Pimentel, Adriano

Volcano observatories (VOs) around the world are required to maintain surveillance of their volcanoes and inform civil protection and aviation authorities about impending eruptions. They often work through consolidated procedures to respond to volcanic crises in a timely manner and provide a service to the community aimed at reducing the potential impact of an eruption. Within the International Airways Volcano Watch (IAVW) framework of the International Civil Aviation Organisation (ICAO), designated State Volcano Observatories (SVOs) are asked to operate a colour coded system designed to inform the aviation community about the status of a volcano and the expected threats associated. Despite the IAVW documentation defining the different colour-coded levels, operating the aviation colour code in a standardised way is not easy, as sometimes, different SVOs adopt different strategies on how, when, and why to change it. Following two European VOs and Volcanic Ash Advisory Centres (VAACs) workshops, the European VOs agreed to present an overview on how they operate the aviation colour code. The comparative analysis presented here reveals that not all VOs in Europe use this system as part of their operational response, mainly because of a lack of volcanic eruptions since the aviation colour code was officially established, or the absence of a formal designation as an SVO. We also note that the VOs that do regularly use aviation colour code operate it differently depending on the frequency and styles of eruptions, the historical eruptive activity, the nature of the unrest, the monitoring level, institutional norms, previous experiences, and on the agreement they may have with the local Air Transport Navigation providers. This study shows that even though the aviation colour code system was designed to provide a standard, its usage strongly depends on the institutional subjectivity in responding to volcano emergencies. Some common questions have been identified across the different (S)VOs that will need to be addressed by ICAO to have a more harmonised approach and usage of the aviation colour code

The Lajes Ignimbrite on Terceira Island (Azores) records the last major pyroclastic density current-forming eruption of Pico Alto Volcano that occurred ca. 21 kyrs ago. This comenditic trachyte ignimbrite contains up to 30 vol% of crystals, mostly anorthoclase. Geochemical investigation of the products collected throughout two key outcrops reveals that major element compositions are poorly variable, whereas trace elements show significant variability, pointing to the presence of a zoned magma reservoir. Thermometry and oxygen fugacity estimations yielded pre-eruptive temperatures of 850-900°C and ∆NNO -2.4 to -1.8. Meltalkali-feldspar hygrometer indicates magmatic H2O contents ranging from 5.8 wt% in the upper part of the reservoir to 3.6 wt% at the bottom, indicating that the magma reservoir (confined at ~4 km depth) was mainly water-undersaturated before the eruption, except for the topmost portion. Two types of anorthoclase crystals were identified. Type 1 crystals show reverse to oscillatory zoning with An contents of 0.4-2.1 mol% and Ba of 200-2000 ppm. They formed in the middle/upper portion of the reservoir, where fractional crystallization processes dominated. Type 2 crystals, mainly present in the less evolved products, are characterized by patchy-zoned cores with large dissolution pockets surrounded by thick oscillatory-zoned rims and show a wide compositional range (An of 0.5-4.7 mol% and Ba of 142-4824 ppm). Their zoning patterns, together with whole-rock and glass compositions of the juvenile clasts, are consistent with the involvement of an anorthoclase-bearing cumulate from the bottom of the reservoir that underwent partial melting. Crystal dissolution was likely induced by the presence of a heat source at depth, without any mass transfer to the eruptible magma as suggested by the lack of petrographic and chemical evidences of mixing between the resident comenditic trachyte and a mafic/intermediate magma. The thermal instability generated convective plumes that were responsible for the admittance of crystals from the cumulate level into the intermediate portions of the magma reservoir and possibly acted as trigger of the explosive eruption.

The island of Terceira (38.5◦N, 27.2◦W) in the Azores Archipelago boasts a wealth of well- dated recent volcanic products; it is therefore the ideal location to improve the resolution of the full-vector geomagnetic field record for the Mid-Atlantic Ocean. We investigated 21 (sub-)sites sampled from 10 different cooling units; six of these cooling units are from the Holocene, the other four are approximately coeval with the age of known geomagnetic excursions. After successfully determining the palaeodirections from all but one site, we applied the multimethod palaeointensity approach to all sites. Here, we used three different palaeointensity methods: the IZZI-Thellier protocol, the Multispecimen technique, and the calibrated pseudo-Thellier method. For the Holocene sites four robust palaeointensity estimates were obtained, they mostly agree with data from the nearby island of Sa ̃o Miguel and data from the Iberian Peninsula and West Africa. Two older sites, dated at 25.7 and 46.3 ka, produced low palaeointensities: 24.4 and 24.1 μT, respectively. As both the IZZI-Thellier and pseudo-Thellier methods yielded technically acceptable results for these sites we used them to improve the pseudo-Thellier calibration relation and establish its applicability to lower palaeointensities. Sixty per cent of all sampled cooling units/ages in our study yielded a technically successful palaeointensity estimate; this unusually high success rate is attributed both to our sampling strategy and the implementation of the multimethod approach.

The AD 1761 eruption on Terceira was the onlyhistorical subaerial event on the island and one of the lastrecorded in the Azores. The eruption occurred along the fis-sure zone that crosses the island and produced a trachybasaltlava flow and scoria cones. Small comenditic trachyte lava domes (known as Mistérios Negros) were also thought by some to have formed simultaneously on the eastern flank of Santa Bárbara Volcano. Following a multidisciplinary approach, we combined geological mapping, paleomagnetic, petrographic, mineral and whole-rock geochemical and structural analyses to study this eruption. The paleomagnetic dating method compared geomagnetic vectors (directions and intensities) recorded by both the AD 1761 lava flow and Mistérios Negros domes and revealed that the two events were indeed coeval. Based on new data and interpretation of historical records, we have accordingly reconstructed the AD 1761eruptive dynamics and distinguished three phases: (1) a pre-cursory phase characterized by decreased degassing in the fumarolic field of Pico Alto Volcano and a gradual increase of seismic activity, which marked the intrusion of trachybasalt magma; (2) a first eruptive phase that started with phreatic explosions on the eastern flank of Santa Bárbara Volcano, followed by the inconspicuous effusion of comenditic trachyte (66 wt% SiO2), forming a WNW-ESE-oriented chain of lava domes; and (3) a second eruptive phase on the central part ofthe fissure zone, where a Hawaiian to Strombolian-style erup-tion formed small scoria cones (E-W to ENE-WSW-oriented)and a trachybasalt lava flow (50 wt% SiO2) which buried 27houses in Biscoitos village. Petrological analyses show thatthe two batches of magma were emitted independently without evidence of interaction. We envisage that the dome-forming event was triggered by local stress changes induced by intrusion of the trachybasalt dyke along the fissure zone, which created tensile stress conditions that promoted ascent of comenditic trachyte magma stored beneath Santa Bárbara Volcano.

Faial is one of the most volcanically active islands of the Azores Archipelago. Historical eruptions occurred on the Capelo Peninsula (westernmost sector of the island) during A.D. 1672–1673 and more recently in A.D. 1957–1958. The other exposed volcanic products of the peninsula are so far loosely dated within the Holocene. Here, we present a successful attempt to correlate scoria cones and lava flows yielded by the same eruption on the Capelo Peninsula using paleomagnetic data from 31 sites (10 basaltic scoriae, 21 basaltic lava flows). In the investigated products, we recognize at least six prehistoric clusters of volcanic activity, whereas 11 lava sites are correlated with four scoria cones. Dating was conducted by comparing our paleomagnetic directions with relocated Holocene reference curves of the paleosecular variation of the geomagnetic field from France and the UK. We find that the studied volcanic rocks exposed on the Capelo Peninsula are younger than previously believed, being entirely formed in the last 8 k.y., and that the activity intensified over the last 3 k.y. Our study confirms that paleomagnetism is a powerful tool for unraveling the chronology and characteristics of Holocene activity at volcanoes where geochronological age constraints are still lacking.