Formed by a buttress made up of eruptive rocks, the volcanic complex of Serra de Todo-o-Mundo appears to have been formed by a lava flow, resembling a half-cone.
"The Serra de Todo-o-Mundo is formed by a buttress made up of eruptive rocks (dolerites) from an altitude of 200 metres, which are exploited in the northern section of the mountain range for the production of gravel. This buttress, which comprises the «volcanic complex of the Serra de Todo-o-Mundo» and which «appears to be made up of a sill or a lava flow», describes a semicircle, a shape that is best seen from the top of Montejunto or along the road that connects Cercal to Caldas da Rainha, and this peculiar shape seems to be «in a possible relationship with a (volcanic) chimney». This phenomenon is not, in fact, strange to the morphostructural unity of the western coast, in which, although, as we have seen, limestone sedimentary rocks dominate, there are records of eruptive phenomena, of which the Lisbon-Mafra volcanic complex stands out, origin is located between the end of the Cretaceous and the beginning of the Cenozoic.