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.
The Eocene environment
The same volcanic activity that produced the basalt flows also created the conditions for one of the world's most spectacular fossil sites. Nearby, the town of Bolca is famous for its Lagerstätte—a sedimentary deposit with exceptionally preserved fossils. During the Eocene, this area was a tropical lagoon, part of the Tethys Ocean. The volcanic eruptions periodically released huge clouds of ash that settled on the seafloor, creating an anoxic environment devoid of oxygen. This suffocated marine life and buried it in fine limestone mud, preventing decay and scavenging.
The result is a perfectly preserved census of a 50-million-year-old ecosystem. Over 250 species of fish, including early angelfish and moonfish, have been identified, many with skin and organs still intact. The fossils also include plants like palm trees, as well as insects and reptiles. These finds, together with the basalt flows, provide details about the Eocene world in this region—a warm, tropical sea with violent volcanic episodes. In some locations, like San Giovanni Ilarione, the slow cooling of thick lava flows caused the rock to contract and fracture, forming fields of striking hexagonal basalt columns.