Somewhere roughly in the middle of the Indian Ocean is the deepest dent in Earth’s gravitational field – the place where Earth’s gravitational pull is the weakest. That’s because there is less mass under that spot on our planet – but why?
Many possible explanations have been put forward, but proving any of the theories has turned out to be tricky. Now Debanjan Pal and Attreyee Ghosh, of the Centre for Earth Sciences at the Indian Institute of Science in Bengaluru, India, think they have the answer. They reconstructed the last 140m years of plate tectonic movements and the stirring in the underlying mantle that accompanied the rearrangement of Earth’s crustal jigsaw.
Their results, published in Geophysical Research Letters, show that ancient chunks of oceanic plate have sunk through the mantle underneath the African continent, causing strong downwelling in the mantle there. To counteract this downwelling, plumes of hot and less dense mantle are upwelling underneath the nearby Indian Ocean. Their model also shows that the oceanic slabs reached the lower mantle about 30m years ago, and that that neighbouring upwelling plume under the Indian Ocean was established about 20m years ago. Roll forward another few tens of millions of years and this upwelling plume will have moved on, but, right now, this is the lightest spot on Earth.
