Greinar (Vísindafélag Íslendinga) - 01.01.1977, Blaðsíða 97

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stance r from the centre is p(r) =p0(r0/r)3. Now replace the
fluid by a dense nanocrystal whose equilibrium pressure p0
is far above the surrounding lithostatic pressure. Then the
crystal tends to expand with the pressure p0. In equilibrium
the radial pressure then falls very rapidly with distance in
the elastic medium, so that the nanocrystal will be kept within
a thin shell of high pressure. This shell of local high pressure
is then in no disagreement with a far lower general lithostatic
pressure.
We shall now quote some experimental data on the densi-
fication of silica glass under high pressure. According to (73),
SiOo-glass was irreversibly densified by a pressure up to 250
kbars. The threshold for a significant densification of this
kind was found to be about 60 kbars. It is well-known that
silica glass can be irreversibly densified by the application
of high pressure but an explanation of this effect seems not
to have been given. We suggest that the densification is due
to polymorphic transitions of the nanocrystals, of which the
glass is taken to consist, cf. (72). “In general, when the glass
is heated under pressure, still denser material results” (73).
This may mean both that rise of temperature eases the at-
tainment of the polymorphic equilibrium, or that by lowering
of viscosity the outer pressure is applied more directly to the
individual nanocrystal, while that might be hampered by
stresses in a glass of higher viscosity.
Wackerle, 1962, found shock densification from 2.2 to 2.4
-2.5 g/cm3 after recovery from a pressure of about 250 kbars
(73). “All investigators report that the densified material is
very stable at room temperature but anneals very quickly at
temperatures above few hundred degrees” (73). This shows
that by sufficiently high viscosity of the “glass”, the densified
nanocrystals are unable to expand to lighter forms, although
the external pressure is low. They are then surrounded by a
high-pressure shell as we have inferred above.
From this result we infer that the relatively low lithostatic
pressure at the oceanic Moho is per se no fundamental argu-