Although we rarely stop to consider it, Earth’s magnetic field is an important part of our daily lives. The magnetic field protects our planet from charged particles emitted by the Sun; high doses of these particles can cause malfunctions in the electrical grid and in satellites.
But thanks to complex processes unfolding deep within Earth, the magnetic field is ever changing. To predict what it might do in the future, scientists are looking deep into the past.
This is no easy task—intentionally kept human records of the magnetic field exist for only the past few hundred years. To see further back in time, scientists rely on “accidental” records, as well as on volcanic and sedimentary rocks, said Lisa Tauxe, a geophysicist at the Scripps Institution of Oceanography. Tauxe is a coauthor of a new study that used ancient artifacts to analyze changes in Earth’s magnetic field thousands of years ago—as far back as 7600 BCE. The study was published in the Proceedings of the National Academy of Sciences of the United States of America.
These accidental records, Tauxe explained, can be created when certain materials are heated to very high temperatures. When they cool, magnetic minerals within the material are frozen in place, providing a snapshot of the direction and strength of Earth’s magnetic field at the time.
Clues from the Past
Ancient pottery is a robust and widely used way to study the history of Earth’s magnetic field. But in the context of geologic history, pottery poses a problem for researchers who want to delve deeper into the past. That’s where the present study comes in.
“In this study, what we did was to try to push the record back beyond pottery,” said Tauxe.
These researchers wanted to see whether they could obtain information about the magnetic field from flint, one of the most common materials used to make stone tools. It is thought that ancient humans deliberately heated the flint to make it easier to work with, said study coauthor Anita Di Chiara, a paleomagnetist at Scripps and at Italy’s National Institute of Geophysics and Volcanology.
Heated flint dates back much further than pottery, likely to around 50,000 years ago, so it could potentially increase our understanding of the magnetic field’s past by a substantial amount. Di Chiara said that obtaining data from flint is difficult because it is generally not very magnetic. However, in this study, researchers were able to obtain data from tiny amounts of impurities in the flints.
Using artifacts from Jordan, including both flint and pottery, researchers found that the magnetic field around 7600 BCE was only two thirds the strength of today’s field but just 600 years later had greater strength than today’s field. Then, around 5200 BCE, it weakened again.
Today, said Tauxe, the strength of Earth’s magnetic field is dropping very quickly. Although this change isn’t necessarily catastrophic, Tauxe said it could cause problems for some types of technology. “Our electrical grid and satellites will become more vulnerable to solar storms.… We need to build our infrastructure with that in mind—that we’re losing the protection of the magnetic field.”
Although Tauxe said the overall strength of the field won’t be very low for another 500 or so years, she noted that one region of the world, an area over parts of South America and the South Atlantic Ocean, is already experiencing an unusually weak magnetic field. This weak spot, called the South Atlantic Anomaly, can leave satellites vulnerable to charged particles, which can cause glitches and malfunctions.
Although studying the magnetism of ancient objects is important for understanding the magnetic field’s history, it could also help us understand human history. Erella Hovers, a professor of prehistoric archaeology at the Hebrew University of Jerusalem, said that studying the magnetism of ancient artifacts can be very useful in an archaeological context. Such study can help archaeologists determine the relative ages of ancient human-made objects. This method is especially useful for objects that are beyond the scope of carbon dating, which is effective only for things that are less than 50,000 years old.
For example, she said, archaeomagnetism allows scientists at older sites to “see if the features [they’re] studying were created at one particular point in time, which would suggest something about the rapidity of the site being formed, or whether it’s just a big hodgepodge of things that were mixed in place and were originally formed in different periods.”
Both Tauxe and Di Chiara emphasized the collaborative nature of this work and noted the importance of partnerships between geophysicists and archaeologists in learning more about Earth’s magnetic history.
—Hannah Thomasy (@HannahThomasy), Science Writer
Thomasy, H. (2021), Ancient flint tools reveal Earth’s changing magnetic field, Eos, 102, https://doi.org/10.1029/2021EO163691. Published on 27 September 2021.
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