The earliest known evidence for ancient knowledge of magnetism is the writing of the sixth century BCE Greek philosopher Thales of Miletus, who described the attraction between a magnetite-rich lodestone and fragments of iron.
However, there is some circumstantial evidence that an understanding of magnetism existed in Mesoamerica before even then: a magnetized bar unearthed at the Olmec site of San Lorenzo in southern Mexico has been dated to 1400-1000 BCE; a strong magnetic anomaly on the snout of an animal sculpture at Izapa has been suggested to have been a deliberate inclusion during its manufacture; and strong magnetism has been measured on the surface of a group of large sculptures at Monte Alto in Guatemala known as the “potbellies”.
The thing that first grabbed people’s attention about the potbellies’ magnetic anomalies a number of decades ago is that they seem to be consistently located in the same areas of the body on the sculptures. However, the magnetic anomalies remained largely anecdotal: previous measurements were done with everyday magnetic tools, and no complete survey of the sculptures was done scientifically.
However, a group of researchers from Harvard, Yale, MIT and other institutions recently examined the sculptures with more precise magnetic measurement equipment. And in an article in the Journal of Archaeological Science titled “Knowledge of Magnetism in Ancient Mesoamerica: Precision measurements of the potbelly sculptures from Monte Alto, Guatemala“, they announced a rather stunning result: “significant magnetic field anomalies” were discovered in close association with two particular body regions – the navel (on full body depictions) and “the right temple forward of the ear” (on head sculptures). And, that this appeared to be an intentional part of their manufacture.
We confirm the observations of Malmström (1997) that all three colossal heads that have significant anomalies exhibit the strongest signals over the right temple and cheek areas. Similarly, we find that all four magnetized full body sculptures have a strong signal overlapping the navel.
[Statistical analysis shows that] for both types of sculptures, the consistent colocation of…anomalies with specific anatomical features is inconsistent with a random distribution at the P < 0.01 level.
Furthermore, analysis of the magnetic patterns indicated that the magnetism was probably caused by “a lightning-induced electrical current across the rock surface” which predated the manufacture of the sculptures.
According to the researchers, the “apparently intentional colocation of carved anatomical features and pre-existing magnetized regions” implies that the sculptors had knowledge of magnetism, and had some sort of method and/or tools that allowed them to detect the presence of anomalous magnetic fields.
What could those tools have been? Measurements of the magnetic fields on the sculptures showed that the anomalous areas were “sufficient to visibly deflect a magnetic compass needle suspended within up to ∼10 cm of the surface.”
A small number of known Mesoamerican artifacts can plausibly be used as magnetic compasses to detect the magnetic anomalies on the Monte Alto sculptures, although they have not been reported at the site. Nevertheless, their presence at earlier sites elsewhere in Mesoamerica opens the possibility that the Late Preclassic inhabitants of the Pacific coast were aware of the properties of lodestones. Most intriguingly, the hematite-rich bar M160 from San Lorenzo was able to align to Earthstrength magnetic fields with <1° precision. Such a device would have readily indicated the locations and morphologies of magnetized regions on the sculptures if it had been available to the ancient Monte Alto sculptors.
Among other known ferromagnetic artifacts, mirrors manufactured from a range of Fe-bearing minerals have been found throughout Mesoamerica. In particular, mirrors made from the minerals magnetite and hematite may potentially be used as lodestones to indicate the direction of magnetic fields. Such mirrors have been recovered from several sites in southern Mexico, indicating manufacture and trade of these objects during the transition between the Early and Middle Preclassic periods
Furthermore, the researchers say, previous suggestions that a lodestone may also have been used to create the lightning-associated petroglyphs at Petroglyph Lake (Oregon, USA), the Providence Mountains (California, USA), and Grimes Point (Nevada, USA) “raises the possibility that knowledge of magnetism was extensive across the North American continent”.
It should be noted, however, that not all of the sculptures surveyed by the research group showed magnetic anomalies. As such, they note that magnetism may not have been the primary consideration in choosing material for a potbelly sculpture. Instead, “if a magnetic anomaly was detected on a preselected boulder, the makers of the sculpture then oriented the carving to place the anomaly near a specific location.”
Overall, however, the researchers concluded that their field measurements of magnetic anomalies on the group of sculptures provides “robust evidence that knowledge of magnetism existed in the Americas by the second half of the first millennium BCE.”