Capelinhos – The Eruption

“The Capelinhos volcano is the image of the process in which magma, rising from the interior of the earth, traverses the crust and emerges at its surface (…). It is the factory of the landscape that manifests itself in the Azores, the origin of the ground on which we tread, the matter of our dream to be (…)”

Duarte Belo (2008)

    

In the registry book of the Capelinhos Lighthouse, where the notes of the dedicated lighthouse keeper Tomás Pacheco da Rosa are found, it can be read that on September 24th of 1957, two earthquakes were registered, and three more the following morning. On September 25th, the quakes became more frequent, although their intensity never surpassed intensity III of the Mercalli scale, and on the following day, more than 40 events were registered. Between the early afternoon of that day until 5h45 of September 27th, a constant tremor was felt. Then the quakes began to diminish. About one hour later, at 6h45 in the morning, the first signs of a new volcano began to emerge from the ocean.

On September 27th of 1957, about 1km from the Capelinhos Lighthouse, the volcano with the same name came into eruption. This phenomenon arose in the ocean, from a depth of between 20 and 60 meters, with the emission of volcanic ash and water vapour. This first phase of the eruption took on Surtseyan features (submarine phase) and was characterized by the projection of enormous jets of volcanic ash and dense clouds of water vapour. The columns of ash reached a maximum height of 1.4km and the columns of water vapour reached up to 4km. Volcanic ash is the most common material in these types of eruptions and is the result of the pulverization of high-temperature lava (approximately 1100˚C to 1200˚C) by cold ocean water, transforming thermal energy (heat) into kinetic energy (movement).    

 

The records of the activity of this volcano are quite detailed. It can be read in some annotations by various authors that, during 33 hours (beginning observations at dawn on October 6th), 11 episodes of highly violent explosions were registered, the longest of which lasted four hours and twelve minutes, uninterrupted. It was precisely on October 6th that, due to the change in the direction of the wind and an increase in ash emission, the areas of Norte Pequeno and Canto were badly affected and eventually evacuated. The following can be read in some records – “(…) during the night, a terrifying blackness caused complete obscurity (…). There was a strong smell of sulphuric gas and other sulphur compounds in the air (…). After the storm had passed, these places, once covered in pasture and crops, had transformed into an unhuman and desolate place, as if a great black blizzard had covered everything (…).” (First report of the Capelinhos eruption. Ribeiro, O. and Brito, R.S).     

 

The accumulation of volcanic ash created a small, new island in the shape of a horseshoe, which was baptized by the population as New Island or Holy Spirit Island. This island reached a maximum height of approximately 100 meters and a diameter of 800 meters. During this eruptive phase, the explosions alternated with periods of calm, during which there was the sinking of the cone that eventually led to the submersion of the small New Island. However, the continuous emission of volcanic ash created new emerged areas, which eventually became connected to the coastline of Faial Island by the creation of an isthmus. The submarine phase of this eruption was accompanied by magnificent electrical manifestations, a phenomenon visually similar to thunderstorms, but that occurred due to the friction of the volcanic particles projected into the atmosphere. These manifestations are best seen at night and there are photographic records of these taken up to 20km away, from the city of Horta. At the end of 1957, when damages were evaluated, it was verified that ash had covered an expansion of 23.5 km2 with a thickness that varied between 5 and 60 cm.

 

In mid-December, an ephemeral subaerial (terrestrial) phase occurred, with alternating periods of effusive and explosive activity. On the eastern side of the cone, a small fracture appeared from which several fountains of incandescent lava, rising up to 15 meters in height, were produced. The volcanic material began to be emitted from three chimneys, where each Strombolian explosion succeeded the next with intervals of only a few seconds. A new collapse of one of the slopes caused the incandescent material to come back into contact with the ocean water, resulting in the volcano taking up submarine characteristics once again, until May 1958. For some authors, the presence of water does not necessarily define the two different phases of this eruption. Rather, they argue, this distinction is defined by the physical differences in the volcanic apparatus and the material emitted. 

 

On the night of the 12th to the 13th of May, 1958, there was an intense seismic crisis where approximately 450 earthquakes were registered. These quakes left Capelo and Praia do Norte almost completely destroyed and in some locations, the intensity of the quakes reached level X on the Mercalli scale. This seismic crisis, which continued until June of the same year, destroyed houses and opened fractures in the ground measuring up to 1 meter in width. Approximately 1,037 houses were damaged and 5,000 people displaced. A fracture also appeared at the bottom of the caldera of the central volcano of Faial (Caldeira of Faial), which led to the draining of the water in the lagoon once found there. This water, which was heated as it drained to greater depths, triggered phreatic explosions that coved the bottom of the caldera in white mud (pulverized pumice stone) and created a fumarole field that remained active for a few months after the eruption.

From this point onwards, the Capelinhos volcano was never the same, taking on exclusively subaerial characteristics of the Strombolian type. The eruption began presenting periods of high explosivity, with the projection of incandescent lava more than 500 meters into the air, interspersed with effusive periods, characterized by the emission of lava flows of various viscosities. As of September 1958, the explosions began to diminish in intensity and the eruption finally ended on October 24th of that year.

 

For thirteen months, scientists from all over the world admired and studied this volcano, which would become the first submarine volcano to be duly studied and documented throughout its entire eruption. The accumulation of 174 million m3 of emitted volcanic material led to the formation of a unique landscape with particular characteristics. The cone of the Capelinhos volcano reached a height of circa 160 meters and added an area of 2.4 km2 to the island of Faial – the New Lands. 

When the eruption of the Capelinhos volcano ended, the process of landscape construction also ended, and the process of landscape destruction began immediately after, driven by external agents such as the ocean, wind and rain. These factors have been the main causes of the erosion of this volcanic landscape. In the periods between volcanic eruptions, erosive agents push back coastlines, thus moving towards a state of equilibrium in the landscape. This process was already visible along the old coastline in this location and which is made up of the dismantled volcano of Costado da Nau (now left as a fossil sea cliff by the eruption of the Capelinhos volcano). Because the last phases of the Capelinhos eruption were characterized by the emission of lava flows, it would be expected that these volcanic products, capable of creating solid, compact rock, would protect this new landscape from the erosive action of the ocean. In actuality, this solidified and compact rock does not possess a solid foundation and instead the foundation of ashes is easily eroded.  The solidified lava flows, without support, eventually collapse. Several scientists predicted that the basalt that solidified within the chimney could become a volcanic needle (a subvolcanic structure revealed when material surrounding the chimney is eroded or partially eroded) – an element which is now already visible.

 

The erosion rates of this recent landscape were extremely high in the years following the eruption, with this process being most prominent on the western slopes, reaching up to 300 m/year in this quadrant in 1959. Between 1976 and 1981, erosion rates were approximately 6 m/year, extremely high values in the years following the eruption, but that tends to decline over time. This deceleration in the rate of erosion is due to several factors, two of them being more evident. First, as a result of this erosive process, material that is stripped off the landscape accumulates to form peddle and sand beaches around the main cone, which helps diminish the erosive effect of the ocean waves on the cliff bases of the volcano. Secondly, the process of palagonitization, the alteration of volcanic ash over time, results in material being compacted and forming a new rock – tuff – that is more resistant to erosion. The last topographic survey of this volcano was carried out in 2007 and revealed an average rate of coastal erosion of approximately 23m/year. However, in recent years, abrupt moments of rock fall have also resulted in recessions of around 1 to 1.5 m/year. Consequently, today only 0.5654 km2 remain of the initial landscape created by this volcano – one-fourth of its original size.

 

 

Bibliography:

Topografia e temperaturas do vulcão dos Capelinhos (setembro,1962). Forjaz, V.H.

Cone dos Capelinhos em 1981. Machado, F.; Freire, T.

Evolução Geomorfológica do ‘’Mistério do Vulcão dos Capelinhos’’ – 1957-2007. Forjaz, V.H.