The free air gravity anomaly and depth are sampled at 2‐km intervals along two long, reasonably straight ship tracks across the Atlantic Ocean. The resulting series are then processed as if they were time series, and filters are obtained to predict the gravity observations from the bathymetry. More than half the energy in the gravity field can be predicted by this means, and that which cannot emphasizes unusual structures beneath the sea floor. More information can be obtained by comparing the gravity and the bathymetry after both series have been Fourier‐transformed. Isostatic compensation begins when the wavelength exceeds about 100 km and increases with increasing wavelength. The results are compared with predictions from various simple models and agree best with a model in which the topography results from variations of crustal thickness and the plate thickness is little greater than 10 km when the compensation occurs. These observations can be understood if the topography results from large‐scale intrusions into the lower crust within tens of kilometers of the spreading center. Though such a model for a slowly spreading ridge differs from most of those which have been previously put forward and must be regarded with skepticism until it is supported by evidence from other sources, it appears to be compatible with the limited information now available.

The broad topographic domes, or plateaus, of East Africa and Afar are characterized by long-wavelength negative Bouguer gravity mtomalies and volcanically active rift valleys. Gravity and topography data from the East African and Afar plateaus and data from the stable cratonic regions to the west were subdivided into 17 smaller regions to study the variation of elastic plate thickness within part of the African continent, its relation to rifting processes within these intracontinental plateau regions, and compensation mechanisms for the broad uplifts. Assuming that loads at the surface, within, and beneath the base of a thin elastic plate contribute to the observed Bouguer gravity anomalies, the wavelength dependence of the coherence between gravity and topography was used to determine the effective elastic plate thickness in each of the subregions. Estimates of elastic plate thickness were found to be 64-90+ km in the stable cratonic areas, provided that surface and subsurface loads are uncorrelated. Lower estimates (43-49 km) were obtained in the largely unfaulted regions encompassing the broad uplifted plateaus and the narrower Darfur dome to the west of the Afar plateau. Estimates of 21-36 km correspond to regions that include the severely faulted and commonly volcanically active Kenya, Western, and Ethiopian rift valleys as well as unfaulted regions adjacent to the rift valleys. We attribute the smallest estimates of elastic plate thickness (21-36 km) to averaging unfaulted topography with mechanically weakened topography within the severely faulted Kenya, Western, and Ethiopian rifts. The linear transfer function between gravity and topography within the uplifted East African plateau region at wavelengths longer than 1000 km can be explained by a dynamic uplift mechanism and associated heating of the thermal lithosphere above a convecting region within the asthenosphere. These isostatic and dynamical compensation mechanisms are consistent with existing geological and geophysical data and with constraints on the timing of volcanism and uplift within the East African plateau region.

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A part of the Ninety East ridge near the equator was examined in 1971 by seismic profiling and gravity and magnetic observations. In the area examined, the topography of the ridge consists of blocklike or en echelon mountainous masses. A fracture zone trending north‐south parallel to the overall trend was found along the eastern margin of the ridge topography. This fracture zone probably marks the principal boundary between the central Indian Ocean plate and the Wharton basin plate. The free air gravity anomalies associated with the Ninety East ridge are small, and thus the mass of the ridge must in some way be compensated at depth. The Ninety East ridge may have originated as a result of emplacement of gabbro and serpentinized peridotite beneath normal oceanic crustal layers. The lower density of the gabbro and serpentinized peridotite with respect to normal mantle at equivalent depths provides for both the uplift of the ridge and its compensation at depth.

Abstract.Cravitational interaction is the weakest among the four known forces in the universe. The particular gravity equipotential field that coincides with sea level is called the geoid, and satellite data have revealed anomalies in its pattern that are puzzling to explain. INTRODUCTIONGravitational interaction is the weakest among the four known forces in the universe: electromagnetism, the strong force, the weak force, and gravity. The strong and weak forces are important at atomic and subatomic distances. Gravity, however, although weak, seemingly reaches out over great distances and has molded the observed patterns of galaxies and the arrangements and shapes of their constituents. To our normal senses it is the force that causes objects to fall to the ground at a rate that accelerates at 980 cm s -2 (-32 feet s -2) because of the attraction from the mass of our planet. The same force keeps the Moon in orbit around our Earth. The gravitational attraction between two point masses appears to be only proportional to the product of those masses (Newton's law) and inversely to the square of their distance apart. However, when one or both of these masses becomes extremely great, and/or their velocity approaches that of the speed of light, other terms are then needed to account for the properties of gravity (such as Einstein's gravitational theory). The history of our understanding of gravity parallels that of many topics of scientific inquiry. Proclamations are made; evidence is discussed, rejected, or accepted; and new ideas and modifications are presented. For the last 15 years the concept of "dynamic topography" has been the accepted explanation for why anomalies in the Earth's geoid (Plate 1) have no obvious relationship to the planet's topography, magnetic anomalies, or plate tectonics. In this review, evidence is given that dynamic topography perhaps does not play the dominant role in contributing to the gravity field that it usually is assumed to. In addition, an alternate interpretation for the deep structure of the Earth is presented. BACKGROUNDThe "logical" fact that heavy objects fall faster than lighter objects was written (---340 B.C.) by Aristotle

The utility of combining geoid, gravity, and vertical gravity gradient measurements for delineation of causative mass anomalies is explained and compared with spatial and spectral methods for depth estimation. Depth rules for various source geometries are reviewed and new rules developed for geoid, gravity, and vertical gravity‐gradient data. Both spatial and frequency‐domain methods are discussed. Simple ratios of single observations of different data types (e.g., geoid, gravity, or vertical gravity gradient) are shown to provide information comparable to the traditional spatial and frequency analyses of one data type alone.

Gravity models of oceanic trenches computed prior to the advent of plate tectonic concepts fell into two classes of solutions: (1) if a homogeneous mantle density was assumed, then the gravity models required an abnormally thin oceanic crust in the first 100 km seaward to the trench axis; or (2) if the oceanic crust was not thinned, then high‐density mantle was included beneath the Moho near the trench axis. In contrast to the gravity models, however, seismic refraction studies near trench axes and on the seaward trench slope have generally observed normal oceanic crustal thicknesses and normal mantle velocities. This apparent conflict between the refraction data and the gravity interpretations can be resolved by taking into account density anomalies in the descending lithosphere. A theoretical density model for the downgoing slab was computed from thermal and petrologic data and was then compared with the observed gravity data of Hayes (1966) and the refraction data of Fisher and Raitt (1962) over the Chile trench at 23°S. Considerations of pressure‐temperature data suggest that the oceanic crust and the lithosphere may transform to eclogite (3.55 g/cm3) and garnet peridotite (3.38 g/cm3), respectively, at depths as shallow as 30 km in the descending slab. This model predicts density anomalies of +0.08 to +0.28 g/cm3 at depths between 30 and 80 km and +0.04 to +0.024 g/cm3 between 80 and 150 km and predicts a mean density anomaly of +0.05 g/cm3 at depths between 150 and 300 km. Incorporation of these high density mantle zones into a two‐dimensional gravity model allows a gravity solution that is in much better agreement with the refraction data seaward of the Chile trench than earlier gravity models, which assumed a homogeneous mantle.