Earth’s Strongest Gravity Anomaly Hides Under Antarctica, Not Where Scientists Thought

For decades, textbooks and science articles have pointed to the Indian Ocean as home to Earth’s deepest geoid low, the term scientists use for depressions in the planet’s gravity field. Pull up a standard map of Earth’s geoid and you’ll see a prominent blue depression in those tropical waters. Case closed, right?

Not quite. The strongest nonhydrostatic geoid depression on the planet sprawls across the Ross Sea, tucked between Victoria Land and Marie Byrd Land in Antarctica. New research has traced this massive feature back 70 million years and revealed how hot rock rising from near Earth’s core has been reshaping the frozen continent since before the dinosaurs went extinct.

The confusion comes down to what is being measured. Standard geoid maps show the planet’s full gravity field relative to a smooth, spinning Earth shape. But when scientists subtract the part caused simply by Earth’s rotation, what’s left (the nonhydrostatic signal) tells a different story. That’s when Antarctica’s massive depression emerges as the undisputed heavyweight.

Scientists Petar Glišović and Alessandro M. Forte wanted to know where this Antarctic geoid low came from. Their research, published in Scientific Reports, uses a back-and-forth nudging technique to reconstruct mantle flow backward through time, tracking the movement of rock deep underground all the way back through the Cenozoic Era: the last 66 million years of Earth’s history.

A Depression on the Move

Antarctica’s geoid low has existed for at least 70 million years, but it hasn’t stayed put. At the dawn of the Cenozoic, around 65 million years ago, the strongest depression sat over the South Atlantic Ocean. Then, between 40 and 30 million years ago, something dramatic happened. The depression shifted rapidly toward its present position over the Ross Embayment. Over the following millions of years, from about 35 million years ago to today, the depression grew 30 percent stronger.

What caused this migration? Deep beneath the Northwest Antarctic margin, ancient slabs of ocean floor have been sinking for millions of years. These cold, dense slabs (remnants of tectonic plates that dove beneath the continent during subduction) created massive downwellings of material that plunged toward Earth’s core. Some of this rock now rests at the core-mantle boundary, more than 1,800 miles below the surface.

But cold, sinking slabs explain only half the story. The models revealed an unexpected broad upwelling of hot rock rising from the deepest part of Earth’s mantle, pushing upward directly beneath the geoid low. This upwelling has been active throughout the Cenozoic and probably started even earlier, back in the Mesozoic Era when dinosaurs still roamed.

Earlier research assumed Antarctica’s geoid depression came mainly from those cold, heavy slabs pulling downward. Some scientists thought contributions from shallower depths came from hydration-induced buoyancy above the subducting plates. Glišović and Forte’s reconstruction tells a different story. The rising material comes from the D-double-prime layer, a distinct zone just above where the mantle meets the core. The results point strongly to heat from deep within Earth as the main driver, rather than just water-rich material from shallower depths.

The Balance Shifts

For the first 35 million years, contributions from different mantle depths rose and fell, making the geoid low fluctuate in strength. The deepest mantle provided a stable foundation, accounting for 30 to 50 percent of the depression. But around 35 million years ago, the balance changed. Material from mid-depths started contributing less while shallower hot rock grew more important. Today, this upper material accounts for 40 percent of the geoid low’s total strength.

The ascent of hot rock from the deep mantle into shallower regions over the past 35 million years amplified Antarctica’s geoid depression by 30 percent. As that hot upwelling rose closer to the surface, it made the geoid anomaly stronger rather than weaker: a counterintuitive result that speaks to the complex physics of how mass and buoyancy interact inside a churning planet.

To verify their reconstruction, the researchers looked at Earth’s rotation axis. When massive amounts of mantle material shift position inside the planet, they change Earth’s moment of inertia and cause the rotation axis to wander, a phenomenon called True Polar Wander. The models predicted a sharp hairpin turn in the axis path around 50 million years ago. According to the authors’ comparison, this prediction matches in timing and character the trajectories derived from paleomagnetic data, which records how Earth’s magnetic field direction changed through time.

The reconstruction also lines up with geology at the surface. Sixty-five million years ago, the modeled downwelling system extended from the southern Atlantic to the Weddell Sea margin, matching geological evidence of volcanic activity along the South Scotia Ridge during that period. The timing of major shifts in the geoid low corresponds with the final breakup of Australia and Antarctica between 58 and 50 million years ago, when seafloor spreading accelerated along the Southeast Indian Ridge.

What It Means for Antarctica

The hot mantle upwelling beneath Antarctica may help explain why parts of the continent sit at such high elevations despite being covered in miles of ice. West Antarctica’s elevated topography and the towering Gamburtsev Subglacial Mountains in East Antarctica both sit above this rising upwelling of deep rock. The results suggest that deep-mantle buoyancy may have contributed to supporting this high topography alongside lithospheric processes, including the stretching and thinning that occurred during ancient rifting events.

The major transition between 40 and 30 million years ago raises an intriguing question. Antarctic glaciation began around 34 million years ago, transforming a relatively temperate continent into the frozen wasteland we know today. Changes in the geoid can alter relative sea level by changing the height difference between the ocean surface and the solid Earth. The timing overlaps, but the authors stress that testing whether mantle-driven sea level changes helped create conditions for ice sheet formation will require future modeling that couples mantle flow, geoid changes, and sea-level physics.

Antarctica’s geoid low emerged from cold ocean slabs sinking toward the core and hot rock rising from the depths: two opposing forces that created the strongest nonhydrostatic geoid depression on the planet. That depression has persisted for at least 70 million years, but its evolution tells a story of Earth’s restless interior, where rock flows on geological timescales and reshapes the surface world above.

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