Jökull - 01.12.1994, Page 32

was recently revisited by Sigmundsson (1990, 1991).
An improved mathematical model was constructed
and tested with the most recent geological evidence of
uplift, using then current ideas about the physical
state of the crust and mantle. A value of 1019 Pa s was
derived for the upper bound of the viscosity of the as-
thenosphere in Iceland. This is almost 100 times
lower than viscosity values commonly accepted for
the upper mantle (Tushingham and Peltier, 1992). The
low viscosity has several interesting consequences
with regard to present crustal movements in Iceland.
There are indications that glaciers in Iceland were
small at the time of settlement in the 9th century. In
the cold period that began around 1200 AD (Berg-
þórsson, 1969) the glaciers gradually advanced and
reached their maximum extent at the end of the last
century. Since then they have been retreating, espe-
cially after 1930 following a distinct warming of the
climate. The area of the largest glacier, Vatnajökull
(Fig. 1), has been reduced by 300 km: and the corre-
sponding reduction in its volume is 182 km3 (Sig-
mundsson and Einarsson, 1992). Crustal movements
due to this unloading can be estimated with the pa-
rameters derived earlier for the Icelandic lithosphere
and asthenosphere (Sigmundsson, 1990, 1991; Sig-
mundsson and Einarsson, 1992). The results indicate
that the land around Vatnajökull should currently be
rising at a rate of 5-15 mm per year. This rate is well
measurable within a decade using geodetic methods.
Tilt changes consistent with the predicted uplift were
detected by repeated measurements of the lake level
at Lake Langisjór at the SW edge of Vatnajökull in
Fig. 1. Map of the Homafjörður and Vatnajökull region in SE Iceland. GPS-survey points are shown, also the gravity refer-
ence pointno. 701.
1. mynd. Kort af mœlisvœðinu við suðausturjaðar Vatnajökuls. Mœlipunktar GPS-mœlinganna eru sýndir, einnig viðmiðunar-
punktur þyngdarmœlinganna, punktur 701.
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JÖKULL, No. 44