Rit (Vísindafélag Íslendinga) - 01.06.1971, Blaðsíða 135

CRUSTAL STRUCTURE OF ICELAND
135
16. SOME SUGGESTED FURTHER RESEARCH
The seismic refraction data that have been presented here, and
their interpretation and correlation with subsurface temperature data,
seem to open new promising paths of research on processes at mid-
ocean ridges. Iceland may be one of the very few places where such
work can be carried out in sufficient detail to obtain significant re-
sults. At the hottom of the ocean it is doubtful whether such work can
be carried out in sufficient detail with present day techniques. The
heat flow measurements on ocean ridges may he subject to large
variations due to convective movement of water in the presumably
highly permeable crestal zone (Pálmason, 1967b). Such conditions
could be detected in a drillhole several tens or hundreds of meters
deep, but could hardly be detected with the presently available tech-
nique of measuring heat flow through the ocean bottom. The cor-
relation of subbottom temperatiu*es with other geophysical and geo-
chemical data would thus be difficult at the ocean bottom, but can
be of critical importance.
For the study of processes beneath high-temperature hydrothermal
areas the indicated association of seismic boundaries with a certain
temperature interval would be of great importance, if it tums out to
be generally valid. The high-temperature thermal areas in Iceland
are all located in the active volcanic zone, where the 2—3 boundary
may be in the process of being created. The increase in density which
presumably is taking place at the 2-3 boundary may lead to increased
permeability near the boundary which in turn would promote water
convection. Such convection in a permeable medium starts easier at
high temperatures than at low temperatures because of the change
in the physical properties of the water (Goguel, 1953). Of great in-
terest in this respect is that recent studies of microearthquakes in the
volcanic zone of Iceland indicate that these occur mainly close to or
just below the 2-3 boundary (S. Björnsson, 1970). Theoretical
studies of hydrothermal circulation systems on the basis of presently
available data would be of great value, and might throw further light
on the nature and the heat source of the high-temperature systems.
A correlation of seismic and gravity data will no doubt in the
future lead to a better picture of the low density anomaly in the
upper mantle under Iceland. More data on the depth to layer 4 are
needed in northern and eastem Iceland as well as on the plateau sur-