On February 16th, 2017 we learned about

Ancient ceramics found to mark fluctuations in Earth’s magnetic field

Three thousand years ago, a potter stamped a jar with a royal seal, inadvertently recording a bit of geological data that we’ve only recently been able to interpret. The seal inscribed in the fired pottery wasn’t the mystery— rulers near what is now Jerusalem regularly demanded tax payments in the form of stamped jars of oil or wine. The hidden record trapped in these pieces of pottery was actually tiny, magnetically-sensitive bits of iron, which captured a snapshot of the Earth’s magnetic field at the moment the clay was hardened.

Polarized particles in pottery

The Earth has a magnetic field that is thought to be generated by large amounts of iron in the planet’s core. This field helps us with everything from keeping our compasses working to blocking damage to our atmosphere from solar wind. There’s been some concern over the last 180 years when measurements found that the magnetic field seemed to be weakening, which is not a trend we’d want to see continue, lest we end up with a thin atmosphere like what you have on Mars. However, figuring out how strong this trend was was difficult, because we only have data from the date the magnetometer was invented, 180 years ago.

This brings us back to our stamped pots. When those pots were made, their clay contained ferrous particles that were sensitive to the Earth’s magnetic field. When the clay was still soft, those particles had enough range of motion to orient themselves to the field’s alignment. When the pottery was fired in a kiln, the particles positioning became locked and preserved by the hardened clay, a bit like what’s been found in dead, magnetized cockroaches. By looking at the alignment of these particles, researches were able to estimate the strength of the planet’s magnetic field at different times.

Putting the pieces together

However, to compare those measurements to our more recently measured data, researchers also needed some kind of chronological reference point. Fortunately, the stamps on each pot corresponded to specific rulers, and each time there was a political upheaval, a new stamp would be introduced, giving researchers reference points sometimes as precise as a 30-year time span. They could then build a timeline of magnetic field measurements, checking on what the Earth was up to 3000 years ago.

The good news is that there was a lot of fluctuation in the Earth’s magnetic field. Over the 600 years recorded in ancient pottery, the strength of the field both increased and decreased, suggesting that what we’re now experiencing is a natural fluctuation, and not necessarily an indication of a one-way trend towards a less magnetic future. The next question is why there’d be this much fluctuation, but that seems to be beyond the scope of the pottery at this point.

Source: Iron Age Potters Carefully Recorded Earth's Magnetic Field — By Accident by Rae Ellen Bichell, The two-way

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