An Analysis of the Gravity Field Over the Hawaiian Islands 
in Terms of Crustal Structure 1 
William E. Strange, George P. Woollakd, and John C. Rose 
During the period October 1963 to Decem- 
ber 1964, some 750 gravity stations were estab- 
lished by the Hawaii Institute of Geophysics on 
islands of the Hawaiian Chain. About 600 sta- 
tions were established on the major islands of 
Oahu, Molokai, Lanai, Kahoolawe, and Maui, 
and about 136 others on many of the smaller 
islands and islets from Nihoa to Midway. In 
addition, more than 500 stations have been 
established by personnel of the U. S. Geologi- 
cal Survey on the islands of Hawaii, Molokai, 
Maui, Lanai, Kauai, and Niihau. These results 
are presented in detail elsewhere in this issue. 
A number of shipboard gravity surveys, which 
are also reported in greater detail elsewhere in 
this Issue (Rose and Belshe, p. 374) have pro- 
duced a large amount of gravity data from 
the ocean areas surrounding the major islands 
at the southeast end of the Hawaiian Ridge. By 
using the shipboard data in conjunction with 
the land data, a composite anomaly map of a 
portion of the Hawaiian Ridge between Oahu 
and Maui was prepared and is presented in 
Figure 1. 
This wealth of new gravity data, combined 
with the large increase in other forms of geo- 
logic and geophysical knowledge concerning the 
Hawaiian area, now makes possible a meaning- 
ful interpretation of the gravity data in terms of 
the gross structure of the Hawaiian Swell. We 
present in this paper a picture of the structure 
of the Hawaiian Swell which, it is believed, not 
only fits the observed gravity field but also is 
compatible with all other available geologic and 
geophysical information. 
SUMMARY ON DENSITY INFORMATION 
As is well known, it is usually possible in 
interpreting gravity data to construct a number 
1 Hawaii Institute of Geophysics Contribution No. 
99. 
of different mass distribution models, all of 
which can equally well account for the observed 
gravity field. A meaningful gravity interpreta- 
tion must define a model which not only will 
satisfy the observed gravity field, but will also 
be compatible with known densities and avail- 
able geologic and seismic information on struc- 
tural variations at depth. Therefore, before de- 
scribing the mass distribution model used to 
explain the observed gravity field of the Ha- 
waiian Islands, the data which were considered 
in establishing the density values will be dis- 
cussed. 
Direct measurements of densities of rock of 
the Hawaiian Islands began with the work of 
Washington ( 1917 ) , which was summarized by 
Woollard (1951). Goranson (1928) quoted a 
measurement by E. S. Shepherd on a typical 
block of pahoehoe having a density of 2.0 g/cc. 
Kinoshita et al. (1963) report that the dry 
density of 63 samples from the denser part of 
flows on the island of Hawaii ranged from 1.8 
to 3.0 g/cc and averaged 2.3 g/cc. Measure- 
ments on flows for the island of Oahu carried 
out at the Hawaii Institute of Geophysics gave 
dry densities varying between 2.3 and 2.9 g/cc. 
Some dense olivine basalts from the island of 
Hawaii have densities lying between 2.8 and 3.1 
g/cc. An amphibolite from the Koolau caldera 
on Oahu gave a density of 3.0 g/cc, while a 
weathered eclogite had a density of 2.8 g/cc. 
Manghnani and Woollard (p. 291 in this issue) 
found that most of the cores from the solidifying 
materials on the lava lake in Alae Crater, Ha- 
waii, have densities of 2. 5-2.8 g/cc. James 
Moore (unpublished) of the U. S. Geological 
Survey sampled lavas along the rift zones off the 
coast of the island of Hawaii. He found that the 
vesicle space and the size of vesicles decreased 
with water depth until at about 1.0 km below 
sea level there were essentially no vesicles. The 
change in density noted was from 2.2 g/cc at 
381 
