Using bricks produced in Mesopotamia more than three thousand years ago, scientists have developed a method that allows them to determine how our planet’s magnetic field has changed and evolved over time. This, in turn, can help us make better predictions about its current and future behavior.
The results and description of the research were published in the journal Proceedings of the National Academy of Sciences (DOI: 10.1073/pnas.2313361120).
Magnetic field
The Earth’s magnetic field is a kind of shield that surrounds the planet and protects it from solar winds and cosmic radiation. Currently, the most popular dynamo theory supported by the strongest evidence (the magnetohydrodynamic dynamo theory) posits that the magnetic field is generated by streams and rotating masses of molten iron located at the core of our planet.
But the Earth’s magnetic field is not constant, it changes with time. These changes may be the result of external factors such as solar wind. It may also depend on changes occurring within the planet. Satellites provide new data on current changes in the Earth’s magnetic field, but to capture its long-term evolution, other methods must be used.
Changes in the magnetic field can be recorded by various materials present on the planet’s surface. For example, in lava, the magnetic field existing at a given moment is preserved. Minerals sensitive to magnetic fields align with the Earth’s magnetic field and maintain this position. Scientists can use these rocks as a record of the magnetic field.
Bricks from Mesopotamia
A team of scientists led by archaeologist Matthew Howland from Wichita State University in the USA decided to examine bricks in Mesopotamia. The scientists wanted to know if the magnetic field left any traces in the building materials.
Scientists examined 32 clay brick fragments from Mesopotamia. Each one of them was marked with the name of the king who ruled at the time the bricks were made, so they knew approximately when the bricks were made.
They then carefully cut a small piece from each brick and used a magnetometer to measure the alignment of the microscopic iron oxide grains embedded within them. This technique allowed them to reconstruct the magnetic field on a large scale over a period of approximately 2,000 years, from the third millennium to the first millennium BC.
“We often rely on dating methods such as radiocarbon dating to determine the chronology of ancient Mesopotamia,” explains archaeologist Mark Al-Tawil of University College London. However, some of the most common cultural remains, such as bricks and pottery, cannot be easily dated because they do not contain organic materials. He adds that this work now helps provide an important basis for dating that allows others to benefit from absolute dating using archaeological magnetism.
abnormality
In the next step, the researchers compared their results with other magnetic field reconstructions obtained from archaeological magnetic studies. Thanks to this, it was possible to confirm the presence of a magnetic anomaly in what is now known as Iraq at that time. The researchers determined that the magnetic field was unusually strong during the period from 1050 to 550 BC. Furthermore, the analysis revealed short fluctuations during the reign of Nebuchadnezzar II between 604 and 562 BC, showing that the Earth’s magnetic field could change significantly over short periods of time.
“The well-dated archaeological remains of the rich cultures of Mesopotamia, especially bricks inscribed with the names of specific kings, provide an unprecedented opportunity to study changes in field strength with high temporal resolution, tracing changes that occurred over several decades or even less,” says Lisa Tookes of Scripps Institution of Oceanography, USA.
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