Jökull - 01.12.1961, Qupperneq 45

mate solubility of silica from basaltic rock is
25% to 35% of that of amorphous silica. This
appears a reasonable ratio.
Laboratory tests on the solubility of silica
from samples of basalts have been carried out
in Reykjavík. The results confirm the above
interpretation.
An important property of dissolved silica is
that is precipitates extremely slowly from weak
oversaturatecl solutions. Tliis is mainly the case
for saturations below 200 ppm. Thus, water
saturated at one temperature and cooled to a
lower temperature will not precipitate the silica.
A polymerization takes place but the colloids are
kept in a fine dispersion for a long time and
are chemically reactive in much the same way
as dissolved matter. The main implication is
that thermal water which has been saturated at
one temperature and is cooled by conduction
losses or flashing will keep the initial silica con-
tent reactive in spite of the temperature loss.
This rather anomolus behaviour of the silica
implies that relation (5) can be applied in order
to obtain a semi-quantitative estimate of the
base temperature. The method is of practical
importance in cases where it is possible to ob-
tain samples of water from hydrothermal sys-
tems. The amount of silica in the sample is
indicative of the base temperature although the
sample has been cooled by conduction losses.
Some thermal springs with a temperature less
than 50 "C and a flow of one or two
liters per second represent leaks from hydro-
thermal systems witli liigh Irase temperature.
The small flow implies large conduction losses
on the way to the surface and the springs may
be rather insignificant. But the amount of silica
in the water reveals the high base temperature
and furnishes a positive evidence as to the pros
pects of the area. The method has been of some
practical importance in Iceland.
In conclusion, it may be mentioned that some
of the gases issued by thermal springs in Iceland
appear to furnish some evidence as to the base
temperature. It has been observecl that CO2
and H2S are practically absent in springs having
Map showing locations of relevant thermal areas and non-thermal boreholes in Iceland.
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