Fogo Volcano Research Articles

Gas geochemistry of hydrothermal fluids of the S. Miguel and Terceira Islands, Azores

We present here for the first time a complete dataset of the chemical and isotopic compositions of fumarolic fluids collected on the S. Miguel (Fogo and Furnas volcanoes) and Terceira (Pico Alto volcano) Islands. The data are analyzed and discussed, to provide both a comprehensive picture of the thermodynamic conditions of the hydrothermal systems on these two islands, and to give new insight into the origins of these fluids, for a better understanding of the geodynamic context of the Azores archipelago. For S. Miguel Island, the gas equilibria in the H2–CO2–CO–CH4–H2O system suggest temperatures of the hydrothermal system reservoirs from 190°C to 280°C for the Fogo volcano fumaroles, which are 30–35°C higher than those measured for geothermal wells. The equilibrium temperatures estimated for the Furnas volcano hydrothermal system are from 200°C to 275°C. Equilibrium vapor at a temperature of ∼190°C is inferred for the fumarolic fluids discharged on Terceira Island.The He isotopic composition of ∼9.6R/Ra measured in the fumaroles of Terceira suggests that a plume-like source, presumably from the lower mantle, feeds this hydrothermal system. The relatively low 3He/4He ratios (from 5.21 to 5.35R/Ra) and higher CO2/3He ratios of S. Miguel fluids suggest an addition of ∼45% of radiogenic He and ∼30% of crustal CO2 to a plume-like composition as at Terceira.A mainly meteoric origin is inferred for the fumarolic water, whereas the unreactive gas species (He, Ar, N2) arise from mixing processes between an atmospheric-like component and a magmatic component. On S. Miguel Island, the estimated magmatic fluid composition in terms of the ratios of N2/40Ar (62±6) suggests a plume-like mantle source. Deep derived N2 isotope compositions characterized by very negative δ15N values (δ15N⩽−14‰) are inferred for the fluids of the Terceira Island. Also S. Miguel fluids are compatible with the same source with a possible involvement of crustal-derived Nitrogen. These values are more negative than the typical compositions of both the upper mantle and plume-like mantle, which thus supports the possible presence of a 15N-depleted source, presumably from the lower mantle, in the region.

Physical geology of the Fogo volcano (Cape Verde Islands) and its 2014–2015 eruption

The geological development of the Fogo island volcano commenced in the early Quaternary, and much later during the Last Glacial stage this involved a mega‐scale lateral collapse of the former edifice. This later event created a large caldera‐like landform open to the east, the floor of which is known as the Chã, and subsequently within this a strato‐volcanic cone has grown. The last phase of volcanic activity started in late 2014 and persisted for 77 days. It had a devastating impact on the lives of the 1000 plus people who were living within the ‘caldera’, since two large villages and a smaller one were each totally destroyed in a matter of days by the advancing lavas. In addition, large areas of cultivated land, upon which the inhabitants were dependent for their livelihood, were enveloped by lava. The eruption proved to be of a greater magnitude than the immediately preceding one of 1995, when a mass evacuation was necessary but as only a few buildings were affected, resettlement followed. Unfortunately the much greater devastation to the human environment makes it doubtful whether any significant resettlement will be possible after the 2014–2015 event.

Morpho-structural evolution of a volcanic island developed inside an active oceanic rift: S. Miguel Island (Terceira Rift, Azores)

The evolution of volcanic islands is generally marked by fast construction phases alternating with destruction by a variety of mass-wasting processes. More specifically, volcanic islands located in areas of intense regional deformation can be particularly prone to gravitational destabilisation. The island of S. Miguel (Azores) has developed during the last 1Myr inside the active Terceira Rift, a major tectonic structure materializing the present boundary between the Eurasian and Nubian lithospheric plates. In this work, we depict the evolution of the island, based on high-resolution DEM data, stratigraphic and structural analyses, high-precision K–Ar dating on separated mineral phases, and offshore data (bathymetry and seismic profiles). The new results indicate that: (1) the oldest volcanic complex (Nordeste), composing the easternmost part of the island, was dominantly active between ca. 850 and 750ka, and was subsequently affected by a major south-directed flank collapse. (2) Between at least 500ka and 250ka, the landslide depression was massively filled by a thick lava succession erupted from volcanic cones and domes distributed along the main E-W collapse scar. (3) Since 250kyr, the western part of this succession (Furnas area) was affected by multiple vertical collapses; associated plinian eruptions produced large pyroclastic deposits, here dated at ca. 60ka and less than 25ka. (4) During the same period, the eastern part of the landslide scar was enlarged by retrogressive erosion, producing the large Povoação valley, which was gradually filled by sediments and young volcanic products. (5) The Fogo volcano, in the middle of S. Miguel, is here dated between ca. 270 and 17ka, and was affected by, at least, one southwards flank collapse. (6) The Sete Cidades volcano, in the western end of the island, is here dated between ca. 91 and 13ka, and experienced mutliple caldera collapses; a landslide to the North is also suspected from the presence of a subtle morphologic scar covered by recent lava flows erupted from alignments of basaltic strombolian cones. The predominance of the N150° and N75° trends in the island suggest that the tectonics of the Terceira Rift controlled the location and the distribution of the volcanism, and to some extent the various destruction events.

Diffuse volcanic gas emission and thermal energy release from the summit crater of Pico do Fogo, Cape Verde

We report the first detailed study of diffuse emission of carbon dioxide (CO2), hydrogen sulfide (H2S), helium (He), and hydrogen (H2) from the summit crater of Pico do Fogo volcano, Cape Verde. Diffuse CO2, H2S, He, and H2 gas fluxes were measured at 57 sampling sites and ranged up to 12,800, 13, 1, and 6 g m−2 day−1, respectively. Soil temperature measurements at each sampling site were used to evaluate the heat flux. Most of the summit crater shows relatively high CO2 efflux, with highest values close to the fumarolic area, suggesting a structural control of the degassing process. In contrast, H2S effluxes were negligible or very low at the summit crater, except close to the fumarolic area where anomalously high CO2 efflux and soil temperatures were also measured. We estimate total CO2, H2S, He, and H2 diffuse gas fluxes of 219 t day−1, 25, 4, and 33 kg day−1, respectively. Based on a H2O/CO2 mass ratio of 1.52 measured at the fumaroles, we estimate a diffuse steam flux from the summit crater of approximately 330 t day−1. The enthalpy of this steam is equivalent to a heat flux of about 10.3 MW. The diffuse gas emission and thermal energy released from the summit crater of Pico do Fogo volcano are comparable to those observed at other volcanoes. Sustained surveillance of Pico do Fogo using these methods will be valuable for monitoring the activity of one of the most active volcanoes in the Atlantic Ocean.

Intraplate seismicity across the Cape Verde swell: A contribution from a temporary seismic network

We present an analysis and characterization of the regional seismicity recorded by a temporary broadband seismic network deployed in the Cape Verde archipelago between November 2007 and September 2008.The detection of earthquakes was based on spectrograms, allowing the discrimination from low-frequency volcanic signals, resulting in 358 events of which 265 were located, the magnitudes usually being smaller than 3. For the location, a new 1-D P-velocity model was derived for the region showing a crust consistent with an oceanic crustal structure. The seismicity is located mostly offshore the westernmost and geologically youngest areas of the archipelago, near the islands of Santo Antão and São Vicente in the NW and Brava and Fogo in the SW. The SW cluster has a lower occurrence rate and corresponds to seismicity concentrated mainly along an alignment between Brava and the Cadamosto seamount presenting normal faulting mechanisms. The existence of the NW cluster, located offshore SW of Santo Antão, was so far unknown and concentrates around a recently recognized submarine cone field; this cluster presents focal depths extending from the crust to the upper mantle and suggests volcanic unrest. No evident temporal behaviour could be perceived, although the events tend to occur in bursts of activity lasting a few days. In this recording period, no significant activity was detected at Fogo volcano, the most active volcanic edifice in Cape Verde.The seismicity characteristics point mainly to a volcanic origin. The correlation of the recorded seismicity with active volcanic structures agrees with the tendency for a westward migration of volcanic activity in the archipelago as indicated by the geologic record.

Reducing volcanic risk on Fogo Volcano, Cape Verde, through a participatory approach: which outcome?

Abstract. This research paper presents the outcomes of Work Package 5 (socio-economical vulnerability assessment and community-based disaster risk reduction) of the MIAVITA (MItigate and Assess risk from Volcanic Impact on Terrain and human Activities) research programme conducted on Fogo Volcano, Cape Verde. The study lasted for almost 3 years (May 2010 to January 2012), of which most of the time was spent in the village of Chã das Caldeiras, situated within the 9 km wide caldera of the volcano inside Fogo Natural Park. The objectives of the programme included assessment of the vulnerability of the community at risk in terms of livelihoods, access to resources, and power relations between the local people and the different public and private institutions. These are important factors that need to be investigated in order to understand the root causes of vulnerability of the local people. This case study shows that the voluntary exposure of people to volcanic threats is linked to daily access to sources of livelihood, especially agriculture and tourism. This is despite the perception of people of the risk to their lives and properties. In order to counter the factors of vulnerability, the study also aimed to identify and enhance local capacities. To achieve such an objective, a participatory three-dimensional mapping (P3DM) activity was conducted to facilitate the dialogue between the local people and the different stakeholders as well as to prepare plans and measures to reduce volcanic risk. The P3DM was a half success considering that it has not yet led to an operational plan which takes into account the local capacities. The main reasons included (1) the non-participatory aspect of the project at the beginning which should have identified priorities for people and let them lead the project to ensure the sustainability of (2) deep conflicts within the community which complicated the focus group discussions around the 3-D map, and the difficulties in involving more marginalised people like women and the youth, and (3) the fact that volcanic risk is not a priority for the people, who are more concerned with daily difficulties due to unsustainable livelihoods, a lack of access to water, land tenure, and the restrictions by the Fogo Natural Park administration and the municipal officials. Still, the study was successful in creating a space for dialogue between the local people and the outside stakeholders such as the Natural Park Administration, the Civil Protection, and the Municipality of Santa Catarina, who have all participated actively during the course of the project.

Characterization and evaluation of hydrothermally influenced clayey sediments from Caldeiras da Ribeira Grande fumarolic field (Azores Archipelago, Portugal) used for aesthetic and pelotherapy purposes

Caldeiras da Ribeira Grande fumarolic field is located in the Fogo Volcano complex, Sao Miguel Island (Azores Archipelago, Portugal). These are associated with a geothermal system of high enthalpy that allows the existence of singular thermal muds or peloids that are continuously mixed with water inside a manmade pool. The peloids produced by the maturation (mixture) in situ are extracted from this boiling-mud pool and used directly in a local thermal centre (the “Banhos da Coroa” centre), which exists since 1811, for the relief of pain associated with rheumatic diseases and skin disorders treatment. Until this investigation, the application of sediments from Ribeira Grande for pelotherapy ends was only evaluated by human perception. Thus, this work aims the physical, mineralogical and geochemical characterization of the peloids collected inside the boiling-mud pool and the evaluation of their potential to be used for aesthetic and therapeutic purposes. The assessment of the volume of clays available for exploitation is also estimated. Sediments from Caldeiras da Ribeira Grande are naturally heated (above 36 °C in most of the cases) and present pH up to 5. The X-ray fluorescence analyses revealed a high content on Si and S, which are useful for therapeutic purposes and the risk inherent to potential toxic elements’ concentrations was evaluated. The X-ray diffraction data showed the presence of clay minerals, sulphates (alunite) and feldspars. Additionally to these features, silty-clay fraction also exhibits interesting features, such as suitable abrasivity and high plasticity. A detailed field survey was carried out at the Caldeiras da Ribeira Grande boiling-mud pool, including the measurement of the sediments thickness at 45 points and the determination of the ratio wet to dry sediments weight. This allowed to estimate a volume of sediments available for exploitation of 84 m3, or about 36 ton of dry sediments.

Groundwater composition in perched-water bodies in the north flank of Fogo volcano (São Miguel, Azores): main causes and comparison with river water chemistry

A set of 46 springs located in the north flank of Fogo volcano (Sao Miguel, Azores) were sampled to characterize groundwater chemistry. The majority of the discharge from volcanic rocks with acidic character (95 %), at an elevation range of 200–400 m a.s.l., and the average discharge are usually not higher than 2 L/s. Nevertheless, the discharge in a few springs from trachytic lava flows can be higher (21.3 L/s). A group of 12 surface water samples were also collected twice along the Ribeira Grande River. Groundwater average temperature ranges between 19.6 and 20.7 °C, and it is possible to depict two groups of discharges: thermal waters (16.7 %), with a temperature range from 27.8 to 71.2 °C, and cold waters, with temperature under 20 °C. In this latter group, temperature is inversely proportional to elevation. The average pH and conductivity range, respectively, between 5.7 and 7.8 and 291–425 μS/cm. Nevertheless, higher conductivity values were measured in boiling pools. Groundwater mainly belongs to the Na–HCO3 to Na–Cl types. The major-ion relative composition of the cold springs shows a decreasing order for cations as Na > Ca > K > Mg and for anions HCO3 > Cl > SO4. In thermal discharges the relative composition is, respectively, Na > Ca > K > Mg and SO4 > HCO3 > Cl. The average nitrate content ranges from 2.09 to 2.57 mg/L comparing all the sampling periods, and the cumulative probability plot shows that about 75 % of the samples present values lower than 1.98 mg/L. Results show clearly that the higher nitrate measurements are observed in river waters and springs sampled in areas where pasture lands are dominant.

Volcanic risk assessment: Quantifying physical vulnerability in the built environment

This paper presents structured and cost-effective methods for assessing the physical vulnerability of at-risk communities to the range of volcanic hazards, developed as part of the MIA-VITA project (2009–2012). An initial assessment of building and infrastructure vulnerability has been carried out for a set of broadly defined building types and infrastructure categories, with the likelihood of damage considered separately for projectile impact, ash fall loading, pyroclastic density current dynamic pressure and earthquake ground shaking intensities. In refining these estimates for two case study areas: Kanlaon volcano in the Philippines and Fogo volcano in Cape Verde, we have developed guidelines and methodologies for carrying out physical vulnerability assessments in the field. These include identifying primary building characteristics, such as construction material and method, as well as subsidiary characteristics, for example the size and prevalence of openings, that may be important in assessing eruption impacts. At-risk buildings around Kanlaon were found to be dominated by timber frame buildings that exhibit a high vulnerability to pyroclastic density currents, but a low vulnerability to failure from seismic shaking. Around Fogo, the predominance of unreinforced masonry buildings with reinforced concrete slab roofs suggests a high vulnerability to volcanic earthquake but a low vulnerability to ash fall loading. Given the importance of agriculture for local livelihoods around Kanlaon and Fogo, we discuss the potential impact of infrastructure vulnerability for local agricultural economies, with implications for volcanic areas worldwide. These methodologies and tools go some way towards offering a standardised approach to carrying out future vulnerability assessments for populated volcanic areas.

Spring geochemistry in an active volcanic environment (São Miguel, Azores): Source and fluxes of inorganic solutes

Mineral waters were monthly sampled in selected springs from Furnas and Fogo trachytic central volcanoes (São Miguel, Azores, Portugal). Water temperatures between 15.1°C and 90.2°C, characterize poorly mineralized waters of Na–HCO3 and Na–HCO3–Cl types.According to the spring location, two watersheds were selected in each volcano to evaluate solute fluxes and chemical weathering rates (Fogo volcano: Ribeira Grande river — RRG; Furnas volcano: Ribeira Quente river — RRQ). Na, Mg, K and Ca fluxes in groundwater represented 43%, 60%, 46% and 57% of the total (subsurface plus surface fluxes) in RRG, and respectively 43%, 53%, 46% and 49% in RRQ. Average HCO3 flux is ten times higher in RRQ (130.1×106mol/yr) compared to RRG (13.8×106mol/yr), reflecting the volcano degassing and the lower ratio between groundwater and river water fluxes. Based on these values, total CO2-consumption by weathering ranged from 1.5×106mol/km2/yr (RRG) to 4×106mol/km2/yr (RRQ).TDS load varied between 3772t/yr (RRG) and 15388t/yr (RRQ), and the ratio between values in groundwater and in surface water is respectively equal to 0.72 and 2.04. The associated chemical weathering rates in groundwater were 206t/km2/yr (RRG) and 399t/km2/yr (RRQ). Coupled to river water, these values indicate that total chemical weathering rates are respectively equal to 493t/km2/yr and 594t/km2/yr. A similar approach developed for the entire archipelago showed that the chemical weathering rates due to groundwater are in the range from 33 to 321t/km2/yr, being partially controlled by the age of the islands.Results of the present study point out to the need to consider groundwater solute fluxes due to weathering when establishing geochemical budgets.

Seismic swarms, fault plane solutions, and stress tensors for São Miguel Island central region (Azores)

The central region of Sao Miguel Island is one of the most seismically active areas of the Azores archipelago. A revised analysis of the seismicity distribution at this region has, for the first time, shown that the seismicity is clustered in two distinct areas: the area around Fogo Volcano (Fogo) and the area around Congro maar (Congro), with each area having a highly localized swarm activity. From a total of about 15,000 events in the period from 2002 to 2010, 78 best located events were selected to make fault plane solutions using P-wave polarities and amplitude ratios. This set of fault plane solutions, and another six subsets derived from it, were inverted for the best fitting stress tensor. The stress tensor using all the 78 fault plane solutions is characterized by a subhorizontal σ1 striking WNW-ESE and a σ3 striking NNE-SSW, consistent with the regional stress field for this region. A similar result, using only the fault plane solutions located in the Fogo area, was obtained. On the other hand, for the Congro area, a local stress field seems to be superimposed on the regional field: subhorizontal σ3, striking NNE-SSW, and a near-vertical σ1. The same stress regime persists in the first 5 km depth, probably related to the upwelling of thermal fluids. The rising fluids generate horizontal extension at shallow depths, which favour the opening of cracks and the circulation and ascension of hydrothermal fluids. The stress regime deeper than 5 km is more uncertain; however, it is indicative of a compressional regime. Thus, it can be conclude that the smaller Fogo area appears to be dominated by the normal regional stress field while the high active Congro area seems to have a different, highly heterogeneous stress field dominated by local conditions.

Tsunami deposits in Santiago Island (Cape Verde archipelago) as possible evidence of a massive flank failure of Fogos volcano

Massive flank failures of volcanic edifices generate tsunami waves. These low-frequency but high magnitude hazards remain poorly documented because of the scarcity of observations. Offshore deposits are studied only by geophysical surveys and the failure rheologies are poorly constrained. Marine conglomerates found at unusually high elevations in Hawaii and in the Canary Islands were previously interpreted as being the result of giant tsunami waves generated by massive flank failures. This study focuses on the search for, and interpretation of, sedimentary evidence of a tsunami around the coastline of Santiago Island, which is located 55 km east of the active Fogo volcano. The Bordeira caldera in Fogo Island, which opens to the east, was formed by the Monte Amarelo flank collapse, and the active edifice is now nested in the failure's scar. The only evidence of a tsunami was found north of Tarrafal Bay. The deposits display many similarities with the tsunami conglomerate described in Hawaii and the Canary Islands: erosive contact with the substratum; rip-up clasts of paleo-soil; marine bioclasts never found in growth or live position; internal organisation into different facies with distinct characteristics of clast-size (up to boulders), sorting (from moderately to very poorly sorted) and clast-fabrics (landward or seaward orientations). This organisation suggests that the tsunami consisted of two main waves. The absence of tsunami deposits elsewhere on Santiago Island, coupled with the relatively low elevation of the conglomerate in Tarrafal (< 15 m), indicate retrogressive failures, rather than a single massive failure. The marine fauna in the tsunami conglomerate is not typical of warm fauna such as the Senegalese fauna (interglacial stages). The age of the tsunami is bracketed by the ages of a coral branch in the conglomerate (123.6 ± 3.9 ka U–Th age) and a post-collapse lava along the Bordeira caldera in Fogo (86 ± 3 ka K–Ar age).

Monitoring of fumarole discharge and CO2 soil degassing in the Azores: contribution to volcanic surveillance and public health risk assessment

Fluid geochemistry monitoring in the Azores involves the regular sampling and analysis of gas discharges from fumaroles and measurements of CO2 diffuse soil gas emissions. Main degassing areas under monitoring are associated with hydrothermal systems of active central volcanoes in S. Miguel, Terceira and Graciosa islands. Fumarole discharge analysis since 1991 show that apart from steam these gas emissions are CO2 dominated with H2S, H2, CH4 and N2 in minor amounts. Mapping of CO2 diffuse soil emissions in S. Miguel Island lead to the conclusion that some inhabited areas are located within hazard-zones. At Furnas village, inside Furnas volcano caldera, about 62% of the 896 houses are within the CO2 anomaly, 5% being in areas of moderate to high risk. At Ribeira Seca, on the north flank of Fogo volcano, few family houses were evacuated when CO2 concentrations in the air reached 8 mol%. To assess and analyse the CO2 soil flux emissions, continuous monitoring stations were installed in S. Miguel (2), Terceira and Graciosa islands. The statistical analysis of the data showed that some meteorological parameters influence the CO2 flux. The average of CO2 flux in S. Miguel stations ranges from 250 g/m2/d at Furnas volcano to 530 g/m2/d at Fogo volcano. At Terceira Island it is about 330 g/m2/d and at Graciosa 4400 g/m2/d.

Seasonal tropospheric influence on SAR interferograms near the ITCZ – The case of Fogo Volcano and Mount Cameroon

The potential of differential SAR interferometry (DInSAR) to measure volcanic ground deformation is widely recognized, despite several limitations still hindering its use in operational volcano monitoring, one of the most critical being water vapor change in the troposphere. In this paper we investigate tropospheric influence on SAR interferograms for two African active volcanoes strongly affected by the oscillation of the Inter-tropical convergence zone (ITCZ). Fogo Island (∼40,000 inhabitants), located in the southwestern part of the Cape Verde archipelago, is a 30-km-wide active volcano that last erupted in 1995. Mount Cameroon, with approximately 300,000 people living in its immediate surroundings, is the most active volcanic center of the 1600 km-long Cameroon volcanic line, counting seven eruptions over the last century. We analyze 72 SLC ASAR images of Fogo, acquired by ENVISAT from June 2005 to December 2007, and 14 SLC ASAR images of Mount Cameroon, acquired from July 2004 to January 2008. A total of 274 two-pass interferograms, computed from the SLC images, were used for fringe counting and least-squares data adjustment, allowing the estimation of a relative phase delay for each image. We then compare the InSAR-retrieved phase delays with two independent calculations of precipitable water vapor (PWV) in the troposphere, using MODIS and GPS, and observe that all time-series, for both regions under study, match up the ITCZ seasonal oscillation. We conclude that most (if not all) of the phase delays observed are due to water vapor change in the troposphere.

Seasonal cycles of seismic velocity variations detected using coda wave interferometry at Fogo volcano, São Miguel, Azores, during 2003–2004

Fogo volcano is an active central volcano, with a lake filled caldera, in the central part of São Miguel Island, Azores, whose current activity is limited to hydrothermal manifestations such as active fumarolic fields, thermal and CO 2 cold springs and soil diffuse degassing areas. It is affected by important active tectonic structures, with high seismic activity and practically continuous micro-seismicity. A recurrent feature from the seismicity observed in volcanic regions is the occurrence of clusters of similar earthquakes, whose origin can be attributed to the repeated action of a similar source mechanism at the same focal area. Doublets/multiplets were identified in this study within a catalogue of small magnitude (usually < 3) volcano tectonic events recorded in 2003–2004 by a selection of stations around Fogo volcano. All events have been cross-correlated and pairs whose waveforms exhibited a cross-correlation coefficient equal to or higher than 0.9 were analysed using the coda-wave interferometry technique. Subtle velocity variations found between events highlight a seasonal cycle of the velocity patterns, with lower velocity in winter time and higher velocity during summer months. Those results, together with quantitative differences between the same doublets at different stations, exhibit an excellent correlation with rainfall. A seasonal effect can also be broadly seen in the seismicity occurrence, and some of the swarms recorded over the two year period occur during the wettest season or close to episodes of abundant (above average) rainfall. Moreover, temporal and spatial analysis of several swarms highlighted the lack of any mainshock–aftershock sequence and organized migration of the hypocenters. This is suggestive of a very heterogeneous stress field. Vp/Vs is found to be lower than usually observed in volcanic areas, an occurrence likely related to the presence of steamy fluid associated with the geothermal system. Taken together, these observations suggest that pore pressurisation plays a major role in controlling a considerable part of the recorded seismicity. The geothermal fluids around Fogo massif have been identified as derived from meteoric water, which infiltrates through Fogo Lake and the volcano flanks and flows from south to north on the northern flank. All those elements seem to point to a role played by rainfall in triggering seismicity at São Miguel, possibly through pressure changes at depth in response to surface rain and/or an interaction with the geothermal system.

Time scales of formation of zoned magma chambers: U-series disequilibria in the Fogo A and 1563 A.D. trachyte deposits, São Miguel, Azores

High precision measurements of 226Ra– 230Th– 238U disequilibria and Ba concentrations are reported for samples from two chemically zoned trachyte deposits from Fogo volcano, São Miguel, Azores. High-precision U-series disequilibria measurements by plasma ionization multicollector mass spectrometry were performed on pumice lapilli and volcanic glass separates from Fogo 1563 A.D. (∼ 0.14 km 3) and the ∼ 4.7 ka Fogo A (∼ 0.7 km 3) deposits in order to quantify the time scales of magmatic processes. Observed ( 226Ra)/Ba–( 230Th)/Ba relationships in Fogo 1563 are compatible with a conventional instantaneous fractional crystallization model and a pre-eruptive magma residence time of ∼ 50 y. However, the Fogo A data cannot be explained by instantaneous fractional crystallization, and require a prolonged crystallization history. Continuous differentiation models better explain the observed 226Ra– 230Th variations within the Fogo deposits and may be more realistic in general. Such models suggest magma residence times prior to eruption of ∼ 50–80 years for Fogo 1563, and ∼ 4.7 ka for the larger volume Fogo A eruption. These time scales represent liquid residence ages rather than the crystallization ages documented in most previous magmatic time scale studies, and allow constraints to be placed on the time scales necessary for the development of chemical zonation within the Fogo magma chamber. Our results indicate that calculated time scales are relatively insensitive to the precise nature of the continuous differentiation models, and further indicate that meaningful magma differentiation time scales can be obtained despite open system behavior, because the Ra–Th disequilibria are overwhelmingly controlled by feldspar fractionation. Calculated time scales are, however, extremely sensitive to D Ra/ D Ba ratios. We therefore emphasize the crucial importance to better constrain the relative partitioning of Ra and Ba when employing Ra–Th disequilibrium data to constrain the rates and time scales of igneous processes.

The Role of Open-System Processes in the Development of Silicic Magma Chambers: a Chemical and Isotopic Investigation of the Fogo A Trachyte Deposit, Sao Miguel, Azores

The processes operating in the development of chemical zonation in silicic magma chambers have been addressed with a Sr---Nd---Pb---Hf---Th isotope study of the chemically zoned trachyte pumice deposit of the Fogo A eruption, Fogo volcano, Azores. Sr isotopic variation is observed in whole rocks, glass separates and sanidine phenocrysts (whole-rock Sr/Sr: 0 7049---0 7061; glass Sr/Sr: 0 7048---0 7052; sanidine Sr/Sr: 0 7048--0 7062). Thorium isotopic variation is observed in glass separates, with (Th/Th)o ranging from 0 8737 to 0 8841, and exhibiting a negative correlation with Sr isotopes. The Nd, Pb and Hf isotopic compositions of the whole-rock trachytic pumices are invariant and indistinguishable from basalts flanking the volcano. The Sr isotope variations in the whole rocks are proposed to be the result of three distinct processes: contamination of the Fogo A magma by assimilation of radiogenic seawater-altered syenite wall rock, to explain the Sr and Th isotopic compositions of the glass separates; incorporation of xenocrysts into the trachytic magma, required to explain the range in feldspar Sr isotopic compositions; and posteruptive surface alteration. This study emphasizes the importance of determining the isotopic composition of glass and mineral separates rather than whole rocks when pre-eruptive magmatic processes are being investigated.

Tidal modulation of seismic noise and volcanic tremor

We present seismic data from Fogo Volcano (Cape Verde Republic) that document a tidal control on the amplitude of seismic noise and volcanic tremor. The effect is detectable in the raw data by simple eye inspection. We identified the main periodicity in the noise modulation, which coincides with the tidal component M2. The signal duration of 40 days, together with a sampling rate of 1440 samples per day (seismic r.m.s. time series) allowed the resolution of M2 from the neighbor solar frequencies that are often related to environmental effects. Spectral analysis shows that the modulation affects selectively the frequency range 2.0 Hz–3.0 Hz, which we interpret as volcanic tremor. White noise from 1.5 Hz to 7 Hz is also modulated. Comparison with meteorological data reinforces the conclusion that the effect is not due to environmental parameters.

InSAR observations of the 1995 Fogo, Cape Verde, eruption: Implications for the effects of collapse events upon island volcanoes

Fogo volcano, Cape Verde Islands, erupted in April 1995 after 43 years of dormancy. About 46 106 m3 of lava erupted during 7.5 weeks from vents on the SW flank of Pico do Fogo into Cha das Caldeiras. Interferograms obtained from 1993–1998 ERS SAR data show ground deformation due to the feeder dike but lack evidence for any volcano‐wide deformation related to volume changes of a shallow magma reservoir. This suggests that Fogo is fed from a relatively deep, mantle‐lithospheric source (&gt;16.5 km depth), consistent with petrological data. This supports the concept that Cha das Caldeiras is the fill of a collapse scar created by a giant landslide, and not a collapse caldera overlying a central, shallow magma chamber. The disturbance of the magmatic system by giant landslides may explain why some oceanic island volcanoes with high magma supply rates currently lack well‐developed shallow reservoirs.

Improvement in TDEM sounding interpretation in presence of induced polarization. A case study in resistive rocks of the Fogo volcano, Cape Verde Islands

A Time Domain Electromagnetic (TDEM) survey was carried out in and around the caldera of the Fogo volcano, Cape Verde Islands, to detect the low resistive structures that could be related to groundwater. A sign reversal in the sounding curves was encountered in central-loop measurements for the soundings located in the centre of the caldera along three main radial profiles. The negative transients are recorded in the early channels between 6.8 and 37 μs. Negative values in an early time transient is an unusual field observation, and consequently the first step was to check the data to ascertain their accuracy and quality. In the second step, three-dimensional (3D) effects are evaluated and ruled out in this zone, while an Induced Polarization (IP) phenomenon is observed using Direct Current (DC) sounding measurements. In the third step, the IP effect is called upon to explain the TDEM distortions; a Cole–Cole dispersive conductivity is found to be adequate to fit the field data. However, the more relevant one-dimensional (1D) model is recovered when both central-loop and offset-loop data are jointly taken into account, thus indicating that an effect of dispersive conductivity is necessary to explain the field data. The 1D electrical structure exhibits four layers, with decreasing resistivity with depth. Only the first layer is polarizable and its Cole–Cole parameters are m=0.85, c=0.8 and τ=0.02 ms for chargeability, frequency dependence and time constant, respectively. However, the Cole–Cole parameters deduced from TDEM forward modelling remain different from those deduced from DC/IP sounding. In this volcanic setting, this IP effect may be caused by the presence of small grains of magnetite and/or by the granularity of effusive products (lapillis). As a conclusion, it is shown that a modelling using different TDEM data sets is essential to recover the electrical structure of this area.