Assessing paleomagnetic shallowing in basalts using magnetic fabrics

Imagine the Earth is wearing a giant, invisible magnetic coat. This coat isn't just a static blanket; it's a dynamic, shifting field generated deep inside our planet's core, acting like a massive compass needle that points north. For scientists, this magnetic field is a time machine. When rocks form, tiny magnetic minerals inside them act like microscopic compasses, freezing in place to record exactly which way the magnetic field was pointing at that exact moment in history. By studying these "frozen compasses" in rocks from different eras, researchers can map out how the Earth's magnetic field has changed over millions of years, helping us understand the planet's deep interior and even reconstruct how continents have drifted.

However, there's a tricky problem with reading these ancient compasses. Sometimes, the direction the rock records doesn't match where the compass should have pointed. It's as if the needle is tilted or "shallow," pointing more toward the horizon than it should. Scientists have long known that sedimentary rocks (like mud and sand) often suffer from this because the magnetic grains can get squished or tilted as the sediment settles. But there's a second, often overlooked suspect: lava flows. When molten rock erupts and cools, it's not just a solid block; it's a flowing river of magma. As it moves, the magnetic minerals inside can align with the flow, creating a hidden "fabric" or texture. The big question is: could this flow-induced alignment be tricking the magnetic compass, making the recorded direction look wrong even in rocks that are supposed to be perfect records?

This is exactly the mystery Pedro Fernandes da Silva tackles in his study of a historic lava flow from 1761 on Terceira Island in the Azores. He decided to investigate whether the way the lava flowed created a magnetic "fabric" that was distorting the ancient compass directions. To do this, he didn't just look at the magnetic direction; he also mapped the internal "grain" of the rock using a technique called Anisotropy of Magnetic Susceptibility (AMS). Think of AMS as a way to feel the texture of the rock, detecting if the magnetic minerals are lined up in a specific direction, much like how wood grain runs along a board.

The study focused on a well-exposed wall of this 1761 lava flow, where the team collected 16 samples along a 30-meter stretch. They first confirmed that the rocks were made of titanomagnetite, a common magnetic mineral, and that they held a stable, primary magnetic record from when the lava cooled. Then, they compared the direction of the magnetic field recorded in the rocks (the paleomagnetism) against the strength and direction of the rock's internal fabric (the AMS).

The results revealed a clear and fascinating connection. The team found that as the magnetic fabric became more "aligned" or anisotropic (meaning the minerals were more strongly oriented by the flow), the recorded magnetic direction started to shift. Specifically, there was a linear relationship: the stronger the fabric alignment, the more the magnetic inclination (the angle pointing down into the Earth) became shallower. In the most extreme cases, the inclination dropped by about 20 degrees compared to the less aligned samples. Furthermore, the magnetic declination (the compass direction) shifted clockwise, closely following the rotation of the rock's internal magnetic axes.

When the researchers split their samples into two groups—one with lower fabric alignment and one with higher—they saw the difference clearly. The low-alignment group recorded a steeper, more "correct" angle that matched global magnetic models for that time period. The high-alignment group, however, showed a significantly shallower angle and a shifted direction. This suggests that the flow of the magma itself physically twisted the magnetic record. The paper argues that this isn't just a minor glitch; it's a significant source of error that has likely been missed in many studies.

Ultimately, the paper concludes that magnetic fabric analysis is a crucial tool that shouldn't be ignored. It acts like a quality control check for paleomagnetism. If you see a magnetic record that looks a bit "off" or shallow, checking the rock's internal fabric can tell you if the rock itself is the culprit. By understanding how the lava flowed and how that flow aligned the magnetic minerals, scientists can better assess the reliability of their ancient compass readings and avoid misinterpreting the history of Earth's magnetic field.