How Did The Depression In The Indian Ocean Arise?

There is a hundred meter deep dimple in the middle of the Indian Ocean discovered in 1948. It is officially called the "Indian Ocean geoid low." Proximately, it exists because the density of the planet there is different than it is in adjacent places causing gravity in its vicinity to be weaker.

Scientists think may be related to a magma plume that arose 20 million years ago or so when the ocean between the Indian subcontinent collided with Asia as continent drift sent it moving North. But, this theory for how it formed, tested in simulations, isn't a perfect fit the data:

[I]t completely fails to reproduce the powerful mantle dynamic plume that erupted 65 million years ago under the present-day location of Réunion Island,” he said. “The eruption of lava flows that covered half of the Indian subcontinent at this time — producing the celebrated Deccan Traps, one of the largest volcanic features on Earth — have long been attributed to a powerful mantle plume that is completely absent from the model simulation.”

Another issue, Forte added, is the difference between the geoid, or surface shape, predicted by the computer simulation and the actual one: “These differences are especially noticeable in the Pacific Ocean, Africa and Eurasia. The authors mention that there is a moderate correlation, around 80%, between the predicted and observed geoids, but they don’t provide a more precise measure of how well they match numerically (in the study). This mismatch suggests that there may be some deficiencies in the computer simulation.”

The paper and its introductory summaries are as follows:

Abstract

The origin of the Earth's lowest geoid, the Indian Ocean geoid low (IOGL) has been controversial. The geoid predicted from present-day tomography models has shown that mid to upper mantle hot anomalies are integral in generating the IOGL. Here we assimilate plate reconstruction in global mantle convection models starting from 140 Ma and show that sinking Tethyan slabs perturbed the African Large Low Shear Velocity province and generated plumes beneath the Indian Ocean, which led to the formation of this negative geoid anomaly. We also show that this low can be reproduced by surrounding mantle density anomalies, without having them present directly beneath the geoid low. We tune the density and viscosity of thermochemical piles at core-mantle boundary, Clapeyron slope and density jump at 660 km discontinuity, and the strength of slabs, to control the rise of plumes, which in turn determine the shape and amplitude of the geoid low. 

Key Points

* Employing time dependent global mantle convection models since the Cretaceous we simulate the origin of the enigmatic Indian Ocean geoid low

* Plumes forming along the edges of the African Large Low Shear Velocity province (LLSVP) control the regional geoid in the Indian Ocean

* These plumes, in turn are generated by lower mantle Tethyan slabs that perturb the African LLSVP

Plain Language Summary

The origin of the deepest geoid on Earth, the Indian Ocean geoid low (IOGL), is debated. Several competing hypotheses exist, amongst which, a recent study employing tomography models suggested that hot anomalies at mid to upper mantle depths are crucial in generating this elusive feature. Assimilating plate motion in global mantle convection models from the Mesozoic till the present day, we attempt to trace the formation of this geoid low. We show that flow induced by downwelling Tethys slabs perturbs the African Large Low Shear Velocity province and gives rise to plumes that reach the upper mantle. These plumes, along with the mantle structure in the vicinity of the geoid low, are responsible for the formation of this negative geoid anomaly. Exploring a wide model parameter space, such as the density and intrinsic viscosity of the thermochemical piles, Clapeyron slope and density jump at 660 km depth, strength of slabs, we show that plumes are integral in generating the IOGL. The contribution of lower mantle Tethys slabs is secondary but also necessary in generating this geoid low.

Debanjan Pal, Attreyee Ghosh, "How the Indian Ocean Geoid Low Was Formed" Geophysical Research Letters (May 5, 2023). 

We don't know everything. One of the things we are continuing to deepen our understanding of are the details of our own planet's composition and inner structure. Improvements in sophisticated geo-sensors and the computational power necessary to run models of how the geology has evolved over time are giving us better answers to these questions.