Jökull - 01.12.1994, Blaðsíða 39

10 years time. The time required for significant gravi-
ty changes to occur is somewhat longer.
Viscosity is one of the most important parameters
in geodynamics. This applies particularly in Iceland,
where reduced viscosity beneath the crust is consid-
ered to reflect temperatures close to the melting
range, and be related to partial melting and magma
generation in the upwelling mantle plume. Experi-
ments to estimate the viscosity are not easy to con-
duct, however. Measurements of crustal response to
sudden changes in stress or loads are among the best
ways to study viscosity. Thus a high velocity of diver-
gent motion measured around the Krafla area in the
period 1987-1990 was interpreted as continued re-
sponse to the large scale rifting that took place there
in 1975-1984 (Foulger et al., 1992; Heki etal., 1993),
as a result of the existence of a viscous layer underly-
ing an 8-30 km thick, elastic plate. The viscosity in
the layer is consistent with values obtained from the
studies of post-glacial rebound.
There is some evidence that the uplift rates around
Vatnajökull are even higher than the model calcula-
tions indicate. Jahn et al. (1993) measured an exten-
sive GPS-network in N - and E-Iceland in 1987. One
of the points occupied was in Homafjörður, about 20
km from the SE-edge of Vatnajökull. The same point
was occupied during the EUREF GPS-campaign in
1990. Comparison between the two determinations of
the vector Akureyri-Hornafjörður showed changes
that could be interpreted as the result of 8 cm uplift of
the Hornafjörður point during the period 1987-1990.
Imsland (1992) documents changes in relative sea
level that could indicate a similar rate of uplift. A
small skerry, Hellnasker, within the Hornafjörður la-
goon emerges about 75 cm above sea level at low tide
at present. Before 1950 this skerry never showed,
even at the lowest tidal levels. Crustal uplift is consid-
ered at least a contributing cause, even though other
changes in the lagoon could also be involved. This
observation indicates a maximum uplift rate of a little
less than 2 cm per year.
ACKNOWLEDGEMENTS
The GPS field work was funded to a large degree
by the University of Iceland Research Fund. The Nat-
ural Environment Research Council in the U. K. pro-
vided the 3 GPS receivers. The Iceland Geodetic Sur-
vey provided one vehicle and the town of Höfn gave
valuable logistical support. WRJ is grateful to G. Wil-
helmi and G. Jung-Jacoby for their help in the field, to
G.W. Hein, Universitat der Bundeswehr, Neubiberg,
and to Guðmundur Pálmason, Orkustofnun, Reykja-
vík, for lending him their LaCoste-Romberg gravime-
ters, to B. Richter and P. Wolf, Institut fiir Ange-
wandte Geodásie, Frankfurt/M. for computing the
solid earth tidal corrections and to Peter Smilde,
Mainz for helping with his gravity evaluation pro-
gram. Leó Kristjánsson and Bryndís Brandsdóttir
read the manuscript critically and suggested valuable
improvements.
REFERENCES
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Völksen, and P. Einarsson 1995. Plate boundaiy de-
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Einarsson, P. 1991. Earthquakes and present-day tecton-
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Einarsson, P. 1993. Benchmarks of GPS-measurements
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Einarsson, T. 1966. Late- and post-glacial rise in Iceland
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