A planetary event frozen in stone
In the Lessinia region of the Venetian Prealps, layers of dark volcanic rock tell the story of a dramatic global event. Around 45 to 50 million years ago, during the Eocene epoch, this area was a hotbed of volcanic activity. Fissures in the Earth’s crust poured out fluid basaltic lava, which spread across the ancient seafloor. These eruptions were a local phenomenon; they were part of a larger pattern of volcanism across what is now Europe. The significance of Lessinia's basalt, however, lies in its timing. As this lava cooled and solidified, it recorded the Earth's magnetic field at a moment it was undergoing a complete reversal.
Every few hundred thousand years, the Earth's magnetic field weakens and its polarity flips—magnetic north becomes south and south becomes north. The process is driven by fluid dynamics in the planet's liquid iron outer core. As the Lessinia lavas erupted, tiny magnetic minerals within the molten rock, primarily magnetite, acted like microscopic compass needles. They aligned themselves with the planet's magnetic field. When the rock cooled below the Curie point (about 580°C for magnetite), this magnetic orientation was locked in place. The successive lava flows in Lessinia erupted over a long enough period to record the entire transition. Lower, older layers of basalt show a normal magnetic polarity, while the highest, youngest layers show a reversed polarity. The layers in between document a chaotic, weakened field as the poles shifted.