Greinar (Vísindafélag Íslendinga) - 01.01.1977, Síða 92

90
Fig. 15, the magma would, during the incubation time, only
penetrate into the root where H>0, and its penetration into the
root will increase with H. If the root is more acidic than the
magma, the likelihood of differentiation by assimilation increases
with H, and the mystery of the rise of maximum differentia-
tion towards the Kverkfjöll centre would evaporate, together
with any chemical evidence for the “greatest Hot Spot on the
earth”.
But we should not leave the matter at this stage. Heis-
kanen’s normal “crust” — a continental one by definition and
according to density — is just a model for making isostatic re-
ductions possible. Instead of this or similar models, the only
precise isostatic reduction could be made on the basis of a
world map, on which is shown the density and thickness of
layer above a “level of compensation”, the definition of which
is not too clear. But Heiskanen’s model may be a good approxi-
mation to this ideal, for layers above the Moho, say.
The Icelandic gravity field was quite clearly formed at the
same time as the uplift of the Lower Tertiary plateau basalt
in the Second Main Tectonic Phase of the country, i.e. in
about the Middle Oligocene, cf. Chapter 2, having broadly a
similar outline as the country (71) and (53), and present paper,
Chapter 2. On the basis of the existing isostatic equilibrium after
the Tertiary Second Main Tectonic Phase of the country (Oli-
gocene time, cf. Chapter 2), I considered it reasonable in (57,
71), and I still do so, to explain the topographic mound by
plastic crustal thickening under compression at some crustal
depth. A root would then automatically compensate for the
mound (cf. the uplift of folded mountains, Chapter 8), and
Fig. 15 would be correct in principle. We do not see any other
sensible way of explaining, in particular, the shape of the
reasonably smoothed mound.
On the other hand, Heiskanen’s normal “continental crust”
of density 2.67 g/cm3, must not be taken literally, neither for
Iceland nor for any other area. Further, whether truely con-
tinental or not, the lower part of the crust need not by general