Greinar (Vísindafélag Íslendinga) - 01.01.1977, Blaðsíða 83

Chapter 6
THE FORMATION OF BASALTIC MAGMA AND THE
EVIDENCE ON THE CONSTITUTION OF THE UPPER MANTLE.
THE NATURE OF THE MOHO.
THE CONCEPT OF NANOCRYSTALS AND THEIR
ÍNFERRED ROLE
CONTENTS
Experimental evidence on the high-pressure densification of silica glass
is interpreted on the basis that the “glass” consists actually of crystals
in the 20—30 angstrom range, about the smallest crystals possible. By a
law of elasticity, much higher pressure than the lithostatic one may form
around such small crystals and keep them in a dense polymorphic state.
It is considered that the Moho marks the change to such dense “nano-
crystals”. The Upper Mantle is considered to be of nearly basaltic com-
position and in a dense nanocrystalline state. Frictional heating at faults
may change the dense bonded nanocrystals into loose and lighter ones of
mineralogical equilibrium with a basaltic melt — leading to the formation
of basaltic magma in an expanding chamber, that will, in the end, open
the fracture from below. Seismic data are thought to indicate that by
general rise of temperature, the Moho will move deeper, causing a shallow-
ing of the ocean floor. Such increase in temperature could be caused by
a general organic sedimentation in a relatively shallow ocean, and would
lead to a transgression.
In the middle of the 19th century there was a dispute on
the question whether there were two layers of magma or only
one. Could acidic rocks really be a differentiate of a single,
basaltic magma? It was gradually agreed that there was only
one general magma layer, the basaltic one.
This general fluid layer was no serious problem at the time,
the earth was simply molten below the crust.
Seismology and the theory of isostasy stirred the peace in
that paradise. There was no molten basalt layer. On the con-
trary: at the Moho, much denser rocks than basalt must take
over, and form the mantle. So, also, basaltic magma must be
a derivate of something else, namely of rocks of density 3.3
g/cm3 or a little more. Eclogite and peridodite came especi-
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