In the Indian Ocean, there is an anomaly. The gravity there is lower (compared to what we'd expect) than anywhere else on the planet, and now, a team of researchers may have figured out why.
According to Newton's law of universal gravitation, objects attract other objects with a force determined by their masses and the square of the distance between them. In simple terms, the more mass the more gravity, the closer to the object the more you will be affected by (and affect) it.
Though you may think of gravity on Earth as uniform, it isn't. This is because the mass of the planet is not evenly distributed like a solid rubber ball. Instead, we have mountains and trenches, and a whole lot of other objects and mysterious structures way below our feet, and bulge out at the equator, causing a varied gravity across the surface.
We know that gravity differs across the planet. However, there are areas where gravity differs from what we would expect to find in that area, which is known as a gravity anomaly.
One such area is in the Indian Ocean, where the gravity is lower compared to what we would expect than at anywhere else on Earth. The problem has puzzled scientists for some time. We know the principles: lower gravity means that mass is missing. However, we didn't know what had caused this lack of mass.
A team from the Centre for Earth Sciences at the Indian Institute of Science believe they have the answer. According to the team's paper, which modeled tectonic plate and global mantle convection over 140 million years, the anomaly may be caused by ancient tectonic plates which have sunk down underneath the Indian Ocean. As a result, they believe a plume of hot mantle – less dense than the sunken crust – rose above it.
"Flow induced by downwelling Tethys slabs perturbs the African Large Low Shear Velocity province and gives rise to plumes that reach the upper mantle," the team write in their paper. "These plumes, along with the mantle structure in the vicinity of the geoid low, are responsible for the formation of this negative geoid anomaly."
The paper is published in Geophysical Research Letters.