Ultrasonic Velocities — Manghnani and Woollard 
293 
asmuch as most of the magmas connected with 
Hawaiian volcanic eruptions are believed to 
originate at the depths of earthquake foci at 
or about 60 km, the seismic velocities below 
the Moho are important from the point of view 
of the chemical composition of the upper man- 
tle. Although it is not the purpose of this report 
to discuss the pros and cons of various models 
of the upper mantle, it can be said that the 
presence of olivine-rich nodules in some of 
the Hawaiian flows, the increasing transmission 
velocity of the olivine basalts (ankaramite) 
with increasing olivine content (see also Birch, 
1961), and the seismic velocities of the layer 
below the Moho discontinuity (8. 2-9-0 km/sec) 
all indicate that the upper mantle below the 
Hawaiian Archipelago is most probably com- 
posed of material resembling dunite. The 
planned high pressure and temperature meas- 
urements of velocities of the dunites and olivine- 
rich basalts at the Hawaii Institute of Geo- 
Fig. 1. Velocity versus density relationship in Ha- 
waiian rocks. 1 and 2 , amphibolitic rocks; 3 , eclogite; 
4 , hawaiite; 5 , ankaramite; 6 , vesicular basalt; 1 , py- 
roxenite; 8 , tholeiite. 
FIG. 2. Velocity versus olivine content in basalts. 
physics should clarify some of the unsolved 
questions. 
Amphibolitic rocks (Fig. 1, 1 and 2) and 
an eclogite (Fig. 1, 3) having higher densities 
than basalt show higher velocity values, as 
stated earlier. Although the curves of Figure 1 
have been drawn on the basis of apparent 
porosity values as deduced from wet and dry 
density determinations, these are obviously not 
all true porosities and permeability is a param- 
eter that is also incorporated. The effect of this 
factor will require further study. In gross form, 
though, the velocity-density relationships shown 
for volcanic rocks are essentially- correct. As 
might be expected, the velocity in a rock is sig- 
nificantly lowered when the porosity increases. 
The dispersion in the three curves can be at- 
tributed to the differences in mineral composi- 
tion, crystallinity of the material constituting 
the rock, the size and geometry of vesicles 
present, and the undetermined effect of permea- 
bility due to microfractures. 
Horizontal variations in seismic velocities of 
the upper layers of the basaltic lavas on the 
Hawaiian Islands, as observed by the Hawaii 
Institute of Geophysics group (Furumoto and 
Woollard, p. 313 in this issue) and others, are 
real and thus signify variations not only in 
mineralogical composition and density but in 
vesicularity and permeability as well. Problems 
pertaining to the degree of vesicularity to be 
expected at depth can best be understood by 
studying the variation in seismic velocity in 
material of variable vesicularity and density. 
The ideal approach to the solution of this prob- 
