The stratovolcano Sete Citades and his circular caldera, 5,000 meters in diameter and a depth of 300-500 meters, occupy west of São Miguel. Its highest point, Pico da Cruz, is 856 meters, or 3,000 m above the basalt plateau forming the underbody of the Azorean archipelago.
The bottom of the caldera is occupied by two connected lakes, the Lagoa azul and the lagoa verde and post-caldera structures.
Legend tells that the two lakes, green and blue, were created by the tears of a princess and her lover, a shepherd, during their separation ... the color of the water recalling the color of their eyes.
The stratovolcano is formed by a stack of lava flows (ankaramite, basanite, alkali basalt, hawaiite, mugearite and tristanite) and pyroclastic fallout.
The oldest subaerial rocks include trachyte domes and tristanite, and flows exposed at the base of the wall of the caldera, and the coast to the west and south of the volcano. Flows of trachyte, located at the base of the NO wall of the caldera, is dated to 210,000 years; another of tristanite, located on the west coast to Ponta da Ferraria, date of 74,000 years.
The stage, which gave the caldera its present size, is dated to 22,000 years, and follows the issuance of trachytic pumice.
São Miguel / Azores - Geological Map of stratovolcano Sete Citades - doc. in Geology of Three Late Quaternary Stratovolcanoes on Sao Miguel, Azores – by R.Moore
At least twenty-two post-caldera eruptions have occurred, including six in his enclosure, with a high phreato-magmatic component; and 16 on the flanks of the volcano.
Eruptions in the caldera are relatively recent : six vents, whose eruptions formed cones and rings of pumice, occupy a position almost circular, following a likely concentric fracture contemporary to the formation of the caldera.
From north to south and anticlockwise, are presented Seara Cerrado da Ladeira to the north, dated 3050 BC., The caldera do Alfreres, 2050 BC, the Caldeira Seca, dated 1444 (?), two domes of trachyte with their top occupied by a lake, lagoa Rasa, dated of the year 380, and the lagoa de Santiago, dated of the year 90. the cinder cone Cerrado das Freiras is built to the east of lagoa azul in 1444.
In 1713, an eruption of the western flank gave the Pico das Camarinhas and Ponta da Ferraria in 1110.
Since 1638, six submarine eruptions took place off the west coast, the last in 1880.
Special landscapes and an eruption just after the arrival of the first Portuguese settlers gave the birth of various legends.
Of these, seven cities established in the caldera have been destroyed by the eruption that followed the arrival of the first Portuguese ... in conjunction with one of the possible positions of the mythical Atlantis.
It has in any case inspired the comic "The enigma of Atlantis", by Edgard P. Jacobs.
About the comic "The enigma of Atlantis" :
On holiday in the Azores, Professor Mortimer made an astounding discovery. When exploring a chasm called "O foro do diabo" (the Devil's Hole), he found an unknown metal with amazing radioactive and luminescent properties. He think of Orichalcum, this mysterious metal that the Atlanteans, in Plato's, considered as valuable as gold. The discovery is important enough to warn that his friend, Captain Blake. The latter joins him in São Migue and events rush. Someone steals the specimen that Mortimer had left in his villa. Blake and he did that time to see a strange craft through space at the speed of light, disappearing into the night. Increasingly intrigued, the two men decided to redo an expedition to "Foro do diabo" to found new samples. They are accompanied by a guide, Pepe and his assistant in the guise, which hides the adventurer Olrik their mortal enemy. The latter was hired by a foreign power to recover the precious ore. The descent into the dangerous calderas begins. This dive into the bowels of the earth will cause them to live their most extraordinary adventure. Elucidating a centuries-old mystery, they will leave to the discovery of Atlantis, its people and its enemies, explore the lost continent and risk their lives to save the peace in the Atlantean Empire. They brave many dangers to return to our world where Atlantis remains an unsolved riddle ... The least of these dangers are not Olrik ...