Analysis of Gravity Field — STRANGE, WOOLLARD, and Rose 
383 
the surface to 2.9 g/cc at 1 km below sea level. 
At greater water depths the densities reached a 
maximum of about 3-0 g/cc. 
It should be noted that all the density meas- 
urements described above are on small samples 
and thus give only the density of the individual 
rock samples. In addition there are negative 
contributions to the bulk density of the island 
masses by the vugular-type porosity associated 
with lava tubes and intraflow voids. It should 
also be noted that the densities given are dry 
densities. On the basis of the densities of non- 
vesicular samples, the grain densities of Ha- 
waiian rocks should range upward from 2.9 
g/cc to more than 3.0 g/cc, depending upon 
the percentage of olivine present. If we adopt 
a reasonable whole rock grain density of 3 0 
g/cc, we can, if we assume perfect permeability, 
compute the relation between dry density and 
wet density as a function of porosity. The as- 
sumption of perfect permeability is justified, as 
it is known from ground-water studies that 
water table coincides closely with sea level in 
the Hawaiian Islands and is essentially inde- 
pendent of surface elevation. The volcanic flow 
material both above and immediately below sea 
level therefore appears to be not only porous 
but quite permeable. Under these conditions dry 
density values should be approached above sea 
level and wet density values below sea level, 
where the majority of the vesicles are filled with 
water. On the basis of the derived relation be- 
tween dry and wet density values to be expected 
(Fig. 2), there should be a discontinuous den- 
sity change of between 0.2 and 0.3 g/cc at sea 
level. This factor, which has been generally 
overlooked in past analyses of the gravity field 
DRY DENSITY 
FIG. 2. The relation of water-saturated density and 
dry density basalt as a function of porosity. 
of the Hawaiian Ridge, places some important 
restraints on the density distribution which can 
be assumed. If the bulk density of the island 
mass were 2.3 g/cc below sea level, as derived 
by Woollard (1951), then the mean density 
above sea level must be only about 2.0 g/cc,. 
and this is the density which should be used 
to reduce land gravity observations to sea level. 
On the other hand, if the mean dry density 
above sea level is 2.3 g/cc, as determined by 
Kinoshita et al. (1963), then below sea level 
the density should be about 2.55 g/cc. 
One can test the probable density above sea 
level by comparing the relation between eleva- 
tion and the Bouguer gravity anomalies in an 
area of broadly varying changes in surface ele- 
vation. Table 1 shows the effect on the maxi- 
mum Bouguer anomalies for the island of Ha- 
TABLE 1 
Relation of Bouguer Anomalies to Volcanic Peak Elevations on 
the Island of Hawaii as a Function of Density 
PEAK 
| BOUGUER ANOMALY 
DIFFERENCE 
( mgal ) 
ELEVATION 
(ft) 
<y = 2.3 g/cc 
(mgal) 
o' ■== 2.0 g/cc 
( mgal ) 
Mauna Loa 
13490 
+ 305 
+ 370 
+ 65 
Maun a Kea 
12380 
+ 301 I 
+ 360 
+ 59 
Kohala 
4026 
+ 305 
+ 324 
+ 19 
Kilauea 
3642 
+ 313 
+ 330 
+ 17 
