Greinar (Vísindafélag Íslendinga) - 01.01.1976, Síða 20

18
Quist and Marshall (14). Watanabe (19) has measured the resis-
tivity in basalt containing some but unknown amount of water at
temperatures up to 1500°C. He finds resistivities between 3 and
30 Om in the temperature range 700-800°C, which is in good
agreement with values expected from Figure 7.
Water from the Icelandic high temperature areas has ionic
strength in the narrow range of 10-20 meq, corresponding to about
0.5-1.0 g/1 NaCl (S. Amórsson, personal communication). Some
of the low temperature springs have similar salinity, but others
are containing water with somewhat lower ionic strength. Only
the high temperature areas close to the shore on the Reykjanes
peninsula contain much more saline water similar to sea water in
salinity.
If we compare the curves in Figure 7 with the results obtained
in the previous section, it becomes obvious that the resistivity
(10-1000 Om) of rocks in the upper part of layer 3 cannot be
explained by conduction in solid dry rocks alone. The temperature
of dry basalt must be at least 500°C to show such a low resistivity.
Hence it is essential that these rocks contain some amount of water.
If we assume 150-300 Qm for the resistivity in the upper part
of layer 3 west of the neovolcanic zone, 70 Qm for the overlying
layer and 150-300°C for the temperature at the 2-3 layer boun-
dary, then the porosity must be around 0.01 - 0.02 in layer 3 and
about 0.04 in layer 2 according to the results of Parkhomenko
et al. (15), shown in Figure 7.
The anomalous high resistivity of 1000 Qm at the site Hérad
in Eastern Iceland can be satisfactorily explained with lower tem-
perature. In this area there is one temperature gradient hole giv-
ing the lowest observed thermal gradient in Iceland 37°C/km (4).
Assuming porosity around 0.01, this low temperature gradient can
explain the observed resistivity of 1000 Qm down to 5 km. Beneath
about 5 km we have to assume still lower porosity. This is in very
good agreement with laboratory measurements on porosity of ba-
saltic rocks from boreholes and from surface samples made by
Pálsson (20). He finds porosity around 0.01-0.03 for dolerite and
many samples of fine grained or pore-filled basalts.
In the active zone of spreading and volcanism the resistivity of
layer 3 is much lower or 10-20 Qm. This does not mean that layer