Greinar (Vísindafélag Íslendinga) - 01.01.1977, Side 77

Chapter 5
FORMATION OF SEISMIC LAYERS AND
SEISMIC BOUNDARTES
CONTENTS
Seismic boundaries are in many cases not identical with stratigraphic
boundaries. The former may go horizontally through dipping plateau
basalts, or regularly through folded mountains. Such seismic boundaries
go undisturbed through a complex structure of various rock types, and
must have been f o r m e d after even major tectonic disturbances. In
some such cases, one is led to the conclusion that the lowest of such
boundaries, the Moho, must also strike through various rock types, and
suggests a polymorphic change after transition to the “nanocrystalline”
state which is discussed in Chapter 6.
In Chapter 4 we have already pointed out the great effect
of deep burial and age on the properties of rocks. We con-
sidered, in particular, magnetization, in light of the fact that
magnetic or domain bonds are much weaker than lattice bonds,
and inferred that the former have yielded in rocks as young
as 60 My and so changed the magnetization.
Seismic Layer 3 in Iceland was mentioned in that connec-
tion, and it seemed at least possible that its magnetization
becomes viscous under the effect of non-hydrostatic stress, and
that the Stardalur anomaly was due to such magnetization.
A further possible change in the basalts, forming Layer 3,
and very possibly also in the Oceanic Layer, is the extermina-
tion of the weakness and space consumption represented by
lattice dislocations. Such dislocations have become better and
better known in later years, and are now considered to be very
common in minerals. These dislocations of the atoms, or im-
perfections in the lattice, are known to be much weaker than
the regular lattice itself, and to wander under non-hydrostatic
stress (52), and may become more or less expelled from a
lattice in that way.