The Santa Bárbara Formation (SBF) results from the last major explosive eruption of 7C volcano and is related to the final phase of caldera-enlarging. Its deposits are mainly preserved along a narrow area between the caldera rim and the coastline (<5 km distance), while a large portion of the pyroclastic products were deposited into the ocean (Kueppers et al., 2009; Porreca et al., 2018).

This complex pyroclastic sequence (dated at 15,740 ± 200 uncalibrated ¹⁴C years BP; Queiroz, 1997) is divided into three distinct members: Lower, Middle, and Upper. The Lower Member consists of a succession of ash beds alternating with fine-grained pumice lapilli beds, deposited mainly on the SW flank of the caldera, which is interpreted as ash fallout and dilute pyroclastic density current (PDC) deposits from short-lived unstable columns. The Middle Member corresponds to pumice and lithic-rich ash-lapilli tuffs, found on the SW and NE sectors, interpreted as deposited from high particle concentration PDCs (Figure 2A). The Upper Member is a pumice lapilli deposit with a narrow distribution along the NE flank, which records fallout from a sub-Plinian eruption column (Figure 2B,C, 3A). Several intercalated ash-rich horizons indicate that the eruption column at times experienced unsteadiness. The detailed stratigraphy and eruption history of the SBF are described in Queiroz et al. (2015) and more recently in Porreca et al. (2018).

A distinctive feature of the SBF is the occurrence of black trachybasaltic inclusions, also known as magmatic mafic enclaves (MME; Kueppers et al., 2009), within white or banded trachytic pumice clasts (Figure 3C,D). These are found in the Middle and Upper Members and increase upward (in both frequency and size) throughout the pyroclastic sequence (Porreca et al., 2018). The MME have been interpreted to result from magma mingling following intrusion of trachybasaltic magma into a trachytic magma reservoir (Queiroz, 1997; Kueppers et al., 2009; Porreca et al., 2018). Upon contact, the trachybasalt was quenched and experienced brittle fragmentation. The resulting fragments (MME) served as favorable sites for bubble nucleation and growth. The combined effect of thermodynamic instability and volatile exsolution, leading to increase in the magmatic pressure of the reservoir, acted as the eruption trigger and subsequent enhancement of the explosivity (see Paredes-Mariño et al., 2017; Porreca et al., 2018; Laeger et al., 2019).

The proximal deposits of the SBF outcropping on the flanks of 7C edifice were first investigated by Queiroz (1997). Kueppers et al. (2007, 2009) and ultimately Porreca et al. (2018) reinvestigated the SBF combining field methods (geological mapping and stratigraphic analysis) with laboratory analyses (granulometry, componentry, and scanning electron microscope imaging) to reconstruct the eruption history in more detail. To that end, deposit characteristics (e.g., grain size and particle shape) were investigated to decipher different lithofacies and ultimately constrain transport and deposition conditions. All outcrops on the 7C edifice are considered proximal. Given the prevailing winds from westerly directions (see Pimentel et al., 2006; Cole et al., 2008; Gaspar et al., 2015), a detailed field survey was extended along the north and south coasts of São Miguel Island to verify the presence of mid-distal deposits of 7C belonging to the SBF (Figure 1). This fieldwork was concentrated along coastal cliffs as outcrops inland are rare due to: (1) substantial urbanization, (2) widespread distribution of younger (<16 ky) basaltic tephra and lava flows from the Picos Fissure System, (3) widespread distribution of younger (<5 ky) trachytic fall deposits related to 7C and Fogo volcanoes and (4) luxuriant vegetation cover.

Seven separate samples of pumice clasts were collected at four locations (Figure 1): (1) outcrop A of PDC deposits from the SBF (Middle Member) on the south flank of 7C near the village of Candelária (samples 7C01 and 7C02; Figure 2A), (2) outcrop B of pumice fall deposit from the SBF (Upper Member) on the NE flank of 7C near Santa Bárbara village (7C03 and 7C04; Figure 2B,C), (3) outcrop C of a pumice fall bed at Ponta do Cintrão peninsula, containing pumice clasts that are texturally very similar to the ones of SBF (7C05 and 7C06; Figure 2D,E) and (4) outcrop D of a younger pumice fall deposit (P17) on the south flank of 7C caldera close to the viewpoint Vista do Rei (7C07; Figure 2F). The sample from outcrop D was included in this study to check for geochemical variability of 7C products. All collected pumice clasts were in the size range of coarse lapilli. Pumice clasts from outcrops A, B, and C visually contain millimeter-sized MME (Table 1).

Several dozen pumice lapilli clasts were investigated macroscopically for their bubble and MME textures. Two instruments hosted at the Department for Earth and Environmental Sciences, Ludwig-Maximilians-Universität (LMU) Munich, Germany were used to investigate polished surfaces of selected pumice clasts from outcrops B and C (Cameca SX100 electron probe microanalyzer) as well as selected ash grains (scanning electron microscope HITACHI SU5000) operating at secondary electron mode with an acceleration voltage of 15 kV.

Major element compositions of glasses from pumice clasts were determined using a JEOL JXA 8200 Superprobe at the Institut für Geochemie und Petrologie, ETH Zürich, Switzerland. Operating conditions were as follows: 15 kV accelerating voltage, current of 10 nA, counting times of 20 s on peak and 10 s on background and a defocused beam of 20 μm to minimize alkali migration. The analyses were organized such that the most potentially mobile elements (Na, K) were measured first.

Trace element compositions of groundmass glass were measured by LA-ICPMS at ETH Zürich using a 193 nm ArF Excimer laser from Resonetics coupled to a Thermo Element XR ICPMS. Analyses involved a spot size of 29 μm with an output energy of the laser beam maintained at ∼3.5 J/cm². Raw counts per second were reduced using identical methods to Szymanowski et al. (2015) with the MATLAB-based SILLS software (Guillong et al., 2008). NIST-610 glass was used as primary standard and GSD-1G as a secondary standard to monitor instrumental performance. Data were reduced using the appropriate SiO₂ content (from microprobe measurements) as internal standard. Long-term (>5 years) reproducibility of a range of homogeneous glass standards indicates precision typically better than 5% for elements with elemental abundances significantly (i.e., >2x) above detection limit. The full data are provided as online Supplementary Materials.

In order to constrain the eruptive source parameters and wind conditions we used the tephra fallout model of the GIS-based VORIS 2.0.1 tool (Felpeto et al., 2007) to reconstruct the dispersal and thickness of the pumice fall deposit of the Upper Member of the SBF. Tephra fallout simulations were performed with an advection-diffusion model that assumes that above the vent the mass is distributed along a vertical line following the approximation proposed in equation 3 by Armienti et al. (1988) to the model of Suzuki (1983). Away from the vent the model assumes that the transport of particles is controlled by: (1) advective effect of the wind, (2) diffusion due to atmospheric turbulence and (3) terminal settling velocity of the particles (see Folch and Felpeto, 2005; Felpeto et al., 2007).

Input parameters were obtained from published data and when unavailable through the solution of the inverse problem, i.e., by best-fitting simulation results with field data to reconstruct the pumice fall deposit (cf. Macedonio et al., 2008; Scollo et al., 2008; Bonasia et al., 2010, 2011; Moiseenko and Malik, 2019). Three tephra fallout scenarios were simulated to obtain the eruptive source parameters, wind conditions and produce volcanic hazard maps. All three scenarios assume a single-spot vent location in the center of today’s 7C caldera, as the widespread cover by deposits related to post-caldera volcanism (<16 ky) does not allow to constrain the exact location of the vent(s) of the Santa Bárbara eruption.

Scenario 1 reconstructs the proximal pumice fall deposit of the Upper Member of the SBF on the NE flank of 7C volcano. To reproduce its narrow distribution a unidirectional wind profile (blowing from WSW) was assumed following the deposit dispersal. Wind intensity was obtained by best-fit with field data. Eruptive source parameters were tested in pre-defined intervals (erupted volume 0.1–0.4 km³; column height 10,000–20,000 m) until the combination of values agreed with field data. Scenario 2 reconstructs the proximal and mid-distal distribution and thickness of the pumice fall deposit, including the pumice lapilli bed at Ponta do Cintrão peninsula. To reproduce the field observations a complex wind profile was used, with directions varying from SW-blowing wind at lower altitude to WNW-blowing wind at higher altitude. Wind intensity was obtained through best-fit. Eruptive source parameters were changed within the same pre-defined intervals as in scenario 1 until their combination fitted proximal and distal field data. Finally, scenario 3 simulates the potential impact of an explosive eruption with the same eruptive source parameters of scenario 2 under wind conditions blowing from NW, i.e., toward the city of Ponta Delgada. This aims to reproduce the worst-case scenario of a future sub-Plinian eruption of 7C volcano (Cole et al., 2008; Queiroz et al., 2008), with the highest impact on the island’s key infrastructure and economy, giving the predominant westerly blowing winds (see details of wind statistical analyses in Pimentel et al., 2006; Cole et al., 2008; Gaspar et al., 2015). Table 2 summarizes the main input parameters used in the simulation of the three scenarios.

The deposits of the SBF outcrop discontinuously along the flanks of 7C volcano, except on the SE sector where they are probably covered by basaltic products from the Picos Fissure System (see Porreca et al., 2018). It is likely that the SBF products were deposited over the entire volcanic edifice. However, due to significant erosion, vegetation cover and younger overlying deposits, the general stratigraphy and sedimentary features have been deciphered by the analysis of isolated outcrops. Stratigraphic correlations were facilitated by the presence of MME in juvenile pumice clasts, which increase in frequency and size toward the top of the pyroclastic sequence (Queiroz et al., 2015; Porreca et al., 2018).

The SBF Upper Member is only found in an area of approximately 10 km² on the NE flank of 7C. It is a typical fall deposit (Figure 2B,C, 3A) with clast-supported fabric of well-sorted (σϕ 1.2), coarse-grained (Mdϕ -3.5) angular pumice lapilli clasts that are generally light-colored, occasionally banded, and contain MME up to 15% of the volume. Larger pumice clasts occur occasionally (average maximum size of the three largest clasts up to 15 cm). Centimeter-sized angular lithic clasts (up to 10% of total volume; basaltic and trachytic lava fragments and hydrothermally altered clasts) and minor obsidian chips are also present (Figure 3B).

In some outcrops, this deposit can be subdivided into two pumice fall beds separated by an ash-rich layer (12 cm thick, Figure 2B,C). The basal bed is thicker (up to 1.4 m), coarse-grained (Mdϕ -3.5) and more widespread than the top bed. The basal pumice lapilli bed does not show significant grain size variation, which suggests stable eruptive conditions (for more details see Porreca et al., 2018). The thin ash-rich layer found between the two pumice beds is weakly stratified and is interpreted as recording unsteadiness of the eruption column, possibly related to a small partial column collapse and generation of dilute PDCs. The top pumice lapilli bed is generally less than 1 m thick, overall finer-grained (Mdϕ -1.3) and shows slight fining upward. In the upper part of the top bed we occasionally find several thin layers of coarse-ash to fine-grained lapilli clasts. While the fining-upward trend indicates waning of the eruption column, these thin layers indicate a repetitive pattern of localized partial erosion and/or reworking of by dilute PDCs or strong winds during the final stages of deposition of the Upper Member. The narrow area of distribution of the Upper Member of the SBF, the small number of outcrops and the rather homogeneous thickness and grain size do not allow for the reconstruction of reliable isopachs and isopleths.

In 2008, we found one additional depositional area at Ponta do Cintrão peninsula (altitude of 103 m a.s.l.), on the north coast of São Miguel Island (i.e., on the north flank of Fogo volcano) >25 km east of 7C caldera (Kueppers et al., 2009, Figure 1). Here a pumice lapilli bed (up to 30 cm thick, Figure 2D,E) shows visually identical features to those reported for the proximal pumice fall deposit of the Upper Member of the SBF. It is a clast-supported, well-sorted pumice fall bed, with yellowish, angular, lapilli-sized pumice clasts (average maximum clast size of the three largest clasts is 4 cm) distinctive for the striking presence of small black inclusions. Lithic clasts (primarily basaltic lava fragments) are subordinate and up to 2 cm size. All characteristics of this pumice lapilli bed point to a primary deposit. The upper and lower limits are well defined by paleosols (Figure 3D). The underlying paleosol is fine-grained, 10–15 cm thick and of beige color. The overlying paleosol is considerably thicker (>70 cm) and contains (sub-)rounded pumice lapilli.

Later, Ponte (2013) described several outcrops of a locally restricted pumice lapilli bed along the north flank of Fogo volcano with similar features to the pumice bed described in this study. The area covered extends for about 9 km between the Ponta do Cintrão peninsula and São Brás village, at altitudes between 54 and 233 m a.s.l. (see Supplementary Figure S1). It is described as a fall deposit (up to 15–25 cm thick) with yellowish lapilli-sized pumice clasts (maximum size of 4 cm with abundant black scoriaceous inclusions), dispersed lithics (1–2 cm across) and small obsidian fragments. Ponte (2013) stated that it was not possible to correlate this bed to the stratigraphy of Fogo volcano. The textural description matches our findings.

Pumice lapilli clasts from the proximal SBF outcrops (A and B; PDC and fall deposits, respectively) and the mid-distal bed at Ponta do Cintrão peninsula (outcrop C) share the same textural features (Kueppers et al., 2009; Fahrner, 2012; Ortenzi, 2014). The pumice clasts are light-colored (whitish or yellowish), although banded clasts also occur. They are almost aphyric (up to 2 vol% phenocryst content) and highly vesicular (up to 80 vol% vesicularity, Figure 4A), exhibiting two populations of bubbles and high bubble connectivity (Figure 4A,B). While few large bubbles (up to 10 cm long, bubble aspect ratio 2:1) can be found in the largest clasts, the dominant bubble population is commonly < 150 μm long. Approximately 30% of the pumice clasts contain mafic magmatic enclaves (MME). They are variable in size and shape, ranging from single (sub-)millimeter-sized, scoriaceous fragments to MME-enriched areas up to 10 cm in large pumice clasts. The MME are microcrystalline and poorly vesicular, commonly showing diktytaxitic texture (Figure 4C,D) interpreted as evidence of quench crystallization (see Paredes-Mariño et al., 2017; Porreca et al., 2018; Laeger et al., 2019). In many pumice clasts the interior wall of large bubbles is dotted with angular MME. No MME are found within the small bubbles. By contrast, the pumice clasts from outcrop D of a recent pumice fall deposit (P17) are angular, whitish and almost aphyric but do not contain MME. The textural features of pumice clasts as well as of MME from outcrops A, B, and C are very similar, confirming that the Ponta do Cintrão pumice bed can be correlated with the Upper Member of the SBF, as already indicated by similar lithofacies. In order to further confirm this hypothesis, the geochemistry of pumice clasts from 4 different outcrops has been investigated.

All normalized glass major elemental compositions from this study plot within the trachyte field of a classical total alkali vs. silica (TAS) diagram, similar to other compositions reported from 7C volcano (Beier et al., 2006; Queiroz et al., 2015; Johansson et al., 2017). Samples from SBF outcrops A and B, and Ponta do Cintrão outcrop C (7C01 to 7C06, see Figure 1 for location) overlap in all major elements, while sample 7C07 from outcrop D is distinguished on the basis of a higher SiO₂ content and lower CaO, MgO, and TiO₂ (Figure 5 and Table 3). Recently published glass major elements from pumices clasts (with and without flow bands) of the SBF (Laeger et al., 2019) overlap with our samples 7C01–7C06. The observed spread is due to the focus of Laeger et al. (2019) on the analysis of banded pumice clasts, which contain filaments of a less evolved composition, due to magma mixing/mingling. Glass geochemistry data from Fogo and Furnas volcanoes (Johansson et al., 2017) are also shown for comparison and clearly reveal different compositions (Figure 5).

In terms of trace elemental abundances, a similar behavior is observed with samples 7C01–06 showing identical compositions while sample 7C07 is compositionally distinct (Figure 5; the complete trace element dataset is given in Supplementary Table S1). The compositional difference between samples 7C01–06 and 7C07 is preserved across the range of geochemical affinities, being readily observed in compatible (e.g., Ba, Sr), incompatible (e.g., Zr, Hf, Nb) and moderately incompatible (e.g., Li) elements. The low Ba content in sample 7C07 is notable, with values approximating the analytical limits of detection (∼3 ppm Ba). While somewhat unusual, these extremely low Ba contents in sample 7C07 were measured in two different analytical sessions where trace elemental abundances of all samples showed good agreement between the two sessions. A final difference comes in the total abundances of the rare earth elements (REE) which are significantly higher (418–691, avg. 583 ppm) in sample 7C07 than in the six other samples (273–432, avg. 333 ppm).

Overall, the geochemical results (Figure 5) are strongly supportive of the correlation of the outcrop at Ponta do Cintrão peninsula with the proximal Upper Member deposit of the SBF, as assumed on the basis of textural features and field lithofacies data. The glass composition of sample 7C07 from a younger (<5 ka) pumice fall deposit shows that compositionally distinct magmas have been erupted from 7C volcano. This further underscores the significance of the identical geochemistry shown in samples 7C01–06.

Spatial distribution and thickness variation of the Upper Member pumice fall deposit of the SBF allowed us to constrain the eruptive source parameters and wind conditions during the final phase of this eruption. Three different fallout scenarios were simulated, always assuming a vent located in the center of the present summit caldera of 7C volcano (see Methodology section for details).

The tephra fall map of scenario 1 (Figure 6) takes into consideration the proximal outcrops of the Upper Member pumice fall deposit along the NE flank of 7C, assuming a unidirectional wind profile blowing from WSW (253°) and wind intensities increasing upward from 4 to 32 m/s. The solution of the inverse problem for the proximal deposits estimated an erupted volume of 0.19 km³ and a column height of 12,000 m (Table 2). The simulation results are in good agreement with the observed distribution and thickness (up to 3 m) of the Upper Member outcrops (Porreca et al., 2018).

The tephra fall map resulting from scenario 2 (Figure 7) reconstructs a larger spatial distribution of the SBF Upper Member by including the proximal deposits and the mid-distal pumice lapilli bed recognized at Ponta do Cintrão. To reproduce the observed deposit distribution and thickness, a more complex wind profile (obtained through best-fit) was used in the simulation. Wind directions vary from SW (230°) at lower attitude (2,000 m a.s.l.) to WNW (290°) at higher altitude (18,000 m a.s.l.), while wind intensities range from 2 to 32 m/s (see Table 2). These wind directions are in good agreement with the prevailing westerly blowing winds (Pimentel et al., 2006; Cole et al., 2008; Gaspar et al., 2015). Eruptive source parameters obtained by solving the inverse problem yielded a tephra volume of 0.27 km³ and column height of 17,000 m (Table 2) for the final phase of the eruption, related to the deposition of the Upper Member. The simulation adequately reproduces the distribution and thickness of the proximal pumice fall deposit, the measured thickness at Ponta do Cintrão outcrop (30 cm) and the rather homogenous thickness (25 to 15 cm, negatively correlated with increasing distance from 7C volcano) along the north flank of Fogo volcano described by Ponte (2013; see Supplementary Figure S1). Although not shown in the map, the simulation predicts that the entire north coast of São Miguel Island is affected by tephra fall (>5 cm thick).

Finally, the tephra fall map from scenario 3 (Figure 8) simulates the potential impact of a sub-Plinian eruption of 7C, under a unidirectional wind profile blowing from NW (322°), using the same wind intensities (2–32 m/s) and eruptive source parameters (erupted tephra volume 0.27 km³; column height 17,000 m) obtained for scenario 2 (Table 2). Considering the predominant westerly wind directions, this is a highly plausible future scenario. The simulation predicts that the capital city of Ponta Delgada, located ∼12 km SE of 7C volcano, can be affected by 0.5–1 m of tephra fall.

There is no doubt that a future explosive eruption of 7C would severely impact the village of Sete Cidades located inside caldera, but also all villages along the volcano’s flanks. However, the finding of a mid-distal portion of the Upper Member of the SBF deposits provides evidence that a future explosive eruption with similar characteristics has the potential to strongly impact most of the island. Importantly, all major infrastructure (main harbor, airport and hospital) are concentrated in the island’s capital, the city of Ponta Delgada, only ∼12 km downwind from 7C caldera. The recent (<5 ky) eruptive history of 7C was marked by at least 17 trachytic intra-caldera explosive eruptions, some of which of sub-Plinian dimensions, and 12 basaltic flank eruptions (Booth et al., 1978; Queiroz, 1997; Cole et al., 2008; Queiroz et al., 2008, 2015). This high eruptive frequency makes 7C the most active volcano in the Azores in the recent geological past. The last intra-caldera explosive eruption (P17) occurred ∼700 years ago (Queiroz et al., 2008) and took place prior to the settlement of the Azores in the first half of the 15th century (Beier and Kramer, 2018). Based on available age constraints, the average eruption recurrence time of around 300 years has already been exceeded (Gaspar et al., 2015). Although 7C is closely monitored since 1998 by a network of seismic stations and episodic GPS/GNSS campaigns, seismic activity is low when compared with Fogo and Furnas volcanoes. The last seismic unrest occurred in 1998 inside the caldera and to the NW (Queiroz et al., 2015). Since then, 7C volcano has been relatively quiet.

Next, we discuss the impact of tephra fallout in proximal and mid-distal areas according to the three scenarios considered in the present work. All of them have a severe impact on São Miguel Island. In proximal areas (i.e., on the flanks of 7C), thick tephra fallout (but also PDCs, not simulated in this study) can have devastating consequences with similar impact assumed for the three scenarios. Buildings and infrastructure (roads, power lines, etc.) are expected to become damaged/destroyed by dynamic pressures (PDC) or load (tephra fall). Ground transportation and radio communication can be disrupted. Severe damage to vegetation, agricultural areas and water sources is also expected. Health hazards including respiratory problems due to ash and volcanic gasses can affect the local population (for a detailed review of the impacts of volcanic ash see Wilson et al., 2012, 2014). The different impacts on medial and distal areas, based on simulation results, are presented next.

Scenario 1 assumes a small sub-Plinian eruption with winds blowing from WSW (Figure 6) as reconstructed only from the distribution of the proximal pumice fall deposit of the Upper Member of the SBF. In this case, 0.19 km³ of tephra are raining out from a 12,000 m high eruption column (Table 2) and cover the NE flank of 7C (between João Bom and Capelas villages) with up to 3 m thick deposits. Most of the material is going to be deposited into the ocean and form potentially thick and coherent pumice rafts. Depending on wind, wave and currents, pumice rafts may remain afloat for weeks to months (cf. Jutzeler et al., 2014) and will severely affect – if not halt – marine operations along the north coast of São Miguel and make important fishery ports (such as Rabo de Peixe) non-operational. Volcanic ash would shut down the international airport at Ponta Delgada as well as cause the partial closing of the airspace over the Azores, affecting inter-island traffic. At constant wind direction and intensity, the ash plume can reach Europe around NW Spain (∼1500 km distance) and affect flights between Central/SW Europe and Central/South America. As seen in the case of the 2010 Eyjafjallajökull eruption (Iceland) volcanic ash reached central Europe in less than 2 days (Gasteiger et al., 2011) and caused several days of airspace closure. In this scenario, the central and eastern part of São Miguel is not affected by tephra fallout and can likely serve as a retreat area in case of evacuation, similar to the case of Montserrat (West Indies) in 1995 when the population was evacuated to the northern part of the island (Kokelaar, 2002). Of the three considered scenarios, this is the one with the smallest impact on São Miguel and major infrastructure (hospital, port) are likely not affected.

Scenario 2 accounts for a larger sub-Plinian eruption with winds blowing from SW at lower altitude and from WNW at higher altitude (Figure 7) as reconstructed based on the proximal and mid-distal pumice fall deposit of the Upper Member of the SBF. Approximately 42% more tephra (0.27 km³) is being erupted compared to scenario 1 from an eruption column up to 17,000 m high. The proximal deposits are expected to extend further westwards to Mosteiros and cover most of the inhabited areas along the north coast of São Miguel (affecting > 50,000 persons in the villages of Capelas, Rabo de Peixe, Ribeira Grande, Porto Formoso, and Maia). These villages can be affected by 10–25 cm of tephra fall, which is enough to collapse ∼50% of the weakest types of roofs in São Miguel (Booth et al., 1983; Pomonis et al., 1999; Cole et al., 2008). As the area directly affected by tephra fall is substantially larger and comprises approximately half of São Miguel Island, human movement will be impaired by cutting off the main road along the north coast. The pumice deposit can likely have a severe impact on the two geothermal power plants located in the north slope of Fogo volcano. Marine operations would be affected to a comparable degree as in scenario 1. Volcanic ash would shut down the international airport at Ponta Delgada as well as cause the partial closing of the airspace over the Azores, affecting inter-island traffic. At constant wind direction and intensity, the ash plume can reach Europe near Portugal (∼1500 km distance) within 2 days and shut down flights between Europe and Central/South America as well as flights to the Canary Islands. In this scenario, the south part of São Miguel is not affected by tephra fallout and can serve as a retreat area in case of evacuation of the villages located on the north coast. The area of land that can still be used for human activities is significantly reduced, yet, the hospital and main harbor may continue to operate under emergency conditions.

Scenario 3 is based on a hypothetical future eruption with the same eruptive source parameters of scenario 2, under wind conditions blowing toward Ponta Delgada (Figure 8) The eruptive source parameters (erupted volume 0.27 km³; column height 17,000 m) obtained from the reconstruction of the pumice fall deposit of the Upper Member of the SBF, through the solution of the inverse problem, are within the same range of values of the parameters of recent (<5 ky) intra-caldera eruptions of 7C (see Cole et al., 2008; Queiroz et al., 2008). This suggests that this pumice fall deposit can be used as a proxy for a future sub-Plinian eruption of 7C. Furthermore, in this case, more than 100,000 persons live in areas that can be affected by up to 1 m of tephra fall, including the capital Ponta Delgada (with Fajã de Cima, Fajã de Baixo, and São Roque in the direct periphery) and Lagoa, the third-largest city (15,000 inhabitants). This thickness of tephra is enough to cause the collapse of almost all buildings, including modern constructions reinforced with concrete (Spence et al., 2005; Cole et al., 2008). In this scenario the airport, the main harbor and critical infrastructure, such as the hospital are not operational, preventing immediate evacuation measures or arrival of first aid goods. At constant wind direction and intensity, the ash plume would reach the Madeira archipelago (∼1000 km distance) and the Canary Islands (∼1300 km) within 1 or 2 days and affect air traffic to these islands, in Northern Africa and between Europe and South America.

Although 7C volcano did not erupt in historic times, the 1563 eruption of Fogo volcano (Walker and Croasdale, 1970; Aguiar et al., 2018) and the 1630 eruption of Furnas volcano (Cole et al., 1995) provide evidence of the severe impact that sub-Plinian eruptions had on São Miguel in the past when the population density was much smaller. In the case of a future sub-Plinian eruption of 7C under the predominant westerly blowing winds, the local impact on the island itself will be catastrophic, likely leaving a long-term effect, with economic consequences for the entire Azores archipelago. As illustrated by scenario 3, the island of São Miguel with an estimated population of > 137,000 inhabitants, cannot be reached by major transportation devices, making evacuation or delivery of supplies very difficult. In the light of this and nonetheless the relatively low probability of such an event, this worst-case scenario should be part of hazard assessment strategies.

The volcanic hazard potential of 7C therefore should not be underestimated and the probability for future explosive eruptions should not be ignored. This work represents a first attempt to perform a realistic assessment of the impact of a major future explosive eruption of 7C on the island of São Miguel. The insights obtained here may be adopted as features of hazard assessment for other volcanic ocean islands prone to explosive eruptions. In general, this study has evidenced the importance of investigative prowess through synergetic studies of field work, textural/geochemical analyses and numerical simulations. A small number of additional, mid-distal to distal outcrops has proven to boost the precision of efforts to precisely constrain source parameters of past eruptions. In future, a supra-disciplinary approach involving scientists working on subaerial deposits, coring seafloor sediments and investigating ice cores should push our current limits of understanding of past eruptions on volcanic ocean islands or in remote positions.