Mt. Etna is an unusual volcano in that most of the world's volcanoes occur on constructive and destructive plate boundaries while Etna is formed on a unique boundary between two continental plates, the European and the African plates, which are pushing towards each other due to convection currents in the mantle beneath the Earth's crust, and will eventually eliminate the Mediterranean.
This map highlights the boundary on which Etna lies and shows global plate boundaries. Notice also that the same boundary, going up through Italy, is responsible for Mt. Vesuvius
The following diagram shows how the pressure applied by the plates on each other has created faults under the Mediterranean. Magma has then forced its way up to the surface to form volcanoes.
The diagram below shows the basic structure of a volcano and if you refer to the national park map you will see many examples of domes (side vents).
The volcano is intitially formed by the escaping lava depositing heavier loads close to the crack in the Earth forming layers in the characteristic cone shape.