A gravity reconnaissance of the island of Oahu

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Abstract
A network of 30 gravimeter stations was established on the island of Oahu in the fall of 1948. A study of these gravity data in conjunction with those of the U. S. Coast and Geodetic Survey on the island and the observations of Vening Meinesz in the adjacent deep‐sea areas indicates the following: (1) The mean specific gravity of the island block down to the ocean floor is 2.3. (2) The regional Bouguer anomaly values corrected for the mass differential of the sea water and island lavas at the sea stations show an anomaly of −75 mgals on the island. (3) Of this deficiency in attraction, part may be related to a thinning of the crystalline crustal layer because of the withdrawal of magmatic material at depth to furnish the lavas and a possible magma chamber at depth. (4) The balance of the anomaly presumably is related to surface subsidence whereby lavas and sediments of 2.3 specific gravity are brought into juxtaposition with oceanic crustal material of 2.95 specific gravity. (5) Regional subsidence of the island to date on the above basis has amounted to between 8,000 and 10,600 ft. (6) To achieve isostatic equilibrium (remove the +50 mgals regional isostatic anomaly) an additional subsidence of approximately 7,000 ft will be required. (7) Local anomalies of (+) 110 mgals occur over the ancient Koolau and Waianae volcanic vents, but no appreciable anomaly is found to be associated with the more recent volcanic vents as Diamond Head, Koko, the Punch Bowl, or Salt Lake. The explanation of this phenomenon appears to lie in the difference in the relative sizes of the early and late vents and the difference in the petrology of the throat material present and its density contrast with the surrounding volcanic flow material. (8) Computations as to the depth to which a maximum density contrast of 0.6 gm/cc exists between volcanic throat material and flow material to satisfy the 110 mgals anomaly indicate subsidence to a depth of 9,000 ft below the ocean floor. This value is in general agreement with the subsidence determined from the analysis of the regional anomalies (8,000 to 10,600 ft). (9) Since it is improbable that the early lavas were vesicular, these figures on degree of subsidence probably represent minimum values. (10) None of the classical mechanisms for achieving isostatic equilibrium under the Airy and Pratt theories of isostasy appear valid for this type of geologic structure.

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