Terceira Island - Geography - Physical Geography

Physical Geography

The island of Terceira consists of four overlapping stratovolcanoes built over a geologic structure called the Terceira Rift: a triple junction between the Eurasian, African and North American tectonic plates. These volcanic structures rise from a depth of over 1,500 metres (5,000 ft) from the floor of the Atlantic Ocean. Radiocarbon dating of eruptive units, in support of geologic mapping, has improved the known chronology of Middle to Late Pleistocene and Holocene volcanic activity on the island of Terceira, Azores, defining the east-to-west progression in stratovolcano growth. These have commonly resulted in a classification of the eruptive events into the following structures:

• Cinco Picos Volcanic Complex – the oldest on Terceira (defined by the Serra do Cume), which completed its main sub-aerial activity by about 370–380,000 years ago. Collapse of the upper part of the stratovolcanic edifice formed an enormous 7 × 9 km caldera about 370,000 years ago. Post-caldera eruptions of basalt from cinder cones on and near the caldera floor and trachytic pyroclastic flow and pumice fall deposits from younger volcanoes west of Cinco Picos have refilled much of the caldera producing an almost-uninterrupted fertile plain.
• Guilherme Moniz Volcanic Complex – the southern portion, in the central part of the island, began erupting about 100,000 years later (about 270,000 ka) and produced trachyte domes (approximately 808 m/2651 ft in altitude), flows, and minor pyroclastic deposits for another 100,000 years (until at least 111 ka). The highest point along the caldera rim reaches 623 metres (2,044 ft). The northern portion of the Caldera is less well exposed, but reflects a similar age range. The northwest portion of the caldera was formed sometime after 44 ka. Several well-studied ignimbrites that blanket much of the island likely erupted from Guilherme Moniz Volcano.
• Pico Alto Volcanic Complex – Center, a tightly spaced cluster of trachyte domes and short flows, is a younger part of Guilherme Moniz Volcano. Stratigraphic studies and radiocarbon analysis suggest that most of the Pico Alto eruptions occurred during the period from about 9000 to 1000 years BP.
• Santa Barbara Volcanic Complex – the youngest stratovolcano on Terceira, began erupting prior to 29,000 years ago, and has been active historically, comprises the western end of the island and at its highest point is 1,023 m (3,356 ft). This stratovolcano is surrounded by several domes and coulee trachyte formations that occupy the volcano's caldera and along various alignments of the volcano's flanks.
• Fissural Zone – Connecting the eastern portion of Santa Bárbara, the western frontier of Pico Alto and Guilherme Moniz exist a grouping of fissural volcanoes and basaltic cones (Hawaiian and Strombolian), the youngest of which formed about 15,000 years ago. The only historical eruptions occurred in 1761, along a fissure on the eastern face of Santa Bárbara, and in 1867 and between 1998–2000 from submarine vents off the western coast (Serreta (Azores)). The groupings of volcanic structures on the island are aligned along a northwest-southwest and west-northwest-east-southeast orientation that extends to the submarine vents/volcanoes towards the eastern basin of Graciosa, including geomorphological alignments of smaller volcanic structures (including both basaltic and trachytic cones).

Island tectonics are highlighted by two great faults in the northeast corner (the Lajes and the Fontinhas faults), the Lajes Graben has been responsible for severe seismic events. Historically, both faults have produced earthquakes that are relatively shallow, strong and responsible for the destruction of property in the northeastern corner (specifically in 1614 and 1841). During the 1614 event, whose epicenter was located along the Lajes fault eight kilometers offshore, recorded magnitudes for the earthquake were between 5.8 and 6.3 on the Richeter scale (Wells and Coppersmith,1994). Both the Lajes and Fontinhas faults are separated by three kilometers). A second, less-developed, graben is located on the southeast of the Santa Bárbara stratovolcano. This feature is marked by trachyte domes and crosses from the coast at Ponta do Queimado (from the historical basin of Serreta (Azores)) to the cliffs, faults, basaltic lava and fissural eruptions near the center of the island.

The western part of Terceira Island is more heavily forested than the eastern part, due to the prevailing westerly winds bringing increased precipitation to that side, resulting in forests of Cryptomeria (Cryptomeria japonica). Other geomorphological points of interest include the plains of Achada, the mounts near Santa Bárbara, the small lakes of Lagoa das Patas and Lagoa da Falca. The northern coast is an area marked by evidence of volcanic activity with several "mistérios" (lava fields), the swimming pools of Biscoitos, while the centre of the island is highlighted by the Algar do Carvão and Furnas do Enxofre (dormant and active volcanic forms) that are popular with tourists and geologists. Most of the island is ringed by coastal cliffs about 20 m (60 ft) high, except on the south coast near Angra do Heroísmo. Here, an eruption of basaltic lava in shallow water formed the tuff cone of Monte Brasil, which protects and shelters the harbor of the island's capital. The cone is about 1 km (0.6 mi) in diameter and rises 205 m (673 ft) above the western side of the harbor.