Ancient rocks contain evidence of the Earth’s magnetic field. That’s why it’s disconcerting

A 3.7 billion-year-old record of our planet’s ancient magnetism has been unearthed, providing evidence that The Earth’s magnetic field it already existed very early in history. This finding, however, is quite surprising. Rocks approaching 4 billion years old are difficult to find; most were recycled Earthof tectonic activity, sliding into the mantle through subduction zones […]