What is Earth’s gravitational hole and how it formed – 7/6/2023 – Science

When the Earth is studied in elementary school, teachers explain to us that its shape is that of a sphere flattened at the poles. Later we are told that its gravity is 9.8 m/s².

The reality is that the Earth is similar to a potato: it is not a homogeneous sphere, but a geoid with many irregularities. This also assumes that its density varies in each region of the globe and that, therefore, gravity is not the same everywhere.

On Earth there are parts where the standard value of gravity varies. This variation is called an anomaly. Anomalies can be positive —when the severity value (g) is greater than the standard— or negative — when it is less.

This is where the concept of the “gravitational hole” comes in.

This region has “an enormous gravitational anomaly, the most important on Earth,” Gabriela Fernández Viejo, PhD in Geology from the University of Oviedo, told BBC News Mundo.

The expert, however, warns that this “hole” is not an area where things sink, nor objects fall faster. And it’s not a visible hole either.

Gravity gauges on ships detected this anomaly decades ago. Since then, more sophisticated satellites have refined the calculations.

But there was no clear explanation why this phenomenon occurred.

A recent study shows very precisely how this anomaly could have occurred.

What is it

This “gravity hole” is the lowest point on Earth’s geoid. It is located in the Indian Ocean, south of the Indian subcontinent.

It is a circular depression that is 105 meters below mean sea level and with an area of ​​over 3 million square kilometers.

The name by which experts know it is Low Geoid of the Indian Ocean (IOGL) and there are numerous hypotheses about how this space that registers the lowest gravity on the planet could have formed.

But there is a basic starting point.

If we remember what we learned in elementary school, gravity is proportional to mass. Thus, less mass implies less gravity.

From this premise — that in the area of ​​the “gravitational hole” there is less mass — all the geophysicists’ explanations started. But there is no consensus on why this smaller amount of mass.

Many hypotheses, but none complete

“The models available so far explained the lack of mass in the Indian Ocean based on the fact that there were a series of oceanic plates that were subducted when they collided”, says Fernández.

The crusts in this area are ancient and have their origin in ancient Tethys, an ocean that was between the continents of Gondwana and Laurasia in the Mesozoic era, a period between 250 million and 66 million years before the emergence of the Indian Ocean.

When the Indian plate broke away from the supercontinent Gondwana to collide with the Eurasian plate, the Tethys plate (which formed an ocean between them) sank.

The seismic speeds are known to geologists and can be explained by the different densities and temperatures of the planet’s layers. Fernández explains that “the only data we have from the interior of the Earth are those referring to seismology”.

And precisely in this the previous models failed.

“They said that this gravitational anomaly was due only to the slabs and did not explain other things, such as the seismic speeds in the region”, says Fernández.

Geoscientists Debanjan Pal and Attreyee Ghosh, from the Indian Institute of Sciences and authors of the most recent research on this phenomenon, argue that “previous studies looked at the current anomaly and were not concerned with how it came about”.

a new model

Pal spent years trying to explain the origin of this anomaly.

With advances in computing, he managed to create a model that, in Fernández’s opinion, “is the most convincing, it explains the seismic velocity data, why it occurred over time and the tectonic plate movements that occurred and when it may end the phenomenon”.

Pal’s team simulated 19 different scenarios for tectonic plate movement and changes in Earth’s mantle over the last 140 million years.

For this, they used different parameters, such as the viscosity or density of the mantle, the temperature, the resistance of the plates or the deformation time.

In each simulation they used different values ​​of these parameters and compared the result with the data that actually exist, that is, with the real geoid of the Earth observed by the satellites.

In six of the simulations, the shape and extent of the geoid below the Indian Ocean closely matched the actual data.

This means that, after analyzing 19 different possible scenarios, the results of six of them coincided with what is observed today in nature.

Why does it happen

If, for previous models, the oceanic plates of Tethys were fundamental, in the study by Pal and Ghosh their contribution “is necessary to generate the anomaly, but it is secondary”.

When the Indian plate broke away from the supercontinent Gondwana to collide with the Eurasian plate, the Tethys plate, which formed an ocean between them, sank into the mantle. This was already known from previous studies.

But now another part of the planet comes into play: East Africa.

Over tens of millions of years, the cooler Tethys plate “slid into the lower mantle and moved towards Africa, where it interacted with a region of hot magma, specifically under East Africa,” says Fernández.

From this interaction between a colder plate and a hotter plate, a disturbance is generated, a kind of column that, in turn, moved back to the Indian Ocean, where the gravitational anomaly is currently located.

This moving material is known as “mantle plumes” and is hot, less dense magma. It is precisely because of these characteristics that it rises above the rest of the materials.

Fernández emphasizes that “in other areas of low density and low gravity, it was possible to observe the presence of mantle feathers and, therefore, it was possible to say that the cause of this lower gravity was a less dense material. But in the Indian Ocean this was not so evidently, it was not known where the less dense material came from”.

“What Pal and Ghosh do is show that mantle plumes exist because they come from somewhere else,” he says.

In the expert’s opinion, the new model developed by the Indian center “adapts to geological history, objective data and mantle convection models”.

And, according to Fernández, it refines the theory of plate tectonics.

This report was originally published here.