Compressional Wave Velocities in Rocks — Christensen 
43 
kb for the 9 cores of basalt is 6.34 km/sec. This 
is 0.52 km/sec lower than the mean reported by 
Birch (1961) for 15 specimens of diabase and 
0.87 km/sec lower than the mean for 9 speci- 
mens of gabbro. The relatively low velocities of 
the basalts are due in part to their low densities. 
This is illustrated in Figure 1, where 10 kb 
velocities have been plotted against densities 
for eclogites, gabbros, diabases, and basalts re- 
ported by Birch (i960), Kanamori and Mizu- 
tani (1965), and this paper. 
Birch (1961) has shown that compressional 
wave velocity is not a single-valued function of 
density, but also depends upon the mean atomic 
weight (m) of a rock. Birch’s straight line solu- 
tions for mean atomic weights of 21 and 22 are 
shown in Figure 1. The points in Figure 1 sug- 
gest that important chemical differences (i.e., 
different values of m) which influence elastic 
properties may be present in basic rocks. Eclo- 
gites and basalts appear to have relatively high 
values of mean atomic weight. Lower values are 
suggested for diabases and gabbros. 
Mean atomic weights calculated from chem- 
ical analyses are highest for basalts and eclogites 
and lowest for gabbros. In Table 3 values of m 
have been calculated from average chemical 
analyses for basalts, gabbros, diabases, and eclo- 
gites. Since the mean atomic weight of a rock 
is usually a measure of its iron content (Birch, 
1961), total iron contents calculated from the 
Density, gm/cm 3 
Fig. 1 . Velocity at 10 kilobars versus density for 
basic rocks. 
reported percentages of FeO, and Fe 2 0 3 are 
also given for the rocks in Table 3. 
Figure 1 and Table 3 show that Birch’s cor- 
relation of compressional wave velocity with 
density and mean atomic weight holds remark- 
ably well for basic igneous rocks. Basalt, which 
is free of glass and vesicles, falls near the line 
for m — 22. This agrees with chemical analyses 
of basalt. Laboratory measured velocities and 
chemical analyses of eclogite also suggest mean 
atomic weights near 22. Lower mean atomic 
weights and intermediate densities of gabbros 
and diabases produce compressional wave ve- 
TABLE 3 
Mean Atomic Weights of Basic Rocks 
rock 
MEAN ATOMIC 
WEIGHT 
PERCENT TOTAL 
IRON 
REFERENCE 
Norite 
21.5 
6.71 
Nockolds (1954) 
Pyroxene gabbro 
21.6 
7.33 
Nockolds (1954) 
Gabbro 
21.7 
6.83 
Clarke ( 1966) 
Olivine gabbro 
21.7 
7.70 
Nockolds (1954) 
Hornblende gabbro 
21.8 
7.99 
Nockolds (1954) 
Diabase 
21.8 
8.72 
Clarke (1966) 
Olivine diabase 
21.9 
9.03 
Clarke (1966) 
Tholeiitic olivine basalt 
21.9 
9.23 
Nockolds (1954) 
Tholeiitic basalt 
22.0 
9.05 
Nockolds (1954) 
Olivine-rich alkali basalt 
22.1 
9.74 
Nockolds (1954) 
Plateau basalt 
22.1 
10.10 
Clarke (1966) 
Alkali basalt without olivine 
22.2 
8.76 
Nockolds (1954) 
Alkali basalt 
22.2 
8.99 
Nockolds (1954) 
Eclogite 
22.2 
10.58 
Coleman et al. (1965) 
