Gravity Relations in American Samoa and the Society Islands'

BECAUSE OCEANIC ISLANDS in general have an underlying volcanic platform which can be expected to include a primary feeder pipe containing 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 presumably having a filling of denser material. Certainly the early study of Oahu by Woollard ( 1951 ) shows that the pipe contribution can amount to more than +100 mgal. To satisfy such a high anomaly requires a density contrast of about 0.5 gm/ cc. As there is a geologic restriction on the density of both the platform mater ial 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 exceeds +300 mgal. 1£ 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 distribution 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 (1964a, 1964b), 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