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4 cm of extension is localized near the Hrafnagjá fissure. The line
524-D8 which crosses Hrafnagjá, however, does not show any
lengthening (Figure 5).
The geodimeter measurements to date indicate the following:
1. Survey lines l-to-4 km long can be repeatably measured to
precisions of 5-to-6 mm standard deviation.
2. Measurements on calibration lines show that three different
geodimeters measure absolute distances with differences up
to 18 mm, but the errors are systematic and can be removed.
3. Systematic changes in the length of a group of survey lines
in the rift zones of 20 mm or more are probably tectonically
significant, e.g. the 1967—1970 changes 25 km northeast of
Hekla Volcano, and the 1970-1973 changes across the north
end of Lake Thingvellir.
4. A possible, but not well supported, interpretation can be made
that small recoverable strains in the rift zones shift in loca-
tion over periods of a few years.
5. No detectable strike-slip motion has occurred on Almannagjá
during 1967-1973. The lines 5001-D15 and D16-D15 cross
Almannagjá obliquely in the opposite sense, and neither line
shows significant change.
6. The net apparent extension evident from the 1967-1973
geodetic results is not inconsistent with the spreading rates
indicated by geologic and magnetic data.
The roles of various possible driving mechanisms for plate tec-
tonics are unresolved. However, on-going subsidence and extension
of the rift zones in Iceland is evident from vertical and horizontal
geodetic results (2,4, this paper). These data suggest a tensile
necking of the lithosphere at diverging plate boundaries rather
than a direct push apart by shallow dike injection.
Deformation data in Iceland are critical to understanding the
dynamic processes of rifting, and much more extensive surveying
over longer periods of time needs to be done.