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The connection between gravity and oceanic water depth was realized more than a century ago. In 1859, J. H. Pratt derived a formula relating anomalous vertical deflection and sea level height on the coast of India to the depth of the adjacent Indian Ocean. Because the predicted deflections (5′–20′ for a mean depth of 4.8 km) were not measurable, Pratt later (in 1871) inferred an excess of mass below the ocean basins, thus prefiguring the concept of isostasy in marine geology. With the accurate measurement of marine gravity on submarines by F. A. Vening Meinesz in the 1920s and 1930s, isostasy was verified and later [e.g., Watts and Ribe, 1984] related to the thickness and flexural rigidity of mechanical lithosphere.