Hawaii as a Site for the Moho Hole — Woollard 
273 
as is indicated by both gravity data and its 
crustal thickness of 43 km for a surface elevation 
of 900 m, Durango has a subnormal gravity field 
(-25 mgal isostatic anomaly) and subnormal 
crustal thickness (43.4 km) for its surface ele- 
vation of 2200 m. The same anomalous rela- 
tions are noted in the middle Rocky Mountain 
region, where the seismic measurement para- 
lelling the Continental Divide actually shows 
a thinner crust than is found in the adjacent 
High Plains area of Wyoming. This anomalous 
change in crustal thickness with surface eleva- 
tion here is also substantiated by phase veloc- 
ity dispersion studies of the crust (Meyer, 
Steinhart, and Woollard, 1958; Ewing and 
Press, 1959; Steinhart and Meyer, 1961; and 
Woollard, 1962). As both the plateau of Mex- 
ico and the Rocky Mountain area have been 
subject to eustatic rise since Miocene time, it 
appears that this uplift has been the result of 
crustal expansion without any significant deep- 
ening of the crust-mantle interface, and with- 
out any observable decrease in the velocity of 
the mantle such as is apparent in the Basin and 
Range area from regional seismic transmission 
studies. These relations all suggest reversible 
exchanges of mantle and crustal material with 
an appreciable difference in volume and density 
between the two phases and, as indicated, 
would explain eustatic uplift and also crustal 
subsidence such as is noted in the Gulf Coastal 
Plain without any observable warping of the 
crust-mantle interface. The tectonic implica- 
tions of the Kennedy and Hess hypotheses, 
however, are different. One would have changes 
in surface load governing the thickness of the 
crust and the depth to the crust-mantle inter- 
face; and the other has movement of surface 
load responding to changes in surface elevation 
created by changes in crustal thickness as a 
result of hydration or dehydration effects at 
depth. 
Summary 
The geophysical evidence relating to the 
mantle can be summarized as follows: 
1. The mantle does appear to vary in its 
physical properties on a regional basis. 
2. There is evidence for variations in the 
structure and composition of the upper mantle 
with depth. 
3. The mantle in many areas is characterized 
by anisotropic seismic transmission. 
4. The physical properties of the mantle are 
influenced by anomalous heat flow. 
5. There is evidence that the physical prop- 
erties of the basal layer of the crust are related 
to those of the underlying mantle. 
6. There is evidence that the basal layer of 
the crust and mantle might represent different 
phases of the same material which is controlled 
either by static pressure and temperature condi- 
tions at depth or the addition or subtraction of 
water at depth. Either mechanism could result 
in an increase or decrease in the volume of 
crustal material with consequent crustal uplift 
or subsidence. 
7. Gravity data in conjunction with seismic 
data suggest that isostasy is a real phenomenon 
for all crustal blocks having a radius of 100 km 
or more, and that apparent regional departures 
from isostasy are related to the mean density of 
the crust rather than to any actual departure 
from hydrostatic equilibrium between the crust 
and mantle. The sign of the anomalies appears 
to be due to the proximity effect, whereby the 
effect of the near-surface mass distribution pre- 
dominates over that of the deep-lying mass dis- 
tribution providing isostatic compensation. 
8. A process whereby the crust-mantle inter- 
face and changes in surface elevation, mass 
transfer, and tectonic processes involving 
changes in crustal mass and volume are auto- 
matically accommodated through a reversible 
crust-mantle transformation would resolve the 
problem on how isostasy is maintained. Al- 
though the resulting mass distribution does not 
allow for the gravitational effect of sub-mantle 
variations in mass, the gravity contribution from 
such deep-seated mass distributions may be nil. 
As yet there is no evidence that velocity values 
greater than 8.5 km/sec indicate any increase in 
density. 
THE MANTLE AS A GEOLOGICAL ENTITY 
The geological identification of the mantle is 
dependent upon laboratory studies of the physi- 
cal and chemical properties of rocks. Not only 
