The tuff cones, also known as hyaloclastite cones, are associated with surtseyan basaltic eruptions of hydromagmatic type. The explosiveness of these eruptions results from the fact that the pipeline is submerged in shallow waters, in most cases at sea, but also at lagoons.
They show often a well-defined conical shape, such as the Ilhéu de Vila Franca (islet in São Miguel Island), the Monte da Guia (Faial Island) and the Monte Brasil (Terceira Island). The surtseyan tuff cones present a great compactness, clear stratification, presence of load figures and, when altered, the pyroclastics have a yellowish or brownish colour.
Photo: Ilhéu de Vila Franca