Earth’s gravity isn’t uniform. While generally spherical, its gravitational field more closely resembles a bumpy potato, with areas of slightly stronger and weaker pull. One of the most significant of these weaker areas, a “gravity hole” beneath Antarctica, is not only present but is also demonstrably strengthening, according to new research. The anomaly, formally known as the Antarctic Geoid Low, is driven by slow movements of rock deep within the Earth over tens of millions of years.
“If we can better understand how Earth’s interior shapes gravity and sea levels, we gain insight into factors that may matter for the growth and stability of large ice sheets,” says geophysicist Alessandro Forte of the University of Florida. While the variations in gravity are small in absolute terms, they can have measurable effects on ocean surfaces.
The Earth’s geoid – this bumpy representation of the gravitational field – exists because gravity is directly linked to mass and the distribution of mass inside the planet is uneven. Different rock compositions possess different densities, creating these subtle variations. Where gravity is weaker, the ocean surface sits slightly lower, as water flows towards areas of stronger gravitational pull. The sea-surface height around Antarctica is measurably lower than it would be without this gravity hole.
Mapping the Invisible: Earthquake Waves as a Window into Earth’s Interior
Researchers, including Professor Forte and Dr. Petar Glišović from the Paris Institute of Earth Physics, have created a detailed map of the Antarctic Geoid Low and traced its evolution throughout the Cenozoic Era, spanning the last 66 million years. Their approach relied on a global scientific project that combined earthquake recordings with physics-based modeling to reconstruct the three-dimensional structure of Earth’s interior.
“Imagine doing a CT scan of the whole Earth, but we don’t have X-rays like we do in a medical office,” Forte explains. “We have earthquakes. Earthquake waves provide the ‘light’ that illuminates the interior of the planet.” Seismic waves from earthquakes travel through the Earth, altering speed and direction as they encounter materials of varying compositions and densities. By analyzing these changes, scientists can infer the structure of the planet’s interior.
The team used this earthquake data to construct a 3D density model of Earth’s mantle, which was then extrapolated into a map of the entire planetary geoid. This map closely matched the gold-standard gravity data collected by satellites, validating the accuracy of their modeling approach.
A 70-Million-Year History of a Strengthening Anomaly
The next challenge was to rewind time, assessing how the geoid has evolved since the early Cenozoic, approximately 70 million years ago. The researchers fed their map into a physics-based model of Earth’s mantle convection, simulating the planet’s geological activity over that timeframe. They then ran the model forward to see if it could reproduce the geoid observed today.
The model also needed to account for changes in Earth’s rotational axis, known as True Polar Wander. The simulation successfully reproduced both the current geoid and the observed polar wander, suggesting a high degree of accuracy in its representation of the geoid’s evolution.
The results revealed that the Antarctic Geoid Low isn’t a recent phenomenon. A gravitational depression has existed near Antarctica for at least 70 million years. However, its position and strength began to change dramatically around 50 million years ago, coinciding with a significant shift in polar wander.
Subduction and Upwelling: The Forces Shaping Antarctica’s Gravity
According to the model, the anomaly initially formed as tectonic slabs subducted beneath Antarctica and sank deep into the mantle, altering the planet’s gravity field at the surface. Over time, a broad region of hot, buoyant material began to rise upward, becoming increasingly influential over the past 40 million years and strengthening the geoid low.
This strengthening may be linked to the glaciation of Antarctica, which began in earnest around 34 million years ago. The geoid influences sea level; as the geoid lowered around Antarctica, the local sea surface would have followed, potentially contributing to the growth of the ice sheet. This connection remains speculative and requires further investigation.
However, the research highlights the interconnectedness of various geodynamic processes – from mantle convection to the geoid and the motion of the poles. These processes can influence each other in complex ways.
While subtle, the gravity hole under Antarctica serves as a reminder that even the slowest processes deep within the Earth can have a lasting impact on the world above. The research, published in Scientific Reports, provides a deeper understanding of the forces shaping our planet and the potential implications for ice sheet stability and sea level change.
