Sete Cidades Massif

Sete Cidades Massif (Maciço de Sete Cidades)
Massif (Maciço)
The emblematic view of the Sete Cidades Massif is highlighted principally by the Lagoa das Sete Cidades crater lake
Official name: Maciço de Sete Cidades/Vulcão de Sete Cidades
Named for: Sete Cidades (Ponta Delgada)
Nickname: Vulcão de Sete Cidades
Country	Portugal
Autonomous Region	Azores
Islands	Eastern Group
Location	Azores Platform, Mid-Atlantic Ridge, Atlantic Ocean
Municipalities	Ponta Delgada
Landmark	Lagoas de Sete Cidades
Highest point	Cuumeiras
- location	Serra Devassa, Ponta Delgada, São Miguel
- coordinates	37°52′11″N 25°46′48″W / 37.86972°N 25.78000°W / 37.86972; -25.78000
Lowest point	Sea level
- location	Atlantic Ocean
- elevation	0 m (0 ft)
Biomes	Temperate, Mediterranean
Geology	Alkali basalt, Tephra, Trachyte, Trachybasalt
Orogeny	Volcanism
Period	Holocene
Location of the Sete Cidades Massif on the island of São Miguel

Sete Cidades Massif is a stratovolcanic complex, referring to a polygenetic volcano and caldera, located in western part of the island of São Miguel, in the Portuguese archipelago of the Azores. More recognizable for the Lagoa das Sete Cidades at its centre, the volcanic complex includes centuries of geomorphological structures that include lava domes, cones, lava flows and maar geomorphology that have marked its history.

The almost circular caldera is a nested structure formed as a result of three separate caldera collapse episodes, each associated with explosive eruptions. These caldera collapse episodes occurred following explosive eruptions that formed the Risco, Bretanha and Santa Barbara pyroclastic fields.

The first phase occurred 35,700 years ago, and ceased with the collapse of the principal volcano. A secondary phase began around 28,750 years ago and was responsible for the collapse of the northwest portion of the primitive caldera. Around 15,740 years ago the last phase, marked by the collapse of the northern and northeastern portions of the crater occurred. In each of these formational phases, lapilli fallout (resulted from Plinian eruptions) and the pyroclastic flow deposits were transformed into ignimbrites, from melting.

The Sete Cidades Massif has experienced 17 intra-caldera eruptions within the last 5000 years, which makes it the most active in the archipelago. The intra-calderan activity has been essentially marked by Trachyte sub-Plinian and Plinian eruptions, with some having hydromagmatic characteristics. These formations date to 35,700 B.C., 28,750 B.C. and 15,740 B.C. Numerous smaller explosive eruptions then occurred between these paroxysmal events. In the last 5000 years, 17 explosive eruptions took place within the Sete Cidades volcano, resulting in fallout and hydromagmatic eruptions. Location of these eruptions vary, but they generally took on a southerly dispersal. Yet, there is geological evidence for pyroclastic density currents associated with three or four explosive events, but high concentrations of pyroclastic flows are sparse. It may be likely that: