Tuzo and Jason: The Two Hidden Continents Deep Inside Earth Shaping Its Magnetic Field

An Awkward Fact

Humanity has sent probes 25 billion kilometers from Earth—to the edge of the Kuiper Belt, where sunlight is as faint as a distant candle. Yet the deepest we have ever drilled is just over 12 km. Soviet scientists spent nearly 20 years on the Kola Peninsula and stopped at 12,262 meters—temperatures deep in the crust were hot enough to soften drill bits, and pressures made rock flow like toothpaste.

25 billion kilometers outward; 12 kilometers inward. That is the gap in humanity's exploration of "the far away" versus "right below our feet."

Earth's radius is 6,371 km. We have directly touched only 0.2% of it. For the remaining 99.8%, we rely on indirect methods—seismic waves, magnetic anomalies, gravity variations, and laboratory simulations of high pressure and temperature.

And hidden within that 99.8% of darkness are two continents.

Not the familiar kind—granite and basalt plates drifting atop the asthenosphere. These two continents sit 2,900 km underground, pressed against the core-mantle boundary, made of material hotter, denser, and chemically different from the surrounding rock. They even have names: the one beneath Africa is called Tuzo, and the one beneath the Pacific is called Jason.

Each is about the size of a continent on Earth's surface.

The Discovery of Tuzo and Jason

The story begins with seismic waves.

Seismic waves are the most reliable messengers of Earth's interior. When a large earthquake occurs, waves penetrate the planet, refracting, reflecting, and changing speed at the boundaries of the core, mantle, and crust. Global seismic stations record arrival times and intensities, letting scientists infer internal structure—like a doctor using ultrasound.

In the late 20th century, as global seismic networks densified, an anomalous signal emerged: at the very bottom of the mantle, just above the core, two regions showed markedly slower seismic wave speeds. Shear-wave velocities dropped by about 2-5%, while the regions were astonishingly large—thousands of kilometers across, accounting for 3-9% of the mantle's total volume.

They were named Large Low-Shear-Velocity Provinces, or LLSVPs. Low shear-wave velocity implies hotter, softer, or chemically distinct material.

The two provinces sit nearly symmetrically: one beneath Africa, one beneath the Pacific—almost exactly antipodal. Their nicknames honor two scientists: Tuzo, after Canadian geologist J. Tuzo Wilson (a key contributor to plate tectonics, proposer of the "Wilson Cycle"), and Jason, after American geophysicist W. Jason Morgan (a founder of plate tectonics and mantle plume theory).

Two names, two continents, two chapters of Earth's history.

What Are They?

This is the most fascinating question—and still unresolved.

Hypothesis one: They are primordial relics of Earth.

When Earth formed 4.5 billion years ago from the solar nebula, heavy elements (iron, nickel) sank to form the core while lighter elements (silicon, oxygen, aluminum) floated up to form the mantle and crust. But the process was imperfect—some denser material may have gotten "stuck" at the base of the mantle, forming Tuzo and Jason. If true, they are fossils of Earth's infancy: 4.5 billion years old, never stirred by mantle convection—older than any surface rock.

Hypothesis two: They are a graveyard of ancient oceanic crust.

Plate tectonics continuously recycles oceanic crust—created at mid-ocean ridges, subducted at trenches. Most subducted crust is heated, mixed, and remelted in the mantle, but some may sink all the way to the mantle's base, piling up into dense chemical anomalies. If true, Tuzo and Jason are the tombs of oceans lost over billions of years—Panthalassa, the Paleo-Tethys, and others that opened and closed during supercontinent cycles.

Hypothesis three: Both.

The latest view favors "thermochemical structures"—differing in both temperature and composition. They may have a primordial core, progressively wrapped and accreted by subducted oceanic crust, growing like a snowball.

Whatever the answer, one thing is certain: Tuzo and Jason are not ordinary geological structures. They are the largest heterogeneities inside Earth, two giant obstacles in the mantle convection system, a physical embodiment of some deep order in the planet's 4.5-billion-year history.

How Do They Shape the Magnetic Field?

In February 2026, a University of Liverpool team published a study in *Nature Geoscience* revealing a new role for Tuzo and Jason: shaping Earth's magnetic field.

Earth's magnetic field is generated in the outer core—a ~2,260-km-thick layer of liquid iron. Flowing liquid iron acts like a dynamo, generating currents and hence a magnetic field: the "geodynamo." The energy driving it comes from latent heat and light elements released by inner-core crystallization, plus core cooling—and the cooling rate depends on the temperature of the mantle above the core.

That is where Tuzo and Jason come in.

The Liverpool team's finding in one sentence: the temperature at the core-mantle boundary is not uniform.

Tuzo and Jason are hotter than the surrounding mantle. Directly beneath them, the top of the core is hotter than elsewhere. This temperature difference alters the flow of liquid iron: in cooler regions (outside the provinces), the iron churns vigorously, actively participating in the geodynamo; in hotter regions (beneath them), buoyancy driving weakens and flow stagnates.

Picture a pot of boiling soup. If you place two heat-insulating pads at the bottom, the soup above them stops churning—heat is blocked and convection suppressed. Tuzo and Jason are two insulating pads beneath the core's soup.

Combining paleomagnetic data (magnetic records preserved in ancient rocks) with supercomputer simulations, the team reconstructed magnetic field behavior over the past 265 million years. They found:

Some components of the magnetic field have remained relatively stable for hundreds of millions of years, while others changed dramatically.

The stable parts correspond to the locations of Tuzo and Jason—like two nails pinning certain field features to fixed geographic locations. The changing parts correspond to the regions between them—where liquid iron flows freely and field patterns keep reorganizing.

The End of the "Perfect Bar Magnet"

This discovery has a deeper implication: it overturns a long-standing assumption.

For decades, geophysicists studying paleomagnetism assumed that averaging Earth's magnetic field over a long enough time (say, millions of years) makes it behave like a perfect bar magnet—magnetic poles aligned with the rotational poles, a perfect dipole distribution.

This assumption is a cornerstone of paleomagnetism. It lets us infer ancient continent positions from magnetic directions recorded in old rocks, reconstructing plate motion history—Pangaea's formation and breakup, continental drift trajectories, ancient climate belts.

But the Liverpool study shows: this assumption may not be entirely correct.

Tuzo and Jason make the thermal structure above the core persistently asymmetric, systematically distorting the field away from a perfect dipole. And the distortion is not random—it stays stable over hundreds of millions of years, anchored by the provinces' fixed positions.

This means previously reconstructed ancient continent positions may need fine-tuning. Studies relying on paleomagnetism—paleoclimate, paleobiogeography, natural resource distribution—could face cascading effects.

A rock structure buried 2,900 km down can influence our understanding of surface climates 300 million years ago. This is the butterfly effect of Earth science—except the butterfly's wings are the size of Africa.

The Inverse Problem: From Effects to Causes

The methodology behind Tuzo and Jason is itself worth pondering.

We cannot see them directly. We can only measure: changes in seismic wave speeds passing through them, subtle surface magnetic anomalies, tiny deviations in Earth's gravity field. Inferring the "cause" 2,900 km underground from these "effects" is a classic inverse problem.

Inverse problems are everywhere in science. Doctors infer tumor locations from X-ray projections (CT scans); astronomers infer exoplanets from tiny dips in stellar brightness (transit method); seismologists infer subsurface structure from station waveforms.

AI researchers face inverse problems too. Interpretability research on large language models is essentially inferring internal structure from outputs—which neurons do what, which activation directions correspond to which concepts. We cannot see the model's "thinking"; we can only guess from input-output relationships.

The Tuzo and Jason research offers a paradigm worth borrowing: cross-validation across multiple independent data sources. Seismic waves reveal their shape and location; paleomagnetic data reveal their influence on the field; supercomputer simulations test whether these influences are physically self-consistent. Any single data source can be misread, but when three independent methods point to the same conclusion, confidence rises sharply.

This mirrors the "multi-probe approach" in AI interpretability: activation analysis, causal interventions, behavioral tests, and structural analysis cross-validating the same conclusion. A single probe can deceive you; the probability of many independent probes deceiving you simultaneously is far smaller.

The Invisible Continents

Tuzo and Jason remind me of something: science's most thrilling moments often come not from seeing something new, but from realizing that something always present had been overlooked.

Before continental drift was accepted, people looked at how South America's east coast fit Africa's west coast for centuries and saw only coincidence. Before plate tectonics was established, people knew earthquakes and volcanoes clustered in belts, but no one realized they were different expressions of the same machine.

So it is with Tuzo and Jason. The seismic velocity anomaly signal appeared in the late 20th century, but interpreting "velocity anomaly" as "continent-sized thermochemical structures," and further as "long-term shapers of the magnetic field," required spanning multiple disciplines: seismology, mineral physics, geomagnetism, paleomagnetism, computational geodynamics.

Each discipline saw only one part of the elephant. The true picture emerges only when all the blind people piece their information together.

Epilogue: The Distance Beneath Our Feet

Sometimes I wonder how long Tuzo and Jason have existed 2,900 km down. If they truly are primordial relics, they are older than the oldest surface rocks—older than Australia's Jack Hills zircons (4.4 billion years), older than the Moon. They witnessed Earth cool from a ball of magma into a planet, the atmosphere shift from hydrogen-helium to nitrogen-oxygen, the birth of the oceans, the origin of life, every mass extinction and every evolutionary radiation.

And they have never been touched.

We launch probes to Mars, Jupiter, and the edge of the solar system, seeking the secrets of distant worlds. But Earth's greatest secret may have always lain 2,900 km beneath our feet—two continent-sized bodies of hot rock, quietly shaping the magnetic field that protects all life on this planet.

25 billion kilometers outward; 12 kilometers inward. Perhaps the true far distance is not in the sky, but beneath our feet.

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References:

• University of Liverpool (2026). "Scientists discover hidden deep-Earth structures shaping the magnetic field." *Nature Geoscience*.
• Garnero, E. J., McNamara, A. K., & Shim, S.-H. (2016). "Continent-sized anomalous zones with low seismic velocity at the base of Earth's mantle." *Nature Geoscience*, 9(7), 481-489.
• Shrestha, R. (2022). "LLSVPs: Mysteries in the Deep Mantle." EGU Geodynamics Blog.
• McNamara, A. K. (2019). "A review of large low shear velocity provinces and ultra low velocity zones." *Tectonophysics*, 760, 199-220.