278 
to root ratio of 1:6 for conditions of hydro- 
static equilibrium. 
3. The changes in crustal root for changes in 
surface elevation based on the above ratio can 
be reconciled closely with gravity data for 
changes in surface elevation, assuming isostatic 
conditions. 
4. Tholeiitic basalts appear to be derived 
directly from mantle material which rises with 
possible differentiation but little change in 
physical properties to within a few thousand 
feet of the surface in primary volcanic pipes 
on many oceanic islands. 
3. Tholeiitic basalts predominate over alkalic 
basalts by a factor of 1000:1. 
6. The volcanic history of the Hawaiian Is- 
lands suggests a progressive differentiation of 
flow material from tholeiitic basalt to trachyte 
and andesite with time. 
7. Mantle-like material is trapped at shallow 
depth in all primary pipes along the Hawaiian 
Swell and also in Samoa. 
8. Presumably the mineralogy of this trapped 
mantle-like material is not the same as that 
existing in the mantle, since it recrystallized 
under low pressure and temperature conditions 
and lost certain constituents through eruption 
and gaseous dissemination into the surround- 
ing rocks. 
9. All oceanic islands do not appear to be 
characterized by tholeiitic basalts, and there is 
no evidence on many islands for primary pipes 
containing mantle-like material; the pipes have 
either a lower density rock filling, or there is 
no density contrast with the surrounding flow 
material. 
10. A firm case cannot be made for varia- 
tions in mantle material from geologic data, 
although geophysical data suggest that such is 
the case. 
11. A dunitic mantle could provide an ade- 
quate source material for the oceanic crust if 
the basaltic upper layer either has a porosity 
of about 21% or contains abundant glass. 
HAWAII AS A SITE FOR THE MOHO HOLE 
Arguments for drilling to the Moho are 
many, and range from determining whether it 
is similar to chondritic meteorites to obtaining 
PACIFIC SCIENCE, Vol. XIX, July 1963 
a better understanding of the isostatic mech- j 
anism. It is not germane to the present report j! 
to review these arguments or to discuss their 
validity. Our purpose is to review the scientific l 
arguments for locating this operation in the 
Hawaiian area rather than elsewhere. It goes 
without saying that practical considerations, 
such as the depth of crust to be drilled, the 
depth of water, distance from a supply base, 
logistic support, local weather and sea condi- 
tions, labor supply, and other factors affecting 
costs, and the chances of a successful operation 
will play a role in deciding the drilling site. 
Even if all these factors were not equal between 
two potential sites, there would be other fac- 
tors of a scientific nature that might well jus- 
tify the selection of one site over another. The 
writer believes that these additional scientific 
benefits in the Hawaiian area make it the logi- 
cal site for the proposed hole to the' mantle. 
Some of these have been touched on in the 
previous discussion. However, before taking 
these up, the practical considerations will be 
reviewed briefly. 
Depth of the mantle 
The seismic crustal measurements in the vi- 
cinity of Hawaii by Shor and Pollard (1964) 
show that about 125 miles north of Maui the 
mantle has a subnormal depth of about 9 km 
below sea level. This is about 4 km less than 
the normal depth of about 13 km encountered 
in the Pacific Ocean for the depth of water 
(2380 fm), and 3 km less than that commonly 
encountered in the Atlantic Ocean. As this depth 
to the mantle has been verified by subsequent 
seismic refraction measurements made by West- 
ern Geophysical Company, it is probably realis- 
tic. The question naturally arises as to why the 
mantle should lie at a subnormal depth here and 
whether there is some associated factor that 
will mitigate against normal mantle material 
being present. From Figure 1, which shows the 
bathymetry in the area, it is seen that the NSF- 
recommended site (marked by a cross) lies on 
the south slope of the Hawaiian Arch and 
about 50 miles north of the burled extension 
of the Molokai Fracture zone near where it 
disappears beneath the North Hawaiian Trench. 
There is nothing in the bathymetry, therefore, 
