Rit (Vísindafélag Íslendinga) - 01.06.1971, Side 127

CRUSTAL STRUCTURE OF ICELAND
127
Fig. 47. Seismic boundaries and possible temperature-depth relations in the crust
beneath lceland.
is peridotite from the upper mantle that is undergoing partial melting
and producing a basaltic magma. Vogt et al. (1969) also suggested
that the base of the oceanic layer under the ridge crests is at the melt-
ing range of basalts for relatively fast spreading ridges. The above
mentioned hypotheses assume that mantle convection is in operation
with its rising limbs under the crestal zones of the mid-ocean ridges.
As can be seen in Fig. 47 the inferred temperature conditions in
the crust in Iceland are consistent with the hypothesis that the 3-4
boundary is close to the melting range for basalts, at least in SW-
and W-Iceland. In N- and E-Iceland this boundary is prohably some-
what below the melting range. As pointed out hy Oxburgh and Tur-
cotte (1968) and others, a partial melting in layer 4 would affect
the ratio between longitudinal and transversal wave velocity. De-
tailed studies of this ratio might thus provide significant data for the
interpretation of the 3-4 houndary.
It is of some interest to compare the seismic structure of Iceland
with similar data that have been obtained in Hawaii (Hill, 1969).
The crust and upper mantle differ considerably in the two areas, but
both are characterized by basaltic volcanism and the building up of
a huge pile of basaltic material. The upper few km of the crust in
Hawaii have P-wave velocities corresponding to layers 0, 1 and 2 in
Iceland. A distinct layer with velocity 6.5 km/sec does not appear to