Abstract
Sete Cidades is an active central volcano on the western part of São Miguel. The geological record reveals that subaerial activity started more than 250 ka ago. Stratigraphic units defined for Sete Cidades deposits reflect major events in the history of the volcano and are organized into two main groups: the Inferior Group and the Superior Group. Caldera formation resulted from three major paroxysmal events that occurred at about 36, 29 and 16 ka ago. Analysis of the eruptive history of Sete Cidades shows that effusive or moderately explosive eruptions, of Hawaiian and/or Strombolian styles, were located on the slopes of the central volcano. Conversely, trachytic explosive activity is mostly centred inside the caldera involving, in a first stage, predominantly Plinian and sub-Plinian phenomena, changing about 5 ka ago to a dominant hydromagmatic style. Trachytic effusive eruptions are represented by domes and associated lava flows that crop out in the inner caldera walls and on the western slopes of the volcano. Offshore submarine activity is represented by the historic Surtseyan eruptions of 1638 and 1811. In the last 5 ka Sete Cidades was the most active central volcano in the Azores with 17 explosive eruptions predominantly with hydromagmatic character.
Rising about 3 km from the average depth of the seafloor of the Azores Platform, Sete Cidades volcanic massif is located in the western part of São Miguel (Fig. 7.1). This region is dominated by an active central volcano, which has developed a roughly circular caldera at its summit. It is bounded by the sea on three of its flanks, and on the southeastern flank it joins with the Picos Fissural Volcanic System (PFVS) (Ferreira et al. 2015).
Location of Sete Cidades massif on São Miguel island.
The central volcano of Sete Cidades began to form several hundred thousand years ago. The transition from a submarine type of activity to a predominantly subaerial regime dates from more than 250 ka ago (Moore 1983). In the first phase it is assumed that this volcano may have been an individual island and its joining to the central volcanoes to the east occurred through the development of the basaltic fissure volcanism associated with the PFVS. Current volcano-related phenomena are restricted to CO2 soil diffuse degassing in some areas of the caldera floor and at Mosteiros Village, together with thermal springs offshore of Mosteiros and Ponta da Ferraria (Viveiros et al. 2015).
Despite the fact that Sete Cidades volcano is located in an area affected by important regional tectonic structures, seismic activity can be considered low when compared with the other two active central volcanoes of the island, Fogo and Furnas. The last major seismic swarm occurred in 1998 and, during about four months, hundreds of low-magnitude earthquakes were recorded by the seismic monitoring network. The largest event had a magnitude of 3.2 and some of the seismic signals indicated shallow hydrothermal or magmatic movements.
Geomorphology
Central volcano with caldera
Sete Cidades is a volcano that was formed from the accumulation of pyroclastic deposits and lava flows. It originated from a central vent that produced numerous eruptions of variable style, magnitude and duration. This volcano has an average subaerial basal diameter of about 12 km and currently has a maximum altitude of 845 m, at the Pico da Cruz geodetic monument (Fig. 7.2). It occupies a surface area of approximately 110 km2 and its subaerial volume is about 45 km3.
Sete Cidades central volcano: main locations and volcanic structures referred to in the text. Numbers denote the volcanic centres inside the caldera, named in the bottom left; diamonds show volcanic centres on the slopes; squares are localities; circles are sites; and triangles are geodetic monuments.
At the summit of Sete Cidades there is a well-developed caldera (Fig. 7.3), roughly circular, with a diameter of about 5 km and an average depth of 350 m. This impressive depression occupies an area of 15 km2 and has a volume of about 5.2 km3, and is bounded by steep sub-vertical walls, with heights ranging from a maximum of 400 m in the area of the Remédios geodetic monument to a minimum of about 30 m close to the Seara cone. At the foot of these walls thick accumulations of talus frequently occur.
Sete Cidades caldera general view taken from SW showing several intracaldera cones and lakes.
Several secondary volcanic cones (i.e. pumice cones, maars and domes) and four lakes occur inside the caldera. Two of these lakes – the Lagoa Azul (3.56 km2) and the Lagoa Verde (0.83 km2) – are linked. Their drainage is via an artificial tunnel built in 1937, since natural water drainage was insufficient to prevent the flooding of Sete Cidades village during periods of intense rainfall. This tunnel is 1.3 km long and was cut through the northern base of the caldera wall; it discharges water into a stream. The other two lakes, Lagoa de Santiago (0.03 km2) and Lagoa Rasa (0.25 km2), are much smaller and are confined inside craters.
The outer flanks of the central volcano have an average slope of about 12°, rising to 18° near to the caldera rim, where a greater accumulation of pyroclastic deposits occur owing to its proximity to the eruptive vents within the caldera. On those upper slopes, where the thickness of loose pyroclastic materials is greatest, streams have incised deep valleys. These streams on the flanks form a radial pattern around the summit caldera, though in some cases their orientation is modified by faults and/or secondary volcanic structures.
The coast around Sete Cidades is generally characterized by large sea cliffs with average heights of the order of 50 m, but rising in places to 220 m in height.
Scoria cones
These are the most common volcanic edifices on Sete Cidades massif and were formed as a result of the accumulation of basaltic scoria expelled during single eruptions of Strombolian and/or Hawaiian styles. Most of these vents are concentrated on the west and SE slopes of the central volcano (Fig. 7.4), reflecting a clear tectonic control. In the SE sector they define alignments that form ridges of scoria cones associated with the PFVS (Ferreira et al. 2015).
Main volcanic structures of Sete Cidades massif.
Pumice cones
These are monogenetic edifices with a stratified internal structure, made up of pumice with dimensions varying between blocks, lapilli and ash. Inside the Sete Cidades caldera are two pumice cones: Caldeira do Alferes and an unnamed cone located to the west of Lagoa Verde (see Fig. 7.4). A third pumice cone, Caldeira Seca, combines features of a tuff cone, since it was built as result of an eruption with a dominant hydromagmatic style. This generated large amounts of pumice-ash that at the end of the event produced pumice lapilli and blocks that added several metres to the cone. Pumice cones are characterized by basal diameters larger than those of scoria cones.
Maars, tuff rings and tuff cones
These forms or their remnants are relatively common on Sete Cidades, and their genesis was conditioned by the existence of water at the time of eruption. According to Queiroz (1997), the Lagoa de Santiago crater located inside the caldera, and the two craters of Lagoa do Canário, which are close to the caldera rim on the SE flank, are maars (see Fig. 7.4). The well-preserved form of Seara is a tuff cone. Considered to be tuff rings are the Lagoa Rasa, the remnants of three craters surrounding Lagoa Verde and the Península, all being found within the caldera. Along the NW coast of Sete Cidades are remnants of two basaltic tuff rings or tuff cones, which form the islets of Mosteiros and Frades.
Tectonic structures
The dominant fracture system of Sete Cidades has a NW–SE trend (Fig. 7.5) and is related to the regional tectonic regime of the Terceira Rift (e.g. Machado 1959; Queiroz 1997; Carmo et al. 2015; Madeira et al. 2015; Miranda et al. 2015). In this context, the Mosteiros Graben on the NW sector of the volcano is a prominent structure with a dominant extensive component, forming scarps with displacements of up to 70 m. It cuts the caldera and extends towards the SE, controlling the alignment of numerous scoria cones (Queiroz 1997).
Main tectonic structures in Sete Cidades massif (adapted from Carmo et al. 2015).
Less evident but also present are east–west fractures that are considered to be associated with deep faults in the oceanic crust, parallel to the direction of the Azores–Gibraltar Fracture Zone (e.g. Krause & Watkins 1970; Laughton et al. 1972; Searle 1980). It is recognized (e.g. Gaspar 1996; Queiroz 1997) that some central volcanoes in the Azores islands (e.g. Caldera volcano in Graciosa island) were built at the intersection of east–west fractures with the regional NW–SE trend. At Sete Cidades the east–west direction is well expressed in alignments of trachytic domes and scoria cones in the western sector of the volcano (see Figs 7.4 & 7.5) and, in conjunction with the NW–SE regional system, may have determined the location of the central volcano. NE–SW-trending faults are particularly well developed on the south and SW flanks of the volcano, where their orientation controls the direction of some valleys.
The distribution of pumice cones and maars, together with the configuration of some dykes, suggests the existence of arcuate fault(s) inside the caldera and it is assumed that the various vents are positioned at their intersection with regional NW–SE and east–west faults (Queiroz 1997). A more comprehensive analysis of tectonic features in Sete Cidades region is presented by Carmo et al. (2015).
Sete Cidades volcano stratigraphy
The first publication on the stratigraphy of Sete Cidades was by Booth et al. (1978), and focused on the analysis of deposits younger than 5 ka. Later Moore (1983) published the first stratigraphy for the whole volcano, grouping into units the various deposits, which were subsequently included within the stratigraphy of São Miguel as a whole in his 1:50 000 scale geological map (Moore 1991a).
Research by Queiroz (1997) led to the publication of a revised stratigraphic interpretation. With reference to activity during the last 5 ka, it was found that the spatial distribution of the deposits, their internal structures and the nature of their components, both juvenile and lithic clasts, justified the revision of the stratigraphy first proposed by Booth et al. (1978). Furthermore, the observation that the principal reference unit employed by Moore (1983, 1991a) included deposits generated by different eruptions, separated in time by several thousands of years, led to a re-assessment of the whole stratigraphy.
The volcano stratigraphy described in this chapter is based on correlations established between more than 100 geological sections and on the absolute dating of specific units (Queiroz 1997). In generic terms it defines an Inferior Group, comprising a set of undifferentiated deposits, and a Superior Group, divided into six units, which are defined according to the main volcanic events that have marked the eruptive history of Sete Cidades (Table 7.1).
Sete Cidades volcano stratigraphic scale
Inferior Group
The Inferior Group includes all deposits older than the Risco Formation (about 36 ka), which is the lowest subunit of the Upper Group. The oldest dated rocks exposed in the Sete Cidades region have a K–Ar age of 210±8 ka (Moore 1990) and correspond to a trachytic flow cropping out at the base of the northern caldera wall.
The scarcity of accessible exposures and the weathered nature of the deposits make it difficult to establish reliable correlations to enable a detailed stratigraphy of this group to be determined. In fact, with some exceptions, the volcano-stratigraphic sequences representing this period in the history of Sete Cidades volcano are only identified in the inner walls of the caldera and in the highest sea cliffs (e.g. cliffs of Risco and Ginetes in the west, Ponta da Ferraria and Mosteiros in the NW and Ponta da Costa in the NE; Fig. 7.6).
Deposits of the Inferior Group at Ponta da Costa sea cliff.
Above the trachytic lavas at the base there is a sequence that comprises pyroclastic deposits (i.e. ignimbrites, surge deposits and pumice fallout layers) that show a high degree of alteration. Some deposits have a hydromagmatic origin and are related to submarine eruptions, like the basaltic tuffs of the Frades islets near Ponta do Escalvado. At the top of the unit there is a succession of highly vesicular aa lava flows.
Superior group
The Superior Group corresponds to the more recent phase in the volcano's history, which started about 36 ka ago. The identification of deposits related to major volcanic events in this period has led to the definition of six subunits.
Risco Formation
The deposits that constitute the Risco Formation are dated at 35.740±0.810 ka ago, according to 14C analysis of the charcoal fragments found within deposits produced by pyroclastic density currents (PDCs) (Queiroz 1997). The formation reaches a thickness of about 6 m in the Risco area, with good exposures in the south and SW sea cliffs of Sete Cidades and in deeply incised stream valleys on its southern and northern flanks. This unit includes pumice fall layers, surge deposits and ignimbrites that have been erupted during a paroxysmal event. As a result of changes in eruptive styles that occurred during the eruption, this unit is subdivided into three members (Fig. 7.7).
Selected sections of Risco Formation in outcrops around the volcano. Inset: location of the sections.
The Lower Member is observed only locally and has been eroded typically by subsequent PDCs that formed the overlying units. It consists of stratified pumice lapilli and ash fallout layers, with relatively low lithic clast content and a few mafic and feldspar crystals. In some places the top has a layer of coarse ash with cross-bedding and scattered small pieces of carbonized tree trunks.
The main body of the unit is the Intermediate Member and comprises an ignimbrite formed by several flow units, whose total thickness is over 8 m in some valleys on the south coast and 5 m in the Risco sea cliffs. At the Risco site four flow units were identified that differ in their degree of compaction, the size of their juvenile constituents and their lithic clast content. The basal unit is partially welded and has blocks of dark glass, often flattened, and few lithic clasts set in an ashy matrix. The subsequent flow units are less consolidated, have dark pumice blocks and lapilli with a few feldspar crystals and lithic clasts with dimensions up to 50 cm, which are dispersed in a matrix of fine ash (Fig. 7.8a).
Deposits from Risco Formation: (a) detail of partially welded ignimbrite from Intermediate Member; (b) lapilli fallout layer from Upper Member with light grey pumice at the base and dark dense juvenile clasts at the top.
The Upper Member is marked by the presence of a distinctive lapilli fallout layer observed in several outcrops on the slopes of the volcano; it attains a thickness of 25 cm in the vicinity of Bretanha and is characterized by a base of light grey pumice lapilli, sometimes with dark bands, and a top composed of dark dense juvenile fragments and loose feldspar crystals (Fig. 7.8b). It is overlaid by another quite indurated ignimbrite, which is composed of grey-white to dark pumice lapilli and abundant lithic clasts, set in an ash matrix.
Ajuda Formation
This unit includes volcaniclastic deposits and lava flows produced in a time interval of the order of 6–8 ka, between 35.740±0.810 ka BP and 28.750±0.240 ka BP, these being the ages of the formations that both underlie (i.e. the Risco Formation) and overlie (i.e. the Bretanha Formation) the Ajuda Formation.
Outcrops of this unit are scarce and the few recognizable units consist chiefly of pumice fallout deposits and some localized hydromagmatic tuffs, basaltic scoria and lava flows. In some places deposits from the subsequent Bretanha Formation lie almost directly on top of the ignimbrites of the Risco Formation, revealing important unconformities. Several thick deposits of reworked material at this stratigraphic level also point to significant erosion during this time interval.
Bretanha Formation
This formation reaches about 6 m in thickness at the northern rim of the caldera and is present in virtually all sectors of the central volcano. It has been dated at 28.750±0.240 ka BP, based on the 14C analysis of charcoal fragments found at its base (Queiroz 1997). It comprises PDCs and pumice fallout deposits. Taking into account the changes in style during the eruptive process, the Bretanha Formation is separated into Lower, Middle and Upper members (Fig. 7.9).
Selected sections of Bretanha Formation in outcrops around the volcano. Inset: location of the sections. Legend: the same as shown in Fig 7.7.
The Lower Member consists of greyish and yellowish-white pumice ash layers with a thin lens of very fine pumice lapilli. In some places this unit is not present and in other places it shows a significant variation in thickness between outcrops separated by only a few tens of metres. This fact, together with the poor sorting of the ashes and the presence of thin discontinuous lenses of rounded fine lapilli, points to deposition from PDCs. In one section on the NE slope of the volcano, this member includes a thick fines-poor ignimbrite with rounded pumice and obsidian blocks and lapilli rich in feldspar phenocrysts (Fig. 7.10a).
Deposits from Bretanha Formation: (a) detail of the fines-poor ignimbrite from the Lower Member; (b) pumice lapilli fallout layer from the Intermediate Member below the block and ash flow deposits of the Upper Member; (c) detail of the top deposit from the Upper Member where the characteristic reddish to salmon colour of the juvenile blocks and ashes is visible; (d) aspect of the breccia deposits that crop out interbedded with the block and ash flow units of the Upper Member.
Field relations show that this deposit was topographically controlled as it is filling a pre-existing valley, and resulted from a localized segregation of finer matrix and accumulation of the large blocks. The large vitreous clasts have shapes that indicate that they were still hot and fluid during emplacement.
The Intermediate Member attains 1 m in thickness at Lomba das Chaves cliffs and is composed of greyish-white pumice lapilli fallout (Fig. 7.10b). The lower part has well-sorted and angular pumice lapilli with some fine lithic clasts and scarce dark dense juvenile fragments. Towards the top of the unit the concentration of these dark dense juvenile fragments increases along with the size of the lapilli.
The Upper Member comprises several block and ash flow units and breccias, and corresponds to the most voluminous part of the formation (see Fig. 7.10b). The basal flow unit often includes at its bottom a thin layer (up to 10 cm) of compact ash with cross-bedding structures. Considering the internal characteristics of these deposits and their extensive dispersion, they provide a good stratigraphic reference – or marker – level. They contain dense juvenile fragments of the block- and lapilli-sized material, with a dark and reddish colour and abundant lithic clasts set in an ashy matrix. The lower flow units have a dark colour, are very compacted and have a massive and homogeneous internal structure. An exception is an outcrop in the Escalvado area, which has crude cross-bedding stratification. The top of the deposit usually shows distinctive reddish or salmon-pink colours, is less compacted and is characterized by the presence of large juvenile blocks with cauliflower-like surfaces and rare lithic clasts, supported in an ashy matrix (Fig. 7.10c). In the Mosteiros Graben area, these flow units are interbedded with thick breccia deposits consisting almost exclusively of large lithic blocks in a grey ash matrix (Fig. 7.10d).
Lombas Formation
This unit comprises pyroclastic deposits and lava flows related to the eruptive activity that took place over the approximately 13 ka that separated the paroxysmal events represented by the Bretanha and Santa Bárbara formations. The reduced dispersion of the deposits and/or their high degree of alteration prevented good stratigraphic correlations being established. However, it is possible to identify some eruptions and establish partial stratigraphic correlations in different sectors of the volcano. The Lombas Formation comprises 12 units (LM1 to LM12), mostly pumice fallout, forming sequences more than 10 m in thickness in the SW sector of the central volcano (Fig. 7.11). Locally interbedded basaltic lava flows and scoria layers also occur associated with scoria cones on the slopes of the volcano, a fine example being the Escalvado cone. This formation also includes the basaltic tuffs of the Mosteiros islets, and deposits reach a maximum thickness of up to 3 m in the coastal cliffs between Mosteiros and Escalvado (Fig. 7.12).
Selected sections of Lombas Formation in outcrops around the volcano. Inset: location of the sections. Legend: the same as shown in Fig 7.7.
Deposit from Mosteiros islets eruption (labelled a) in the NW sea cliffs of Sete Cidades region. At the base of the outcrop is a block and ash flow unit of the Bretanha Formation (labelled b).
Santa Bárbara Formation
This unit provides one of the best reference levels for Sete Cidades volcano, as it has widespread dispersion and its deposits have distinctive features. It contains whitish-yellow pumice with inclusions of scattered, dark and vesicular, possibly juvenile, fine fragments. These scoriaceous inclusions are typically 0.5–1 cm and can be observed in most pumice lapilli and blocks from all the subunits of this formation. This feature allowed this unit to be recognized and enabled field correlations to be made. Abundant feldspar crystals are also a characteristic of this important formation. This pyroclastic sequence was erupted during a paroxysmal event, is dated at 15.740±0.200 ka BP by 14C (Queiroz 1997) and includes, from bottom to top, PDCs and fallout layers, ignimbrites and pumice fallout deposits that form, respectively, the Lower, Intermediate and Upper members (Fig. 7.13).
Selected sections of Santa Bárbara Formation in outcrops around the volcano. Inset: location of the sections. Legend: the same as shown in Fig 7.7.
Deposits of the Lower Member are observed as a set of interbedded layers of fine pumice ash and lapilli whose contacts are usually gradational. This unit is rich throughout in lithic clasts and the pumice vary in colour from white to yellow, brown and dark grey. It is well exposed in several sections on the western part of the central volcano, reaching a maximum thickness of 80 cm in the area of Candelaria. Some of the beds show lateral variation in thickness and cross-bedding structures and are sometimes partially or totally eroded by the overlaying ignimbrites. On the NE slopes of the volcano a deposit more than 1.5 m in thickness is interpreted as a proximal facies of this unit (Fig. 7.14a). The basal layer is composed of relatively well sorted and angular pumice lapilli and blocks with abundant lithic clasts. Above, several interbedded layers with diffuse boundaries are mostly composed of poorly sorted fine to medium rounded pumice lapilli and ash. The finer-grained layers are especially rich in lithic clasts and present characteristic cross-bedding structures that point to deposition from PDCs.
Deposits from Santa Bárbara Formation: (a) units from the Lower Member; (b) ignimbrite from the Intermediate Member; (c) flow units from the Intermediate Member (labelled 1) below pumice fall units from the Upper Member (labelled 2).
The Intermediate Member consists of ignimbrites and ignimbritic breccias, whose total thickness is more than 10 m in some palaeovalleys (Fig. 7.14b). Different flow units are made up of rounded pumice ash, lapilli and blocks that can reach more than 30 cm in size (some with black pumice or vitreous cores), loose feldspar crystals, obsidian chips and lithic clasts. In general, flow units differ in the size of their constituents, the greater or lesser percentage of lithic clasts and the presence or absence of a fine ash matrix supporting the larger clasts. Many of the layers contain remains of carbonized tree trunks. In proximal exposures on the northern caldera rim, near the Seara cone, thick breccia deposits crop out, composed mainly of large lithic clasts, which are found associated with the ignimbrites. They are interpreted as resulting from the segregation of the large clasts from moving PDCs.
The Upper Member is exposed in the Santa Bárbara region, where it reaches about 2 m in thickness and is composed of a fallout deposit of coarse pumice lapilli, with some lithic clasts (Fig. 7.14c). The pumice is white to light yellow, angular and is generally well sorted.
Lagoas Formation
The Lagoas Formation comprises the more recent deposits of the Sete Cidades massif, younger than 16 ka, which originated in vents both inside the caldera and on the flanks of the central volcano. Most of the eruptions produced volcanic structures that contain lakes in their craters.
This formation is divided into two members, the Cascalho Negro and Pepom, based on a change in the nature of eruptive activity from mostly magmatic to predominantly hydromagmatic. More precisely, however, the distinctive Fogo A deposit is used to define the boundary between these two members. It was erupted from Fogo Volcano and is an important stratigraphic marker across most of São Miguel (Wallenstein 1999; Wallenstein et al. 2015). Radiometric dating of charcoal from this unit gave ages of around 4.550 ka BP (Booth et al. 1978) and 4.230±0.150 to 4.520±0.090 ka BP (Wallenstein 1999).
Cascalho Negro Member includes products of the volcanic activity that occurred during a time interval of the order of 10 ka. In this unit 11 events are recognized and their deposits are named CN1–CN11, which correspond to trachytic pumice fallout sediments, domes and related block and ash flow deposits, basaltic pyroclasts and lava flows, and basaltic tuffs. The best outcrops are observed on the western slopes of the volcano (Fig. 7.15).
Selected sections of Cascalho Negro Member from Lagoas Formation in outcrops from the western slopes of the volcano. Inset: location of the sections. Legend: the same as shown in Fig 7.7.
Trachytic pumice fallout constitutes deposits CN2, CN6, CN7, CN9 and CN10. Most of these are exposed as deeply altered fine layers of pumice lapilli. The youngest, CN10, is the most well preserved and widespread, provides a useful reference level in the Sete Cidades region and is observed as far away as Ponta Delgada. CN10 has a very distinctive internal structure, being a very well-stratified deposit with thin beds of grey ash separating comparatively thicker layers of pumice lapilli, often with indistinct reverse grading. Lithic clasts increase in concentration towards the tops of these layers, where they become the dominant fraction, conferring a typical change in colour from brownish-white to dark brown. Outcrops of the proximal facies in the middle of the deposit show two layers with coarse lapilli and blocks, separated by 10 cm of homogeneous ash. A thick soil developed on top of the deposit and is locally eroded.
CN3 includes a set of trachytic domes, lava flows and associated block and ash flow deposits exposed on the west flank of the central volcano. From limited outcrops it cannot be established whether all the domes were generated during the same eruptive phase or in different stages closely spaced in time. The domes and lava flows have abundant phenocrysts of feldspar set in a fine grey matrix. The block and ash flow deposits have extensive dispersion (observed in outcrops between Várzea and Lomba dos Gagos) and are made up of dark brown fragments of dense juvenile material, rich in feldspar, included in an ash matrix. In some places (e.g. north of Lomba dos Gagos) the deposit consists of products of several flow units with the basal unit being characterized by a high content of lithic clasts.
CN5 is a deposit of gritty ash with some dispersed dense juvenile blocks. It is dark-coloured and rich in loose crystals, which are similar to the dome-forming materials. It is probably related to an explosive phase at the end of the dome-forming eruptions.
CN4 and CN11 are deposits of basaltic scoria and lava flows related, respectively, to the formation of the Pico da Várzea and Lagoa do Pilar scoria cones. It has not been possible to correlate the basaltic CN1 lava flows and the CN8 basaltic ash with a source vent or vents.
Pepom Member is well exposed and overlies the Fogo A deposit. It is mainly composed of trachytic pumice deposits, which originated inside the caldera, and mantles most of Sete Cidades region. This unit also includes basaltic scoria and lava flows produced from vents on the outer slopes of the central volcano. Booth et al. (1978) identify 12 trachytic explosive eruptions younger than Fogo A from the Sete Cidades caldera. Queiroz (1997) discriminates between the deposits of at least 17 events from the same period that are sequentially named from P1, the oldest, to P17, the youngest, and demonstrate that all of the products resulted from explosive trachytic activity with the main differences being the magnitude of eruption and the relative proportions of magmatic to hydromagmatic phases (Fig. 7.16). The main features of the eruptions are described in Queiroz et al. (2008) and Cole et al. (2008), and are summarized in Table 7.2. According to their eruptive character, they may be subdivided into three main groups (Queiroz 1997).
Selected sections of Pepom units in exposures around the volcano. Inset: location of the sections (adapted from Queiroz et al. 2008).
Eruptions with relatively constant alternating styles of hydromagmatic and magmatic activity, which formed the deposits of P1, P8 and P12. Overall these units resulted from pumice fallout and granulometric variation of the juvenile clasts together with changes in lithic clasts content produced stratified deposits. This internal stratification is especially well defined in P8. Fine discontinuous lenses of ash with internal cross-bedding that point to small localized dilute PDCs are found in scarce outcrops of P1 and P8. Additionally P1 has a distinctive fine-grained ignimbrite at the top of the unit that is only observed in outcrops on the SW slopes of the volcano, in Candelaria village region. Where this flow filled valleys, the deposits reach 20 m in thickness and include abundant tree moulds up to about 20 cm in diameter
Eruptions with relatively constant hydromagmatic characteristics produced deposits P2, P4, P9, P13, P16 and P17. All of these deposits derive from fallout, and the ash fraction is dominant with some fine layers of interbedded lapilli. The P2 eruption built the Seara tuff cone. P9 deposits have a distinctive feature with the presence of some discontinuous lenses of lithic-rich compact brown ash with pinch-and-swell and cross-bedding structures, suggesting the occurrence of surge-like deposits interbedded with the fallout units. P17 is the youngest deposit from the intracaldera activity and was erupted about 600 years ago, forming the Caldeira Seca tuff and pumice cone.
Eruptions with short magmatic initial stages and with subsequent alternating magmatic and hydromagmatic phases, the importance of the latter increasing with time, forming the deposits of P3, P5, P6, P7, P10, P11, P14 and P15. They have basal layers composed essentially of pumice lapilli fallout and then they evolve to a stratified upper part made from alternating lapilli and ash fallout layers. P11 is distinctive as it is the most voluminous deposit of the Pepom Member and is one of the most widespread, being identified in outcrops in Ponta Delgada where it attains an average thickness of 20 cm. P14 and P15 show very similar internal stratification and are difficult to distinguish in exposures where only one is present, although P15 is more voluminous and widespread. Overall the remaining units are less well exposed and result probably from lower-magnitude events.
Volcanic parameters for the last 5 ka eruptions located inside the caldera (in Queiroz et al. 2008)
Flank eruptions
Alongside the trachytic explosive activity which was centred in the caldera, more than 12 basaltic eruptions have occurred on the outer flanks of the central volcano during the last 5 ka (Queiroz 1997; Queiroz et al. 2008). Most of these were concentrated SE of the caldera and are related to the Strombolian activity from PFVS (Ferreira et al. 2015), such as Pico do Carvão, dated at 1.280±0.150 ka BP (Moore & Rubin 1991), and Pico das Ferrarias, dated at 0.980±0.090 ka BP (Moore & Rubin 1991).
Other prominent scoria cones formed in this period were Pico de Mafra and Ginetes. Their age is uncertain but stratigraphically they are positioned between the P1 and P2 deposits. In addition to basaltic scoria, both events erupted lava flows and those from Pico de Mafra formed the Mosteiros lava delta.
The most recent flank eruption built the cone of Pico das Camarinhas at the edge of the western sea cliffs of Sete Cidades, producing a scoria cone and lava flows that formed the lava delta at Ponta da Ferraria. The products of this eruption were dated at 0.840±0.060 ka BP (Moore & Rubin 1991). P17 is the only deposit that covers Pico das Camarinhas and its dating confirms the stratigraphy, namely, that little time elapsed between the eruption of these two eruptive events.
Historical eruptions
Since the settlement of São Miguel in the first half of the fifteenth century there have been no on-land eruptions in the Sete Cidades region. Three events took place offshore, however, near the west coast of the central volcano (Fig. 7.17).
Location of submarine historical eruptions.
According to historical reports, in 1638 a submarine eruption occurred and activity was observed to last for about 25 days (Agostinho 1960; Chaves 1960; Weston 1964). Although the precise position of the vent is unknown, Agostinho (1960) points to a location about 3 km west of Ponta da Candelaria, based on bathymetric data and contemporaneous accounts. It involved the emission of dark ash and blocks that built a cone which reached about 120 m above sea-level. This cone disappeared after some months owing to marine erosion.
In 1811 two submarine eruptive phenomena occurred in different locations and 4 months apart (Chaves 1960; Agostinho 1960). The first occurred in February 1811, about 1.7 km SW of Ponta da Ferraria. Accounts mention that the eruption was visible for 1 week but did not form any structure above sea-level, only producing a submarine bank. Later, in June and from a vent located to the north of the previous event, a Surtseyan eruption created a short-lived islet named Sabrina. A description on the volcanic activity at that time was made by Captain S. Tillard of the Royal Navy ship Sabrina, after which the temporary islet received its name (Tillard 1812). It was located about 1.7 km NW of Ponta da Ferraria and was also removed by erosion after a few months. Seismic activity that preceded and accompanied these eruptions caused major destruction in villages on the SW coast of the island.
No accounts mention the deposition of ash inland for any of these three eruptions and field work has not revealed any deposit that could correlate with these events.
Petrology
The rocks of the Sete Cidades Volcanic System define an alkaline magmatic series (Fig. 7.18) with a potassic signature (Fig. 7.19) and include basalts, hawaiites, mugearites (and shoshonites), benmoreites and trachytes (Queiroz 1997). The observed compositional variation of the magmas, in terms of major and trace elements, agrees with an evolution process dominated by fractional crystallization at low pressures in a magma chamber considered to be located at about 3–5 km depth below the central volcano (Machado 1972; Queiroz 1997; Beier, et al. 2006). The major and trace element correlations, either positive or negative, can be explained by fractionation of the mineral phases identified: olivine, pyroxene, plagioclase, amphibole, biotite, oxides, alkali feldspar and apatite (Queiroz 1997).
Sample classification according to the total alkali/silica diagram of Le Bas et al. (1986).
Potassic signature of most of the samples according to the Na2O/K2O diagram classification of Middlemost (1975).
In terms of petrography, alkali basalts have phenocrysts of olivine, clinopyroxene and plagioclases that occur also in the groundmass together with oxides. In hawaiites phenocrysts observed include olivine, titaniferous clinopyroxene, plagioclase, amphibole and iron oxides, whereas in the groundmass clinopyroxene and oxides are dominant. Mugearites have a similar mineralogy showing in some cases the presence of apatite phenocrysts. Trachytes present generally well-defined crystals of alkali feldspar, biotite and amphibole in a matrix with similar minerals together with oxides and apatite.
Samples from units older than 16 ka, specifically rocks formed before the last caldera-forming event, range across the alkaline series whereas the younger rocks are located as end members of basaltic and trachytic fields, revealing a compositional gap as the intermediate members are missing (see Fig. 7.18). Since c. 16 ka ago basaltic lavas have originated from vents on the slopes of the volcano while trachytes have originated from inside the caldera. The only exceptions are the trachytic domes and flows and associated PDC deposits, which were erupted on the western flank of the volcano.
The development of a shallow magma chamber is consistent with the spatial distribution of the different eruptive centres of trachytic and basaltic nature (sensu lato). It is assumed, therefore, that magmas differentiating in such a reservoir are responsible for the trachytic explosive events of the central volcano. Conversely, the liquids with basaltic composition ascend directly from deeper zones feeding the monogenetic edifices on the slopes of Sete Cidades (Queiroz 1997; Beier et al. 2006). Under certain conditions these basaltic magmas are injected into the magma chamber itself, disturbing the existing physicochemical equilibrium and working to trigger some of the explosive eruptions that are generated from the central volcano (Queiroz 1997).
A significant feature is the clear compositional variation recorded during the paroxysmal eruptions of the Risco and Bretanha formations, related to the two first caldera-forming stages. The event that produced the Risco Formation evolved from trachytic magmas generating the pumice fallout layers of the Lower Member and the ignimbrite units of the Intermediate Member. Then, at the base of the Upper Member, a fallout layer shows the compositional variation from trachytic pumice lapilli to shoshonitic dense juvenile clasts, composition that continued to the flow units at the top of this member. Similarly the Bretanha Formation shows compositional change in the deposits. The Lower and Intermediate members’ juvenile materials are trachytic while the deposits from PDCs that originated the Upper Member have a mugearitic composition. Overall both eruptive processes suggest the extraction of liquid from a compositionally stratified magmatic reservoir.
In the case of the deposits of the Santa Bárbara Formation that are associated with the most recent caldera-forming event, no chemical change is observed. However, petrographic analysis reveals that the dark scoriaceous inclusions observed within the trachytic pumice clasts relate to more mafic material, which is likely to have been juvenile. It is proposed that magma mixing was the eruptive mechanism involved (Queiroz 1997).
Eruptive history
Construction of the central volcano
Sete Cidades central volcano began to form several hundred thousand years ago, rising from the c. 2000 m-deep Azores seafloor platform (Needham & Francheteau 1974). The transition from submarine activity to a predominantly subaerial regime probably dates back more than 250 ka, taking into account:
the age of 210±8 ka BP obtained for the oldest rocks cropping out (Moore 1983); and
the occurrence of subsidence phenomena related to the NW–SE regional tectonic system, as observed in the case of Fogo Volcano (Muecke et al. 1974; Wallenstein et al. 2015).
Subaerial volcanic activity at Sete Cidades was marked initially by predominantly effusive events, of Hawaiian and/or Strombolian character (Walker 1973; Queiroz 1997), as evidenced by the thick succession of lava flows exposed in the cliffs in the Bretanha area, on the north sector of the central volcano. Extrusion of lava flows of intermediate to evolved chemical composition was occasionally interrupted by explosive activity, the importance of which appears to have increased through time. In fact several exposures observed in sea cliffs and in the inner walls of the present caldera show towards the top an increasing number of trachytic pumice layers, including fall deposits, ignimbrites and surges, which were generated during sub-Plinian and Plinian episodes.
The eruption of volcanic products with variable composition – between basalts and trachytes related by low-pressure fractional crystallization – indicates a shallow magma chamber, probably developed at the intersection of NW–SE and east–west fracture systems. These contributed significantly to the formation of the polygenetic volcano. According to Machado (1972) the magma chamber developed at an average depth of 5 km with a thickness of 2–3 km. Also Beier et al. (2006) located this reservoir at a depth of 3.5 km based on petrological criteria.
It is assumed that at an early stage this structure may have constituted an island, standing apart from the other volcanoes to the east. The joining of Sete Cidades to Fogo was achieved through the development of a ridge of basaltic scoria cones of the PFVS along the SE flank of the central volcano. The volcanological evolution of the Sete Cidades massif at that stage in its development still raises many questions. In fact, the exact time when the PFVS started to form is unknown. It is also still necessary to clarify the origin of the thick trachytic flows and pyroclastic deposits that are exposed in the sea cliffs in Rocha da Relva region and which possibly constitute the remnants of an old and highly eroded polygenetic volcano located to the south of Sete Cidades.
Genesis of the caldera
The genesis of Sete Cidades caldera has been discussed by several authors. Machado (1957) considered that this depression was formed by several explosive and collapse episodes. Walker (1984) concluded that the caldera resulted from incremental subsidence over time and through small events, since he did not recognize the existence of a pyroclastic deposit that was associated with a caldera-forming event. In contrast, Moore (1990, 1991b) attributed the genesis of the caldera to a single paroxysmal episode that occurred about 22 ka ago. Beier et al. (2006) argued that, whilst accepting the dates proposed by Queiroz (1997) for the three caldera-forming events (i.e. 16, 29 and 36 ka), joined them together into a single caldera-forming stage.
Queiroz (1997), from stratigraphically consistent arguments developed through analysis of the field data, considered that the morphology of the caldera indicates a complex evolutionary process, reflecting the sum of different paroxysmal phases intercalated with lower magnitude intracaldera eruptions. The model put forward by Moore (1990, 1991b) is based on a set of stratigraphic correlations that cannot be substantiated (Moore 1983, 1991a). The stratigraphic system proposed for the volcano by Moore is based on the identification of an eruptive sequence formed by pyroclastic flows (locally welded), fall and surge deposits, named the ‘outflow deposit’, which he considered to be linked to caldera formation c. 22 ka ago. This age was set by taking into account the carbon dating of charcoal found in deposits located two palaeosols above the ‘outflow deposit’, which gave an age of 21.160±0.180 ka BP (Moore & Rubin 1991). This does not agree with the absolute dating established in Queiroz (1997). Indeed, the stratigraphic position of the deposit dated by Moore & Rubin (1991) is below the Bretanha Formation, for which the age determined was of the order of 29 ka.
The issues raised above also extend to relative chronology. Moore (1983, 1991a) identified deposits that are in fact quite different in age as belonging to his ‘outflow’ eruptive sequence. For example, both the pyroclastic flow deposits of the Bretanha Formation (c. 29 ka), which crop out in the cliffs between Pico do Escalvado and Mosteiros village, and the partially welded ignimbrites of the Risco Formation (c. 36 ka) exposed at the south cliffs of the central volcano were considered by Moore as lateral equivalents of the ‘outflow’ deposits observed at the base of the section near to the geodetic monument of Seara in the caldera rim. Actually, the deposits of Bretanha Formation are present in this section several units above the ‘outflow’ deposit.
It is concluded that a model for the caldera formation that is based on the existence of a single paroxysmal episode c. 22 ka ago cannot be supported. In contrast, from the data presented in this account, it is proposed that there were several paroxysmal events related to the evolution of the caldera. The model presented for the genesis of the caldera proposes at least three major eruptive phases that led to the caldera collapse (Queiroz 1997).
Starting from a polygenetic structure with approximately conical shape (Fig. 7.20a), this model places a likely origin of the first caldera collapse around c. 36 ka ago, associated with the paroxysmal event that produced the deposits of the Risco Formation. At this point, the eruption of a considerable amount of material resulted in the development of extensive ignimbrites and pyroclastic surges. The compositional variation found in the deposits, ranging from trachytic at the base to mugearitic at the top, are suggestive of the emptying of a compositionally zoned magma chamber. As a result there was probably a collapse of the main edifice, so forming a depression with an average diameter of c. 3.5 km (Fig. 7.20b).
Illustrative sketch of the Sete Cidades caldera evolution: (a) edifice of the central volcano; (b) formation of the first caldera; (c) first stage of caldera infilling and flank eruptions; (d) second stage of caldera formation with collapse of the NW sector of the volcano; (e) second caldera infilling stage and flank eruptions; (f) third stage of caldera formation with enlargement of the north and NE sectors of the depression.
The volcanic activity that followed corresponded to the Ajuda Formation and involved predominantly basaltic effusive activity of Hawaiian/Strombolian styles. This was located on the flanks of the central volcano, along radial and/or regional fractures. This is evidenced by the lava flows that cover the deposits of the Risco Formation along the north and NW coasts, their spatial distribution confirming that the original caldera was located in the SE part of the current depression.
In the same period, intracaldera volcanism – strongly controlled by evolved magmas from a shallow reservoir – was characterized by both explosive events of sub-Plinian/Plinian styles and by effusive extrusion of thick lava flows and domes. Such a process probably contributed to the filling of the depression, whose altitude at its base may have been 500 m because of the accumulation of deposits (Fig. 7.20c).
The second stage of the caldera formation occurred at approximately 29 ka, following a violent eruption that was responsible for the collapse of the NW sector of the volcano. It was initially marked by the development of a substantial vertical eruptive column from which a layer of trachytic pumice fall was deposited to the north. The event continued with the formation of several block and ash flow units of mugearitic composition. The process involved, locally, the deposition of lag breccias, in particular towards the NW and fines-poor ignimbrites. The compositional range from trachytic at the beginning of the eruption to less evolved compositions towards its end again indicates an emptying of a compositionally zoned magmatic reservoir.
The thickness of the Bretanha Formation flow deposits at the NW flank of the central volcano, interbedded in some places with thick deposits of breccia, suggests that the Mosteiros Graben probably existed at this stage and underwent some movement during the eruption.
Overall this episode caused the enlargement of the early caldera to the NW, which acquired an elliptical shape about 5 km long and 3.5 km wide (Fig. 7.20d). This is consistent with the NW–SE direction of the main fracture system. Having a western common wall, the two coalescing calderas at this stage showed distinct SE borders as the latter cut the fill deposits of the former.
There followed a period of more moderate explosive activity, during which intracaldera sub-Plinian/Plinian eruptions were alternating with the basaltic flank eruptions of the Lombas Formation (Fig. 7.20e). This formation includes the scoria cones of Lomba dos Gagos, on the south flank and the Pico do Escalvado on the NW flank. The identification of the Ilhéus dos Mosteiros submarine basaltic tuffs in outcrops on the NW slopes of the central volcano links this Surtseyan eruption to this phase of activity.
The last major phase of the Sete Cidades caldera formation took place about 16 ka ago, corresponding to a paroxysmal event that produced the deposits of the Santa Bárbara Formation. It involved the extrusion of lithic-rich ignimbrites and surges, deposited all around the volcano, and pumice fall at the end of the eruption, mainly deposited on the NE and east flanks.
The presence of basaltic lava flows and scoria between the deposits of Bretanha and Santa Bárbara formations, which are cut by the north and NE caldera walls, indicates that this sector collapsed at that stage (Figs 7.20f & 7.21). This explains the absence of the volcanic centres that generated those deposits. Moreover, the stream valleys on the NE flank of the central volcano are cut by the caldera wall, demonstrating the relative youth of this part of the structure when compared with other sectors of the rim.
View of the north and NE parts of the caldera wall, formed after the last paroxysmal event. At the bottom the deeply carved valleys in the infilling deposits and the SE caldera wall, which are controlled by NW–SE regional faults, are noticeable.
Throughout its history, Sete Cidades caldera was affected by strong erosional processes. Directly linked to seismic and/or volcanic causes, or simply the result of the action of external agents, such phenomena have certainly contributed to the retreat of the caldera walls and have played an important role in its evolution. The older segments of the caldera wall, in the SE, reveal the existence of landslide scars and deeply incised streams, some of which are controlled by NW–SE regional faults (see Fig. 7.21).
The post-caldera volcanism
About 16 ka ago the shape of the Sete Cidades caldera approached that of the present day and the following eruptions did not contribute to major changes in its morphology. Field evidence of intracaldera activity between 16 and 5 ka is scarce, and the few deposits observed resulted from predominantly magmatic explosive activity of sub-Plinian scale. It was at this stage that several dome-forming eruptions took place on the western flank of the volcano, producing mostly thick deposits from block and ash flows.
The change from mainly magmatic events to ones with a strong hydromagmatic character centred in the caldera defines the Pepom Member in the stratigraphy. Despite the absence of absolute dating for that change, it is likely to have occurred close to 5 ka ago, as the establishment of prevailing hydromagmatic signature in the deposits is positioned stratigraphically at the level of the Fogo A deposit (Booth et al. 1978). To explain the important role of water in the eruptive styles, it is considered that at this stage a well-developed hydrological system was formed, and geomorphological analysis of the caldera supports the presence of a lake occupying most of that depression.
It is notable that a synchroneity between the change from magmatic to hydromagmatic activity at c. 5 ka BP occurs, not only on Sete Cidades, but also on Fogo and Furnas (see Guest et al. 1999, 2015; Wallenstein et al. 2015). Whether or not this represents external forcing, such as for example the post-Pleistocene increase in eustatic sea-level at c. 7 ka BP (Fleming et al. 1998) and/or rainfall variation, is unknown because of a lack of detailed palaeo-environmental research in the Azores. The influence of environmental change on volcanic activity in the Azores is an interesting area for future research.
During this period, the trachytic volcanic activity produced several volcanic edifices inside the caldera and covered the surrounding landscape with pumice deposits mainly of fallout origin. Among these P11 was the most voluminous deposit, reaching the area where the city of Ponta Delgada is located with thicknesses of about 20 cm (Gomes 2013). At the same time subaerial basaltic events were located on the flanks of the central volcano. The regional tectonic trend appears to explain the preferential distribution of most of these monogenetic vents, as may be observed in the ridge of scoria cones originating close to the SE caldera rim and extending SE for more than 5 km. The Pico do Carvão and Pico das Ferrarias cones, which formed about 1 ka ago (Moore & Rubin 1991), are part of this ridge.
Since the settlement of the island, almost 600 years ago, no eruptions have occurred that are associated with the Sete Cidades volcanic plumbing system. The only events in the area were the three submarine eruptions in 1638 and 1811 considered to be related to the system that feeds basaltic activity on the flanks of the volcano. They were located offshore to the west and on the NW coast and were characterized by Surtseyan eruptive styles.
Final remarks
Throughout its history Sete Cidades central volcano recorded significant paroxysmal eruptions related to caldera-forming stages. The physical and chemical characteristics of these events show diagnostic features that enable an understanding of the volcano's plumbing system to emerge. Taking into account volcanic activity over the last 5 ka, Sete Cidades is considered to be the most active central volcano in the Azores archipelago with the highest number of eruptions. In that period the geological record shows at least 17 trachytic explosive events of sub-Plinian type with strong hydromagmatic characteristics, generated from vents located in the caldera. In addition, more than 12 basaltic eruptions have occurred over the same time span, either subaerial (of the Hawaiian and/or Strombolian types, on the flanks of the central volcano) or submarine (of the Surtseyan type) near to the coast.
The identification and analysis of the events that took place in the Sete Cidades region in the past, including the observed eruptive styles, their frequencies and magnitudes, are essential in assessing the volcanic hazard. This has been discussed in Queiroz et al. (2008) through the definition of an ‘event tree’, following the methodology of Newhall & Hoblitt (2002). Cole et al. (2008) modelled in a probabilistic manner the ashfall from P11 and P17 considering that these eruptions are, respectively, the worst-case and best-case scenarios for a future event. Gaspar et al. (2015a), based on parameters from past events, such as eruptive frequencies and styles on Sete Cidades, defined the areas that are likely to be affected by the different volcanic products (i.e. lava flows, fallout deposits, PDCs), producing hazard zonation maps.
Other hazards are likely to affect Sete Cidades region in the future. People and properties are, for instance, vulnerable to both earthquakes and landslides. Even though seismic activity in the area has been relatively low when compared with the other two active central volcanoes of the island, there are important seismogenic zones close to Sete Cidades that have generated strong earthquakes with damaging effects on the region (Gaspar et al. 2015b; Silva et al. 2015). The superficial deposits that cover most of the landscape in the region are made from unconsolidated pyroclasts, which are susceptible to landslides in particular on the caldera walls, the sea cliffs, steep slopes and deep valleys and the fault scarps of the Mosteiros Graben (Marques et al. 2015).
Acknowledgments
The work on Sete Cidades volcano was undertaken under the auspices of the Centro de Vulcanologia e Avaliação de Riscos Geológicos and was the target of Project EXPLORIS (Explosive eruption risk and decision support for EU populations threatened by volcanoes) and funded by EU FP5, contract no. EVR1-CT-2002-40026. Catarina Goulart and José Medeiros gave important support in DEM and graphics. The authors are thankful to David Chester, Angus Duncan and Paul Cole whose valuable suggestions and corrections helped to improve the chapter.
- © 2015 The Author(s). Published by The Geological Society of London. All rights reserved
References
Chapter 7 Eruptive history and evolution of Sete Cidades Volcano, São Miguel Island, Azores
