Greinar (Vísindafélag Íslendinga) - 01.01.1977, Side 99

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waves, Press (74) has found for oceanic regions an increase
in density from the Moho down to the Low velocity layer. This
means also increase in rigidity, in spite of the probable in-
crease in temperature. It seems to follow that if, due to oc-
casional raising in temperature in the uppermost mantle, such
as a thick blanket of organic deposition would cause, the con-
ditions for dense nanocrystals are only found deeper than be-
fore, the Moho will move deeper, while the formerly dense
material above the new Moho will expand. This corresponds
to shallowing of the sea, coupled with a transgression.
With time the organogen blanket becomes denser and less
heat-insulating. The Moho rises somewhat, which tends to
create a regression. But in particular the shear in a global
stress field renews dense nanocrystals at a shallower depth,
which will be the main cause of a major regression, as we
discuss further in Chapter 9.
The working hypothesis of a basaltic upper mantle in a
nanocrystalline state will now be considered. Optically the
material would appear to be glass, and according to seismic
findings it is solid. We must then think of bonded nanocrystals,
as they would result in the process of formation. As indicated in
(72) they would be “bonded efficiently together in a manner
analogous to that found in twinned crystals”.
A disruption of the twinning bonds demands much less energy
than a disintegration of the lattice of the nanocrystals. Thus,
it demands less energy to convert the mass into a viscous por-
ridge of loose nanocrystals than to melt the mass entirely.
Referring again to the dissipation of strain energy by flow-
age at a depth of a few km in the common case of shallow
earthquakes in the ridge areas, wre realize that a disruption of
the bonding of the nanocrystals, the first step in magma
formation, can only be expected if the stress field is of such
intensity — or the strain accumulation of such a rate that
the strain is first turned into heat by plastic yield at a depth
where the temperature is not far below magma temperature.
This demands an adjustment of plasticity and the strength of
the stress field.
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