Jökull - 01.12.1975, Page 12

Fig. 6. Illustration of the
various stages in the
growth of a tablemoun-
tain (tuyas).
Mynd, 6. Hin ýmsu stig
í myndun stapa.
The base area of that cupola is about 3 km2.
Volcanic eruptions in Katla trigger the most
catastrophic jökulhlaups. In a few hours the
discharge reaches a maximum of 100 to 200
thousand m3/s. A volcanic eruption cannot
melt ice at this rate. Thus a water reservoir
must be released. Further studies of this re-
servoir are required. Thorarinsson (1975 b)
suggested that magmatic heat starts melting ice
some time before the eruption breaks out. Re-
peated geodetic surveys might detect the result-
ing formation of a water cupola. This may turn
out to be one way of preclicting jökulhlaups
from Katla.
CONCLUSION
The cause of jökulhlaups from the interior
of glaciers has been examined theoretically.
1 0 JÖKULL 25. AR
The theory defines the conditions for the exist-
ence of subglacial water reservoirs and the
stability of the various types of reservoirs.
Water reservoirs which trigger jökulhlaups
may be formed at subglacial geothermal areas.
This explains the jökulhlaups from Grímsvötn
and the ice cauldrons 10 km north-west of
Grímsvötn. The geothermal areas at Kverkfjöll,
Torfajökull, and Mýrdalsjökull are also poten-
tial sources of jökulhlaups of this type. Glacier
growth at marginal geothermal areas may actu-
ate jökulhlaups.
Stable water reservoirs may from beneath a
flat or a convex glacier surface. Accumulation
of water into the reservoirs would not cause
jökulhlaups. Possible sites for stable reservoirs
can be pointed out with the aid of a topo-
graphic map of the glacier. Geophysical surveys
shoulcl be focused on craters and calderas like