Greinar (Vísindafélag Íslendinga) - 01.01.1977, Side 100

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On this basis we understand that the shieldvolcano magmas,
produced during the severe climatic spells towards the end of
the last ice age, seem definitely to come from greater depth
than the fissure magmas of postglacial time.
New important data concerning the formation of magma
have been given by Capaldi et al. (76). These authors have
discovered radioactive disequilibria between Ra and Th short-
and intermediate-lived isotopes belonging to the 2:i2Th and
238U decay series. Ra was found to be enriched in relation
to Th in the erupted fraction of the magmas (material from
Etna and Stromboli erupted since 1971), by processes of
magma formation, differentiation, and eruption. The first
major event of Ra enrichment was dated as 30-40 years for
each volcano. In other words the time from the beginning of
magma formation to the eruption is 30-40 years for both
volcanoes.
Our interpretation of these data is, that not only are the
twinning-like bonds destroyed, the heavy nanocrystals change
into a light polymorph because of the increase in temperature,
and decrease in viscosity. These new nanocrystals are equili-
brium crystals for the respective litho-hydrostatic pressure
and temperature, and are no oscillators, and it appears that
Th does not fit into the new lattice, and is expelled.
The lightening of the nanocrystals means expansion of the
affected mass, and it appears that the process goes on imtil
the pressure on the wall of the self-made vertical reservoir
opens the shear fracture from below, whereupon the litho-
hydrostatic pressure in the quasi-plastic or elastic walls of
the reservoir has thrust practically all the magma up into the
fracture and eventually up to the surface.
But Th must not just be expelled into the surroundings of
each new equilibrium crystal, for Th must be left behind. It
must be extracted from the eruptive fraction.
There seems to be only one way of doing this: pretty large
and heavy crystals must also be formed, into which the Th
fits, and these crystals must then settle to the bottom of the
magma reservoir.