Why does gravitational force decrease over the top of a salt dome?

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salt dome Figure 1: Interrelationships of salt structures (see text)

Gravitational force decreases over the top of a salt dome because of the density contrast between the salt core and the surrounding rock strata.1

In geological settings, salt is significantly less dense than the consolidated sedimentary rocks—such as sandstone—that typically surround and bury it. Because gravity measurements are sensitive to local variations in subsurface density, the presence of this lower-density salt mass creates a negative gravity anomaly compared to the denser, surrounding geological environment. Essentially, the salt dome acts as a mass deficiency in the subsurface, resulting in a measurable decrease in the gravitational pull directly above it.12