Greinar (Vísindafélag Íslendinga) - 01.01.1977, Blaðsíða 94

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tions of differentiation in individual eruptions, one must first
keep in mind the height of the eruption locality on a smoothed
topographic map. The smoothing in Fig. 14 is not necessarily
the most appropriate one for geochemical studies, although
just the isostatic point of view should be used as a basis. (In
(71) I pointed out that T=20 km would be a better choice
than T=30 km). Somewhat smaller or greater smoothing than
in Fig. 14, might possibly be better. And, finally, a smooth root
is only an approximation, to be used until sufficient petro-
chemical material is available to infer such deviations from
the smooth root as there quite possibly may be.
The inference of “acidic” material in the lowest Iceland
crust does not necessarily mean a continuous acidie layer, but
at any rate a rather dense distribution of acidic material.
Further study must reveal, whether one could possibly infer
a sunk sialic layer (cf. also the discussion of ocean floors in
a later chapter).
There is still nothing to compel us to change the conclusion
in 1950 (59) of a general layer of “potential” „primary basal-
tic magma” under Iceland. We now come to the question: What
means potential, in physical terms?
To begin with, we should remember, what was stressed in
Chapter 3, that the time factor of the order of a million bet-
ween laboratory- and natural processes, may exclude labora-
tory tests of some natural processes. But natural processes
are also evidence, as soon as we have recognized them1)
Secondly, not all processes are reversible. This, again, is con-
nected with a much more fundamental rule, i.e. that develop-
ment or history is usually a uni-directional process, and, fin-
ally, this is a consequenee of a fundamental physical law, the
2nd law of thermodynamics.
1) Too much importance can be given to laboratory results. When theory
lags behind, they lead to such a clash, as there has been between geo-
logy on one hand, and seismology and isostasy on the other. The theory
of the nanocrystals shows that there need not have been any clash, if
more attention had been given to theory at the time when high-pressure
experiments began.