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Why is the geoid irregular instead of a smooth surface?

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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 not a smooth, regular surface because it is an equipotential surface that must remain everywhere perpendicular to the local pull of gravity. Because Earth’s internal mass is not distributed uniformly, the gravitational force varies from place to place, causing the geoid to undulate rather than form a perfect sphere or ellipsoid.123

These irregularities, known as gravity anomalies, are primarily caused by:

Internal Density Variations: Differences in density within the Earth, such as local buried-mass concentrations, create variations in gravitational attraction.12
Mantle Dynamics: Height variations in the geoid are closely linked to mantle convection and temperature differences at depth, which also correlate with plate tectonics and plate boundaries.3
Topographic Differences: The significant elevation differences between continents and seafloors contribute to the complex shape of the geoid.1

Because of these factors, the geoid can deviate by as much as 100 meters above or below a standard reference ellipsoid.3