Low points in the geoid ― the shape the Earth's surface would take if it was entirely covered by motionless water ― arise from anomalies in the Earth's mantle where ancient tectonic plates have sunk, concludes a study published online this week in Nature Geoscience. The findings explain the low points in the geoid that are found around the Pacific Ocean, including troughs in the Indian Ocean, the Ross Sea off Antarctica, northeast Pacific and west Atlantic Oceans.

Sonja Spasojevic and colleagues used a model of the dynamics of Earth's mantle to identify anomalies in mantle velocities associated with the geoid low points. They attribute these unusual velocities to the presence of ancient tectonic plates in the deep mantle, where they form 'slab graveyards', and to the effect of their chemical make-up on the overlying mid-to-upper mantle.