Rit (Vísindafélag Íslendinga) - 01.06.1967, Page 51

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DISCUSSION
S. Thorarinsson: According to Walker’s “steady status”
hypothesis the belt of active volcanism in Iceland has re-
mained approximately uniform during the Tertiary and
Quaternary. I think, however, that there is no doubt that
during the last million years or so the active belt has nar-
rowed so much that the difference in width can not be ex-
plained by drift.
S. Björnsson: What are the geological reasons for believing
that the basalt pile must be 20—30 km thick?
H. Sigurdsson: More than 10 km stratigraphic thickness
of basalt lavas is exposed above sea level in eastern and west-
ern Iceland, showing down-dip thickening in the east. It is
therefore reasonable to expect equal thickness of basalt lavas
below sea level. Walker has also pointed out, that the present
rate of production of lavas “would be enough to produce a
layer more than 20 km thick over the present area of Ice-
land since the beginning of the Tertiary”.
S. Björnsson: How stable are the zeolite zones? Can the
horizontality of these zones be regarded as an evidence of
zeolite formation only after cessation of tilting of the basalts
or have older horizons of zeolites been washed out after the
basalts were tilted?
H. Sigurdsson: The zeolite zones have been connected with
the rise of geoisotherms, and on tilting or further burial of
the lava pile the lower zeolite zones will rise and replace the
upper, lower-temperature mineral assemblages. There is in
my opinion no reason why the higher-temperature minerals
should not wholly replace lower-temperature assemblages,
given enough time to attain equilibrium. On the other hand,
if a basalt series containing zeolites were lifted above the zeo-
lite-forming environment, the displaced zeolite zones should
be preserved indefinitely. The fact that snch displaced zones