Rit (Vísindafélag Íslendinga) - 01.06.1971, Side 120

120
GUÐMUNDUR PÁLMASON
tion of acid volcanic rocks in Iceland it may be expected that this
applies roughly to other parts of Iceland also.
In the Icelandic refraction results the low and varying velocity in
layer 0 is douhtless due mainly to a relatively large porosity. This
layer is very inhomogeneous as far as seismic velocities are concerned
and it has in some of the more active volcanic areas heen found
to have a downwards increasing velocity. The measured velocities
range from about 2.1 km/sec to about 3.4 km/sec. An average value
would be close to 2.8 km/sec. The lowest velocities are found in the
surface volcanic products in the active volcanic zone, e.g. at Reykja-
nes, Thingvellir, Torfajökull, Mývatn and Kelduhverfi. This layer is
traced out to practically zero thickness in the Tertiary areas to the
east and west of the volcanic zone, and it therefore appears likely
that it represents largely a stratigraphic boundary characterized by
relatively porous formations above and denser, less porous formations,
below. The volume of layer 0 has earlier (sect. 11.7) heen shown to
be approximately what would be expected for a period of 106 years
at the present rate of production of volcanic rocks in Iceland.
The interpretation of the boundary ’between layers 1 and 2 is more
uncertain. There is little doubt that both layers are essentially flood
basalts. The boundary is nearly horizontal and shows no marked
relationship to the dips of the surface basalts. In southeastern Ice-
land the lower layer is found at tlie surface (Pálmason, 1963) and
consists here of a mixture of flood basalts and acid and basic intru-
sions. Perhaps the 1—2 boundary is related to zeolite zones as sug-
gested by Einarsson (1965).
Refraction measurements in the Faeroe Islands (Pálmason, 1965)
have shown that the Tertiary basalts there are also divided into two
layers, with P-wave velocities of about 3.9 and 5.0 km/sec. The
boundary appears to correlate with the stratigraphic division of the
basalt pile into three formations. The lower velocity is obtained in
the uppermost formation, while the higher velocity is found in the
two lower formations. Noe-Nygaard and Rasmussen (1968) have
published a petrological study of the Faeroe basalts and conclude that
the uppermost series, corresponding to seismic layer 1, consists of
ohvine tholeiitic lavas, while the two lower series, corresponding to
the seismic layer 2, consist of aphyric (lower) and porphyritic (up-
per) quartz tholeiitic series. Density measurements (Saxov and
Abrahamsen, 1966) have given average values of 2.90, 2.82 and 2.87
g/cm3 for the lower, middle and upper stratigraphic series, respec-