Seismic Refraction Studies — Furumoto and Woollard 
319 
km/sec, was made over a plug in the caldera 
area of Koolau Volcano. The plug had intruded 
into a layer having a velocity of 4.63 km/sec. 
The rift zone measurement, showing a velocity 
of 7. 6-7.7 km/sec, was made on a profile paral- 
leling the northwest rift zone of the Koolau 
Volcano. The northwest rift zone was outlined 
by a local gravity anomaly high (+50 mgal) 
which indicates an abnormal mass distribution. 
It is not clear whether the 1. 6-1.1 km/sec 
layer which occurs at a depth of about 5.5 km 
under the rift zone is the true mantle or, more 
likely, is a differentiate of the mantle marking 
the magma chamber that furnished the Koolau 
Mountain volcanics. 
To the south of Oahu a crustal traverse, al- 
though with incomplete measurements, indi- 
cated mantle depth to be close to 21 km. If 
the same slope associated with the basal crustal 
layer’s upper surface applies to the crust-mantle 
interface, the mantle velocity is at least 8.4 
km/sec. 
It should be mentioned that the occurrence 
of intrusive bodies, such as the plug in the 
caldera, complicates refraction measurements on 
volcanic islands. The relatively small area of 
the islands, precluding long refraction spreads 
on land, imposes another complication. There- 
fore, it is not surprising that the present meas- 
urements, incomplete as they are, are the first 
to come up with values on the depth to the 
mantle. Previous studies of volcanic islands, 
such as those made by Officer et al. (1952) on 
Bermuda, by Raitt (1952) on Kwajalein and 
Bikini, and by Gaskell and Swallow (1953) on 
Funafuti and Nukufetau, have defined only the 
depth to the volcanics and, in a few cases, to 
the upper crustal structure. All failed to give 
information on crustal thickness or to define 
the seismic nature and boundaries of the vol- 
canic pipes and rift zones present. 
SUMMARY 
On the Hawaiian Arch the depth to the man- 
tle is about 10.0 km on the average and locally 
is as shallow as 9 km. On the shelf area north 
of Maui a velocity comparable to the velocity 
of the Moho is found at a depth of 5.8 km. 
Magnetic data suggest this shallow depth is re- 
lated to an intrusive. On the island of Oahu 
high velocity material of 7.7 km/sec is also 
found at a shallow depth (5 km). This is re- 
lated to a known volcanic rift zone marked by 
high gravity and magnetic values. South of 
Oahu along the axis of the Hawaiian Ridge a 
depth of 20-23 km is defined for the mantle. 
Between the Hawaiian Ridge and the Hawaiian 
Arch in the area of the Hawaiian Deep the 
depth to the mantle is about 13.0 km. The 
above relations are summarized by a composite 
section across these three features, as shown in 
Figure 3. This section shows clearly that the 
origin of the Hawaiian Deep must be related 
to crustal subsidence beneath the Hawaiian 
Ridge. This is in agreement with current theory 
on the origin of seamounts and atolls, and would 
explain the progressive change from atolls at 
one end of the Hawaiian Ridge (Midway I.) to 
the high relief (13,500 ft) associated with 
Hawaii on the other end of the Ridge. 
REFERENCES 
EATON, J. P. 1962. Crustal structure and vol- 
canism in Hawaii. In: The crust of the Pa- 
cific Basin. Am. Geoph. Union, Monogr. 6 y 
pp. 13-29. 
Gaskell, T., and J. C. Swallow. 1953. Seis- 
mic experiments in two Pacific atolls. Occa- 
sional Papers of The Challenger Soc., Vol. 3, 
13 pp. 
Officer, C. B., M. Ewing, and P. C. Wuen- 
SCHEL. 1952. Seismic refraction measure- 
ments in the Atlantic Ocean, Part IV. Ber- 
muda Rise and Nares Basin. Bull. Geol. Soc. 
Am. 63:777-808. 
Raitt, R. 1952. The 1950 seismic refraction 
studies of Bikini and Kwajalein Atolls and 
Sylvania Guyot. Scripps Institute Report 52— 
38,35 pp. 
1956. Seismic refraction studies of the 
Pacific Basin, Part I. Crustal thickness of 
Central Equatorial Pacific. Bull. Geol. Soc. 
Am. 67:1623-1640. 
Shor, G. G. I960. Crustal structure of the Ha- 
waiian Ridge near Gardner Pinnacles. Bull. 
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and D. D. POLLARD. 1964. Mohole site 
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Res. 69(8): 1627-1638. 
