Rit (Vísindafélag Íslendinga) - 01.06.1971, Page 132

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GUÐMUNDUR PÁLMASON
about 3.1 g/cm3 and that of layer 3 about 2.9 g/cm3. For the purpose
of inferring the most likely values of density in the overlying layers
a comparison will be made with the predictions of Fig. 48 and in-
ferred density differences from combined seismic-gravity interpreta-
tions.
The Nafe—Drake curve (1) predicts a density difference of 0.30
g/cm3 between layers 2 and 3 and a difference of 0.15 g/cm3 be-
tween layers 1 and 2. The Woollard curve (2) predicts a difference
of 0.25 g/cm3 between layers 2 and 3 and a difference of 0.04 g/cm3
between layers 1 and 2. Curve (5) predicts a difference of 0.23 g/cm3
between layers 2 and 3, and a difference of 0.12 g/cm3 between
layers 1 and 2.
As was discussed in sect. 6.7, the combined seismic and gravity
data in Hrútafjördur and Vatnsdalur indicated density differences
of 0.18 and 0.19 g/cm3 respectively between layers 2 and 3. It was
then assumed that layers 1 and 2 were of the same density. If a non-
zero density difference was assumed for layers 1 and 2, the resulting
density difference between layers 2 and 3 would be still smaller than
the values given above. It then appears that, of the crnves in Fig. 48,
the Woollard curve (2) comes closest to agreeing with the above
deductions of density differences in Iceland. This curve gives densi-
ties of about 2.60 and 2.65 g/cm3 for layers 1 and 2, respectively.
The density in layer 0 is no doubt highly variable, as is the P-wave
velocity. It appears likely that density values for the material of this
layer may range from about 2.1 g/cm3, or even lower, to about
2.5 g/cm3.
The most probable densities of the crustal seismic layers in Ice-
land are then summarized in the following table.
Table 14.1.
Layer Average P-velocity km/sec Average density g/cm.3
0 2.8 2.1-2.5
1 4.2 2.6
2 5.1 2.65
3 6.5 2.9
4 7.2 3.1
Using these density values it would be possible to speculate on
isostatic equilibrium and density variations within the mantle. It
is readily obtained, for instance, that for a depth of less than 15 km