294 
DENSITY gm/cc 
Fig. 3= Velocity versus density in cores from Alae 
Crater, Hawaii. 
lem would be to drill through lava material and 
obtain the cores to examine vesicularity, density, 
chemical composition, and other physical prop- 
erties. Figure 3 shows the physical measure- 
ments on some drill cores obtained from Alae 
Crater, island of Hawaii, by the U. S. Geologi- 
cal Survey Volcano Observatory scientists. Al- 
though there is a direct relationship between 
the velocity and density of these cores there is 
significant variation in the velocities of cores 
(Fig. 3, A-9-15, A-10-16, A-10-13 , A-9-12, A- 
11-J , A-7-12 1 etc.) with little or no change in 
density (2.6-2 .7 gm/cc). The chemical compo- 
sition of these cores is about the same (Dallas 
Peck, personal communication), but there is a 
large variation in the glass content in the rock 
material. Because the modulus of rigidity of 
glass is much lower than the average value of 
equivalent crystalline rock material, the veloc- 
ity of propagation of elastic waves will be low- 
ered as the percentage of glass increases. The 
relationship between velocity and glass content 
for these cores is shown in Figure 4. The low 
velocities of the Pacific Ocean upper crustal 
layer (3. 8-4.2 km/sec) reported by Raitt 
(1956), and by Shor and Pollard (1964), and 
verified by the unpublished studies by Western 
Geophysical Company, suggest that the glass 
content may be influencing the seismic velocity 
to a large extent. Certainly, the work by J. 
Moore of the U. S. Geological Survey (per- 
sonal communication) indicates essentially 
zero porosity for basalts extruded on the ocean 
floor. Quick chilling could produce interstitial 
glass, however. 
PACIFIC SCIENCE, Vol. XIX, July 1965 
Fig. 4. Velocity versus glass content in cores from 
Alae Crater, Hawaii. 
ANISOTROPY 
There is a significant degree of anisotropy 
in the transmission of compressional waves in 
Hawaiian rocks. This is shown in Table 1 and 
Figure 1, where velocities obtained in three 
perpendicular directions through the same sam- 
ple are recorded. In some rock types there is 
as much as 8—10% difference between the 
maximum and minimum velocities observed. 
Although anisotropism in vesicular lavas is re- 
lated to dimensional orientation of the vesicles, 
in whole rock material it is related principally 
to mineral orientation. Both olivine and plagio- 
clase feldspars are of special interest in the in- 
terpretation of seismic refraction data. This por- 
tion of the study now underway at the Institute 
of Geophysics is still in its initial stage. 
SUMMARY 
1. Density, rock structure, porosity, permea- 
bility, and glass content control the seismic 
velocities of the vesicular basaltic lavas. 
2. Density, glass content, and mineralogical 
composition of the non-vesicular flows and in- 
trusive? control their seismic velocities and re- 
lated elastic properties. 
3. The alkalic basalts (eg.., trachytlc type) 
have a low velocity of transmission as compared 
with the tholeiitic olivine basalts. 
4. The average value of Poissons ratio for 
Hawaiian basalts is 0.29. 
