314 
PACIFIC SCIENCE, VoL XIX, July 1965 
the western shore of the island of Hawaii the 
Mohorovicic discontinuity is at a depth of about 
16 km (D. Hill, 1964, personal communication). 
An interesting product of the study is the 
existence of high velocity materials at shallow 
depths in the rift zone and in the plug of the 
volcano. The value of 7.7 km/sec is close to the 
arbitrary value of 7.8 km/sec assigned to man- 
tle material. It can be conjectured that mantle 
material has risen through the plug and rift 
zone to shallow places. It can also be postu- 
lated that differentiation in magma chambers 
caused denser material with higher seismic ve- 
locity to settle out. But all these are conjectures 
which can be resolved only by analysis of actual 
samples obtained by drilling in the rift zone and 
plug, or by drilling to depths considered to be 
the normal mantle. 
CONCLUSIONS AND ACKNOWLEDGMENTS 
The seismic refraction project carried out 
over the Koolau Volcano yielded an over-all 
picture of the subsurface structure that fits well 
with gravity and magnetic data. Materials of 
unusually high velocity at shallow depths were 
found in the plug and rift zone. This may very 
well be the first time that quantitative values 
for the internal structure of a volcano have 
ever been published. 
Further investigations by seismic techniques 
are recommended, especially on the eastern 
flank and the southeast rift zone. 
The authors thank the numerous staff mem- 
bers and graduate students of the Hawaii Insti- 
tute of Geophysics who participated in carry- 
ing out the difficult field work of the project. 
The study was carried out under the auspices 
of the National Science Foundation through 
Grant GP-2257. 
REFERENCES 
Malahoff, Alexander, and G. P. Wool- 
lard. 1965. Magnetic Surveys over the Ha- 
waiian Ridge. Hawaii Inst. Geoph. Rept. 65- 
11, June, 1965. 
Stearns, H. T. 1946. Geology of the Hawaiian 
Islands. Terr. Hawaii, Div. Hydrography, 
Bull. 8, 106 pp. 
— - and K. N. Vaksvik. 1935. Geology and 
ground-water resources of the island of Oahu, 
Hawaii. Terr. Hawaii, Div. Hydrography, 
Bull. 1, 479 pp. 
Steinhart, J. S., and R. P. Meyer. 1961. Ex- 
plosion studies of continental crust. Carnegie 
Inst., Washington, Publication 622. 
Woollard, G. P. 1951. A gravity reconnais- 
sance of the island of Oahu. Trans. Am. 
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