Coming to the Point: FAIAL – Azores part 4

Aerial view of the Capelinhos volcano. (© Herbert Terra, via Panoramio)

THE ISLAND

Faial Island (or Fayal in English) belongs to the Central Group of the Azores archipelago and is the nearest of the central Azorean islands to the Mid-Atlantic Ridge. With an area of 173 km² (21 km by 14 km) size-wise it takes exactly the middle of the nine Azorean islands. Its geology is complex enough, it is explained by volcanic products on the one hand and, besides the main Faial-Pico Fracture Zone, by a maze of grabens and crossing faults on the other.

Sketch showing the Faial-Pico Fracture Zone (FPFZ) extending from the Mid-Atlantic Ridge (MAR) almost east through Faial and Pico.

Faial is the most volcanically active island of the Azores Archipelago. Historical eruptions occurred on the Capelo Peninsula (westernmost sector of the island) during 1672–1673 and more recently in 1957–1958. Composed entirely of volcanic materials, the island of Faial is structured around a large complex andesitic-to-trachytic stratovolcano called Maciço da Caldeira with a deep caldera at its center.
The slopes of the Maciço da Caldeira are asymmetrical, a consequence of the differences in age of the formations. The existence of secondary volcanism, and overprinting in relief of tectonic structures, result from processes due to great tensions associated with the seismically highly active FPFZ (Faial-Pico Fracture Zone).

Town and port of Horta, Faial. The fields on the left show the beginning of the “horst and graben” features of the east coast. (© Alberto Garcia Quesada, via Wikimedia)

There are two main fault systems recognized in the Central Group. The dominant FPFZ, striking WNW-ESE, controls the general shape of the islands. The second, less obvious yet important fault system, is trending NNW-SSE. Volcanism is controlled by these tectonics: intersections of the two fault systems determine the location of polygenetic volcanoes, while monogenetic volcanoes lie mainly on WNW-ESE fault lines. One study from 2013 suggests that Faial, along with the nearby Pico Island, is a major locus of extension within the Azores, directly above the imaged hotspot.

Several interesting geomorphological structures resulting from volcanism and tectonic forces have formed the island of Faial:

Elevation model of Faial Island with hydrography. (Annotations by this author)

Faial, Maciço da Caldeira, view into the caldera. (© Carlos Luis M C da Cruz, via Wikimedia)

– The highest part of the volcano Maciço da Caldeira is the peak Cabeço Gordo at 1043 m a.s.l. The volcano is truncated by a crater about 2 km in diameter. At the bottom of it, more than 400 m below the ridge, there is a flat area with water marshes, shrubs, and small cinder cones, surrounded by almost vertical cliffs with a diverse vegetation, including several endemic species of Azorean flora.
– The Falhado da Costa Este is a region deeply crafted by tectonic forces that occupies the entire ENE to NE sector of the island. This formation is characterized by the presence of a complex tectonic structure, dominated by parallel faults mainly of WNW-ESE direction – the local manifestation of the Faial-Pico Fracture Zone. Some of these faults have large tailings, indicative of horst and graben structures (locally called Bumps and sags), flanked by large cliffs of exposed boulders. As a result of this tectonic activity several peripheral cones rose along the fault lines, with their associated lava spills. This is the oldest part of the island (exceeding 800 ka in places).
– The graben of Almoxarife Beach is an imposing formation, plunging into the sea at the small beach that gave the place its name, flanked by faults that continue to release large amounts of gases, mainly CO². This is a basaltic fissural unit; it overlies the SE slope of Caldeira volcano and is covered by Caldeira formation pyroclasts. It comprises hawaiian/strombolian scoria cones and lava flows as well as one Surtseyan cone.

The Pedro Miguel Parish is located in a wide valley (graben) between Lomba dos Frades (horst) and Lomba Grande (horst).(© José Luís Ávila Silveira/Pedro Noronha e Costa, via Wikipedia)

– The Pedro Miguel Graben, located in the island’s northeast, is also characterized by an extensive fault system; the remnants of the original Ribeirinha Central Volcano that first formed the island can be found in this area.

Horta on the SE corner of Faial on a stormy day; seen from Monte da Guia. (José Luís Ávila Silveira/Pedro Noronha e Costa)

– In the southeast, the platform of Faial’s main town Horta is characterized by pyroclastic deposits, lava flows and several strombolian and surtseyan cones.

Morro de Castelo Branco, a volcanic dome on the southern coast of Faial. (via Wikipedia)

– The Morro de Castelo Branco on the southern shore is a volcanic dome formed sometime in the last 10 000 years, linked to the island by a very narrow isthmus. It is one of the only two trachyte domes in Faial.
– Monte da Guia: This geological structure, just opposite the city of Horta, is a 145 m high volcanic cone of submarine origin. Geologically it is formed by several layers of tuff superimposed and laminated parallel to the surface. It is built up by hydro-magmatic action as partially lithified pyroclastic deposits, commonly called palagonitic tuff. The name has been given for Our Lady of Guia, the patron saint of fishermen, whose Chapel on top of the mountain dominates the landscape.

Monte da Guia (with 74 ha), consists of volcanic cones, craters, slopes, cliffs, bays, coves, pebble and sandy beaches, rocky reefs and sea caves. It is a Protected Landscape Area of Faial. (© Parques Naturais)

 

THE VOLCANO

Pre-Caldera period
In the northeast, the volcanic complex of Ribeirinha, dated 800 ka BP, was identified as the oldest part of the island. It is now strongly disrupted by faulting and partially covered by younger volcanism. Immediately to the northwest of it, the volcanic complex of Cedros is next with ~580 ka old. This is the lower unit of the modern volcano, a sequence composed of sub aerial basalt, hawaiite, mugearite and benmoreite lava flows, parasitic scoria cones, and trachyte lava domes that corresponds to the shield volcano phase. From ~400 ka until ~10 ka ago, starting on the western flank of the ancient Ribeirinha volcano, grew this gigantic volcano we see today, essentially composed of lava spills resulting from predominantly effusive eruptive activity. As a result of this mass, and the activity of the FPFZ, major fissure tectonics and volcanic activity occurred on the eastern side of the island ~50 ka ago that formed the Falhado da Costa Este.

Caldera formation
~10 ka ago, there was a change in eruptive style of the central volcano, entering an explosive phase which was responsible for the vast deposits of an ignimbrite that covers one-third of the island. Gradually, the volcano also changed its type: with lots of ash and pyroclastic layers being deposited among lavas, it changed from shield to stratovolcano. The collapse of the caldera appears to have occurred in two distinct episodes: first a sagging of the surface on the mountain top; the second was caused by a violent Plinian eruption. The sinking of the caldera happened at the same time or immediately after this eruption, which covered more than 40% of the island’s surface with a thick layer of pyroclastic materials, mightiest north and east of the eruptive center. Most of the vegetation cover, if not all of it, was destroyed. The eruption was accompanied by powerful floods, or lahars, as a result from heavy rainfall-induced condensation on volcanic dust in the atmosphere and the steepness of the slopes. In the cliffs at Praia do Norte are traces of these mass movements still visible.

Post-Caldera period
Formation of the steep-walled 500-m-deep caldera was followed by the construction of fissure-fed basaltic lava fields and small monogenetic cones. After the caldera formation the volcano has shown relative stability, although its eastern and northern slopes show clear signs of multiple landslides, some of which happened after the settlement of the island. The caldera remains volcanically active, proven by the lava flows inside it from ~1000 years ago. In the western region of Faial is found an ‘en echelon’ alignment of basaltic hawaiian/strombolian and surtseyan cones and lava flows, the Capelo Volcanic Complex, built on the W-slope of Caldeira volcano and forming a peninsula extending to the west. It is younger than 10,000 years (as it overlies the oldest pyroclasts of Caldeira formation time), and includes the 1672-74 hawaiian/strombolian (Cabeço do Fogo) and the 1957-58 surtseyan/strombolian (Capelinhos) eruptions. Several cracks in the caldera floor opened during the seismic crisis of May 1958. This led to the outflow of water from the small lakes into the central cone, triggering violent phreatic explosions and the occurrence of temporary fumarolic activity. The earthquake of 9 July 1998 caused several slope failures in the almost vertical walls of the crater.

Cabeço do Fogo Eruption 1672/73 (then called Cabeço do Rilha Boi)

Cabeço do Fogo, the site of the 1672/73 eruption (the place was then called Cabeço do Rilha Boi)
(via Parques Naturais)

The Capelo/Praia do Norte volcanic event started by premonitory seismicity, and by February 1672 the population of Praia do Norte and Capelo had already abandoned their homes, fearing that they could collapse. An eruption started on a hill then called Rilha Boi located above the village of Praia do Norte. Lava flowed to the north and south, reaching the coast in a few hours and destroying the villages of Ribeira Brava and Praia do Norte in its path. Part of the fleeing people went to the place of Canto (west of the eruption), becoming trapped by the lava flows that reached the north and south coasts. An anonymous account says that the ashes of the eruption covered the whole island and even reached Pico island across the channel. The town of Horta was practically destroyed during this eruption; volcanic ash and pyroclastics rained down on the communities, destroying homes and sterilizing agricultural lands mainly in Capelo and Praia do Norte. Many people must have been injured, and several died during the eruption, but there seem to be no clear records: “…among the people who died… was the monk Friar Manuel da Luz, one of his brothers and a young servant of the convent, that, while watching the eruption, were trapped between two branches of the lava flows where they… miserably perished…“

Capelinhos Eruption 1957-59

Capelinhos eruption 1958. Note the “tiny” lighthouse on the shore, left. (No author found)

A column of ash and steam towers above a new submarine vent off the western coast of Fayal Island in early October 1957. Horizontally traveling base surges form a ring at the base of the eruption column.  The Capelinhos lighthouse, is visible on the center shoreline. Lava effusion was continuous for the last five months of the 13-month-long eruption. (© U.S. Air Force, 1957)

First phase
The eruption site is part of the volcanic complex of Capelo, consisting of about 20 cones of slag and their lava spills over the WNW-ESE volcano-tectonic alignment. From 16 to 27 September 1957, there was a seismic crisis on the island with more than 200 earthquakes.

The second islet is emerging from the depth and already connected to the shore. (No Author given)

Visitors watching the submarine eruption at Capelinhos in 1958. (© Robert Sisson, 1958)

 

 

 

 

 

 

 

 

On 21 September 1957, the sea water began to seethe. Three days later, the activity increased markedly with emission of black fountains of volcanic ash about 1 000 m high (reaching a maximum of 1400 m) and steam clouds rising sometimes more than 4000 m. On September 27, 1957 began a submarine eruption 300 m from Ponta dos Capelinhos. A first small island of finally 800 m in diameter and 99 m high formed from 10 Oct. on, but sank back into the crater on Oct. 29.

Second phase
At the beginning of November, another new island emerged and was soon followed by an isthmus connecting it to Faial. In December a new cone appeared and from the 16th on, the so far explosive eruption changed to mainly effusive pouring out great amounts of lava.

First row: destruction caused by the earthquakes during the Capelinhos eruption. Second row: Ash released by the volcano buried everything throughout Faial (no Author given)

Pyroclastic-surge deposits from the 1957 Capelinhos eruption lap up against a lighthouse near the western coast of Fayal Island. Pyroclastic surges produced by magma-water interaction during the submarine eruption nearly buried the lower floor of the lighthouse building. (© R. V. Fisher, 1979)

Third phase

The eroded pyroclastic cone at the left, now forming the western tip of Fayal Island, was constructed during the 1957-58 Capelinhos eruption. Vigorous erosion by the sea soon truncated the sides of the new cone, forming the steep south-facing cliffs seen here. Beach erosion in the foreground exposes light-colored ashfall deposits from the 1957-58 eruption. (© Rick Wunderman, 1997)

From January to April 1958 sharp jets of ash reappeared, usually accompanied by white or brownish fumes. By March, the houses of the nearest village had been buried up to their roofs (which are still visible today). After the violent seismic crisis in the night from 12 to 13 May, in which there were over 450 earthquakes, the eruption of Capelinhos suffered profound readjustments in the volcanic edifice and tectonic structure. From May 14 , the activity went to Strombolian type, with loud noises, accompanied by infra-sound waves that made her doors and windows rattle throughout the island and sometimes on the nearby islands, and fragments of incandescent lava were ejected to more than 500 meters high. Also that day, there were fumaroles at the caldera floor (central volcano of the island), which emitted steam with the smell of sulfur and boiling mud.

Bay on the North side of Capelinhos volcano. (© Christian Bølling, via Panoramio)

As a result of the eruption, between May and October 1958, the total area of the island increased by approximately 2.5 km². Later this gain was diminished by half because of the very unconsolidated nature of the rocks and the erosive action of the waves. A vent of the volcano still releases steam and volcanic gases at temperatures in the range of 180-200°C.

This is a wonderful video-animation showing the development of the volcano over the eruption time (4:50 min.). The artist is Hugo Visser, and this is a way I would like to see the changes in every volcano when it erupts… thank you!
HERE is a 32-minute documentary (in Portuguese) of the eruption time with lots of authentic footage, by Jose Serra, commemorating fifty years of the volcano of Capelinhos, and containing unpublished images of the eruption and the homeless.
HERE an original video recording from 1957 (5 min)

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Surtseyan or Capelinian?

This was the first submarine eruption that was extensively watched and documented by scientists and residents, under best conditions like transport and communications being readily available. Unfortunately for the Capelinhos volcano, the type name of this eruption was not patented properly at the time and so it happened that, in 1963, when in Iceland the new island of Surtsey emerged in same fashion, that eruption style was named “Surtseyan”. For all intents and purposes, the rightful term should have become “Capelinian” eruption.

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The most catastrophic effects of this event resulted, however, from an earthquake swarm that started on the 12th of May 1958 and lasted up to June. This and the volcanic eruption and ash fall led to the widespread destruction of homes, farmland and pastures, causing half of the population of Fayal to emigrate to the U.S. (However, this demographic breakdown contributed to the improvement of life for the remaining resident population, more job opportunities and better salaries being available.)

The old lighthouse of Capelinhos stands on the former coastline at right. Capelinhos spatter cone rises inside the tuff cone. (© Stromboli online: J. Alean, R. Carniel, M. Fulle)

On the NW tip of Capelinhos: half of the spatter and tuff cones have been already eroded by the sea. Gray lava flows and dikes among reddish ash layers.

 

EARTHQUAKES

Faial (like Pico), sitting astride the very active FPFZ, has to suffer more than its fair share of earthquakes in the Azores. Although they are generally of low magnitude, the vicinity and shallowness of their hypocenters makes some of the stronger ones quite a hazard to the population.

Epicentre of the July 9th, 1998 Faial earthquake and aftershocks.

For example, on 9 July 1998, an earthquake of Mw 6.2 – which produced a MM intensity of VIII (!) for Faial – struck the Islands of Faial, Pico and S. Jorge, causing a total of 8 deaths and 150 injured people. The destruction of about 70% of the building stock of Faial, as well as extensive damage to infrastructure, left over 1500 people homeless. The epicentre was located 10km NE of the town of Horta, the communities of Ribeirinha and Espalhafatos were totally destroyed.

1998 earthquake damage, photo 2004 (© Romeu Vicente, Tiago Ferreira, Rui Maio)

Churches, most of them more than a century old, suffered partial collapse or total ruin as well as the Ribeirinha lighthouse, the building nearest to the epicentre; its outer walls, 50 cm thick and made of stone, fell outwards while the light tower suffered fracturation at mid-height with dextral rotation of 20 cm.
Surface tectonic deformation produced ground cracks up to 10 m long located along some of the main faults. Cracks presented geometry, trends and displacements that agreed with the main families of faults and their kinematics. On the north flank of the central volcano a debris avalanche started on a 200 m wide zone close to the caldera rim mobilizing the ignimbrite deposit that flowed for 1400 m down a valley towards the village of Ribeira Funda. It affected an area of 177.000 m² and displaced 600 000 m³ of material.

 

How is the seismologic monitoring these days?

Reports on seismic and volcanic events in the Azores Islands are available since the time of its discovery in 1427. Throughout the last five centuries several devastating seismic events occurred, causing thousands of deaths each time. Due to the Azores’ seismicity, the first Portuguese analogical seismographic stations were installed there, two in 1902 in Faial (Horta) and S. Miguel and in 1932 one on Terceira Island.

This now composed the entire seismic monitoring network in the Azores for several decades. During the Capelinhos eruption only these three stations existed in the archipelago – but the equipment in Faial was inoperative at the time! – Over 20 years later (01/01/1980), when a devastating M 7.2 earthquake hit the Terceira area, the „network“ was still composed of the same three stations.

Since then, many efforts have been made to implement modern equipment and use efficient scientific research methods. In the nineteen-eighties an analogical network composed of 16 seismographic stations was installed in the islands, the majority of them at the central group because it presented the highest level of seismicity. In 1998, the monitoring system included also 12 digital seismic stations.

Comprehensive hazard mitigation plans that take in account the different requirements for the various island groups of the archipelago are put in place and scientists have been working on the compilation of a homogeneous earthquake catalogue of the Azores region for better risk assessment. In these days, all the available modern methods from digital models to satellite observations is used by Portuguese and international scientists to gain better knowledge of the geologic processes and with that the best possible protection for the people in the Azores.

Capelinhos volcano from the air. This magnificent photo has obviously been scanned from a paper brochure, I would love to see a high res digital version of it!

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
– Relationships between tectonics and magmatism […] Faial Island (2013, paywalled)
– […] volcanic activity on the Capelo Peninsula […] paleomagnetic contribution (2014, paywalled)
– The volcanoes of Azores island: A world-class heritage […] (2005)
– Comemorações do 50º Aniversário do Vulcão dos Capelinhos
– Fogo (Hist. photos in Blog post by Teresa Marques)
– Capelinhos volcano (on chain.eu)
– “Stromboli online” at swisseduc.ch
– Seismic hazard for the Central Group of the Azores Islands (2001)
– URBSIS project/Faial
– Seismicity Studies of the Azores […] July 9, 1998 earthquake (2000?)
– […] volcanic insular shelves: […] Faial Island (2010)

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