Ultrasonic Velocities and Related Elastic Properties 
of Hawaiian Basaltic Rocks 1 
Murli H. Manghnani and George P. Woollard 
Laboratory measurements of the compres- 
sional-wave and shear- wave velocities of various 
rock types give a useful basis for understanding 
the geological significance of observed variations 
in seismic-wave velocities associated with the 
crust and upper mantle (Birch, 1961, 1964). 
The rocks exposed at the surface in Hawaii 
provide excellent material for testing the 
velocity-density relationships and the aniso- 
tropic effects in rocks believed to form the 
crust and the upper mantle under the ocean 
floor. These rocks exhibit wide ranges of poros- 
ity, density, and mineralogical composition, and 
their physical properties represent a number of 
emplacement environments. 
The Hawaii Institute of Geophysics program 
for measuring the elastic properties of the Ha- 
waiian rocks is confined as yet to normal pres- 
sure and temperature conditions, but it is 
planned to expand the program to include 
studies under high pressure and temperature 
conditions to match those at depth down to 
the level of the lower crust and the upper 
mantle. 
The following paragraphs present the results 
of measurements of density and ultrasonic 
velocities for some selected Hawaiian rocks 
which are significant in the interpretation of 
the crustal and upper mantle structure from 
seismic and gravity observations. 
METHOD OF MEASUREMENT 
The pulse technique of measuring compres- 
sional-wave velocities described by Birch (I960) 
was the one employed in the present investiga- 
tion. Shear-wave velocities were measured using 
the technique described by Jamieson and Hos- 
kins (1963). Their method involves the use of 
P-wave transducers, the conversion of the P- 
wave pulse into an S-wave pulse through a 
1 Hawaii Institute of Geophysics Contribution No. 
88 . 
pyrex glass prism, transmittance of the S-wave 
through the sample, and reconversion of the S- 
wave into a P-wave through a similar prism. 
Barium titanate transducers were employed in 
making all measurements. In addition to cores, 
samples cut into cuboids of suitable dimensions 
(2X2X2 inches ) were used for measuring 
velocities in three mutually perpendicular di- 
rections. 
RESULTS AND DISCUSSION 
The elastic properties of some of the Ha- 
waiian basaltic rocks that were studied are given 
in Table 1. The rocks represented are tholeiitic 
and alkaline olivine basalts, trachyte, amphibo- 
lite, ankaramite, dunite, and eclogite. Basalts 
represent the largest group of rocks studied. The 
maximum compressional-wave velocity (7.0 
km/sec) under surface conditions is found in 
an amphibolitic intrusive dike rock having a 
density of 3.0 gm/cc. This rock is found in the 
Koolau caldera. Another specimen (eclogite), 
also from the Koolau caldera, has a rather low 
density (2.81 gm/cc) and velocity (6.2 km/sec). 
In hand specimen the eclogite appears to be 
porous ( > 5 % ) , and under the microscope it 
shows some alteration zones formed as a result 
of reaction with the magma during its eruption. 
Without this contamination the eclogite would 
have had a density and velocity equivalent to 
that found for the amphibolitic rock of the 
Koolau caldera. The significance of high density 
(3.0 gm/cc or higher) and high velocity (7.0 
km/sec or higher) values is evident in that 
these materials truly represent, in terms of seis- 
mic velocities, the constituents of mantle-like 
material in this caldera. An ultrabasic rock with 
a surface density of 3-0 gm/cc and velocity of 
7.0 km/sec, when placed in the upper mantle 
environment under the oceans (approximately 
a depth of 10-12 km), would have a velocity 
of 7. 5-7.8 km/sec. 
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