Flank Collapse Events Research Articles

Seismic signature detection during the 2018 Anak Krakatau flank collapse and tsunami using seismic amplitudes from regional-scale monitoring

On 22 December 2018, the major flank collapse of Anak Krakatau volcano generated a tsunami that struck the surrounding coasts of Java and Sumatra islands in Indonesia without warning. It was later suggested that the corresponding seismic event lacked the body-wave arrivals typical of tectonic earthquakes, causing difficulties for the automated detection system to recognize the event. We explore the possibility of detecting the seismic signature of such events without relying on the arrival times of body waves, by measuring seismic amplitudes in a regional seismic network at the expected arrival times from a fixed, potential source and comparing them to the theoretical attenuation of surface waves. We propose a fast detection method and evaluate the method using seismograms recorded during the flank collapse and tsunami episode as well as several known tectonic earthquakes. Detailed examinations of the detection results confirm the seismic signatures of the flank collapse and teleseismic events as suggested by previous studies. We also find more seismic signatures suggesting the occurrence of two possible smaller collapse events and variations in the eruptive activity related to the major flank collapse, as well as body and surface wave signals from two teleseismic earthquakes that were present during this episode. Finally, we construct a timeline of events during this devastating episode, combining our results with previous studies as well as insights from weather radar observations. With the ability to detect and discriminate various types of seismic events from each other, the detection method can be useful in assisting the existing monitoring and early warning systems in detecting major volcano-related tsunamigenic events.Graphical

Gravity-driven sliding and associated deformations along complex submarine slopes: a laboratory modeling approach based on constraints observed offshore Martinique Island (Lesser Antilles)

Submarine gravity-driven sliding of sediments are common processes in the vicinity of volcanic islands. In the Lesser Antilles arc, the Montagne Pelée volcano on Martinique Island underwent several flank-collapse events during its long-term eruptive history, resulting in debris avalanches. When the debris avalanches entered into the seawater, they were emplaced over the unstable slope of the volcano, triggering a seafloor sediment failure and massive landslides downslope. Using a laboratory modeling approach, we simulated the gravity-driven sliding of a sand layer lying above a silicone layer. The experiments were performed using various slope geometries (slope lengths and number of slope breaks separating the slopes with different angles), under both dry and aqueous conditions, and while varying the amount of additional sand inputs upslope. The resulting deformations were characterized in each experiment in order to compare the obtained structures with those shown by the seismic lines offshore to the west of Martinique Island. During all the experiments, a compressional frontal deformation zone made of several reverse faults formed downslope, often near the slope breaks. Downslope, a portion of the sediments was mostly displaced and poorly deformed in a damping zone, while an extensional deformation zone formed upslope. The displacements of the surficial markers were measured through time to characterize the sliding dynamics. Our study demonstrates that the slope geometry and additional sand inputs primarily favor and increase the sliding deformation, whereas the hydrostatic pressure plays a secondary catalytic role over time. These results provide new constraints on the driving factors and their consequences on gravity-driven sliding in terms of deformations and runout distance over time. This may have a significant impact on the associated hazard assessment related to offshore infrastructures, in a region known for its seismic and volcanic risks.

Temporal evolution of the Fogo Volcano magma storage system (Cape Verde Archipelago): a fluid inclusions perspective

The architecture of the magma storage system underneath Fogo Volcano (Cape Verde Archipelago) is characterised using novel fluid inclusion results from fifteen basanites, spanning the last 120 thousand years of volcanic activity, and encompassing a major flank collapse event at ∼73 ka. Fluid inclusions, hosted in olivine and clinopyroxene, are made of pure CO2, and based on their textural characteristics, are distinguished in early (Type I) and late (Type II) stage. Inclusions homogenize to a liquid phase in the 2.8 to 30.8 °C temperature range. Densities values, recalculated assuming an original 10% H2O content at the time of trapping, range from 543 to 952 kg·m−3, and correspond to entrapment or re-equilibration pressure ranges of 500–595 MPa, 700–740 MPa, and 245–610 MPa respectively for pre-collapse, early post-collapse, and Holocene/historical eruptions. These entrapment pressures are interpreted as reflecting the existence of two main magma accumulation zones at ∼25 km and ∼ 13–21 km depth, and a zone of fluid inclusion re-equilibration at 9–12 km depth. There is evidence of a complex temporal evolution of the magma system. Historical eruptions, and especially the three most recent ones (occurred in 1951, 1995 and 2014–25), bring fluid inclusion evidence for transient, pre-eruptive shallow (9–17 km depth) magma ponding. Early post-collapse (60 ka) volcanics, in contrast, document fast magma transport from ∼25 km, and suggest a reconfiguration of the magma system after the Monte Amarelo collapse event.

The volcanology of Idunn Mons, Venus: The complex evolution of a possible active volcano

Idunn Mons is a shield volcano located in the Imdr Regio large igneous rise, a Venusian equivalent to a hotspot geologic province on Earth. Recent studies using emissivity data from ESA's Venus Express Mission, together with experimental laboratory studies on the alteration of volcanic materials on Venus conditions, suggest that Idunn Mons is a possible site for very recent or even ongoing volcanic activity. In this work we carried out geologic mapping of Idunn Mons to characterize its materials and structures and to establish its geologic evolution. This study suggests that Idunn Mons is analogous to oceanic island volcanoes on Earth, with volcanic constructive phases that alternate with flank collapse episodes, resulting in its flat-topped summit morphology. These flank collapse events can be related to the intimate temporal and spatial relationship existing between Idunn Mons and Olapa Chasma, a rift that dominates the large igneous rise and relates to the volcano evolution, suggesting that this is not simply a candidate site for active volcanism but also for active tectonism.

Remote hydroacoustic-infrasonic detection and characterization of Anak Krakatau eruptive activity leading to, during, and following the December 2018 flank collapse and tsunami

A climactic eruption phase on December 22, 2018, triggered the collapse of the southwest flank and summit of Anak Krakatau stratovolcano, generating a tsunami which struck the coastlines of Sumatra and Java. We employ a selection of remote moored hydroacoustic (H08S, 3307 km; H01W, 3720 km) and infrasonic (IS06, 1156 km; IS07, 3475 km; IS52, 3638 km) stations of the International Monitoring System (IMS) to investigate eruptive activity preceding, during, and after the climactic eruption phase. We observe 6 months of co-eruptive intermittent infrasound at IS06 and powerful infrasound from the climactic eruption on IS06 and IS52. The climactic eruption phase was not detected hydroacoustically, but we observe a ~ 12-day swarm of hydroacoustic signals beginning 24 days before the flank collapse event that we attribute to sustained submarine eruptive activity at Anak Krakatau. We perform hydroacoustic waveform and envelope multiplet analysis to assess event similarity during the hydroacoustic swarm. Hydroacoustic waveforms are not well-correlated, but envelopes with a main pulse duration of ~ 20-s are correlated, with 88.7% of 247 events grouping into two multiplets using a threshold correlation coefficient of 0.75. The repetitive envelopes indicate a repetitive impulsive volcanic process, either underwater submarine explosions or volcanic earthquakes in the solid Earth coupled to the water column from the Sunda Shelf. This study further underscores the potential of remote acoustic technology for detecting and characterizing eruptions at submarine or partially submerged volcanoes.

Computable general equilibrium modelling of economic impacts from volcanic event scenarios at regional and national scale, Mt. Taranaki, New Zealand

The economic impacts of volcanism extend well beyond the direct costs of loss of life and asset damage. This paper presents one of the first attempts to assess the economic consequences of disruption associated with volcanic impacts at a range of temporal and spatial scales using multi-regional and dynamic computable general equilibrium (CGE) modelling. Based on the last decade of volcanic research findings at Mt. Taranaki, three volcanic event scenarios (Tahurangi, Inglewood and Opua) differentiated by critical physical thresholds were generated. In turn, the corresponding disruption economic impacts were calculated for each scenario. Under the Tahurangi scenario (annual probability of 0.01–0.02), a small-scale explosive (Volcanic Explosivity Index (VEI) 2–3) and dome forming eruption, the economic impacts were negligible with complete economic recovery experienced within a year. The larger Inglewood sub-Plinian to Plinian eruption scenario event (VEI > 4, annualised probability of ~ 0.003) produced significant impacts on the Taranaki region economy of $207 million (representing ~ 4.0% of regional gross domestic product (GDP) 1 year after the event, 2007 New Zealand dollars), that will take around 5 years to recover. The Opua scenario, the largest magnitude volcanic hazard modelled, is a major flank collapse and debris avalanche event with an annual probability of 0.00018. The associated economic impacts of this scenario were $397 million (representing ~ 7.7% of regional GDP 1 year after the event) with the Taranaki region economy suffering permanent structural changes. Our dynamic analysis illustrates that different economic impacts play out at different stages in a volcanic crisis. We also discuss the key strengths and weaknesses of our modelling along with potential extensions.

High-resolution K-Ar dating of a complex magmatic system: The example of Basse-Terre Island (French West Indies)

For the first time in the Lesser Antilles volcanic arc, we combine for one island, 123K-Ar ages, with 133 major and trace elements geochemical analyses, in order to better constrain the volcanic history of Basse-Terre Island. In this study, nine new ages have been obtained from the southern part of the island, and complemented with eighty-three new major and trace element analyses of samples collected across the island. The southern part of Basse-Terre Island has been the loci of volcanic activity since the last 200kyr. It is characterized by the construction of the Grande-Découverte Volcanic Complex (GDVC) composed by the Grande-Découverte – Soufrière (GDS) and the Trois-Rivières Madeleine Field (TRMF). After the onset of construction at least at 205±28ka, the GDVC displays strikingly continuous activity between 140±13 and 56±3ka, followed by a 30kyr volcanic hiatus, which is coeval with the hiatus also observed for the TRMF activity. Two new ages of 125±14 and 140±13ka obtained on a lava flow from the Grande-Découverte caldera wall suggest the presence of a depression, resulting of a major flank collapse and/or explosive event, before 140ka. Finally, a new age of 9±6ka, obtained from outcrops exposed on the edge of the Class River, in the north of the GDVC, allows us to calculate channel incision rates between 11 and 56mm/yr. These values are consistent with incision rates determined on other volcanic islands with similar climates.In a broad sense, the petrology and geochemistry of Basse-Terre Island rocks appear fairly homogeneous, with mainly andesite and basaltic-andesite rocks and typical features of volcanic-arc lavas. Nevertheless, in detail, various magmatic processes can be discerned. Most variations are principally controlled by crystal-melt fractionation-accumulation, but major and trace elements also highlight episodic magmatic recharge, involving magma mixing. There are also indications for assimilation of crustal rocks with continental affinity, as well as mantle input of slab-derived fluids. Trace element ratios suggest the presence of at least two different magmatic sources characterized by different partial melting rates and different continental contributions for Basse-Terre Island. Different massifs show a bimodal behavior, with the Basal Complex, the Axial Chain (Piton de Bouillante and Southern Axial Chain), the Monts-Caraïbes volcanoes and the Sans-Toucher volcano in the first group, and the Septentrional Chain, and the Grande-Découverte Volcanic Complex (GDS and TRMF) in the second. Given the unique amount of time-constrained geochemical data, this study provides a complete and detailed investigation of volcanic evolution in the central part of the Lesser Antilles active arc.

Numerical simulation of the 30–45 ka debris avalanche flow of Montagne Pelée volcano, Martinique: from volcano flank collapse to submarine emplacement

We simulate here the emplacement of the debris avalanche generated by the last flank collapse event of Montagne Pelee volcano (30–45 ka), Martinique, Lesser Antilles. Our objective is to assess the maximum distance (i.e., runout) that can be reached by this type of debris avalanche as a function of the volume involved. Numerical simulations are performed using two complementary depth-averaged thin-layer continuum models because no complete models were available in the literature. The first model, SHALTOP, accurately describes dry granular flows over a 3D topography and may be easily extended to describe submarine avalanches. The second model, HYSEA, describes the subaerial and submarine parts of the avalanche as well as its interaction with the water column. However, HYSEA less accurately describes the thin-layer approximation on the 3D topography. Simulations were undertaken testing different empirical friction laws and debris avalanche volume flows. Our study suggests that large collapses (~25 km3) probably occurred in several times with successive volumes smaller than about 5 km3 entering the sea. This result provides new constraints on the emplacement processes of debris avalanches associated with these collapses which can drastically change the related hazard assessment such as the generated tsunami, in a region known for its seismic and volcanic risks.

Climate influence on volcano edifice stability and fluvial landscape evolution surrounding Mount Ruapehu, New Zealand

Large volcanic debris avalanches are triggered by failure of the steep flanks of long-lived composite cones. Their huge deposits change the landscape and drainage pattern surrounding stratovolcanoes for thousands of years. At Mt. Ruapehu, New Zealand, we identified seven major flank-collapse events that produced debris avalanches travelling down pre-existing river catchments for up to 90km from source. In two cases the extreme mass flux into the river valleys led to their complete truncation from the volcano, while four drainage systems were subsequently re-established along similar pathways influenced by regional strike-slip faulting, which caused localized graben formation. In all cases the volcanic debris-avalanche deposits currently form distinctive plateaus at or near the highest topographic elevations of each river valley margin. The timing of the flank failures indicate that inter-eruptive cone destabilization of Mt. Ruapehu is affected by climate change and occurs most commonly during interstadials when glaciers on the cone are in retreat, whereas syn-eruptive collapses are most prominent during cold stages. Dated debris-avalanche deposit levels, along with those of up to four stadial-related aggradational gravel terraces between c. 125 and 18ka, were used to calculate regional uplift rates in this area. Rates of between 0.2±0.1mmyr−1 to 3.8±0.8mmyr−1 are found for four river systems dissecting the central North Island of New Zealand. In three cases incision below the diamicton sequences and into the basement, allowed quantification of sediment-flux rates into the Tasman Sea of 107,000±1,200m3yr−1 to 177,000±3,500m3yr−1 since debris-avalanche emplacement.

Evidence for a new shallow magma intrusion at La Soufrière of Guadeloupe (Lesser Antilles): Insights from long-term geochemical monitoring of halogen-rich hydrothermal fluids

More than three decades of geochemical monitoring of hot springs and fumaroles of La Soufrière of Guadeloupe allows the construction of a working model of the shallow hydrothermal system. This system is delimited by the nested caldera structures inherited from the repeated flank collapse events and the present dome built during the last magmatic eruption (1530AD) and which has been highly fractured by the subsequent phreatic or phreatomagmatic eruptions. Because it is confined into the low volume, highly compartmented and partially sealed upper edifice structure, the hydrothermal system is highly reactive to perturbations in the volcanic activity (input of deep magmatic fluids), the edifice structure (sealing and fracturing) and meteorology (wet tropical regime).The current unrest, which began with a mild reactivation of fumarolic activity in 1990, increased markedly in 1992 with seismic swarms and an increase of degassing from the summit of the dome. In 1997 seismic activity increased further and was accompanied by a sudden high-flux HCl-rich gas from summit fumaroles. We focus on the interpretation of the time series of the chemistry and temperature of fumarolic gases and hot springs as well as the relative behaviours of halogens (F, Cl, Br and I). This extensive geochemical time series shows that the deep magmatic fluids have undergone large changes in composition due to condensation and chemical interaction with shallow groundwater (scrubbing). It is possible to trace back these processes and the potential contribution of a deep magmatic source using a limited set of geochemical time series: T, CO2 and total S content in fumaroles, T and Cl− in hot springs and the relative fractionations between F, Cl, Br and I in both fluids.Coupling 35years of geochemical data with meteorological rainfall data and models of ion transport in the hydrothermal aquifers has allowed us to identify a series of magmatic gas pulses into the hydrothermal system since the 1976–1977 crisis. The contrasting behaviours of S- and Cl-bearing species in fumarolic gas and in thermal springs suggest that the current activity is the result of a new magma intrusion which was progressively emplaced at shallow depth since ~1992. Although it might still be evolving, the characteristics of this new intrusion indicate that it has already reached a magnitude similar to the intrusion that was emplaced during the 1976–1977 eruptive crisis. The assessment of potential hazards associated with evolution of the current unrest must consider the implications of recurrent intrusion and further pressurisation of the hydrothermal system on the likelihood of renewed phreatic explosive activity. Moreover, the role of hydrothermal pressurisation on the basal friction along low-strength layers within the upper part of the edifice must be evaluated with regards to partial flank collapse. At this stage enhanced monitoring, research, and data analysis are required to quantify the uncertainties related to future scenarios of renewed eruptive activity and magmatic evolution.

Role of large flank-collapse events on magma evolution of volcanoes. Insights from the Lesser Antilles Arc

Flank-collapse events are now recognized as common processes of destruction of volcanoes. They may occur several times on a volcanic edifice pulling out varying volumes of material from km3 to thousands of km3. In the Lesser Antilles Arc, a large number of flank-collapse events were identified. Here, we show that some of the largest events are correlated to significant variations in erupted magma compositions and eruptive styles.On Montagne Pelée (Martinique), magma production rate has been sustained during several thousand years following a 32ka old flank-collapse event. Basic and dense magmas were emitted through open-vent eruptions that generated abundant scoria flows while significantly more acidic magmas were produced before the flank collapse. The rapid building of a new cone increased the load on magma bodies at depth and the density threshold. Magma production rate decreased and composition of the erupted products changed to more acidic compared to the preceding period of activity. These low density magma generated plinian and dome-forming eruptions up to the Present. In contrast at Soufrière Volcanic Centre of St. Lucia and at Pitons du Carbet in Martinique, the flank-collapses have an opposite effect: in both cases, the acidic magmas erupted immediately after the flank-collapses. These magmas are highly porphyritic (up to 60% phenocrysts) and much more viscous than the magmas erupted before the flank-collapses. They have been generally emplaced as voluminous and uptight lava domes (called “the Pitons”). Such magmas could not ascent without a significant decrease of the threshold effect produced by the volcanic edifice loading before the flank-collapse.

Experimental generation of volcanic pseudotachylytes: Constraining rheology

Volcanic systems are highly dynamic environments that comprise rocks and magmas, which, in the process of strain localisation (e.g., in catastrophic flank collapse or dome-building events), are candidate materials for the occurrence of frictional melting and the formation of pseudotachylytes. We evaluate the frictional behaviour of a plagioclase-, two-pyroxene- and glass-bearing andesite and introduce an approach to constrain the rheology of frictional melts. Frictional slip at a rate of 1.3m/s under an axial stress of 1.5MPa induces heating at a rate approximating 130°C per metre of slip and frictional melting of the andesite occurs at >1000°C, which corresponds to a peak in shear stress. With continuing displacement, the shear stress decreases by ∼25% from the peak strength and stabilises while the temperature equilibrates to 1230–1290°C. The shear stress for the system is congruent with a non-Arrhenian temperature-dependent frictional melt rheology exhibiting a non-Newtonian viscosity evolving from approximately 104.2 to 103.4Pas. Post-experiment analysis shows a gradation from 1) the host rock, to 2) an extraordinary zone of unmelted, yet viscously deformed material, to 3) a thin, outer region of chemically heterogeneous protomelts, and to 4) an inner region of chemically homogeneous frictionally generated/modified melt in the core of the slip zone. We discuss the role and identification of frictional melting in volcanic systems and the implication of viscous remobilisation of magmatic plugs during frictional slip of glass-bearing volcanic materials.

Barometry of lavas from the 1951 eruption of Fogo, Cape Verde Islands: Implications for historic and prehistoric magma plumbing systems

Fogo is one of the most active oceanic volcanoes in the world. The island was affected by a prehistoric giant lateral collapse that decapitated the summit of the former Monte Amarelo volcano. Subsequent volcanism has partly filled the collapse scar and built up the present-day Cha das Caldeiras plain and the Pico do Fogo stratovolcano. We have conducted a thermobarometric study of historic and prehistoric, basanitic to tephritic rocks in order to gain insight into Fogo's magma plumbing system and the impact of the collapse event on fractionation depths. A main focus was the penultimate 1951 eruption, which produced basanites to tephrites (5.0–8.2wt.% MgO) at two sites south and northwest of Pico do Fogo. Clinopyroxene–melt barometry of phenocrysts yields a well-confined pressure range of 480–650MPa for the final crystallization level. Microthermometric data of CO2-dominated fluid inclusions in olivine and clinopyroxene phenocrysts yield systematically lower pressures of 250–430MPa. Inclusions in cumulate xenoliths yield pressures of 100–290MPa. The combined data indicate pre-eruptive magma storage in the uppermost mantle between 17 and 22km depth and syn-eruptive short-term magma stalling within the lower crust at 8–13km depth. The lower pressures revealed by fluid inclusions in xenoliths may indicate that they originate from pre-1951 magma pulses that stalled and crystallized at variable levels in the crust. There is, however, no petrologic evidence for persistent crustal magma chambers. Clinopyroxene–melt barometric data of other historic and prehistoric eruptions indicate that magma storage and differentiation occurred in the uppermost mantle at pressures between 420 and 870MPa (15–30km depth) throughout the subaerial evolution of Fogo. Our data indicate that fractionation depths decreased significantly during a period of about 100ka representing a strong growth phase of Fogo edifice leading up to the giant Monte Amarelo flank collapse at 123–62ka. This trend seems to have been interrupted by the flank collapse, as early post-collapse magmas stagnated at deeper levels than late pre-collapse ones. Historic eruptions show shallower and broader pressure ranges, indicating an increase in complexity of the magma storage systems. Thus petrologic data indicate that flank collapse events may significantly influence deep-seated magma plumbing systems beneath ocean islands.

Hydrothermal alteration in andesitic volcanoes: Trace element redistribution in active and ancient hydrothermal systems of Guadeloupe (Lesser Antilles)

The mineralogy and the trace element compositions of hydrothermally-altered volcanic materials collected from ash fall deposits and in four debris-avalanche deposits (DADs) at La Soufrière volcano in Guadeloupe have been determined. Phreatic explosions of the 1976 eruption and flank collapse events have sampled various parts of the active and ancient hydrothermal systems of the volcano. Hydrothermal mineral assemblages (smectite + silica polymorphs ± pyrite/jarosite ± gypsum) are typical of rock alteration by low-temperature acid-sulphate fluids. High-temperature mineral assemblages are rare, indicating that phreatic explosions and flank collapse events have sampled mainly the upper parts of the volcanic edifice. Andesitic eruptive products affected by shallow hydrothermal alteration are complex assemblages of volcanic materials (glass, phenocrysts and xenocrysts with complex magmatic histories) of different ages and compositions. The use of incompatible element ratios and REE compositions normalised to an unaltered reference material overcomes the interpretation difficulties related to mass balance effects of alteration processes and the petrologic heterogeneity of the initial material. REE and other incompatible elements (Th, U, Hf, Zr) are mainly concentrated in the glassy matrix of unaltered andesitic rocks. Secondary S-bearing mineral phases (e.g., gypsum, jarosite) that have precipitated from acid-sulphate fluids do not contain substantial incompatible elements (REE, U, Th, Hf, Zr). Compositional variations of incompatible elements in hydrothermally-altered andesitic materials reflect mainly volcanic glass–smectite transformation, which is characterised by (i) strong depletion of alkalis and alkaline earths (Ba, Sr) and first transition series elements (Zn, Cu, Cr, Co, Ni), (ii) immobility of highly incompatible elements (Th, Zr, Hf, LREE) and (iii) strong depletion of MREE and HREE. The sigmoid shape of normalised REE pattern is characteristic of glass–smectite transformation by low-temperature acid-sulphate fluids. This transformation also produces significant variations in U/Th values, which offer the opportunity to date the cessation of hydrothermal alteration and to reconstruct the evolution in space and time of hydrothermal activity in a volcanic edifice.

Age of the El Golfo debris avalanche, El Hierro (Canary Islands): New constraints from laser and furnace 40Ar/ 39Ar dating

The life cycle of ocean island volcanoes is commonly punctuated by large-scale flank collapse events, which may generate far-reaching tsunami. Estimates exist, though are poorly constrained, for worldwide recurrence time intervals of giant landslides. However, individual events are often difficult to date. The El Golfo debris avalanche, on the island of El Hierro (Canary Islands), is a young feature although its exact formation age remains elusive. Using both furnace and laser heating 40Ar/ 39Ar dating techniques on groundmass separates, we obtained new ages for the altitudinally highest lava flows cut by the El Golfo landslide headwall, as well as a post-collapse lava flow erupted from within the embayment. The new data significantly improve limits on the age of the debris avalanche, which we propose occurred between 87 ± 8 ka and 39 ± 13 ka ago. This age range raises doubts on the previously proposed correlation between this event and the glacial to interglacial transition at ~ 130 ka.

The K–Ar Cassignol–Gillot technique applied to western Martinique lavas: A record of Lesser Antilles arc activity from 2 Ma to Mount Pelée volcanism

We present 23 new ages from three volcanic complexes of the Lesser Antilles arc in Martinique Island (French West Indies). These ages obtained with the K–Ar Cassignol–Gillot technique are distributed within the whole Quaternary. They allowed us to reconstruct a detailed history of successive volcanic growth and flank collapse stages. Trois Ilets Volcanism has been active during at least 2 Ma, between 2.35 ± 0.03 Ma and 346 ± 27 ka, with monogenetic volcanoes of basaltic-andesite to andesitic compositions. We here propose that magma mixing, which characterizes this volcanism, could have been initiated between 617 and 346 ka by the activation of arc-parallel and arc-transverse fault systems. Meanwhile, the Carbet complex was active 25 km to the north from 998 ± 14 to 322 ± 6 ka, and was partially destroyed by a flank collapse after 602 ± 10 ka. Together with geochemical data, our ages show that Mount Conil and Mount Pelée volcanoes are parts of the same edifice sharing a single magmatic reservoir. Mount Conil started to emerge before 543 ± 8 ka, and andesites erupted until 127 ± 2 ka, when a flank collapse destroyed the western flank of the edifice, probably triggering the emplacement of Piton Marcel, the last eruption of this first stage. We note that this collapse occurred during the transition from oxygen stages 6 to 5, i.e. during glacial to interglacial change, when eustatic level rapidly increased. After that, and until present, Mount Pelée volcano was built with periods of cone growth intercalated by flank collapse events. We here show that a peak of activity occurred between 550 and 330 ka in western Martinique within the three complexes, which are spaced of 15–25 km. Since 330 ka volcanic activity is limited to the northernmost Mount Conil–Mount Pelée complex. Our data are in agreement with the regional scale observations that the whole recent Lesser Antilles arc was subject to a high volcanic activity since 600 ka, probably linked to an increase in magma production. This permanent establishment of rising magma in regularly spaced batches and tectonically controlled, could explain the individual chemical evolution of each edifice and the different eruptive dynamisms occurring at the same time along the recent arc.

40Ar/ 39Ar geochronology of Holocene basalts; examples from Stromboli, Italy

Absolute chronologies of active volcanoes and consequently timescales for eruptive behaviour and magma production form a quantitative basis for understanding the risk of volcanoes. Surprisingly, the youngest records in the geological timescale often prove to be the most elusive when it comes to isotopic dating. Absolute Holocene volcanic records almost exclusively rely on 14C ages measured on fossil wood or other forms of biogenic carbon. However, on volcanic flanks, fossil carbon is often not preserved, and of uncertain origin when present in paleosols. Also, low 14C-volcanic CO 2 may have mixed with atmospheric and soil 14C-CO 2, potentially causing biased ages. Even when reliable data are available, it is important to have independent corroboration of inferred chronologies as can be obtained in principle using the 40K/ 40Ar decay system. Here we present results of a 40Ar/ 39Ar dating study of basaltic groundmass in the products from the Pleistocene – Holocene boundary until the beginning of the historic era for the north-northeastern flank of Stromboli, Aeolian Islands, Italy, identifying a short phase of intensified flank effusive activity 7500 ± 500 a ago, and a maximum age of 4000 ± 900 a for the last flank collapse event that might have caused the formation of the Sciara del Fuoco depression. We expect that under optimum conditions 40Ar/ 39Ar dating of basaltic groundmass samples can be used more widely for dating Holocene volcanic events.

Coupling of Dispersive Tsunami Propagation and Shallow Water Coastal Response

The key issue of this article is the concept of combining a model dedicated to dispersive large scale propagation of tsunamis with ComMIT, developed and made freely available by NOAA, that is a state of the art tool for tsunami impact studies. First, the main motivation for this approach, namely the need for efficient computation of runup of tsunamis from submarine/subaerial slides and certain types of earthquake, is discussed. Then the models involved are presented. We describe in some detail the dispersive model component which is a Boussinesq type model that is recently developed for tsunami propagation purposes. Finally, the performance and flexibility of the joint model approach is illustrated by two case studies including inundation computations at selected cites. The potentially disastrous, but small probability, flank-collapse event at the La Palma Island is used as an example of slide generated tsunamis where dispersion plays an important role. The second example is a tsunami from a potential inverse thrust fault at the Lesser Antilles. In this case dispersion during propagation is important for some regions, but not for others.

Substrate deformation associated with the Jocotitlán edifice collapse and debris avalanche deposit, Central México

An impressive debris avalanche deposit is preserved at Jocotitlán volcano in Central Mexico. The northern flank of this edifice collapsed ∼ 9690 years B.P. resulting in a 80 km 2-covering clast-supported deposit that lacks substantial matrix, fine, weak or hydrothermally altered materials. The deposit can be subdivided into three morphologically distinct areas which are each accompanied by specific, and often unique, deformation features in the underlying and adjacent volcaniclastic and lacustrine sediments. From these features, a complex history of pre- and syn-avalanche events was reconstructed beginning with edifice-spreading on the weak substrate material prior to and in preparation of part of the flank collapse event. The north-eastern flank in particular was strongly coupled with the deforming substrate material as is still evident in its extensional profile and the unique mode of failure during the catastrophic event resulting in the deposition of what resembles non-volcanic blockslide deposits rather than the typical hummocky volcanic debris avalanche morphology. This latter type of failure occurred at the north-western flank of Jocotitlán volcano where few signs of substrate interactions are preserved in a deposit dominated by large conical hummocks. In addition to substrate response, interaction with pre-avalanche topography in the eastern deposit area facilitated the emplacement of a lobe roughly perpendicular to the flank failure direction, at apparent high emplacement velocity, and with longitudinal ridges as its most striking surface expression.

Volcanic and geochemical evolution of the Teno massif, Tenerife, Canary Islands: Some repercussions of giant landslides on ocean island magmatism

Large‐scale, catastrophic mass wasting is a major process contributing to the dismantling of oceanic intraplate volcanoes. Recent studies, however, have highlighted a possible feedback relationship between flank collapse, or incipient instability, and subsequent episodes of structural rearrangement and/or renewed volcano growth. The Teno massif, located in northwestern Tenerife (Canary Islands), is a deeply eroded Miocene shield volcano that was built in four major eruptive phases punctuated by two lateral collapses, each removing >20–25 km3 of the volcano's north flank. In this paper, we use detailed field observations and petrological and geochemical data to evaluate possible links between large‐scale landslides and subsequent volcanism/magmatism during Teno's evolution. Inspection of key stratigraphic sequences reveals that steep angular unconformities, relics of paleolandslide scars, are marked by polymict breccias. Near their base, these deposits typically include abundant juvenile pyroclastic material, otherwise scarce in the region. While some of Teno's most evolved, low‐density magmas were produced just before flank collapses, early postlandslide lava sequences are characterized by anomalously high proportions of dense ankaramite flows, extremely rich in clinopyroxene and olivine crystals. A detailed sampling profile shows transitions from low‐Mg # lavas relatively rich in SiO2 to lavas with low silica content and comparatively high Mg # after both landslides. Long‐term variations in Zr/Nb, normative nepheline, and La/Lu are coupled but do not show a systematic correlation with stratigraphic boundaries. We propose that whereas loading of the growing precollapse volcano promoted magma stagnation and differentiation, the successive giant landslides modified the shallow volcano‐tectonic stress field at Teno, resulting in widespread pyroclastic eruptions and shallow magma reservoir drainage. This rapid unloading of several tens of km3 of near‐surface rocks appears to have upset magma differentiation processes, while facilitating the remobilization and tapping of denser ankaramite magmas that were stored in the uppermost mantle. Degrees of mantle melting coincidently reached a maximum in the short time interval between the two landslides and declined shortly after, probably reflecting intrinsic plume processes rather than a collapse‐induced influence on mantle melting. Our study of Teno volcano bears implications for other oceanic volcanoes where short‐term compositional variations may also directly relate to major flank collapse events.