As I have promised in my last Azores post on São Miguel – and on request of our readers – here is the next, this time about the delightful island of Terceira.
Ilha Terceira (the name translates as: the third island) is an oval-shaped island in the central group of the Açores archipelago, some 30km long, with 56 000 inhabitants and the Azores’ oldest city, UNESCO Heritage Site and former capital of the Azores, Angra do Heroísmo. It is the third largest island, and abt. 100km NW of São Miguel Island.
Most of the island is ringed by coastal cliffs about 20 m 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 and makes for an attractive view:
90% of the island is built up from the sea ground (at 1 500m depth) by a-million-year-old basalts which overlie the Terceira Rift: the triple junction between the Eurasian, African and North American tectonic plates. The Terceira Rift is 550 km long; it connects the Mid-Atlantic Ridge to the Gloria Fault, and it represents the world’s slowest spreading center, with plate divergence of 2-4 mm/year. (See agimarc’s post Azores part 2 – Tectonics).
The upper 10% of the island are much younger – erupted from the four stratovolcanoes constructed along a prominent, 2-km-wide, ESE-WNW-trending fissure zone or rift zone that cuts across the island. This zone is defined by various scoria cones and lava flow vents and is considered to be the surface expression of this large-scale geodynamic feature, the Terceira Rift.
Historical eruptions on Terceira have occurred in April 1761 from the fissure zone between Pico Alto and Santa Barbara, and in 1867 & 1998-2000 from submarine vents west of Santa Barbara, also two submarine events more to the south of Terceira in 1800 & 1902. Before them, GVP lists 12 eruptions from 6720 BCE on that have been found by researchers using radiocarbon dating and tephrachronology. Most Holocene eruptions have produced basaltic-to-rhyolitic lava flows from the fissure zone transsecting the island. They show surface morphologies typical of viscous magmas such as ridges, convex in the direction of flow, high levees, lava channels and spines. The lava domes are 14-183 m in height, coulees can reach lengths in excess of 2800 m, with widths ranging from 110 to 900 m and thicknesses of 15-70 m.
A significant proportion of the island’s stratigraphy is made up by ignimbrites, attesting to multiple major ignimbrite-forming eruptions during the last 100 ka. The volcanoes grew in east-to-west progression due to the tectonic plate movement and rifting activity.
1) Cinco Picos Volcanic Complex:
The oldest volcano is Cinco Picos in the SE and once it was a steep-sided cone. What remains of it after a big collapse 370 ka ago is an enormous 7×9 km eroded caldera. Its minimal remnants of a rim embrace what is now the largest flat area in the whole of the Azores, an almost uninterrupted fertile plain. Not entirely flat, though – five younger cinder cones are scattered on the caldera floor, the Cinco Picos (five peaks), providing the present name for the volcano. These post-caldera eruptions of basalt from the cinder cones (plus trachytic pyroclastic flow and pumice fall deposits from another volcano west of Cinco Picos) have refilled much of the caldera.
2) Guilherme Moniz Volcanic Complex:
Guilherme Moniz caldera, the successor of Volcano Serra do Morião – the once largest volcano on Terceira – also with a caldera (4 km x 2.5 km), lies to the west of Cinco Picos and along the fissure zone in the central-southern region. The caldera’s northern side has been obscured by layers and cones of later eruptions from the north but to the S and SW the Serra do Morião (632 m) form a rocky semi-circle around it. Guilherme Moniz Volcano has developed with the initial formation of an imposing shield volcano. Subsequently, it evolved into a more explosive character, culminating in the caldera collapse, which occurred less than about 180 ka ago.
One outstanding event made the Caldeira de Guilherme Moniz possibly the largest reservoir of water of Angra do Heroísmo municipality, if not of the entire island: This caldera contained a lake when the adjacent Algar do Carvão (see below) erupted, which was then completely filled and covered by pyroclasts and lava flows. There are several springs now giving water through the lava tubes.
3) Pico Alto Volcanic Complex:
Pico Alto lies north of the fissure zone in the north-central part of the island and contains a Pleistocene polygenetic volcano caldera, almost entirely filled with numerous domes and trachytic coulees. On the SW flank Furnas Enxofre is located, an area of about 7 000 m2 which includes a fumarolic field and areas of diffuse degassing.
Pico Alto has had several ignimbrite-producing eruptions, with sub-plinian/lava dome-forming events occurring before, during, and after ignimbrite volcanism. Despite no historical eruptions are known the volcano’s eruptive activity has been quite recent, occurring last about 1,000 years BP or less.
The Algar do Carvão (Cavern of Coal) is an ancient lava tube or vent inside a volcanic cone. It was created in one of the latest eruptive events from the Pico Alto complex when effusive flows produced very fluid lava that charred existing vegetation. The dating of fossils gives the Algar do Carvão an age of 2148 (+/- 115 years). The existence of the cavern had been known for a long time, but the depth and lack of ambient light made any descent difficult, owing to the vertical access tube, and delayed any real exploration. In 1963, a group of enthusiasts organized a descent using a “chair” platform suspended on a nylon line and, later, a harness. With the advent of portable lighting systems, it was possible to re-examine former observations first-hand, including the most remote and narrow sections of the cavern. The mouth of the Algar consists of a 45 m (148 ft) vertical passage to the interior, that reaches a ramp of debris and gravel. From here there is another decline to the clear waters of the inner pool, approximately 90 m (300 ft) from the level surface.
The cave itself is remarkable for the unique mineralogical characteristics of its silicate* stalactites. It was made accessible to the public in the late 20th century, access is now through an artificial tunnel and a staircase. *Wikipedia mentions silicate but other sources have calcite stalactites for this location. I know that stalactites etc. are commonly of calcitic compounds as calcite is easily soluble in water but, if these are “unique”, they may well be of silicate composition (I am no scientist, so I can just report what I find written).
Furnas do Enxofre, a secondary volcanic fumarole field in the trachytic rocks of the southern sector of the Pico Alta volcano, is an area containing several aggressive volcanic gas vents. The temperatures of some of them are quite high (around 95°C at the surface to about 130°C at half a meter depth). The emissions consist essentially of carbon dioxide (c. 95%), hydrogen sulfide (c. 3%), sulfur dioxide, hydrogen, nitrogen, methane and traces of others. Around some of the vents deposits of sulfur can be seen.
4) Santa Bárbara Volcanic Complex:
The historically active Santa Bárbara is the most geodiverse of all Terceira volcanoes; it occupies the entire western quarter of the island. It also started out as a shield volcano which then has undergone numerous subplinian/lava dome-forming events. Ignimbrite eruptive volumes estimated from on-land deposits are small but considerable amounts of material must have flown into the sea.
The volcano is truncated by two calderas; the youngest of these formed about 15,000 years ago. Subsequently, the inside of the second caldera has seen various eruptions as evidenced by the presence of domes and thick coulees. The eastern flank of Santa Bárbara volcano was involved in the 1761 fissure zone eruptions, when the Mistérios Negros lava domes were formed (see below). Many slag cones and trachytic lava extrusions convert the flanks of the volcano into a “moonscape” and are considered the most spectacular volcanic materials of the archipelago.
Serreta Volcanic Ridge
in Dec. 1998 a submarine eruption started from multiple vents along the Serreta Volcanic Ridge, about 10 km W of Terceira. Vents along the ridge were very active between 12/1998 and 09/1999. Activity then declined to low levels but increased again in late 01/2000. At that time, a yellowish spot appeared at the sea surface above the eruptive area due to the dispersion of a volcanic plume that rose from a new vent located at about 250 m depth. The area of water discoloration reached a maximum diameter of 8 km by the end of Febr. and the eruption ended about a month later. During 1999, basaltic lava balloons were observed floating in the eruptive area. These “balloons” were very hot, gas-rich, lava fragments produced from small submarine lava lakes/fountains. During ascent to the surface, magmatic gas exsolved from the hot fragments, increasing the volume of the balloon while the crust was still glassy and expansible. Once at the surface, interaction between the hot blocks and seawater produced white steam columns that could be seen from land.
5) Fissure Zone:
This is a grouping of fissural volcanoes and basaltic cones (Hawaiian and Strombolian style), the youngest of which formed about 15k years ago. It connects the eastern portion of Santa Bárbara with the western sides of Pico Alto and Guilherme Moniz volcanoes and was the site of the only recorded historical eruption on land.
The AD 1761 eruption evolved in three phases: (1) a precursory phase characterized by decreased degassing in the fumarolic field of Pico Alto Volcano and a gradual increase of seismic activity, which marked the intrusion of trachybasalt magma; (2) a first eruptive phase that started with phreatic explosions on the eastern flank of Santa Bárbara Volcano, followed by the inconspicuous effusion of trachyte (66 wt% SiO2), forming a WNW-ESE-oriented chain of lava domes (known as Mistérios Negros); and (3) a second eruptive phase on the central part of the fissure zone, where a Hawaiian to Strombolian-style eruption formed small scoria cones (E-W to ENE-WSW-oriented) and a trachybasalt lava flow (50 wt% SiO2) which buried 27 houses in Biscoitos village. Petrological analyses showed that the two batches of magma were emitted independently without evidence of interaction. It is thought that the dome-forming event was triggered by local stress changes induced by intrusion of the trachybasalt dyke along the fissure zone, which created tensile stress conditions that promoted ascent of the trachyte magma stored beneath Santa Bárbara Volcano.
6) Lajes Graben
Is located in the northeast of the island and is defined by several scarps in NW-SE orientation. It is caused by the sinking of the portion between two great normal parallel faults, separated by 3 km, in the northeast corner of Terceira, the Lajes and the Fontinhas faults. This graben is still tectonically active and progressively sinking, tilting to the SE. In the past, the Lajes Graben has been responsible for severe and destructive seismic events, specifically in 1614 and 1841 (see description on C.Blog), with great destruction of lives and property. In 1614 (M 5.8 – M 6.3), the epicenter was 8 km offshore along the Lajes fault. (Another, smaller graben is located southeast of the Santa Bárbara volcano. This feature is marked by trachyte domes and crosses from the coast at Ponta do Queimado to the cliffs, faults, basaltic lava and fissural eruptions near the center of the island.)
Lajes Air base: In 1934, the only 6-year-old air field on São Miguel island had to be abandoned because of unfavorable weather conditions, and the new choice was the Lajes Field, parallel to and between the two fault lines. Luckily, no substantial earthquake had its epicenter there since*, and now, it seems not to matter much anymore… Since WW2, Lajes field had also been a US military air base, but now the US military has lost interest in it. Hundreds of US military personnel and hundreds of Portuguese workers have already left the infrastructure, the future of the air base seems to be foggy at best. May the Chinese or any other contender worry about earthquakes and may the Portuguese and American taxpayer cut their losses…
*Although the M7.2 in 1980 did cause fatalities and widespread destruction on Terceira, there was no damage to speak of in the air base. Its epicenter was 30km west of Terceira at 10km depth.
Enigmatic “Cart-Ruts” and other archaeological features
According to a newspaper report, a number of hypogaea (tombs excavated in rocks) were found in Monte Brasil on Terceira, indicating a history of settlement that may date back 2000 years, alluding to human occupation of the island long before discovery by the Portuguese: “Archaeologists from the Portuguese Association of Archeological Research (APIA) believe to have found in Monte Brasil, Angra do Heroismo, Terceira island, a significant number of Carthaginian temples from the fourth century BC, dedicated to the goddess Tanit.“
Another, more mysterious feature are the “cart-ruts” on Terceira and Pico Islands; I am referring to this website Ancient Wisdom:
“The presence of cart-ruts is one of the most unexpected facts to present itself in the search for the first settlers on the Azores. These enigmatic features are found in large numbers around the Mediterranean basin, […] most famously on Malta. As we [on the Azores] have no record of their being made since the ‘official’ discovery of the Azores, we must assume that these were made by a people prior to the Portuguese.” On further internet search one thing seems to become clear: “cart” ruts they are not but – some other suggestions are less improbable – could have been worn in by transport (dragging?) of rock materials, or carved out to allow water to flow (for religious reasons?) In any case, the production of such grooves “requires a large supply of manpower, lots of time and a considerable motivation. As we have no record of such practices by the Carthaginians or Greeks, we probably need to look elsewhere to find the origin of these most peculiar geological features. The Maltese cart-ruts have now shown to have an association with the temple building period c. 3,000 BC. Not only are the stones used to build the temples of the same dimensions as the ‘gauge’ of the tracks, but they are found close to the temples themselves. Again, none of these facts appear to be relevant on the Azores, so, for now at least, the very existence of what is essentially a ‘prehistoric’ feature is one of the island’s most fascinating mysteries.“
Enjoy! – GRANYIA
SOURCES & FURTHER READING
– Bradt Travel Guide Azores (Book by David Sayers, 2010)
– “The Montanheiros”: Algar do Carvão
– Adam Jeffery, Postgraduate Keele University, UK
– Centro de Vulcanologia da Universidade dos Açores
– Monte Brasil, (archaeology) Wikipedia
– Commentary Magazine, on Lajes decision (02/2016)
– Basis for the geological heritage management in the Azores Archipelago (2014, PDF)
– SIARAM – Vulcanismo
– C.Blog – Terceira Island – Vulcanismo
– Domos e coulées da ilha Terceira (Açores):[…] (2006, in Portuguese only, opens download) This seemingly highly interesting work by Adriano H.G. Pimentel cannot be translated by Google – if you know any way to read it in English, please let me know.
– Nature Parks Terceira