Rit (Vísindafélag Íslendinga) - 01.06.1971, Page 116

116
GUÐMUNDUR PÁLMASON
of the crest of the Reykjanes Ridge with its strong, linear magnetic
anomaly.
The relationship hetween the Snaefellsnes volcanic zone and the
associated seismic structure is not quite clear. The structure deduced
from Ráth’s profile WP (cf. Fig. 41) indicates that an appreciable
change in depth to layer 4 may be associated with it, and perhaps
continue in an east-west direction across central Iceland. More data
are, however, needed to confirm this. Near the Snaefellsjökull vol-
cano the seismic data indicate increased depth to hoth layers 3 and 4
in a westward direction. This needs, however, also to be studied
further by offshore profiles.
In northeastern Iceland the volcanic zone appears to be associated
with a weak synclinal form of the 2-3 boundary. No refraction data
are here available yet on the 3—4' boundary, and it is therefore not
possible to say whether the structure associated with the eastem vol-
canic zone in southern Iceland continues into northeastem Iceland.
The low-velocity surface layer (layer 0), which forms the surface
rocks in the volcanic belt of Iceland, has been interpreted as Quater-
nary volcanic rocks, formed after the outpouring of the main bulk
of the Tertiary plateau basalts. It is of interest to compare the volume
of this layer as estimated from the seismic data, with independent 1
estimates on the basis of the present rate of production of volcanic
material from active volcanoes.
The total volume of layer 0 is obtained hy estimating its average
thickness within approximately rectangular areas, 10 longitude times
0.5° latitude. The total volume over the present land area of Iceland
is then found to be about 2 X 104 km3. If the Quatemary period is
taken as 106 years, an average production of 0.02 km3/yr is obtained,
or for the last 10,000 years 200 km3. Estimates of this volume on the
basis of geological ohservations have been given as 250 km3 (Rödvars-
son and Walker, 1964) and 400 km3 (Thórarinsson, 1967). The
latter estimate is for 11,000 years. The seismically determined volrnne
of layer 0 is thus in a fairly good agreement with the expected volume
of volcanic products during 106 years, with the same average rate of
production as during the last 10,000 years.
The fact that the seismic boundary between layers 0 and 1 is very
distinct and can be followed more or less continuously across the vol-
canic belt between the Tertiary basalt areas on both sides, suggests
that, prior to the production of layer 0, there was a relative quiescence