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| Data Series 135: Iowa Isostatic Gravity Anomaly Map |
Northeast Illumination
The colors on this isostatic residual gravity (Hill and others, 1995) 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 30 km (18.65 mi), a crustal density of 2.67 gm/cc, and a density contrast
between the crust and upper mantle of 0.40 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, and so
forth. Positive anomalies (red colors) delineate rocks more dense than the
Bouguer reduction density of 2.67 gm/cc, whereas negative anomalies (blue colors)
result from rocks of lower density.
The grid for this map is available from the data directory as grviso.grd
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