The archipelago of the Azores is located in the middle of the northern hemisphere of the Atlantic Ocean and extends along a west-northwest to east-southeast orientation (between 36.540 North latitudes and 24.531.5 West longitudes) in an area approximately 600 kilometres (373 miles) wide. The islands of the Azores emerged from what is called the Azores Platform, a 5.8 million km2 region that is morphologically accented by a depth of 2,000 metres (6,600 feet).
From a geostructural perspective the Azores is located above an active triple junction between three of the world's large tectonic plates (the North American Plate, the Eurasian Plate and the African Plate), a condition that has translated into the existence of many faults and fractures in this region of the Atlantic. The westernmost islands of the archipelago (Corvo and Flores) are located in the North American Plate, while the remaining islands are located within the boundary that divides the Eurasian and African Plates.
The principal tectonic structures that exist in the region of the Azores are the Mid-Atlantic Ridge, the Terceira Rift, the Azores Fracture Zone and the Glria Fault. The Mid-Atlantic Ridge is the main frontier between the American Plate and the African-Eurasian Plates that crosses the Azores Platform between the islands of Flores and Faial from north to south then to the southwest; it is an extensive form crossed by many transform faults running perpendicular to its north-south orientation, that is seismically active and susceptible to volcanism. The Terceira Rift is a system of fractures that extends from the Mid-Atlantic Ridge to the Glria Fault that represents the main frontier between the Eurasian and African Plates. It is defined by a line of submarine volcanoes and island mounts that extend northwest to southeast for about 550 kilometres (342 miles), from the area west of Graciosa until the islets of the Formigas, that includes the islands of Graciosa, Terceira and So Miguel. Its northwest limit connects to the Mid-Atlantic Ridge, while the southeast section intersects the Gloria Fault southeast of the island of Santa Maria. The Azores Fracture Zone extends from the Glria Fault and encompasses a relatively inactive area to the south of the islands of the Central and Eastern groups north to the Terceira Rift, along a 45 angle. The Glria Fault, for its part, extends 800 kilometres (497 miles) along a linear line from the Azores to the AzoresGibraltar Transform Fault.
The island's volcanism is associated with the rifting along the Azores Triple Junction; the spread of the crust along the existing faults and fractures has produced many of the active volcanic and seismic events, while supported by buoyant upwelling in the deeper mantle, some associate with an Azores hotspot. Most of the volcanic activity has centered, primarily, along the Terceira Rift. From the beginning of the island's settlement, around the 15th century, there have been 28 registered volcanic eruptions (15 terrestrial and 13 submarine). The last significant volcanic eruption, the Capelinhos volcano (Vulco dos Capelinhos), occurred off the coast of the island of Faial in 1957; the most recent volcanic activity occurred in the seamounts and submarine volcanoes off the coast of Serreta and in the Pico-So Jorge Channel. The islands have many examples of volcano-built geomorphology including many of the caves and subterranean lava tubes (such as the Gruta das Torres, Algar do Carvo, Gruta do Natal, Gruta das Cinco Ribeiras), the coastal lava fields (like the coast of Feteiras, Faial, the Mistrio of Prainha or So Joo on Pico Island) in addition to the currently inactive cones in central So Miguel Island, the aforementioned Capelinhos on Faial, the volcanic complexes of Terceira or Plinian caldeira of Corvo Island.
The islands of the archipelago were formed through volcanic and seismic activity during the Neogene Period; the first embryonic surfaces started to appear in the waters of Santa Maria during the Miocene epoch (from circa 8 million years ago). The sequence of the island formation has been generally characterized as: Santa Maria (8.12 Ma), So Miguel (4.1 Ma), Terceira (3.52 Ma), Graciosa (2.5 Ma), Flores (2.16 Ma), Faial (0.7 Ma), So Jorge (0.55 Ma), Corvo (0.7 Ma) and the youngest, Pico (0.27 Ma). Although all islands have experienced volcanism during their geological history, within recorded "human settlement" history the islands of Santa Maria, Graciosa, Flores, and Corvo have not experienced any volcanic eruptions; in addition to active fumaroles and hot-springs, the remaining islands have had sporadic eruptions since the 14th century. Apart from the Capelinhos volcano in 195758, the last recorded instance of "island formation" occurred off the coast of So Miguel, when the island of Sabrina was briefly formed.
Owing to its geodynamic environment, the region has been center of intense seismic activity, particularly along its tectonic boundaries on the Mid-Atlantic Ridge and Terceira Rift. Seismic events although frequent, usually tectonic or vulco-tectonic in nature, but in general low to medium intensities, occasionally punctuated by events of level 5 or greater on the Richter magnitude scale. The most severe earthquake was registered in 1757, near Calheta on the island of So Jorge, which exceeded 7 on the Richter magnitude scale. In comparison, the 1522 earthquake that was mentioned by historian Gaspar Frutuoso measured 6.8, but its effects were judged to be X (Extreme) on the Mercalli intensity scale, and was responsible for the destruction of Vila Franca do Campo and landslides that may have killed less than 5,000 of the inhabitants.
The nine islands that compose the archipelago occupy a surface area of 2,346 km2 (906 sq mi), that includes both the main islands and many islets located in their vicinities. Each of the islands have their own distinct geomorphological characteristics that make them unique: Corvo (the smallest island) is a crater of a major Plinian eruption; Flores (its neighbor on the North American Plate) is a rugged island carved by many valleys and escarpments; Faial characterized for its shield volcano and caldera (Cabeo Gordo); Pico, is the highest point, at 2,351 meters (7,713 ft), in the Azores and continental Portugal; Graciosa is known for its active Furnas do Enxofre and mixture of volcanic cones and plains; So Jorge is a long slender island, formed from fissural eruptions over thousands of years; Terceira, almost circular, is the location of one of the largest craters in the region; So Miguel is the largest island, and is pitted with many large craters and fields of spatter cones; and Santa Maria, the oldest island, is heavily eroded, being one of the few places to encounter brown sandy beaches in the archipelago. They range in surface area from the largest, So Miguel, at 759 km2 (293 sq mi) to the smallest, Corvo, at approximately 17 km2 (7 sq mi).
These islands can be divided into three recognizable groups located on the Azores Platform:
The Eastern Group (Grupo Oriental) of So Miguel, Santa Maria and Formigas Islets
The Central Group (Grupo Central) of Terceira, Graciosa, So Jorge, Pico and Faial
The Western Group (Grupo Ocidental) of Flores and Corvo.
In addition, several sub-surface reefs (particularly the Dollabarat on the fringe of the Formigas), banks (specifically the Princess Alice Bank and D. Joo de Castro Bank, as well as many hydrothermal vents and sea-mounts are monitored by the regional authorities, owing to the complex geotectonic and socioeconomic significance within the economic exclusion zone of the archipelago.