Geography
The lake is situated within the caldera of the Sete Cidades Massif, an ancient volcano built on various layers of ash, pyroclasts and trachyte and basaltic lavas. It is a stratovolcano constructed from alternating phases of explosive and effusive ejecta, from dominantly basaltic pre-caldera eruptions, a trachytic caldera-forming stage and a post-caldera stage consisting of alternating trachytic and basaltic eruptions. The caldera-forming eruption was triggered by a basaltic injection into a shallow trachytic magma chamber.
These geomorphological structures allowed varying hydro-chemical properties and produced many types of springs and water circulation networks (Coutinho et al., 1996). Generally, formations with good hydrodynamic properties include areas with lava flows, basaltic pyroclasts and/or pumice, while solidified Ignimbrites, conditioned by high temperatures and paleo-soils, such as ash, have produced impermeable conditions. The aquifers are dependent on reduced permeability, or secondary volcanoes with hydro-geological significant volumes. Consequently, the accumulation of ash at the bottom of the craters and caldera have conditioned the formation of lakes.
Although, hydrologically, the Lagoa das Sete Cidades is one lake (it is only divided by a bridge), most refer to it in terms of two separate bodies: Lagoa Verde and Lagoa Azul. Literally, the Green Lake and the Blue Lake (respectively), they are named for the reflect pigment of bodies: each side of the lake reflects the sunlight in different colors.
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