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The strength and direction of the Earth’s magnetic field have changed a lot over the millennia. Scientists are eager to study its past patterns to determine how the field might change in the future – a fairly vital area of research, given that this magnetic shield protects us from harmful cosmic radiation.
However, instruments capable of directly measuring the Earth’s magnetic field have only been around for about 200 years. So we need to look to other methods to go further back in time – including, in a new study, artefacts recovered from a site in Jordan, dated to around 8,000 to 10,000 years ago (the Neolithic or New stone Age).
These objects – including ceramic pottery and burnt flint used to make other tools – are special because their creation involved subjecting them to extremely high temperatures.
This heating process and subsequent cooling caused certain minerals and crystals in the artifacts to trap a “frozen” record of what Earth’s magnetic field looked like at the time, a phenomenon known as residual or afterglow magnetization.
“This is the first time that burnt flint from prehistoric sites has been used to reconstruct the magnetic field of their time,” says archaeologist Erez Ben-Yosef of Tel Aviv University in Israel.
“Working with this material expands the possibilities of research tens of thousands of years ago, because humans used flint tools for a very long time before the invention of ceramics.”
The excavation site in Jordan. (Thomas E. Lévy)
The researchers looked at 129 different elements in total, building on previous work that assessed the viability of using flint fragments as a guide to magnetic field strength, which will be incredibly useful for future studies. .
What the team found was a decrease in the strength of the magnetic field during the period of use of the artefacts, followed by recovery over a few hundred years (not long at all in the large diagram). of the history of the planet).
With Earth’s magnetic field currently weakening over time – a potential cause for concern – it helps to know that this has happened before and that doesn’t necessarily mean our protective bubble will disappear over the next few centuries.
“The results of our study can be reassuring: it has happened in the past,” says geophysicist Lisa Ratee of the Scripps Institution of Oceanography.
“About 7600 years ago the strength of the magnetic field was even weaker than it is today, but in about 600 years it gained strength and reached high levels again.”
Our planet’s magnetic field is thought to be generated by convection currents created as molten iron and nickel circulate in the Earth’s outer core, some 3,000 kilometers (1,864 miles) underground – but it doesn’t there’s not much else we’re sure about.
Some of the unanswered questions about this great phenomenon focus on how the magnetic field might relate to processes in the Earth’s atmosphere and changing climate, and how they influence each other.
And while geological studies allow us to go further back in terms of tracking changes in Earth’s magnetism, they do not offer the precision in terms of timing that archaeological analysis does – as this latest study demonstrates.
“The essence and origins of the magnetic field have remained largely unresolved. In our research, we have sought to open a hole in this great conundrum,” says Ben-Yosef.
The research was published in PNAS.
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