Dents in Earth's gravitational field due to plumes - physics-math

Ancient pieces of continental crust that are falling to the bottom of the Earth's mantle could explain mysterious dents in our planet's gravitational field.

Sail towards the centre of the Indian Ocean and you will find yourself losing weight because the Earth's gravitational field is weaker in this region. Similar dents in field strength are found in the north-east Pacific Ocean and the Ross Sea.

These weaknesses are believed to be created by "slab graveyards" – ancient pieces of crust and sediment that were pushed down into the Earth when plates collided and are now falling through the mantle. The slabs are denser than the surrounding mantle, so they have a stronger gravitational pull. As they fall, however, their effect on the gravitational field at the Earth's surface decreases.

But surrounding these areas are even weaker, unexplained dents in the field. Now Sonja Spasojevic from the California Institute of Technology in Pasadena and colleagues say this could be because the movement of the slabs through the mantle forces plumes of less dense material to rise towards the surface. The distribution of the unexplained weak spots simply reflects the pattern of these plumes, they say.

However, there are other ways the slabs may be creating density changes in the mantle, including chemical reactions that may occur between the slab and the mantle. "The new explanation is plausible but there are other possibilities too," says Norm Sleep a geophysicist at Stanford University in California.

Journal reference: Nature Geoscience, DOI: 10.1038/ngeo855