdóttir 1984: The
Grímsvötn geothermal area, Vatnajökull, Ice-
land. Jökull 34:
Einarsson, P. 1978: S-wave shadows in the Krafla
caldera in NE-Iceland, evidence for a magma
chamber in the crust. Bull. Volcanol. 41: 1—9.
Einarsson, P. 1979: Travel times recorded at
Icelandic seismograph stations during the
Reykjanes Ridge Iceland Seismic Project
(RRISP). Science Institute, University of Ice-
land RH-79-10: 33pp.
Einarsson, P. and S. Björnsson 1976: Seismic
activity associated with the 1970 eruption of
volcano Hekla in Iceland. Jökull 26: 8-19.
22 JÖKULL 34. ÁR