Gravity Investigations on the Leeward Islands 
of the Hawaiian Ridge and Johnston Island 1 
Loren W. Kroenke and George P. Woollard 
Through the courtesy of the U. S. Coast 
Guard, who allowed the senior author to ac- 
company the U. S. C. G. Cutter "Plaintree” 
during a base resupply cruise in 1964, it was 
possible to visit many of the small islands and 
atolls along the Hawaiian Ridge extending from 
Nihoa to Midway, as well as Johnston L, to 
make gravity observations. In all, 133 gravity 
stations were established on Nihoa, Laysan, 
Lisianski, Pearl and Hermes Reef, and Midway, 
and twenty-three stations were established on 
Johnston I. All the leeward islands in the east- 
ern half of the Hawaiian chain consist of vol- 
canic peaks. On the chain’s western end, most 
of the peaks are capped by coral reefs to form 
atolls. Although single gravity observations had 
been made on many of these islands by the Air 
Force Air Photographic and Charting Service, 
and some were studied as early as 1948 (Wool- 
lard, 1950), the present series of measurements 
are the first to give a sufficient number of 
observations for a realistic appraisal of the 
anomaly field, in terms of both the absolute 
Bouguer anomaly values and the local gravity 
gradient. 
All observed gravity values were referred to 
the absolute gravity base at the Hawaii Insti- 
tute of Geophysics established by Woollard ( un- 
published), and were adjusted for instrumental 
drift through Woollard s airport gravity bases 
at Midway I. and Johnston I. (Woollard and 
Rose, 1963 ) . The observations were made with 
a low drift-rate Worden gravimeter having a 
range of 2000 mgal. The reliability of the 
values is not uniform and may be no better 
than ± 2 mgal in the observed gravity values 
for the central area representing the area of 
poorest control. Elevations except for those for 
Nihoa were estimated in most cases by visual 
observation of sea level and are believed ac- 
curate to within ±2 ft. Elevations for Nihoa 
1 Hawaii Institute of Geophysics Contribution No. 
96. 
were estimated from an elevation contour map 
with 100-ft contour intervals and could be in 
error as much as 30 ft. Bouguer anomalies 
were computed using a density of 2.3 gm/cc 
in the reductions. The table of principal facts 
for all stations is reported elsewhere (Hawaii 
Inst. Geoph., 1965, Table 9). 
HAWAIIAN ARCHIPELAGO OBSERVATIONS 
The bathymetry surrounding each of the 
islands is similar, with the land surface falling 
off rapidly to the northeast and southwest and 
decreasing over submerged shelf areas elongated 
parallel to the island chain. In general, the is- 
lands have a low elevation and are elongated 
parallel to the direction of the main oceanic 
current systems. The exception to this generali- 
zation is Nihoa I., which has a maximum ele- 
vation of 895 ft and a north shore formed by 
perpendicular cliffs more than 800 ft high. 
Based on the observed shapes of the gravity 
anomalies associated with the volcanic centers 
on the larger islands at the southeast end of 
the chain, and the observed magnitudes and 
gradients observed on these smaller islands, esti- 
mates of the location of the center of the gravity 
highs and the maximum Bouguer anomaly val- 
ues have been made. 
Nihoa 
This island covers an area of 156 acres. 
Rough seas and a landing site rimmed by sea- 
cliffs made landings from a small boat hazard- 
ous as well as difficult. The rocks are princi- 
pally olivine basalts occurring as flows or dikes. 
Because the terrain effect is appreciable and dif- 
ficult to estimate for stations located along the 
ridge top near the cliff edge, and as the eleva- 
tion and meter drift control were weak here, 
the anomalies may have no better than =j=5 
mgal accuracy. The lowest Bouguer anomaly 
value of +245 mgal (Fig. 1) is found at the 
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