Corresponding to Antarctica there is a “gravitational hole” where the gravity force is less compared to other regions of the planet. The anomaly, known as Lower Antarctic Geoid (Antarctic Geoid LowAGL) and located between Victoria Land and Marie Byrd Land, originated 70 million years ago and later became more marked. To reconstruct its origin and evolution, in a study published in the journal Naturewas the University of Florida, in collaboration with the Institut de Physique du Globe de Paris (IPGP). The research highlighted a possible correlation between the accentuation of the anomaly and the glaciation which 34 million years ago led to the formation of the Antarctic ice sheet.
What is the “gravity hole” in Antarctica
The Antarctic “gravity hole” is located at the bay of the Ross Seain Southwestern Antarcticabetween Victoria Land and Marie Byrd Land. There gravitational force is not the same at every point of the earth’s crust, because our planet has an irregular surface and the rocks inside it have variable characteristics. In particular, where there is water or less dense rocks the force of gravity is lower, while where there are reliefs or denser rocks it is greater. The solid that best represents the shape of the Earth, taking into account the local gravitational force and therefore the distribution of masses, is the geoid. The most depressed point of the geoid, where the gravitational force is less than everis located in the Indian Ocean. Even in Antarctica the geoid presents a depression, corresponding to a large area millions of square kilometers. Here the sea level is over 100 m lower of the global average: where the force of gravity is lower, in fact, the surface of the ocean can lower because the water flows towards areas where the force of gravity is greater.
Geological causes and origin: how the “gravity hole” evolved
To reconstruct the origin of the Antarctic anomaly, the researchers used seismic data which reveal the density of the rocky materials that make up our planet. The seismic waves generated by earthquakes change speed depending on the properties of the materials they encounter as they propagate inside the Earth. It was thus possible to build a three-dimensional model of mantle density terrestrial and compare it with i data relating to the force of gravity collected by satellites. In this way, a correspondence was identified between the lower gravitational force in West Antarctica and a lower density in the mantle in this area compared to the surrounding ones. Then, thanks to sophisticated computer models, it was possible to trace back to 70 million years agowhen the “hole” originated, probably due to the sinking into the mantle of a lithospheric plate under Antarctica. Initially the anomaly was weaker than today, but between about 50 and 30 million years ago it intensified due to the very slow movements of the mantle rocks, which being viscous can flow leading to a redistribution of the masses. Own 34 million years ago the glaciation began which led to the formation of a ice cap over Antarctica, which before then was a temperate continent. One goal of the researchers is to demonstrate a link between this event and the intensification of the anomaly. In fact, we said that the decrease in the force of gravity causes the sea level to lower. The decrease in the latter around Antarctica could have influenced the development of the ice sheet.
Geophysicist Alessandro Forte of the University of Florida states:
If we can better understand how the Earth’s interior shapes gravity and sea level, we can gain insights into factors relevant to the growth and stability of large ice sheets.
