Jökull - 01.12.1977, Síða 6

Fig. 2. Sketch map showing namcs, and geological and topographical features in the Vatnajökull
area. (From Thorarinsson et al., 1973).
Mynd. 2. Örnefni og helstu drættir jarðfrœdi á Vatnajökli og umhverfi, sem sést á ERTS-1 gervi-
hnattarmyndinni (mynd 1) (Sig. Þórarinsson o. fl., 1973).
results in a pressure-release of about 14 bar,
wliich, at 3 km depth (the top of a hypothetical
magma chamber, using Krafla (Björnsson et al.
1977) as a model) means a release in pressure
of about 1.3%. This variation might seem of
little consequence to trigger large events. How-
ever, tliat the contrary relationship obtains, i.e.
that eruptions trigger jökulhlaups, is untenable
as a general rulc, for it makes no difference in
terms of hydrostatic pressure at the lake bottom
4 JÖKULL 27. ÁR
wliether the ice in the caldera be molten or
not. However, if an eruption were to take place
before the caldera had been all but filled to
the critical level, it would not trigger a jökul-
hlaup. Conversely, jökulhlaups do occur with-
out accompanying eruptions — since 1934 every
five or six years.
The volcanic history of KVERKFJÖLL, a
central volcano distinguished by a subglacial