Gravity Relations in Samoa and Society Is. — M achesky 
373 
3975 ft. Fringing and barrier reefs surround 
the island. Geologically, Moorea differs from 
Tahiti in that there is an absence of visible in- 
trusive rocks. The primary outcropping rock is 
a basaltic lava with subordinate amounts of 
nepheline-bearing trachytes and some andesites. 
The geologic position of the caldera has been 
fixed northeast of the center of the island. The 
entire area of the caldera is effectively covered 
by a mantle of laterite; consequently, the exact 
nature of the volcanic plug cannot be ascer- 
tained by surface examination. 
The gravity contours ( Fig. 5 ) form a roughly 
triangular shape corresponding to the shape of 
the island. The lowest value of +195 mgal 
was found in the northwest while the highest 
value of +216 was found near the assumed 
center of the caldera. The maximum gradients 
of 11 mgal per mile, outlining probable rift 
zones, occur trending north, northwest, and 
northeast from the gravity high with the mini- 
mum gradient trending southwards. Because of 
the absence of roads, an extensive series of 
observations into the center of the caldera area 
was impossible. Projecting the gravity gradient 
southward, however, would produce a + 225- 
mgal Bouguer anomaly maximum over the 
caldera. The gravimetric center would then be 
in approximately the same position as the ge- 
ologic center described by Williams (1933). 
Inasmuch as the absolute Bouguer gravity 
anomaly values are all similar to those observed 
in Hawaii, it is probable that the island plat- 
form is composed of tholeiitic basalt, but that 
the pipe material is basic rather than ultra- 
basic in character. 
To summarize, the gravity field observed on 
Tahiti and Moorea conforms closely with the 
topography and geology of the islands. A 
steeper gravity anomaly gradient as the main 
caldera on Tahiti-Nui is approached suggests 
a value not unlike those observed in Hawaii 
(+300 mgal). Direct observations were pre- 
cluded by the mountain jungle interior. On 
Moorea observations near the caldera area were 
possible. The gradient, however, was only +11 
mgal per mile, and the maximum Bouguer 
anomaly probably does not exceed +225 mgal. 
This value is only slightly higher than the mass 
effect to be expected for the island platform. 
The pipe effect, therefore, is quite small as com- 
pared with that observed in the Hawaiian Is- 
lands and as projected on Tahiti. 
ACKNOWLEDGMENTS 
The work reported was carried out under 
nsf Grant No. GP 2256, and operations were 
greatly implemented in Samoa by Mr. Floyd 
McCoy, who was carrying out his geologic 
studies at the same time under NSF Grant No. 
gp 2196. 
REFERENCES 
Engel, A., and C. Engel. 1964a. Composition 
of basalts from the Mid-Atlantic Ridge. 
Science 144: 1330. 
1964 A Igneous rocks of the 
East Pacific Rise. Science 146:477. 
Hawaii Institute of Geophysics. 1965. 
Data from gravity surveys over the Hawaiian 
Archipelago and other Pacific islands. Ha- 
waii Inst. Geoph. Rept. 65-4, March 1965. 
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Macdonald, G. A. 1949. Hawaiian petro- 
graphic provinces. Bull. Geol. Soc. Am. 60: 
1541-1596. * 
McCoy, F. W. 1965. The geology of Ofu and 
Olosega Islands, Manu’a Group, American 
Samoa. Unpublished M.S. thesis, University 
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Stearns, H. T. 1944. Geology of the Samoan 
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Williams, H. 1933. Geology of Tahiti, Moorea, 
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