Pico volcano is the youngest in the Azores islands, a kid of just 6000 years. Although it hasn’t been playing with fire in the last few centuries, in a geological time scale he and his siblings have been constantly active during the Holocene. The Island of Pico in the Central Azores group presents 173 square miles of most diverse volcanic landscape – dotted with crater lakes and marshes, with extensive lava fields and caves. It is the most important natural reserve of the Azores. The coast of Pico is dominated by steep slopes with fantastic viewpoints, lava arches and vineyards. Some of the local wines that should be tasted by any visiting volcanoholic are “Magma”, “Basalto” or “Terras d’ Lava” 😉 .
With a size of 46 by 16 km Pico is the second largest island of the Azores. It belongs to the Central Group and is situated 17 km south of São Jorge and just 6 km east of Faial. The main settlements are the capital Madalena in the west, São Roque do Pico on the northern coast and Lajes do Pico in the south of the island; the total population is around 15,000.
As you may remember from our previous Azores posts, all except two of the Azores Islands have developed along and parallel to a secondary spreading rift zone branching off the main Mid-Atlantic Rift to the SE. The oldest volcanism is found in the easternmost parts of the archipelago. The most recent activity happened here, within the central group, at Faial, São Jorge and Pico. Pico Island has risen from the same ridge as Faial, on the Faial–Pico Fracture Zone. The two islands are separated by a narrow 6 km-wide channel. This zone has a high seismicity and probably defines, at its intersection with the MAR, the present position of the Azores triple point.
Pico is in an early stage of volcanic island formation. It all began, as with the other islands, with submarine fissure eruptions, which explains the elongate shape of the emerging island. Upon this, the central volcano Mt. Pico is now superimposed in its western part. The eastern Planalto da Achada (high plain) leads into a completely different world: About 200 cinder cones, craters and lava outcrops of various ages tell of the volcanic history.
The volcanics of Pico are divided into three volcanic complexes. These seem to be defined by location rather than by age, as they all contain rocks of the following epochs:
300 000* yrs: The oldest rocks of the island have been produced by fissure eruptions of what is often referred to as the “Spine”, or the “Linear Volcano”. These can be found at the foot of some coastal cliffs only. On top of those early rocks, on the southern side, came Topo, an ancient shield volcano, now destroyed by erosion/ landslides and covered by newer products. Topo is thought to have been a caldera that collapsed in “trapdoor”-fashion, i.e. the caldera floor has slumped down one side only, being hinged on the other. In modern terms this is the Topo-Lajes Volcanic Complex, located on the central-southern part of the island (color green in the map above).
>230 000 yrs: The volcanic complex of Calheta do Nesquim / Achada Plain (color pink in the map above). This is a fissural zone that stretches length-wise along the spine of the island. It is associated with the main fault systems along the central plateau. It is composed of numerous Hawaiian/Strombolian scoria cones and related lava ﬂows. Lavas from these cones are mostly aa ﬂows, which cascaded down the steep northern and southern slopes of the plateau.
Holocene-historic: The Madalena (or Montanha) Volcanic Complex (color peach in the map above). This includes the formations associated with Pico volcano as well as Holocene activity of the fissure zone in the eastern part. Historical eruptions have been restricted to the flanks of Mt. Pico and to a SE-trending rift zone, the Sao Roque Piedade complex, distinguished by well conserved cones, craters and flows. Lava flowing into the sea for two years (1562-64) formed a new peninsula on the northern coast: Mistério da Prainha. The latest eruptions produced lavas near Santa Luzia and São João (1718, a flank eruption with lava flows that reached both coasts) and Silveira (1720). There are mentions of a submarine eruption off the NW coast in 1963, although its occurrence and nature are debatable.
The tectonic structure of the island is characterized by two fault systems: The main WNW-ESE structures are the dextral faults of Lagoa do Capitão and Topo. They merge to the east, forming a narrow shallow graben (or trench). To the west, the graben is completely covered by the Pico stratovolcano. To the east it is infilled with the cones and lavas of the fissural zone. The faults of the second zone, running NNW-SSE, are much less in number and include normal left lateral, oblique slip faults responsible for the main volcanic eruptions: the Lomba de Fogo-São João fault (basis of the 1718 eruption) and the Santo António volcanic alignment.
There are two main areas of diffuse degassing on the island. Those are associated with the graben between the Fault of the Captain’s Lagoon and the old Topo Volcano. In the area of Silveira there is a source of cold water with high CO2 content. – The southeastern flank of Pico Island is affected by an active large-scale slump (Hildenbrand, et al. 2012b). In order to monitor the evolution of this structure, new GPS and microseismic networks, as well as an inclinometer, were installed in 2012. Three earlier extensive landslide scars have been determined along northern and southern coasts.
The most eye-catching feature on the island is of course the stratovolcano Ponta do Pico (or Montanha do Pico, Serra do Pico, Pico Alto or simply Pico). It is not only the highest volcano of the Azores but the highest mountain in all of Portugal. Moreover, if one takes a broader view, it is also the highest point of the Mid-Atlantic Ridge: measuring from the depths of the contiguous abyssal zone makes the volcanic edifice almost 5,000 meters high.
Those intending to climb Mt. Pico are required to register at Casa da Montanha, a small visitor center at the start of the path. The mountain is 2351 m high, straight out of the sea, which makes for a not very difficult but long and demanding hike to the top. The Center has (of course) a bar and also provides information on the geology, biology, history and climate of the Pico Mountain Natural Reserve.
The almost perfect cone of the volcano is topped by an almost perfectly round, 30m deep flat-bottom crater of ~550 m diameter. Which, in turn, is topped by a nice pointed hat – Piquinho – a 70m-high cinder cone sitting in its eastern part. The foot of Piquinho is also the place where the main fumarolic activity (60°C) of Pico Island is found; others can be seen on the eastern slope. Mt. Pico’s flanks are covered in pahoehoe lavas and cinder cones formed along radial fractures – although recent activity was rather from parasitic cones than from the summit crater. On the southern flank there is a deep avalanche scar that was probably caused by gravity sliding.
Mt. Pico sits on top of the early basalts of the Montanha Volcanic Complex, where it has risen through new fissure eruptions abt. 6000 years ago. Its very fresh morphology and the radiocarbon dates indicate intense activity during the Holocene. The steep cone of Pico has been constructed by repeated eruptions at low rates of effusion and with very little tephra involved. Meaning that not much in the way of explosive events would have occurred here.
Pico is an important composite volcano in an early stage of development. The lavas of all volcanic complexes in the island demonstrate that the majority of the rocks are normal basalts (∼78%), followed by hawaiites (∼20%). Only one lava flow, erupted in 1718 AD, has the characteristics of mugearite and benmoreite. Hawaiite, mugearite and benmoreite are basalts that have evolved with (repose-) time in direction of a more felsic composition, but are not quite at a stage (yet) where they are called trachytes.
Much research has been done in the last half century to find out what’s underneath Pico. Palaeoseismologic, radiometric, paleomagnetic and gravimetric studies have all contributed to understanding the geological processes here. Although there had been suggestions of a shallow and large magma reservoir beneath Pico volcano, this seems NOT to be the case. Nunes et al. say in their 2006 paper: “In this study we have not found any evidence of the existence of such huge magma body. On the contrary, we propose that those discrete low-density bodies correspond to complex intersections of different sets of tectonic lineaments, along which magma tends to intrude, as shallow (2–5 km) and small (10–20 km3) magma pockets.”
Although it is highly unlikely that Pico will produce any devastatingly explosive eruptions, there are other risks that should not be neglected. There is of course the ever present threat of strong earthquakes, and everyone should be prepared for them at any time. Another risk factor in a young volcanic island are landslides. Failures of the upper slopes are able to generate significant tsunamis. Not on a global scale, but they could be threatening to the local shores. Small-scale slope failures, caused by volcanic products moving out towards the sea floor, “probably pose a frequent threat to local populations but one that still remains poorly evaluated.” (R. Quartau et al. in 2015).
* Concerning the age of Pico Island I have stuck with the 300 ka stated on the website of the Azorean Instituto de Investigação em Vulcanologia. However, in a more recent thorough investigation of the submarine shelves of the Faial-Pico Ridge the authors (R. Quartau et al., 2015) found that apparently this ridge is older than previously thought, i.e. more than ~850 000 years. Which is still quite young for a volcanic island of that hight!
Monumento Natural Regional da Gruta das Torres
The Gruta das Torres, located W of Mt. Pico, near Madalena, is the largest lava tunnel known to exist in the Azores; it is roughly 5 km long and up to 15 m high. The interior is rich in geological formations, with various types of lava stalactites and stalagmites, lateral “side-benches”, lava balls, striated walls and long rope-shaped strings of lava. Very interesting is the floor of the cave presenting a variety of forms, such as rope or rough lava, prickly and irregular surface, sometimes with pointed protuberances.
Walls and Wine
There is a reason that Pico island is nicknamed the “Ilha Preta” (Black Island) – as black lava dominates much of the ground as well as all sorts of walls all over the place. The “checkered” vineyards cover long stretches along the N and W coast but can also be found in every nook and cranny in more hilly areas. The dark lava walls (moroicos) form thousands of small, contiguous, rectangular plots (currais).
They protect the vines from wind or salty sea water spray, and they also store the heat to warm the plants in chilly nights. What an ingenious idea! But then, just imagine the amount of back-breaking work that was necessary to ① clear the ground from rock debris and ② build all those miles of lava walls before one even can ③ begin planting. One of the grapes cultivated again (after it had all but disappeared from a disease) is Verdelho, a traditional variety on Pico Island.
The origins of this tradition date back to the 15th century. In some places the old “rilheiras” – ruts in the lava left by the wheels of ox carts carrying the grapes and barrels – can still be seen, and in the harbours and coves along the coast the “rola-pipas” – ramps excavated from the rock for rolling the wine barrels to the boats, remain as witnesses to the old-time wine industry.
After many ups and downs in history, the farmers created a sustainable living from viticulture and make a much-valued wine today. The extraordinary assembly of fields and houses, of wineries and early-19th-century manor houses, wine-cellars, churches and ports was worthy of listing Pico as a UNESCO World Heritage site.
Disclaimer: I am not a scientist, all information in this (and any of my other posts) is gleaned from the www and/or from books I have read, so hopefully from people who do get things right! 🙂 If you find something not quite right, or if you can add some more facts, please leave a comment.
Enjoy! – GRANYIA
SOURCES & FURTHER READING
– GVP, Pico
– Inst. de Investigação em Vulcanologia […]
– Petrology, geochemistry […] Pico Island (2006, paywalled)
– Active tectonics and first paleoseismological results in Faial, Pico and S. Jorge islands (Madeira 2003, PDF)
– Large-scale active slump […] Pico Island (2012, paywalled)
– Geology and volcanic history of Pico Island Volcano (1974, paywalled)
– Gravity anomalies and crustal signature of volcano-tectonic structures of Pico Island (2006)
– The insular shelves of the Faial-Pico Ridge […] (2015)
– Casa da Montanha Visitor Center
– UNESCO Pico Island entry
– The Wine Detective (Blog)
– Webcam Mt. Pico