Rit (Vísindafélag Íslendinga) - 01.06.1971, Síða 119

CRUSTAL STRUCTURE OF ICELAND
119
These calculations show that increasing temperature with depth
may lead to a small decrease in velocity with depth in layer 3. Other
factors, such as compositional changes, may easily outweigh the ef-
fects of temperature and pressure.
12. INTERPRETATION OF THE SEISMIC
30UNDARIES
Evidence for seismic velocities in known rock types comes mainly
from two kinds of data, laboratory and field measurements. Extensive
laboratory measurements have been made on samples of igneous and
metamorphicrocks (HuGHEsandMAURETTE, 1956,1957; Birch, 1958,
1960, 1961; Simmons, 1964a, 1964b; Christensen, 1965, 1966;
Kanamori and Mizutani, 1965; Manghnani and Woollard, 1968;
Volarovich, 1968) under the conditions of temperature and pressure
which exist in the earth’s crust and upper mantle. The velocity ap-
pears to depend primarily on pressure, temperature, porosity, density
and mineral composition. For Hawaiian basalts the elastic moduli
appear to depend strongly on porosity (Manghnani and Woollard,
1968). For most rocks the velocity increases with pressure, more
strongly at low pressures, less than 1—2 kbars, and less at higher
pressures.
Birch (1961) showed that there appears to he a systematic rela-
tionship between P-wave velocity, density and mean atomic weight.
For a constant mean atomic weight the relationship is of the form
VD = a + b • q
where a and b are constants and n is the density. Modifications have
later been proposed to take into account deviations such as are shown
by calcium-rich rocks (Simmons, 1964b).
It is not possible in general to identify crustal rock type on the
basis of velocity measurements alone, since the velocity ranges for
different rock types may overlap. Only broad conclusions can be
drawn, such as that granitic rocks have lower velocities than basaltic
rocks. Additional information is needed to interpret the seismic data
in geological terms.
The surface rocks in Iceland are largely of a basaltic composition.
Walker (1966) found that in eastern Iceland ahout 10% of the
Tertiary volcanic pile was made up of acid rocks. From the distribu-