To the naked eye, observed from space or represented on a globe, our planet appears to be a perfect sphere. In reality, in addition to the deformation that we know with the school expression “flattened at the poles”, the Earth has a relatively irregular surface, with culminating points such as Everest (8,849 meters above sea level) and the Mariana Trench. (almost 10,000 meters below sea level).
A lesser known variability occurs in areas such as the Indian Ocean Geoid Low (IOGL), in southern Sri Lanka, a kind of deformity (a sinkhole with respect to the rest of the spherical surface) in the that there is also a gravitational anomaly or gravitational hole that has attracted the attention of scientists for decades. Now, a team of researchers from the Indian Institute of Science in India seems to have discovered the origin of this geoid and gravitational anomaly.
“The existence of the low Indian Ocean geoid is one of the most outstanding problems in Earth sciences. It is the lowest gravity/geoid anomaly on Earth and until now there has been no consensus regarding its origin,” said Professor Attreyee Ghosh, Assistant Professor at the Earth Sciences Centre, Indian Institute of Science, Bangalore.
In a recent study published in the journal Geophysical Research Letters , Professor Ghosh and colleagues, in collaboration with researchers from the German Research Center for Geosciences (GFZ, Germany), have analyzed the reasons for the lack of mass that causes this anomaly of the geoid.
Although several studies in the past have tried to answer this question, in most cases they limited themselves to attributing it to a tectonic plate process millions of years ago, whereby a part of the Earth was embedded under the surface. current surface layer.
The new study uses numerical models of ‘mantle convection’ to explain the mass deficit. Mantle convection is a type of movement caused within Earth’s mantle or middle layer, where hotter, lighter material rises to the top and cooler, denser material sinks due to gravity. This convective movement within the mantle was driven by seismic tomography models that use seismic waves to obtain a three-dimensional image of the Earth’s interior.
The researchers found that “low-density anomalies,” or the presence of lighter materials in the upper to mid-mantle below the IOGL, were responsible for the low gravity in this region. Mantle plumes or mantle plumes (narrow columns of material from the mantle) or the rise of abnormally hot rocks within the Earth’s mantle can result in low-density anomalies. However, no known mantle plume exists below the IOGL, ruling out this theory.
The new study found that there was hot material rising from the Large Low Shear Velocity African Zoan (LLSVP) or African Super Plume, in the vicinity of IOGL, drifting eastward and ending below IOGL. The deviation is possibly due to the rapid movement of the Indian plate, the researchers argue.
“A low geoid or negative geoid anomaly would be caused by a mass deficit within the deep mantle. Our study explains this trough with hotter and lighter material extending from a depth of 300 km to about 900 km in the northern Indian Ocean, probably coming from the African super plume,” says Professor Ghosh.
“This study is definitely a breakthrough in convincingly explaining the occurrence of the Indian Ocean Low Geoid using a model driven by current density anomalies,” says the Indian Institute of Science. Most existing theories have tried to explain this negative anomaly with the help of cold, dense oceanic plates that sank into the mantle in the past. As part of future work, the researchers would like to investigate the evolution of the low geoid from the past to the present using time-dependent mantle convection models.