Message posted by Peter Merlin on December 08, 2008 at 10:14:49 PST:
Although gravity at the Earth’s surface is very nearly constant, it is slightly greater where dense rock formations lie close to the surface. Gravitational force, therefore, increases over the tops of anticlinal (arch-shaped) folds and decreases over the tops of salt domes. Gravity surveys can be used to measure such geological features as earthquake faults and ancient meteorite impact structures. In large, complex craters with central uplifts and/or rings, gravity anomalies may be complex as well. Rock fracturing and low-density impact melt rocks, suevites and other breccias (mixed, crushed rock) cause negative anomalies. Post-impact crater sediments may also contrast with the pre-impact target rocks. Relatively positive anomalies may indicate that rocks of higher density were uplifted during the cratering process to form the uplifts and rings. Shock lithification of porous rocks may also lead to locally increased density. Gravity measurements can be used to detect buried impact structures and trace the original size of deeply eroded craters where only relics of impactites point to an impact origin. Just such an impact event occurred some 367 million years ago near in the vicinity of Rachel and Area 51. The Alamo bolide impact occurred 367 million years ago, when one or more hypervelocity objects from space slammed into shallow marine waters at a site that is now the Devonian Guilmette Formation of the Worthington Mountains and Schell Creek Range of southeastern Nevada. More information can be found here:
http://www.lpi.usra.edu/meetings/lpsc2006/pdf/2453.pdf
In Reply to: Air Force conducted gravity surveys near 51 in early 80's posted by breeser on December 07, 2008 at 13:26:34 PST:
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