Gravity Relations in American Samoa 
and the Society Islands 1 
L. F, Machesky 
Because oceanic islands in general have an 
underlying volcanic platform which can be ex- 
pected to include a primary feeder pipe con- 
taining recrystallized magma, the gravity field 
observed will reflect both the topographic mass 
effect of the platform rising from the sea floor 
and a contribution from the feeder pipe pre- 
sumably having a filling of denser material Cer- 
tainly the early study of Oahu by Woollard 
(1951) shows that the pipe contribution can 
amount to more than +100 rngal To satisfy 
such a high anomaly requires a density contrast 
of about 0.5 gm/cc As there is a geologic re- 
striction on the density of both the platform 
material and that filling the pipe, one is forced 
to postulate mantle-like material with a density 
of about 3.3 gm/cc near sea level in the pipes 
where such high local anomalies occur. The 
Bouguer anomaly in such pipe areas often ex- 
ceeds + 300 mgal If there were no difference 
between the density of the pipe filling and that 
of the platform, the anomaly would be only 
about +200 mgal 
The absolute value of the Bouguer anomaly 
and the local gravity gradient, therefore, have 
considerable geologic value, and presumably 
could be diagnostic in the study of the distribu- 
tion of tholeiitic basalts, such as are found in 
Hawaii, and the more alkalic basalts, such as 
are found in the Society Islands, because there 
is a significant difference in their respective 
density values. This is especially important since 
recent work, such as that of Engel and Engel 
( 1964^, 1964$), has led to the proposal that 
the major part of all oceanic island platforms 
Is composed of tholeiitic basalt with only a 
veneer of alkalic basalt on the islands. 
To test this hypothesis the writer carried out 
a series of gravity studies in American Samoa 
and the Society Islands, where the predominant 
1 Hawaii Institute of Geophysics Contribution No. 
97. 
lavas at the surface appear to be alkalic basalts, 
for comparison with the data from the Ha- 
waiian Islands. The table of principal facts is 
reported elsewhere (Hawaii Inst. Geoph., 1965, 
Table 10). 
MEASUREMENTS IN AMERICAN SAMOA 
The Samoan Islands form a chain trending 
east to west between 169° and 171° W, at 
about 14°30 / S. They are situated north and 
slightly east of the Kermadec-Tonga Trench 
and lie on the oceanic side of the Andesite 
Line as defined by Macdonald (1949). 
From west to east the islands are Tutuila, 
Ofu, Olosega, and Tau. During July and Au- 
gust 1964, 130 gravity stations were estab- 
lished on these islands, through the use of a 
Worden gravimeter No. 366. All data are based 
on the absolute gravity value, at the Hawaii 
Institute of Geophysics, of 978.9562 gal, as de- 
termined by Woollard (unpublished). 
Tutuila 
Tutuila is the largest island of the Samoan 
group and covers an area of 54 sq miles. The 
water increases uniformly to normal oceanic 
depths north and south of the island, while 
underwater ridges extend to the east and west. 
The island is surrounded by fringing coral 
reefs. Surface geologic investigations show al- 
kalic and tholeiitic basalts, with minor amounts 
of quartz trachyte and andesite. The highest 
elevation, atop one of the quartz trachyte plugs, 
is 2141 ft. A major caldera is situated in the 
center of the island and extends 6 miles east 
to west and 3 miles north to south, as deter- 
mined by Stearns (1944). Well-developed rift 
zones extend northeast and southwest from the 
caldera area. The age of the volcanics is esti- 
mated to be late Pliocene or early Pleistocene. 
A brief summary of the geologic history of 
Tutuila I. as given by Stearns (1944) follows: 
367 
