The caldera has lots of trachytic pumice in its soil deposits, not from the caldera itself but trachytic tephra airfall from the nearby Furnas volcano to the west which is a much more active caldera for sure. The caldera lake was formed 15 000 years ago and that was the last really large eruption, but there been numerous trachytic explosive eruptions since that. Sete Cidades have a long history of both trachytic and basaltic eruptions, flanks mostly produce mafic eruptions making mafic scoria cones, while the trachytic caldera produce explosive pyroclastic eruptions whose pyroclastic flows and tephra falls have laid down pale lapilli and pumice over the entire peninsula, seen in cliff exposures at the coast.