On Earth, gravity's attraction is constant, although our world is not a perfect sphere. A geoid is an undulating map that is covered in lumps and bumps caused by the geology of different densities tugging on neighbouring masses with subtly different degrees of power. Deep under the Indian Ocean, that pull diminishes to a very low level, leaving what is thought to be a gigantic gravity 'hole' that is around three million square kilometres in size and is where the bottom descends into a significant depression. Its existence has been hinted at for some time, and it is one of the most significant gravitational anomalies on Earth. Long ago, ship-based surveys and satellite observations showed that the gravitational tug-of-war between the appropriately termed Indian Ocean geoid low and the nearby gravitational 'highs' was the cause of the sea level drop right off the tip of the Indian subcontinent.
It's unclear exactly what led to this relative weakness. The kind of planetary events that could be involved is now thought to be better understood, according to two researchers from the Indian Institute of Science. Geoscientists Debanjan Pal and Attreyee Ghosh write in their recently published paper, which describes their new working theory, that "all these [previous] studies looked at the present-day anomaly and were not concerned with how this geoid low came into existence." More than 1,000 kilometres (621 miles) beneath the Earth's crust, they think they have found the answer, where the hot, molten rock was churned up around 30 million years ago as the cold, dense remains of an ancient ocean plummeted into a "slab graveyard" beneath Africa. However, unless additional evidence is gathered, their computer model-based findings are unlikely to put an end to the contentious discussion around the causes of the geoid low.
A shipload of scientists from India's National Centre for Polar and Ocean Research set out to install a number of seismometers along the seafloor in order to study the deformation zone. There wasn't much seismic data in the region because it was so far offshore. The findings of the 2018 scan suggested that hot plumes of molten rock were rising up beneath the Indian Ocean and were somehow responsible for its significant dent. But to rebuild the geoid low in its early stages, a longer perspective was required. Pal and Ghosh thus used a model of how tectonic plates crossed Earth's hot, goopy mantle throughout the previous 140 million years to reconstruct the genesis of the huge geoid. At that time, the Indian tectonic plate was just beginning to separate from the supercontinent of Gondwana to start moving north. The Tethys Sea's floor sunk into the Earth's mantle when the Indian plate moved forward, revealing the Indian Ocean behind it.
In order to compare the form of the oceanic low that those models anticipated with observations of the dent itself, Pal and Ghosh ran simulations employing more than a dozen computer models of plate motion and mantle motions. One characteristic was shared by all the models that accurately depicted the Indian Ocean geoid low in its present state: plumes of hot, low-density magma rising up beneath the low. If they climb high enough, these plumes, together with a characteristic mantle structure, are what generated the geoid low, according to Pal and Ghosh. The pair concludes, "Shortly, our results suggest that plumes need to be buoyant enough to come up to mid-mantle depths in order to match the [shape and amplitude of the] observed geoid low."
Around 10 million years after the former Tethys Sea submerged into the lower mantle, the first of these plumes initially emerged approximately 20 million years ago, south of the Indian Ocean geoid low. The low grew stronger as the plumes grew beneath the lithosphere and crept towards the Indian peninsula. The researchers propose that the telltale plumes were propelled up as the Tethys seabed fell into the lower mantle, disrupting the infamous "African blob," because their findings are compatible with some of Ghosh's earlier modelling work from 2017. Some scientists who weren't part of the project, however, aren't persuaded, telling New Scientist that there isn't currently conclusive seismographic proof that the simulated plumes are genuinely there beneath the Indian Ocean. Such information could surface soon, but there's no urgency as the geoid low is predicted to last for many more millions of years.
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