Rit (Vísindafélag Íslendinga) - 01.06.1967, Blaðsíða 42

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such rise is seen on the seismic profiles — rather on the con-
trary (Pálmason, 1963).
It is of interest to compare the rate of production of lavas
in Iceland as indicated by Gibson’s hypothesis 2—4 km thick
lava pile) to the known rate of lava-production in Recent
times. A basalt pile with the area of Iceland, 100000 km2,
and 3 km in thickness gives 300000 km3 as the volume of
the Icelandic basalt plateau according to Gihson’s hypothesis.
The total time of production is hardly less than 30 million
years, giving a rate of production of 100 km3 for every
10000 years in the Tertiary. The post-glacial lavas of Ice-
land, formed during the last 10000 years, cover an area of
12000 km2 with an average thickness of 20—40 m. The pro-
duction of lava during the last 10000 years is then over 300
km3, probably nearer 500 km3, three to five times greater
than the rate of production in the Tertiary as indicated hy
Gibson’s estimated thickness. We are therefore left with two
choices, either volcanicity is three to five times more vigo-
rous today than in Tertiary times or, as the present writer
is inclined to believe, Gibson’s thickness of the lava pile is
much to low. Gibson’s hypothesis demands periodic shifts of
the area of active dyke injection to the west with time, It is
hard to visualize any factors which might cause such shifts.
Finally, Einarsson’s criticism of the hypothesis put forward by
Bödvarsson and Walker, especially as regards dip and gravity
structure, is equally applicable to Gibson’s hypothesis.
It is clear from the above, that a reliable interpretation
of the seismic structure (Pálmason 1963) is not at hand but
greatly needed for our understanding of the structure of the
basalt plateau. Einarsson (1965) pointed out, that the atti-
tude of the seismic layers bears no relation to the dip of the
lavas, but is quasi-horizontal like the zeolite zones. He sug-
gests, that the seismic layers are a reflection of the degree
of alteration of the basalt pile. Recent seismic work on Snæ-
fellsnes (G. Pálmason, personal communication) indicates
that layer 3 is close to the surface in Fródárhreppur