The Filia volcanic vein is located along the Kallonis – Skala Eresou provincial road, near the settlement of Filia, and is an impressive volcanic structure created by the latest volcanic activity on Lesbos, 17 million years ago.
The volcanic structure was formed through the intrusion of fluid magma, of a dacite composition, into pre-existing volcanic rocks that cover the wider area. Fluid magma moved to the upper parts of the crust and the surface, through faults (tectonic openings in the crust), where it solidified from the gradual loss of energy.The rocks surrounding the solidified magma were removed through erosion over geological time, creating the landform that crowns the hill above the settlement of Filia.