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 is characterized for its shield volcano and caldera (Caldeira Volcano) Pico, is the highest point, at , in the Azores and continental Portugal Graciosa is known for its active Furnas do Enxofre and mixture of volcanic cones and plains São 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 São Miguel is the largest island and is pitted with many large craters and fields of spatter cones Santa Maria – the oldest island – is heavily eroded, being one of the few places to encounter brown sandy beaches in the archipelago. These islands can be divided into three recognizable groups located on the Azores Plateau: The Eastern Group () of São Miguel, Santa Maria and Formigas Islets The Central Group () of Terceira, Graciosa, São Jorge, Pico and Faial The Western Group () of Flores and Corvo. São Jorge, Pico and Faial are also collectively called (‘Islands of the Triangle’). Several sub-surface reefs (particularly the Dollabarat on the fringe of the Formigas), banks (specifically the Princess Alice Bank and D. João 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. Geology From a geostructural perspective, the Azores are located above an active triple junction between three of the world's major 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 on 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 Glória Fault. The Mid-Atlantic Ridge is the main frontier between the North American Plate and the African-Eurasian Plates that crosses the Azores Plateau between the islands of Flores and Faial from north to south then