Jökull - 01.12.1994, Page 31

An Experiment in Glacio-Isostasy near Vatnajökull, Iceland,
1991
Páll Einarsson,
Science Institute, University of Iceland, Dunhaga 5, 107 Reykjavík,
Freysteinn Sigmundsson,
Nordic Volcanological Institute, University of Iceland, Grensásvegi 50, 108 Reykjavík,
M. A. Hofton, G. R. Foulger,
University of Durham, South Road, Durham, DHl 3LE, England
and W. Jacoby,
Institut fiir Geowissenschaften, Johannes-Gutenberg-Universitat Mainz, D-55122 Mainz, Germany.
Abstract - Several lines ofevidence suggest low subcrustal viscosity beneath Iceland. Values of
the order oflO19 Pa 5 are compatible with high rates ofcrustal uplift at the end ofthe last glacial
period, and post-rifting stress relaxation measured after the Krafla rifting episode. Present-day
changes in glacial loading of the crust are expected to lead to rapid isostatic adjustments that
should be easily detectable by geodetic methods. The volume ofthe Vatnajökull ice cap has de-
creased by 180 km3 in this century. Model calculations indicate that this decrease should lead to
crustal uplift in the area around the glacier at a rate of 5-15 mm per year at the present time and
tilt away from the glacier. An experiment to measure this uplift was initiated in 1991-1992. A
geodetic network of 10 points was installed in the area SE ofthe glacier and measured by GPS
geodesy. The network contains points near the glacier edge as well as a point at a distance of 50
km, i.e. outside the affected area. Uncertainties in relative positioning are estimated to be about
1 cm in the horizontal components and 2 cm in the vertical. Gravity was also measured at the
geodetic points as well as several older gravity points in the area. Indications of gravity changes
were found, consistent with uplift of several. mm peryear.
INTRODUCTION
It is well known that the crust and upper mantle
respond to changes in surface loads and seek isostatic
equilibrium. In some areas such as Fennoscandia and
Canada the crust is still uplifting in response to the
deglaciation at the end of the last glacial period. The
rate of movement depends on the thickness of the
lithosphere, the load distribution in space and time,
and the viscosity of the underlying asthenosphere.
Similar movements took place in Iceland at the end of
the ice age, but they appear to have been completed in
less than 1000 years. This has been used to infer that
the viscosity of the asthenosphere beneath Iceland is
at least an order of magnitude lower than that beneath
the continental areas (Einarsson, 1966). This problem
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