Gravity is constant on Earth, but our planet is not a uniform sphere. It is covered with lumps and bumps on an undulating map known as a geoid, with geology of different densities pulling nearby masses with subtly different degrees of force.

 

In the depths of the Indian Ocean, this gravitational pull is extremely weakened, leaving a huge gravity 'hole', about three million square kilometres in size, where the seafloor sinks into a vast trench.

The existence of this hole, one of the deepest gravity anomalies on Earth, has been signalled for some time. Ship-based surveys and satellite measurements have long established that sea level at the very tip of the Indian subcontinent is lowering due to gravitational tug between the aptly named Indian Ocean geoid low and the surrounding gravitational 'highs'.

What causes this relative weakening has never been clear. Now two researchers from the Indian Institute of Science think they have a better idea of the types of planetary events that may be involved.

"All these [past] studies have looked at the present-day anomaly and have not dealt with how this geoid dip arose," say geoscientists Debanjan Pal and Attreyee Ghosh in their published paper describing their new working hypothesis.

The researchers think the answer lies more than 1,000 kilometres (621 miles) beneath the Earth's crust, where the cold and dense remnants of an ancient ocean plunged into a 'plate graveyard' beneath Africa about 30 million years ago, stirring up hot molten rock.

But results based on computer models are unlikely to resolve the heated debate about the origins of the geoid low - at least until more data are collected.

In 2018, a shipload of scientists from India's National Centre for Polar and Ocean Research set out to deploy a series of seismometers along the seafloor of the deformation zone to map the region.

Little seismic data had previously been collected in the region because it is so far from shore. The results from this 2018 survey pointed to the presence of clouds of hot molten rock rising beneath the Indian Ocean and somehow contributing to the great collapse.

But a longer view was needed to reconstruct the low level of the geoid in its early stages. Pal and Ghosh therefore traced the formation of the great geoid by modelling how tectonic plates slid over Earth's hot, sticky mantle over the last 140 million years.

At that time, the Indian tectonic plate had begun to break away from the supercontinent Gondwana to begin its northward advance. As the Indian plate advanced, the seafloor of an ancient ocean called the Tethys Sea sank into Earth's mantle, and the Indian Ocean opened up behind it.

Using more than a dozen computer models, Pal and Ghosh simulated plate motion and mantle motions and compared the shape of the ocean low predicted by these models with observations of the subduction itself.

The models that reproduced the Indian Ocean geoid low in its current form all had one thing in common: hot, low-density magma clouds rising from beneath the low. According to Pal and Ghosh, these bubbles, in addition to a distinctive mantle structure, are what create the geoid low if they rise high enough.

"In short, our results suggest that to match the [shape and amplitude] of the observed geoid low, plumes must have enough buoyancy to rise up to mid-mantle depths," the duo write.

The first of these bubbles appeared about 20 million years ago, south of the Indian Ocean geoid low and about 10 million years after the ancient Tethys Sea sank into the lower mantle. The subsidence intensified as the bubbles spread beneath the lithosphere and moved towards the Indian peninsula.

Given that their results are consistent with elements of Ghosh's previous modelling work in 2017, the duo suggest that prominent plumes were pushed upwards after the Tethys seafloor sank into the lower mantle and disturbed the famous 'African blob'.

However, some researchers not involved in the study were not convinced, telling New Scientist that there is as yet no clear seismographic evidence that the simulated plumes are actually present beneath the Indian Ocean.

Such data may soon come to light, and there's really no rush - geoid lowering is expected to continue for millions of more years.

 

Source: https://www.sciencealert.com/