Jökull - 01.12.1974, Blaðsíða 8

Mynd, 4. Likan af jökulbreiðu og jarðhitasvaði, sjá ágrip, Jökulhlaupin.
Fig. 4. a) An idealized cross-section of the ice cap and the subglacial hydrothermal area. The
geothermal area, located in the Neo-Volcanic Zone, is heated by recent hot volcanic structures
and characterized by hot ground and numerous natural-steam holes. The volcanic subsidence
caldera (Grímsvötn) has probably ring fractures along the edges which provide open channels
for downpercolation of water. The water circulates in a permeable flood basalt down to 2
to 3 km, where the base temperature is over 200 °C, and picks up its characteristic chemical
composition (Bödvarsson and Pálmason, 1961). The chemical composition of the water in jökul-
hlaups can be explained if 14 of the water volume has circulated in a closed system beneath
the caldera (Sigualdason, 1965). Outside the caldera there are probably only single fumaroles
and hot ground without display. b) A subglacial lake is located above the geothermal area. If
there were a hydraulic connection between the lake and the bed of the glacier, the map of the
glacier bed would represent the subglacial hydrostatic water pressure P relative to the water
level of the lake; P = pwg (hw — hb), see text for explanation of the symbols. The ice over-
burden pressure at the glacier bed is given as pj = PigHj, see text. The values of p4 are com-
pared with the possible values of P. Both p; and P are plotted from the same datum level, namely
downward from the water level of the lake. p^ attains a maximum value and then decreases to-
ward the edge of the glacier The possible values of P increase continuously toward the edge. P
could exceed pj at a certain distance from the lake. There is therefore a barrier inside this
point. The lake is sealed and P does not exist outside the sealed area. As the water level in
the lake rises the width and height o£ the barrier will decrease. If the water level can rise high
enough, the barrier will be exceeded and some water will escape through the barrier. Hydraulic
connection would then be provided with the area outside the former seal.
6 JÖKULL 24. ÁR