U.S. Geological Survey

South Dakota Isostatic Gravity Anomaly Map

Northeast Illumination

South Dakota Isostatic Gravity Anomaly Map

The colors on this isostatic residual gravity map reflect variations in the Earth's gravity field caused  by density variations in the rocks composing the upper part of the Earth's crust. An isostatic residual gravity grid was derived from the Bouguer gravity anomaly data by removing the gravitational effect of the compensating mass that supports topographic loads. The thickness of this compensating mass was calculated using averaged digital topography by assuming a crustal thickness for sea-level topography of 20 km, a crustal density of 2.67 gm/cc, and a density contrast between the crust and upper mantle of 0.30 gm/cc. The anomalies and patterns on the map reveal features of subsurface geology including the location of buried faults, sedimentary basins, plutons, uplifted basement rocks, etc. Positive anomalies (red colors) delineate rocks more dense than the Bouguer reduction density of 2.45 gm/cc, whereas negative anomalies (blue colors) result from rocks of lower density.

Grids for this map in various formats are available from the data directory as grviso.***

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