Archaeomagnetism Helps Analyze Earth's Magnetic Field

In 2008, Erez Ben-Yosef unearthed a piece of Iron Age "trash" and inadvertently revealed the strongest magnetic-field anomaly ever found. This was sourced from Live Science.

Ben-Yosef, an archaeologist at Tel Aviv University, had been working in southern Jordan with Ron Shaar, who was analyzing archaeological materials around the Levant. Shaar, a geologist at The Hebrew University of Jerusalem, was building a record of the area's magnetic field.

The hunk of copper slag — a waste byproduct of forging metals — they found recorded an intense spike in Earth's magnetic field around 3,000 years ago.

When Ben-Yosef's team first described their discovery, many geophysicists were skeptical because the magnitude of the spike was unprecedented in geologic history. "There was no model that could explain such a spike," Ben-Yosef told Live Science.

So Shaar worked hard to give them more evidence. After they had analyzed and described samples from around the region for more than a decade, the anomaly was accepted by the research community and named the Levantine Iron Age Anomaly (LIAA). From about 1100 to 550 B.C., the magnetic field emanating from the Middle East fluctuated in intense surges.

Shaar and Ben-Yosef were using a relatively new technique called archaeomagnetism. With this method, geophysicists can peer into the magnetic particles inside archaeological materials like metal waste, pottery and building stone to recreate Earth's magnetic past.

This technique has some advantages over traditional methods of reconstructing Earth's magnetic field, particularly for studying the relatively recent past.

Generally, scientists study Earth's past magnetic field by looking at snapshots captured in rocks as they cooled into solids. But rock formation doesn't happen often, so for the most part, it gives scientists a glimpse of Earth's magnetic field hundreds of thousands to millions of years ago, or after relatively rare events, like volcanic eruptions.

Past magnetic-field data helps us understand the "geodynamo" — the engine that generates our planet's protective magnetic field. This field is generated by liquid iron slowly moving around the planet’s outer core, and this movement can also affect, and in turn be affected by, processes in the mantle, Earth's middle layer. So differences in the magnetic field hint at turmoil roiling deep below the surface in Earth's geodynamo.

"We cannot directly observe what is going on in Earth's outer core," Shaar told Live Science. "The only way we can indirectly measure what is happening in the core is by looking at changes in the geomagnetic field."

Knowing what the magnetic field did in the past can help us predict its future. And some studies suggest our planet's magnetic field is weakening over time. The magnetic field shields us from deadly space radiation, so its weakening could lead to a breakdown in satellite communications, and potentially increase cancer risk. As a result, predicting the magnetic field based on its past behavior has become ever more important. But observational data of the magnetic field’s intensity only began in 1832, so it's difficult to make predictions about the future if we only dimly understand the forces that steered the magnetic field in the past. Archaeomagnetism has started to fill these gaps.