Greinar (Vísindafélag Íslendinga) - 01.01.1977, Page 102

100
of the dense nanocrystals. This suggests that at such mantle
depths we have an original earth layer, left over by the crystal
settling differentiation, when metals and heavy minerals sank,
and the lightest and most acidic ones rose to form a global
acid layer over a thick layer of practically “primary” basalt
composition. The composition of the surface layer has been
between granite and pure silica. Mixing with basaltic material
from eruptions or intrusions, ultimately formed the sial of
about 65% SiOo.
We now understand fully the role of the atmospheric circula-
tion in connection with magma formation. Climate which pro-
duces such strong crustal stresses, that the appropriate stress
for plastic movement is at depth of magma temperature, creates
just there frictional heat. This destroys the twinning-like bonds,
and due to the temperature, the viscosity becomes sufficiently
low for the dense polymorphs to turn into light ones. By mild
climate the frictional heat is given off at a shallower depth,
where the low temperature does not allow this process of magma
formation.
To summarize, our general conclusion is then that magma
formation can be expected, if the strain energy is dissipated
by plastic flow at a depth, where magma temperature is found.
This would demand greater surface stresses than those which
lead to shallow earthquakes, as these are connected with plastic
dissipation of strain energy at such shallow depths as 3-5 km.
It then seems questionable, whether such shallow eartquakes
would, in general, be common at a time during which magma
is being formed at a greater depth.
In a paper on The Problems in Radiometric dating, the pre-
sent author (20) has shown that the rapidly cooled thick
crust of pillows extruded at a greater depth than 2160 m in
seawater (super-critical pressure) must be made of a different
kind of glass than the slower cooled glass of the interior of
the pillows. The former contains excess argon the latter not.
We now seem to know the difference of the two types of glass,
the argon-retentive glass is still formed of fine equilibrium
crystals, while the other is not. These nanocrystals have re-