Hawaii as a Site for the Moho Hole — W OOLLARD 
277 
intermittent volcanic caldera. Seismic earth- 
quake focal depths suggest that Kilauea draws 
its magma from a depth of about 60 km and, 
as with all other major Hawaiian volcanoes, the 
late stage lava is now alkalic. 
The high gravity anomalies reported by 
Strange, Machesky, Woollard (p. 350, this is- 
sue) for Oahu and by Strange et al. (p. 381, this 
issue) for the Hawaiian Islands, as a whole, can 
only be explained by high density rock material 
having a density of about 3.1 gm/cc coming 
essentially up to the surface. Seismic refraction 
measurements in the old Koolau caldera near 
the town of Kailua on Oahu ( Adams and 
Furumoto, p. 296 in this issue) show that here 
there is material with a seismic velocity of 
about 7.0 to 7.6 km/sec at a depth of about 
4000-6000 ft. This is similar to the velocity 
of 7.6 km/sec observed for what may be the 
mantle at a depth of 6 km on a seismic refrac- 
tion line along the north shore of Oahu parallel- 
ing the Koolau Mountain Rift zone. In addition, 
deep reflections from about 12,000 ft were ob- 
tained on the west side of the Koolau caldera 
which could represent an inter-crustal magma 
chamber. On another seismic-refraction line 
paralleling the south shore of Oahu, the mantle 
has a velocity of about 9.0 km/sec at a depth 
of about 23 km. Thus, there is good evidence 
for mantle-like material at shallow depth in 
the old Koolau pipe on Oahu. However, this 
pipe, as well as all others (Malahoff and Wool- 
lard, in a forthcoming issue of Pacific Science ), 
is characterized by marked local magnetic anom- 
alies, and there is a problem as to the nature of 
the pipe material. Certainly one would not ex- 
pect dunite. 
In Samoa the gravity measurements likewise 
show a local gravity high of the same magni- 
tude and absolute value (+300 mgal Bouguer) 
over the site of the ancient caldera defined geo- 
logically, with little gravity effect over the later 
volcanic pipes. These data, therefore, suggest 
that the alkalic basalts present are the results 
of magmatic differentiation, and that they con- 
stitute a superficial cover over earlier tholeiitic 
basalt. 
Gravity observations on the Line Islands, 
however, do not bear out the above relations. 
The local anomaly values show no pipe effect 
and have absolute Bouguer values of less than 
+200 mgal. This is true on Washington, 
Christmas, Palmyra, and Fanning islands. John- 
ston Island, which lies on the extension of the 
Line Island Ridge, however, does have values 
that get up to about +270 mgal absolute Bou- 
guer. The relations along the Line Island Ridge, 
therefore, are variable, and either reflect a dif- 
ference in pipe material, or extreme variability 
in the degree of differentiation that has taken 
place in the pipe, or else marked differences in 
the size of the pipes or depth of the primary 
material. To some extent similar results are 
found in the Hawaiian Islands, as on Niihau 
there is no pronounced pipe effect, and the 
Bouguer anomalies do not exceed 290 mgal. 
In this connection, it is of interest that the 
gravity values on Bermuda, while showing a 
smaller local anomaly than on Hawaii ( +80 
mgal), are higher on an absolute scale (+355 
mgal Bouguer) (Woollard, 1954). 
By way of contrast with these results, gravity 
studies of calderas of andesitic volcanoes in 
Japan all show the pipe area to be defined by 
a pronounced gravity minimum of 25-30 mgal. 
A similar relation is observed over granite in- 
trusions on the continents, and the recent re- 
port on the AMSOC hole in Puerto Rico ( Brom- 
ery and Griscom, 1964) shows a local mini- 
mum of 20 mgal over a serpentinite body that 
was once a high density peridotite. Hess (1964) 
feels that the magma in the latter case orig- 
inated within the mantle rather than in the 
crust. Granite presumably originated within the 
crust during mountain orogeny. Andesitic mag- 
mas possibly could be generated in the mantle, 
as suggested by laboratory studies, but as yet 
there are no corroborating data other than 
earthquake foci relations. 
Summary 
The geologic data taken in conjunction with 
physical and geophysical data indicate the fol- 
lowing: 
1. The mantle, in general, appears to be sim- 
ilar to dunite, with seismic velocity of 8.15 
km/sec and a density of 3.33 gm/cc. 
2. The mean density contrast of the mantle 
and crust is 0.475 gm/cc, giving a free board 
