3 min readHere’s what you’ll learn when you read this story:
- The “gravity hole”—a region under Antarctica where gravity is unusually low—began to form at least 70 million years ago.
- Researchers used earthquake data to reconstruct a gravitational map of Earth as it was during the late Cretaceous, giving insight into this phenomenon.
- The progression of seismic processes revealed by the maps showed that the gravity hole was caused by convection in the mantle.
Earth’s gravity keeps us securely on terra firma—it’s ubiquitous and inescapable. But it’s also inconsistent. Even though you won’t find yourself floating like an astronaut or getting crushed by a black hole here on Earth, our planet has significant gravitational anomalies. The largest one is also the most mysterious.
Of the four universal forces—gravity, electromagnetism, and the strong and weak nuclear forces—gravity is actually the weakest. Any object with mass exerts gravity on another object with mass, and each part of that mass also exerts gravity on every other part. Earth’s gravity varies across the planet depending on mass fluctuations related to surface features (like the constantly changing flow of water) and subsurface features (like oozing magma). Changes in those magma-based subsurface features, while still influential today, had even more impact millions of years ago.
Shortly before the asteroid strike that wiped out the non-avian dinosaurs 65 million years ago, tectonic plates shifted beneath the region that is now East Antarctica, lowering the density of the material between the mantle and Earth’s core there. About 30 million years later, rising mantle lowered the density of the geological layers in this region even more. Today, as a result, the gravity in East Antarctica is measurably weaker than anywhere else on the planet.
This peculiar gravitational deficit may seem counterintuitive if you know that, in principle, the strength of gravity in any given area depends on how much mass lies between Earth’s surface and the planet’s core. With an average elevation of just under 10,000 feet, East Antarctica is a fairly high-altitude region, giving it a greater-than-average mass. But as it turns out, the region’s greater bulk doesn’t translate into higher levels of gravity. Much bigger factors are at play below Earth’s surface.
Because Earth’s deep mantle is too inaccessible to study directly, interpreting its past is often the best way to understand its present—and future—geological characteristics. By using earthquake data to simulate ancient tectonic activity, researchers Alessandro Forte and Petar Glišović of the Paris Institute of Earth Physics were able to create a gravitational map of Earth as it looked 70 million years ago. The map offers profound insights into how the gravity hole formed and evolved.
“By integrating seismic, geodynamic, and mineral-physics data, our reconstructions provide a dynamically consistent view of mantle flow beneath Antarctica and offer new insights into the coupling between deep and shallow mantle processes that govern Earth’s [seismic] evolution,” they said in a study recently published in the journal Scientific Reports.
Rewinding geological time (at least, virtually), the researchers saw that early gravitational lows in what’s now East Antarctica were mainly caused by convection. Cold, dense mantle material had already been sinking over a long span of time when, somewhere between 50 and 30 million years ago, there was a slight shift in Earth’s rotational axis. As a result, a swath of warmer and less-dense material, which was once trapped deep in the mantle, made its way upward. As colder material in the mantle continued to sink, and lower density beneath Antarctica drove down the mass of that region, creating the gravity hole.
Forte and Glišović also think the shift in mantle processes that triggered the gravity hole’s formation could have an effect on sea levels and Antarctic ice sheets, as these processes also coincided with Antarctica freezing over about 30 million years ago. Ice volume and tectonic processes can both cause sea levels to rise and fall.
Indeed, the scientists point out that the gravity hole should, at least in theory, have its own effect on nearby sea levels. Antarctica’s gravity hole will interact in complex ways with the future effects climate change, and the researchers suggest there might be hints hiding beneath the ice about just what will occur. What happened in the past may also be able to predict the future.
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Elizabeth Rayne is a creature who writes. Her work has appeared in Popular Mechanics, Ars Technica, SYFY WIRE, Space.com, Live Science, Den of Geek, Forbidden Futures and Collective Tales. She lurks right outside New York City with her parrot, Lestat. When not writing, she can be found drawing, playing the piano or shapeshifting.