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Why can salt domes produce negative gravity anomalies?

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Diagram showing the deflection of the vertical from the geoid to the ellipsoid or spheroid, with labeled lines for geoid, spheroid, rock surface, ocean, and geoid height in a geology context. — geoid Deflection of the vertical from the geoid to the spheroid.

A gravity anomaly represents a difference between the locally observed gravitational force and the theoretically calculated value, often caused by variations in the density of underlying rock formations.1

Salt domes typically produce negative gravity anomalies because the salt core is significantly less dense than the surrounding sedimentary rock strata. As sedimentary rocks are buried deeper, they undergo compaction and cementation, which increases their density. In contrast, the rock salt within a dome maintains a relatively low density—typically ranging from 2.10 to 2.20 g/cm³—regardless of the depth to which it is buried.23

Because the salt core is less dense than the host rock it has pierced, it exerts a weaker gravitational pull than the surrounding material, resulting in a measurable decrease in gravity at the surface. Detecting these negative anomalies is a vital technique in petroleum exploration, as salt domes often create structural traps for oil and gas.42