Acta naturalia Islandica - 01.02.1946, Page 70

04
TRAUSTI EINARSSON
thickness of 50 m on a slope with the inclination 10°. Let h be the
thickness, y the distance of an element of the fluid from the bottom, v
the velocity of this element, ^ the viscosity, q the density of the melt,
and g the acceleration of gravity. Then we have by steady flow, dis-
regarding the influence of walls
cLv
Qg (h — y) sin 10° — ?; ox'
dy
v = — y (h — y/2) sin 10°
v
and at the surface of the flow
g q sin 10° h2
With the figures assumed we find vinax = 23 m per hour. With a
viscosity of 10s the velocity is 2,3 km per hour, but crystallization
would be imperceptible unless the melt was kept at a constant tem-
perature for a very long time.
But the water content of the lava is a faetor which determines
some main characteristics of this type of volcanism and it is easily
seen how it favours the formation of glass.
Qualitatively the effects of the gas content in a magma on its
viscosity are well-known. Quantitative data are, however, scant and
such directly relating to our basic magmas are not available. Data
on melts of other composition are, however, significant and may
suffice for the present purpose. Dry melts, the one of composition
Na2Si03 and the other of Na2Si05 have been compared in the
Geophysical Laboratory of the Carnegie Inst. of Washington with
the same melts under a steam pressure of 200 Atm. In the first case
the liquidus was lowered by 175° C and in the second by 165° C, on
account of the absorbed water content.23) Transferring these figures
into viscosities we might expect that the basic magmas would under
similar conditions retain fluidity at 100—200°C below the corre-
sponding point for a dry magma.
The temperature interval of crystallization for a melt of the com-
position of e.g. augite is only 30—40°C and it is hardly very different
for the here considered magmas. Our wet magmas will then be able
to retain fluidity at a temperature far below that interval. When