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How do uneven mass distributions inside Earth affect the shape of the geoid?

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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.

The geoid is an equipotential surface that represents the figure of Earth, coinciding with mean sea level over the oceans and extending as an imaginary surface through continental areas. Because Earth is not a perfectly homogeneous body, uneven mass distributions—or density anomalies—within the planet cause the geoid to deviate from a regular ellipsoid of revolution.123

These internal mass irregularities create gravity anomalies, which directly affect the geoid's shape. In regions where there is a local mass excess, the gravitational potential is altered, causing the geoid surface to warp outward to maintain a constant potential. Conversely, areas with mass deficiencies result in inward warps.14

Scientists believe these variations are largely related to mantle convection and temperature differences deep within the Earth. The correlation between these gravity anomalies and surface features, such as plate boundaries, supports the understanding that large-scale mantle circulation drives tectonics and shapes the geoid.35