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This subsection contains mostly Miocene volcanic rocks, Tertiary subvolcanic (hypabyssal) rocks, and, on San Clemente Island, marine sedimentary rocks of the Franciscan Complex.
The island is composed of volcanic and subvolcanic rocks formed between the end of the Pliocene and the Holocene periods, and it marks the only place in the Brazilian territory where part of a volcanic cone is still recognizable.
The archipelago primarily consists of highly alkaline and subsatured volcanic and subvolcanic rocks and is part of a volcanic mountain that developed along an east-west oceanic fracture zone.