Furnas volcano located at the eastern tip of the island of Sao Miguel, contains two nested calderas, the youngest of 6 km. wide and an older, larger but less distinct in the local topography. The northeast wall of the former caldera notch the west wall of the caldera Povoação of the ancoent Nordeste volcano.
Sao Miguel - the nested calderas Furnas (dotted and dashed boundaries) - in Geology of Three Late Quaternary Stratovolcanoes on Sao Miguel, Azores – by R.Moore
Activity of Furnas volcano have started here about 100,000 years, with some 85 eruptions recorded; ancient rocks are "tristanites" exposed in the cliffs of Ribeira Quente and cover the flows of the Nordeste volcano.
There are about 30,000 years, a Plinian eruption and the issuance of trachytic ignimbrites are responsible for the formation of the oldest caldera; they will be gradually filled by various eruptions.
Latest Furnas caldera was formed 12,000 years ago, in response to the ejection of trachytic pumice (subaerial volume estimated 7 km ³, but more importantly including the impact in the ocean, impossible to quantified) which can be seen in deposits 10 km. north of the village of Furnas. This ignimbrite unit is called "ignimbrite Povoação." Three cones of tristanite, two of which related to the flows, are dated post-caldera . Similarly, a series of seven domes of trachyte, whose the Pico do Ferro, are located on fractures formed during the collapse of the caldera.
The post-caldera volcanic activity has been significant in the past 5000 years; at least 11 layers of trachytic pumice testify, classified " Furnas A to Furnas J" and the Pico do Gaspar, a trachytic lava dome located to the East of the crater lake Lagoa das Furnas.
Its formation follows several Plinian eruptions accompanied by pyroclastic flows and the formation of two nestled rings of pumice; they were dated respectively 840 + / - 100 and 1441 years latter, which occurred during the Little Ice Age, issued 100 million cubic meters of tephra; it is classified as VEI 4.
The last eruption of VEI 5, dated September 3 to November 2, 1630, resulted in hundreds of dead on the island; it is accompanied by significant emissions of pumice, pyroclastic flows and the formation of a trachytic lava dome, the Diego Preto / Mont Areia, in the center of a ring of pumice, the Cova da Burra.
Azores - São Miguel - Furnas volcano: the dome of the Pico do Gaspar is encircled by a line of trees (between the lake and the summit lava dome) which marks the edge of the ring of pumice. - Photo Rick Wunderman / Smithsonian institute.
A fumarole field with thirty hot springs (15 ° to 60 ° C), and some geysers, is in the village of Furnas, north of the lake.
Geothermal areas are exploited by people to cook their stew, the "Cozido", they do simmer several hours in pots, wrapped in rags, in holes of 60 cm deep.
Among the ingredients composing the "Cozido" include : cabbage, potatoes, carrots, sweet potato, with as meat, chorizo, chicken, beef and black pudding ... invigorating!
Azores - Sao Miguel - Furnas volcano - geothermal area and cooking platform of "Cozido" - photo Geoparque Azores