Destruction as a key to preserving the past? It sounds paradoxical—fires, floods, and war have often wiped out historic records and infrastructure. But destructive events are also a source of knowledge when it comes to the study of Earth’s magnetic field. Items burned during ancient upheavals store geomagnetic information from long ago—and those data can illuminate the timeline of human history.
A research team specializing in archaeology and geomagnetism has collected magnetic information from burned mud bricks and other objects at 15 archaeological sites spread across the southern Levant. These efforts build on the team’s recently published study that examines the direction and intensity of the geomagnetic field in 586 BCE, when Babylonian forces burned the city of Jerusalem. The new research is broader in scope than the paper is, providing detailed geomagnetic data from throughout the region during much of the Levantine Iron Age geomagnetic anomaly, a period of about 500–600 years when the intensity of the geomagnetic field was unusually high.
“They’re doing just really great work,” said Michele Stillinger, a professor of Earth science at Dougherty Family College, University of St. Thomas, who was not involved in the research. “I’m excited to see what kind of results they get and to compare across the different sites.”
Many materials, including rocks and soils, contain minerals that are magnetic. When exposed to intense heat, the material’s internal magnetic signature is erased. This happens to lava during a volcanic eruption, for example, and to clay pottery as it’s fired in a kiln. As the material cools down again, it takes on the characteristics of the geomagnetic field that surrounds it.
Vaknin and his colleagues measured the magnetic direction and intensity of dried mud bricks, a common building material in the ancient Near East that was often burned during times of war. Although other objects (like pottery) are also viable sources of magnetic information, such items are less likely to remain fixed in place than blocks used in construction—and a shift in location and orientation makes it impossible to extract meaningful data on geomagnetic direction.
“A structure that has been fired in place in these destruction events gives you so much more data as to exact moment in time when that destruction occurred,” said Stillinger.
Multipurpose Time Capsule
Because the geomagnetic field is constantly changing direction and intensity, objects that burn at different times will record different geomagnetic signatures. These snapshots of the geomagnetic field sometimes can be linked to well-recorded events in history—such as the destruction of Jerusalem, which historians have narrowed down to August 586 BCE. “We know [the city was burned] in August 586 BCE,” said Vaknin. “There is an argument [over whether] it’s 3 days earlier or 3 days later.”
Such events are fundamental to archaeomagnetic dating, a process that first uses burned material to build a geomagnetic timeline, then uses that timeline to date other archaeological objects through their magnetic properties. This technique is an important complement to more traditional archaeological methods like radiocarbon dating. But Vaknin’s data are also valuable for the study of geomagnetism itself.
All the magnetic information goes into a worldwide database, said Vaknin, and researchers try to reconstruct and build models of the behavior of the geomagnetic field. “It’s a very big enigma, why the geomagnetic field behaves as it does.”
—Alice McBride ([email protected]), Science Writer