If you ever took a general science class then you were probably told that the Earth gravity at the surface has a value of 9.81 m/s2 or 32 ft/s2. This is the gravitational acceleration you should experience at sea level of the surface and as you go up in height the gravity goes down following Newton’s law of gravity, or something along those lines. To an extent this can be used as true to make some calculations easy, like measuring the depth of a well if you were that kind of kid, or figuring out which object will touch the ground first, but as an adult now you may think that sea level is not an accurate representation of a distance from the earth’s center and that the feeling of the gravitational acceleration is different on different parts of the world, so the value is more like an average that you should experience across the globe, and this hold mostly true except for one area.
In the Indian Ocean there is an area of 3 million square miles where the gravity is so weak that the sea level is 106 meters lower than the global average. That is quite a huge difference that has puzzled scientists since 1948. Just for a little reminder the actual shape of planet Earth is not perfectly spherical, but more approximately like an ellipsoid with the poles being flatten down. For simple calculation in science classes we like to refer to Earth as a sphere as it makes calculations easier (trust me this is a common practice in physics when we are not considering accuracy), and the approximations hold up to a degree, but when precision and accuracy are quite important then those details arise and must be taken seriously to make adequate predictions and the use of Newton’s law of gravity is not as reliable and we apply some general relativity into the mix. Although it could be tedious, it is quite important work to keep in mind if you see yourself dealing with satellites as you will need to make adjustments to their orbit to remain up there, also to make GPS corrections, otherwise they would end up crashing. Continuing with our narrative.
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| Map showing the variation of Earth's gravity. The color depict the strength of gravity, the blue dot over India is the gravity hole. Courtesy of ESA's GOCE Satellite (Insider). |
Officially known as the Indian Ocean geoid low, this region has been mapped using computer models of Earth’s gravity and compared with real data to try to understand how the anomaly came to be. After all such a big area could not be a mere coincidence.
Scientists studying the anomaly have thought that something underneath was causing the strange effect, But a new study suggests that they should have been looking around, and not under, the gravity anomaly to solve the mystery of how it formed.
A team of researchers in India claims that the gravity hole was formed by plumes of low-density magma that rose up from Earth’s mantle generated 50 million years ago from a sunken tectonic plate.
The new study was conducted using computer simulations at the Indian Institute of Science in the city of Bengaluru. They plotted 19 different scenarios to simulate the movement of the tectonic plates in the past 140 million years, and only a few of these scenarios account for the gravity hole but none of these scenarios showed the low gravity was caused by something directly underneath. Instead they found the hole was likely shaped by plumes of low density magma.
“As India separated from Africa and smashed into the European plate, the ocean that used to be there, called Tethys, was split apart and squished between the continental plates.”
Future ocean surveys can confirm whether these plumes exist or not, but as of now this is the latest and more insightful explanation we have ever received to explain such an anomaly. Maybe this way of thinking can change the perspective on how to proceed with tectonic plates research.
If you want to learn more about this you can read these articles.
- https://www.dailymail.co.uk/sciencetech/article-12282031/Indian-Ocean-gravity-hole-sea-levels-300ft-lower-experts-think-know-why.html
- https://www.advancedsciencenews.com/mystery-of-gravity-hole-in-indian-ocean-solved/
- https://www.businessinsider.com/scientists-crack-mystery-of-huge-gravity-hole-in-the-indian-ocean-2023-6
- https://www.usgs.gov/faqs/what-geoid-why-do-we-use-it-and-where-does-its-shape-come
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