One of the most impressive volcanic domea of Lesvos is the Portos volcanic dome, which rises imposingly on the Mesotopou-Eresos provincial road near the Pythariou Monastery. It is a dome-shaped hill with steep slopes. The volcanic rock forms impressive intense and dense fissures that create successive prismatic pillar-like forms.
It was formed by fluid magma that penetrated the crust along a rift running E-W and which appears today as a volcanic vein of great thickness. The magma moved upward at low speed, penetrated the older metamorphic rocks of the basement of Lesvos about 300 million years old, obscured the older volcanic formations in the area and created a giant dome without causing a volcanic eruption.
The volcanic structure was created in the last period of volcanic activity on Lesvos 16.5 million years ago. It is the largest of a series of volcanic structures that occur in the same area. The magma cooled and solidified at a relatively shallow depth from the surface. The erosion of the pyroclastic rocks that surrounded the volcanic rocks revealed the impressive dome that dominates the area today.