The occurrence of Mt Barca flank eruption in the evolution of the NW periphery of Etna volcano (Italy)

Geological surveys, tephrostratigraphic study, and ⁴⁰Ar/³⁹Ar age determinations have allowed us to chronologically constrain the geological evolution of the lower NW flank of Etna volcano and to reconstruct the eruptive style of the Mt Barca flank eruption. This peripheral sector of the Mt Etna edifice, corresponding to the upper Simeto valley, was invaded by the Ellittico volcano lava flows between 41 and 29 ka ago when the Mt Barca eruption occurred. The vent of this flank eruption is located at about 15 km away from the summit craters, close to the town of Bronte. The Mt Barca eruption was characterized by a vigorous explosive activity that produced pyroclastic deposits dispersed eastward and minor effusive activity with the emission of a 1.1-km-long lava flow. Explosive activity was characterized by a phreatomagmatic phase followed by a magmatic one. The geological setting of this peripheral sector of the volcano favors the interaction between the rising magma and the shallow groundwater hosted in the volcanic pile resting on the impermeable sedimentary basement. This process produced phreatomagmatic activity in the first phase of the eruption, forming a pyroclastic fall deposit made of high-density, poorly vesicular scoria lapilli and lithic clasts. Conversely, during the second phase, a typical strombolian fall deposit formed. In terms of hazard assessment, the possible occurrence of this type of highly explosive flank eruption, at lower elevation in the densely inhabited areas, increases the volcanic risk in the Etnean region and widens the already known hazard scenario.     

Eruption and mass-wasting-induced processes during the late holocene destructive phase of Nevado del Ruiz volcano, Colombia

The November 13, 1985, eruption was characteristic of the Arenas eruptive stage of Nevado del Ruiz, the most recent of a series of twelve eruptive stages that have occurred in the past 11,000 years. Eruptive sequences, deposits and processes similar to that of 1985 have characterized the behavior of Nevado del Ruiz during three major prehistorical and historical eruptive stages: the approximately 3300-3100 yr. B.P. Hedionda, the 16th century Azufrado, and the mid-1800's Lagunillas eruptive stages, that partly destroyed the present Ruiz summit.According to the interpretation of the stratigraphic record of prehistorical eruptions and historical accounts, almost every recent magmatic event was small or short-lived. Nevertheless, rockslide-debris avalanches and catastrophic debris flows were triggered in all the eruptions owing to slope failures related to specific tectonic features of Ruiz volcano and/or to significant interactions between pyroclastic debris and the ice cap. Evidence for headward retreat of avalanche scarps during multiple eruptions reinforce the case that large slope failures can occur repeatedly at a large-volume volcano like Ruiz without reconstruction of the edifice. The latest Ruiz eruptions that involved rockslide-debris avalanches resemble in part the Shiveluch 1964 event for which evidence of lateral blast deposits are lacking, but differ in part from this type because non-eruptive and mass-wasting processes also triggered rockslide-debris avalanches.Many factors render the cluster of domes of the Ruiz summit unstable, including: (1) deeply dissected troughs opened toward the north-northeast (Azufrado), east (Lagunillas), and south (Recio) flanks; (2) strongly hydrothermally altered north and east flanks of the summit; (3) currently glaciated or recently deglaciated, high cliffs; (4) steep unstable margins of the ice cap on the north and east.Thus, in light of its past behavior, a small eruption or an earthquake might trigger catastrophic rockslide-debris avalanches. Furthermore, such avalanches as well as glacial outburst-floods and ice avalanches could induce debris flows by mobilizing weathered, water-saturated, and unconsolidated rocks or deposits.     

Eruption of kimberlite magmas: physical volcanology, geomorphology and age of the youngest kimberlitic volcanoes known on earth (the Upper Pleistocene/Holocene Igwisi Hills volcanoes, Tanzania)

The Igwisi Hills volcanoes (IHV), Tanzania, are unique and important in preserving extra-crater lavas and pyroclastic edifices. They provide critical insights into the eruptive behaviour of kimberlite magmas that are not available at other known kimberlite volcanoes. Cosmogenic ³He dating of olivine crystals from IHV lavas and palaeomagnetic analyses indicates that they are Upper Pleistocene to Holocene in age. This makes them the youngest known kimberlite bodies on Earth by >30?Ma and may indicate a new phase of kimberlite volcanism on the Tanzania craton. Geological mapping, Global Positioning System surveying and field investigations reveal that each volcano comprises partially eroded pyroclastic edifices, craters and lavas. The volcanoes stand <40?m above the surrounding ground and are comparable in size to small monogenetic basaltic volcanoes. Pyroclastic cones consist of diffusely layered pyroclastic fall deposits comprising scoriaceous, pelletal and dense juvenile pyroclasts. Pyroclasts are similar to those documented in many ancient kimberlite pipes, indicating overlap in magma fragmentation dynamics between the Igwisi eruptions and other kimberlite eruptions. Characteristics of the pyroclastic cone deposits, including an absence of ballistic clasts and dominantly poorly vesicular scoria lapillistones and lapilli tuffs, indicate relatively weak explosive activity. Lava flow features indicate unexpectedly high viscosities (estimated at >10² to 10⁶?Pa?s) for kimberlite, attributed to degassing and in-vent cooling. Each volcano is inferred to be the result of a small-volume, short-lived (days to weeks) monogenetic eruption. The eruptive processes of each Igwisi volcano were broadly similar and developed through three phases: (1) fallout of lithic-bearing pyroclastic rocks during explosive excavation of craters and conduits; (2) fallout of juvenile lapilli from unsteady eruption columns and the construction of pyroclastic edifices around the vent; and (3) effusion of degassed viscous magma as lava flows. These processes are similar to those observed for other small-volume monogenetic eruptions (e.g. of basaltic magma).     

The Holocene Pucón eruption of Volcán Villarrica, Chile: deposit architecture and eruption chronology

The Pucón eruption was the largest Holocene explosive outburst of Volcán Villarrica, Chile. It discharged >1.0 km³ of basaltic-andesite magma and >0.8 km³ of pre-existing rock, forming a thin scoria-fall deposit overlain by voluminous ignimbrite intercalated with pyroclastic surge beds. The deposits are up to 70 m thick and are preserved up to 21 km from the present-day summit, post-eruptive lahar deposits extending farther. Two ignimbrite units are distinguished: a lower one (P1) in which all accidental lithic clasts are of volcanic origin and an upper unit (P2) in which basement granitoids also occur, both as free clasts and as xenoliths in scoria. P2 accounts for ∼80% of the erupted products. Following the initial scoria fallout phase, P1 pyroclastic flows swept down the northern and western flanks of the volcano, magma fragmentation during this phase being confined to within the volcanic edifice. Following a pause of at least a couple of days sufficient for wood devolatilization, eruption recommenced, the fragmentation level dropped to within the granitoid basement, and the pyroclastic flows of P2 were erupted. The first P2 flow had a highly turbulent front, laid down ignimbrite with large-scale cross-stratification and regressive bedforms, and sheared the ground; flow then waned and became confined to the southeastern flank. Following emplacement of pyroclastic surge deposits all across the volcano, the eruption terminated with pyroclastic flows down the northern flank. Multiple lahars were generated prior to the onset of a new eruptive cycle. Charcoal samples yield a probable eruption age of 3,510 ± 60 ¹⁴C years BP.     

The 472 AD Pollena eruption of Somma-Vesuvius (Italy) and its environmental impact at the end of the Roman Empire

Catastrophic sedimentary processes associated with explosive eruptions represent a significant geologic hazard in volcanic areas. Here we report a striking historic example of an intermediate-scale explosive event whose environmental effects were strongly amplified by secondary rapid mass flows and hydrogeologic disasters. The 472 AD Pollena eruption of Somma-Vesuvius (Campania, Italy) took place in the critical period of the fall of the Western Roman Empire. On the basis of an integrated geologic–archaeologic study we point out evidence of human habitation at the time of the eruption, effects induced and recovery time in a wide territory of Campania, and how the eruption significantly accelerated the deterioration of the local society during the Late Ancient age. The eruption began with a pulsating, sustained eruption column, followed by pyroclastic surges and scoria flows. Hydromagmatism acted early in the event, different from the typical Plinian eruptions of Somma-Vesuvius. Specific facies associations of primary and secondary volcaniclastic deposits characterize three depositional domains, including the volcano slopes, the surrounding alluvial plains and the distal mountains of the Apennine Range. Both volcano slopes and distal mountain slopes supplied loose pyroclastic material to the hyperconcentrated floods and debris flows that spread across the alluvial plains. The great impact of secondary volcaniclastic processes arose from: (1) the high vulnerability of the territory due to its geomorphic context; (2) the humid climatic conditions; (3) the hydromagmatic character of the eruption; (4) the decline of land management at the end of the Roman Empire.     

Hazards from pyroclastic density currents at Mt. Etna (Italy)

Despite the recent recognition of Mount Etna as a periodically violently explosive volcano, the hazards from various types of pyroclastic density currents (PDCs) have until now received virtually no attention at this volcano. Large-scale pyroclastic flows last occurred during the caldera-forming Ellittico eruptions, 15–16 ka ago, and the risk of them occurring in the near future is negligible. However, minor PDCs can affect much of the summit area and portions of the upper flanks of the volcano. During the past ~ 20 years, small pyroclastic flows or base-surge-like vapor and ash clouds have occurred in at least 8 cases during summit eruptions of Etna. Four different mechanisms of PDC generation have been identified during these events: (1) collapse of pyroclastic fountains (as in 2000 and possibly in 1986); (2) phreatomagmatic explosions resulting from mixing of lava with wet rock (2006); (3) phreatomagmatic explosions resulting from mixing of lava with thick snow (2007); (4) disintegration of the unstable flanks of a lava dome-like structure growing over the rim of one of the summit craters (1999). All of these recent PDCs were of a rather minor extent (maximum runout lengths were about 1.5 km in November 2006 and March 2007) and thus they represented no threat for populated areas and human property around the volcano. Yet, events of this type pose a significant threat to the lives of people visiting the summit area of Etna, and areas in a radius of 2 km from the summit craters should be off-limits anytime an event capable of producing similar PDCs occurs. The most likely source of further PDCs in the near future is the Southeast Crater, the youngest, most active and most unstable of the four summit craters of Etna, where 6 of the 8 documented recent PDCs originated. It is likely that similar hazards exist in a number of volcanic settings elsewhere, especially at snow- or glacier-covered volcanoes and on volcano slopes strongly affected by hydrothermal alteration.     

Tectonic and structural evolution of Gough volcano: A volcanological model

Morphostructural, stratigraphic and tectonic data indicate that the evolution of Gough volcano is similar to other oceanic intraplate volcanoes, is older than 1 Ma, and is related to a transform fault. At least six evolutionary stages can be distinguished within two major magmatostructural periods dominated by basaltic and trachytic magmas, respectively.The basaltic shield volcano is characterized by a curved, elongated shape in plan and a rift zone with a high density of dykes, combined with a radial intrusive system. The latter is interpreted as being fed by a magma chamber some 4 km below the surface. The activity of the volcano became more centralized at the end of the basaltic period and its slopes became steeper. This corresponds to the development of a shallower and narrower central conduit in the edifice. The basaltic period was terminated by formation of a shield caldera related to the 4 km deep magma chamber. The term “shield caldera” is used for a collapse structure that is postmagmatic, large in comparison with the diameter of the volcano, and delimited by normal faults that do not show a closed circular pattern but rather a series of arcs. In contrast, summit calderas are defined as smaller, circular-shaped, centrally situated, synmagmatic features, related to a central shallow column. During the basaltic period, landslides were generated on the flanks of the edifice as a result of slope stability factors which are not easy to determine at present, and dynamic factors among which the intrusion of magma along a curved zone certainly played a major role.The trachytic period is characterized by comparatively rare pyroclastic deposits and a large volume of thick flows extruded from domes. These extrusions, as well as plugs, formed from vertical cylindrical columns of magma rising from shallow individual magma pockets fed by the main reservoir.     

Recent explosive eruptions and volcano hazards at Soputan volcano—a basalt stratovolcano in north Sulawesi, Indonesia

Soputan is a high-alumina basalt stratovolcano located in the active North Sulawesi-Sangihe Islands magmatic arc. Although immediately adjacent to the still geothermally active Quaternary Tondono Caldera, Soputan’s magmas are geochemically distinct from those of the caldera and from other magmas in the arc. Unusual for a basalt volcano, Soputan produces summit lava domes and explosive eruptions with high-altitude ash plumes and pyroclastic flows—eight explosive eruptions during the period 2003–2011. Our field observations, remote sensing, gas emission, seismic, and petrologic analyses indicate that Soputan is an open-vent-type volcano that taps basalt magma derived from the arc-mantle wedge, accumulated and fractionated in a deep-crustal reservoir and transported slowly or staged at shallow levels prior to eruption. A combination of high phenocryst content, extensive microlite crystallization and separation of a gas phase at shallow levels results in a highly viscous basalt magma and explosive eruptive style. The open-vent structure and frequent eruptions indicate that Soputan will likely erupt again in the next decade, perhaps repeatedly. Explosive eruptions in the Volcano Explosivity Index (VEI) 2–3 range and lava dome growth are most probable, with a small chance of larger VEI 4 eruptions. A rapid ramp up in seismicity preceding the recent eruptions suggests that future eruptions may have no more than a few days of seismic warning. Risk to population in the region is currently greatest for villages located on the southern and western flanks of the volcano where flow deposits are directed by topography. In addition, Soputan’s explosive eruptions produce high-altitude ash clouds that pose a risk to air traffic in the region.     

Toward a revised hazard assessment at Merapi volcano, Central Java

Of 1.1 million people living on the flanks of the active Merapi volcano, 440,000 are at relatively high risk in areas prone to pyroclastic flows, surges, and lahars. For the last two centuries, the activity of Merapi has alternated regularly between long periods of viscous lava dome extrusion, and brief explosive episodes at 8–15 year intervals, which generated dome-collapse pyroclastic flows and destroyed part of the pre-existing domes. Violent explosive episodes on an average recurrence of 26–54 years have generated pyroclastic flows, surges, tephra-falls, and subsequent lahars. The 61 reported eruptions since the mid-1500s killed about 7000 people. The current hazard-zone map of Merapi (Pardyanto et al., 1978) portrays three areas, termed ‘forbidden zone’, ‘first danger zone’ and ‘second danger zone’, based on successively declining hazards. Revision of the hazard map is desirable, because it lacks details necessary to outline hazard zones with accuracy, in particular the valleys likely to be swept by lahars, and excludes some areas likely to be devastated by pyroclastic gravity-currents such as the 22 November 1994 surge. In addition, risk maps should be developed to incorporate social, technical, and economic factors of vulnerability.Eruptive hazard assessment at Merapi is based on reconstructed eruptive history, on eruptive behavior and scenarios, and on existing models and preliminary numerical modeling. Firstly, the reconstructed eruptive activity, in particular for the past 7000 years and from historical accounts of eruptions, helps to define the extent and recurrence frequency of the most hazardous phenomena (Newhall et al., 2000; Camus et al., 2000). Pyroclastic flows traveled as far as 9–15 km from the source, pyroclastic surges swept the flanks as far as 9–20 km away from the vent, thick tephra fall buried temples in the vicinity of Yogyakarta 25 km to the south, and subsequent lahars spilled down the radial valleys as far as 30 km to the west and south. At least one large edifice collapse has occurred in the past 7000 years (Newhall et al., 2000; Camus et al., 2000). Secondly, four eruption scenarios are portrayed as hazardous zones on two maps and derived from the past eruptive behavior of Merapi and from the most affected areas in the past. Thirdly, simple numerical simulation, based on a Digital Elevation Model, a stereo-pair of SPOT satellite images, and one 2D-orthoimage helps to simulate pyroclastic and lahar flowage on the flanks and in radial valley channels, and to outline areas likely to be devastated.Three major threats are identified: (1) a collapse of the summit dome in the short-to mid-term, that can release large-volume pyroclastic flows and high-energy surges towards the south–southwest sector of the volcano; (2) an explosive eruption, much larger than any since 1930, may sweep all the flanks of Merapi at least once every century; (3) a potential collapse of the summit area, involving the fumarolic field of Gendol and part of the southern flank, which can contribute to moderate-scale debris avalanches and debris flows.     

Ubinas: the evolution of the historically most active volcano in southern Peru

Ubinas volcano has had 23 degassing and ashfall episodes since A.D. 1550, making it the historically most active volcano in southern Peru. Based on fieldwork, on interpretation of aerial photographs and satellite images, and on radiometric ages, the eruptive history of Ubinas is divided into two major periods. Ubinas I (Middle Pleistocene >376 ka) is characterized by lava flow activity that formed the lower part of the edifice. This edifice collapsed and resulted in a debris-avalanche deposit distributed as far as 12 km downstream the Rio Ubinas. Non-welded ignimbrites were erupted subsequently and ponded to a thickness of 150 m as far as 7 km south of the summit. These eruptions probably left a small collapse caldera on the summit of Ubinas I. A 100-m-thick sequence of ash-and-pumice flow deposits followed, filling paleo-valleys 6 km from the summit. Ubinas II, 376 ky to present comprises several stages. The summit cone was built by andesite and dacite flows between 376 and 142 ky. A series of domes grew on the southern flank and the largest one was dated at 250 ky; block-and-ash flow deposits from these domes filled the upper Rio Ubinas valley 10 km to the south. The summit caldera was formed between 25 and 9.7 ky. Ash-flow deposits and two Plinian deposits reflect explosive eruptions of more differentiated magmas. A debris-avalanche deposit (about 1.2 km³) formed hummocks at the base of the 1,000-m-high, fractured and unstable south flank before 3.6 ka. Countless explosive events took place inside the summit caldera during the last 9.7 ky. The last Plinian eruption, dated A.D.1000–1160, produced an andesitic pumice-fall deposit, which achieved a thickness of 25 cm 40 km SE of the summit. Minor eruptions since then show phreatomagmatic characteristics and a wide range in composition (mafic to rhyolitic): the events reported since A.D. 1550 include many degassing episodes, four moderate (VEI 2–3) eruptions, and one VEI 3 eruption in A.D. 1667. Ubinas erupted high-K, calc-alkaline magmas (SiO₂=56 to 71%). Magmatic processes include fractional crystallization and mixing of deeply derived mafic andesites in a shallow magma chamber. Parent magmas have been relatively homogeneous through time but reflect variable conditions of deep-crustal assimilation, as shown in the large variations in Sr/Y and LREE/HREE. Depleted HREE and Y values in some lavas, mostly late mafic rocks, suggest contamination of magmas near the base of the >60-km-thick continental crust. The most recently erupted products (mostly scoria) show a wide range in composition and a trend towards more mafic magmas.Recent eruptions indicate that Ubinas poses a severe threat to at least 5,000 people living in the valley of the Rio Ubinas, and within a 15-km radius of the summit. The threat includes thick tephra falls, phreatomagmatic ejecta, failure of the unstable south flank with subsequent debris avalanches, rain-triggered lahars, and pyroclastic flows. Should Plinian eruptions of the size of the Holocene events recur at Ubinas, tephra fall would affect about one million people living in the Arequipa area 60 km west of the summit.Editorial responsibility: D Dingwell     

Swift snowmelt and floods (lahars) caused by great pyroclastic surge at Mount St Helens volcano,Washington, 18 May 1980

The initial explosions at Mount St. Helens, Washington, on the moring of 18 May 1980 developed into a huge pyroclastic surge that generated catastrophic floods off the east and west flanks of the volcano. Near-source surge deposits on the east and west were lithic, sorted, lacking in accretionary lapilli and vesiculated ash, not plastered against upright obstacles, and hot enough to char wood — all attributes of dry pyroclastic surge. Material deposited at the surge base on steep slopes near the volcano transformed into high-concentration lithic pyroclastic flows whose deposits contain charred wood and other features indicating that these flows were hot and dry. Stratigraphy shows that even the tail of the surge had passed the east and west volcano flanks before the geomorphically distinct floods (lahars) arrived. This field evidence undermines hypotheses that the turbulent surge was itself wet and that its heavy components segregated out to transform directly into lahars. Nor is there evidence that meters-thick snow-slab avalanches intimately mixed with the surge to form the floods. The floods must have instead originated by swift snowmelt at the base of a hot and relatively dry turbulent surge. Impacting hot pyroclasts probably transferred downslope momentum to the snow surface and churned snow grains into the surge base. Melting snow and accumulating hot surge debris may have moved initially as thousands of small thin slushflows. As these flows removed the surface snow and pyroclasts, newly uncovered snow was partly melted by the turbulent surge base; this and accumulating hot surge debris in turn began flowing, a self-sustaining process feeding the initial flows. The flows thus grew swiftly over tens of seconds and united downslope into great slushy ejecta-laden sheetfloods. Gravity accelerated the floods to more than 100 km/h as they swept down and off the volcano flanks while the snow component melted to form great debris-rich floods (lahars) channeled into valleys.     

Geology of Tok Island,Korea: eruptive and depositional processes of a shoaling to emergent island volcano

Detailed mapping of Tok Island, located in the middle of the East Sea (Sea of Japan), along with lithofacies analysis and K-Ar age determinations reveal that the island is of early to late Pliocene age and comprises eight rock units: Trachyte I, Unit P-I, Unit P-II, Trachyandesite (2.7±0.1 Ma), Unit P-III, Trachyte II (2.7±0.1 Ma), Trachyte III (2.5±0.1 Ma) and dikes in ascending stratigraphic order. Trachyte I is a mixture of coherent trachytic lavas and breccias that are interpreted to be subaqueous lavas and related hyaloclastites. Unit P-I comprises massive and inversely graded basaltic breccias which resulted from subaerial gain flows and subaqueous debris flows. A basalt clast from the unit, derived from below Trachyte I, has an age of 4.6±0.4 Ma. Unit P-II is composed of graded and stratified lapilli tuffs with the characteristics of proximal pyroclastic surge deposits. The Trachyandesite is a massive subaerial lava ponded in a volcano-tectonic depression, probably a summit crater. A pyroclastic sequence containing flattened scoria clasts (Unit P-III) and a small volume subaerial lava (Trachyte II) occur above the Trachyandesite, suggesting resumption of pyroclastic activity and lava effusion. Afterwards, shallow intrusion of magma occurred, producing Trachyte III and trachyte dikes.The eight rock units provide an example of the changing eruptive and depositional processes and resultant succession of lithofacies as a seamount builds up above sea level to form an island volcano: Trachyte I represents a wholly subaqueous and effusive stage; Units P-I and P-II represent Surtseyan and Taalian eruptive phases during an explosive transitional (subaqueous to emergent) stage; and the other rock units represent later subaerial effusive and explosive stages. Reconstruction of volcano morphology suggests that the island is a remnant of the south-western crater rim of a volcano the vent of which lies several hundred meters to the north-east.     

Stratigraphic framework of Holocene volcaniclastic deposits, Akutan Volcano, east-central Aleutian Islands, Alaska

 Akutan Volcano is one of the most active volcanoes in the Aleutian arc, but until recently little was known about its history and eruptive character. Following a brief but sustained period of intense seismic activity in March 1996, the Alaska Volcano Observatory began investigating the geology of the volcano and evaluating potential volcanic hazards that could affect residents of Akutan Island. During these studies new information was obtained about the Holocene eruptive history of the volcano on the basis of stratigraphic studies of volcaniclastic deposits and radiocarbon dating of associated buried soils and peat. A black, scoria-bearing, lapilli tephra, informally named the "Akutan tephra," is up to 2 m thick and is found over most of the island, primarily east of the volcano summit. Six radiocarbon ages on the humic fraction of soil A-horizons beneath the tephra indicate that the Akutan tephra was erupted approximately 1611 years B.P. At several locations the Akutan tephra is within a conformable stratigraphic sequence of pyroclastic-flow and lahar deposits that are all part of the same eruptive sequence. The thickness, widespread distribution, and conformable stratigraphic association with overlying pyroclastic-flow and lahar deposits indicate that the Akutan tephra likely records a major eruption of Akutan Volcano that may have formed the present summit caldera. Noncohesive lahar and pyroclastic-flow deposits that predate the Akutan tephra occur in the major valleys that head on the volcano and are evidence for six to eight earlier Holocene eruptions. These eruptions were strombolian to subplinian events that generated limited amounts of tephra and small pyroclastic flows that extended only a few kilometers from the vent. The pyroclastic flows melted snow and ice on the volcano flanks and formed lahars that traveled several kilometers down broad, formerly glaciated valleys, reaching the coast as thin, watery, hyperconcentrated flows or water floods. Slightly cohesive lahars in Hot Springs valley and Long valley could have formed from minor flank collapses of hydrothermally altered volcanic bedrock. These lahars may be unrelated to eruptive activity. Received: 31 August 1998 / Accepted: 30 January 1999     

Pyroclastic flow generated by crater-wall collapse and outpouring of the lava pool of Arenal Volcano, Costa Rica

The pyroclastic flow that issued from the Arenal summit crater on 28 August 1993 came from the collapse of the crater wall of the cone and the drainage of a lava pool. The 3-km-long pyroclastic flow, 2.2ǂ.8᎒⁶ m³ in volume, was confined to narrow valleys (30-100 m wide). The thickness of the pyroclastic deposit ranged from 1 to 10 m, and its temperature was about 400 °C, although single bombs were up to 1,000 °C. The deposit is clast-supported, has a bimodal grain size distribution, and consists of an intimate mixture of finely pulverized rock ash, lapilli, small blocks, and cauliflower bread-crusted bombs, in which are set meter-size lava fragments and juvenile and non-juvenile angular blocks, and bombs up to 7 m in diameter. Large faceted blocks make up 50% of the total volume of the deposit. The cauliflower bombs have deep and intricate bread-crust texture and post-depositional vesiculation. It is proposed that the juvenile material was produced entirely from a lava pool, whereas faceted non-juvenile blocks come from the crater-wall collapse. The concentration and maximum diameter of cauliflower bread-crusted bombs increases significantly from the base (rockslide + pyroclastic flow) to the top (the pyroclastic flow) of the deposit. An ash cloud deposited accretionary lapilli in the proximal region (outside of the pyroclastic flow deposit), and very fine ash fell in the distal region (between 5 and 30 km). The accretionary lapilli deposit is derived from the fine, elutriated products of the flow as it moved. A turbulent overriding surge blew down the surrounding shrubbery in the flow direction. The pyroclastic flow from August 1993, similar to the flows of June 1975, May 1998, August 2000, and March 2001, slid and rolled rather than being buoyed up by gas. They grooved, scratched, and polished the surfaces over which they swept, similar to a Merapi-type pyroclastic flow. However, the mechanism of the outpouring of a lava pool and the resulting flows composed of high- to moderate-vesiculated, cauliflower bread-crusted bombs and juvenile blocks have not been described before. High-frequency earthquake swarms, followed by an increase in low-frequency volcanic events, preceded the 1975, 1993, and 2000 eruptions 2-4 months before. These pyroclastic flow events, therefore, may be triggered by internal expansion of the unstable cone in the upper part because of a slight change in the pressure of the magma column (gas content and/or effusive rate). This phenomenon has important short-term, volcanic hazard implications for touristic development of some parts on the flanks of the volcano.     

A facies model for a quaternary andesitic composite volcano: Ruapehu,New Zealand

Ruapehu composite volcano is a dynamic volcanic-sedimentary system, characterised by high accumulation rates and by rapid lateral and vertical change in facies. Four major cone-building episodes have occurred over 250 Ka, from a variety of summit, flank and satellite vents. Eruptive styles include subplinian, strombolian, phreatomagmatic, vulcanian and dome-related explosive eruptions, and extrusion of lava flows and domes. The volcano can be divided into two parts: a composite cone of volume 110 km³, surrounded by an equally voluminous ring plain. Complementary portions of Ruapehu's history are preserved in cone-forming and ring plain environments. Cone-forming sequences are dominated by sheet- and autobrecciated-lava flows, which seldom reach the ring plain. The ring plain is built predominantly from the products of explosive volcanism, both the distal primary pyroclastic deposits and the reworked material eroded from the cone. Much of the material entering the ring plain is transported by lahars either generated directly by eruptions or triggered by the high intensity rain storms which characterise the region. Ring plain detritus is reworked rapidly by concentrated and hyperconcentrated streams in pulses of rapid aggradation immediately following eruptions and more gradually in the longer intervals between eruptions.     

Volcanic hazards from Bezymianny- and Bandai-type eruptions

Major slope failures are a significant degradational process at volcanoes. Slope failures and associated explosive eruptions have resulted in more than 20 000 fatalities in the past 400 years; the historic record provides evidence for at least six of these events in the past century. Several historic debris avalanches exceed 1 km³ in volume. Holocene avalanches an order of magnitude larger have traveled 50–100 km from the source volcano and affected areas of 500–1500 km². Historic eruptions associated with major slope failures include those with a magmatic component (Bezymianny type) and those solely phreatic (Bandai type). The associated gravitational failures remove major segments of the volcanoes, creating massive horseshoe-shaped depressions commonly of caldera size. The paroxysmal phase of a Bezymianny-type eruption may include powerful lateral explosions and pumiceous pyroclastic flows; it is often followed by construction of lava dome or pyroclastic cone in the new crater. Bandai-type eruptions begin and end with the paroxysmal phase, during which slope failure removes a portion of the edifice. Massive volcanic landslides can also occur without related explosive eruptions, as at the Unzen volcano in 1792.The main potential hazards from these events derive from lateral blasts, the debris avalanche itself, and avalanche-induced tsunamis. Lateral blasts produced by sudden decompression of hydrothermal and/or magmatic systems can devastate areas in excess of 500km² at velocities exceeding 100 m s^–1. The ratio of area covered to distance traveled for the Mount St. Helens and Bezymianny lateral blasts exceeds that of many pyroclastic flows or surges of comparable volume. The potential for large-scale lateral blasts is likely related to the location of magma at the time of slope failure and appears highest when magma has intruded into the upper edifice, as at Mount St. Helens and Bezymianny.Debris avalanches can move faster than 100 ms^–1 and travel tens of kilometers. When not confined by valley walls, avalanches can affect wide areas beyond the volcano's flanks. Tsunamis from debris avalanches at coastal volcanoes have caused more fatalities than have the landslides themselves or associated eruptions. The probable travel distance (L) of avalanches can be estimated by considering the potential vertical drop (H). Data from a catalog of around 200 debris avalanches indicates that the H/L rations for avalanches with volumes of 0.1–1 km³ average 0.13 and range 0.09–0.18; for avalanches exceeding 1 km³, H/L ratios average 0.09 and range 0.5–0.13.Large-scale deformation of the volcanic edefice and intense local seismicity precede many slope failures and can indicate the likely failure direction and orientation of potential lateral blasts. The nature and duration of precursory activity vary widely, and the timing of slope faliure greatly affects the type of associated eruption. Bandai-type eruptions are particularly difficult to anticipate because they typically climax suddenly without precursory eruptions and may be preceded by only short periods of seismicity.     

Repetitive mixing events and holocene pyroclastic activity at Pico de Orizaba and Popocatepetl (Mexico)

The Holocene volcanic activity which built up the present terminal cones of Pico de Orizaba and Popocatepetl in eastern Mexico, was characterized by repeated pyroclastic Saint-Vincent type eruptions. Radiocarbon data show that these paroxysmal events occurred at more or less regular intervals, and were followed by moderate activity producing ash and pumice falls and andesitic lava flows from the summit craters.Typical ash and scoria pyroclastic flows exhibit a heterogeneous composition given by the interaction of a dacitic component with a more basic andesitic one. Scoria bombs are characterized by banded to emulsified textures, mineralogical desequilibrium assemblages and linear chemical variations on element-element plots as exemplified by the Loma Grande flow at Pico.Periodic replenishments of the magmatic reservoir could be the major phenomenon that started mixing and consequently triggered the pyroclastic eruptions.     

Origin and stratigraphy of phreatomagmatic deposits at the Pleistocene Sinker Butte Volcano, Western Snake River Plain, Idaho

Sinker Butte is the erosional remnant of a very large basaltic tuff cone of middle Pleistocene age located at the southern edge of the western Snake River Plain. Phreatomagmatic tephras are exposed in complete sections up to 100 m thick in the walls of the Snake River Canyon, creating an unusual opportunity to study the deposits produced by this volcano through its entire sequence of explosive eruptions. The main objectives of the study were to determine the overall evolution of the Sinker Butte volcano while focusing particularly on the tephras produced by its phreatomagmatic eruptions. Toward this end, twenty-three detailed stratigraphic sections ranging from 20 to 100 m thick were examined and measured in canyon walls exposing tephras deposited around 180° of the circumference of the volcano.Three main rock units are recognized in canyon walls at Sinker Butte: a lower sequence composed of numerous thin basaltic lava flows, an intermediate sequence of phreatomagmatic tephras, and a capping sequence of welded basaltic spatter and more lava flows. We subdivide the phreatomagmatic deposits into two main parts, a series of reworked, mostly subaqueously deposited tephras and a more voluminous sequence of overlying subaerial surge and fall deposits. Most of the reworked deposits are gray in color and exhibit features such as channel scour and fill, planar-stratification, high and low angle cross-stratification, trough cross-stratification, and Bouma-turbidite sequences consistent with their being deposited in shallow standing water or in braided streams. The overlying subaerial deposits are commonly brown or orange in color due to palagonitization. They display a wide variety of bedding types and sedimentary structures consistent with deposition by base surges, wet to dry pyroclastic fall events, and water saturated debris flows.Proximal sections through the subaerial tephras exhibit large regressive cross-strata, planar bedding, and bomb sags suggesting deposition by wet base surges and tephra fallout. Medial and distal deposits consist of a thick sequence of well-bedded tephras; however, the cross-stratified base-surge deposits are thinner and interbedded within the fallout deposits. The average wavelength and amplitude of the cross strata continue to decrease with distance from the vent. These bedded surge and fall deposits grade upward into dominantly fall deposits containing 75–95% juvenile vesiculated clasts and localized layers of welded spatter, indicating a greatly reduced water-melt ratio. Overlying these “dryer” deposits are massive tuff breccias that were probably deposited as water saturated debris flows (lahars). The first appearance of rounded river gravels in these massive tuff breccias indicates downward coring of the diatreme and entrainment of country rock from lower in the stratigraphic section. The “wetter” nature of these deposits suggests a renewed source of external water. The massive deposits grade upward into wet fallout tephras and the phreatomagmatic sequence ends with a dry scoria fall deposit overlain by welded spatter and lava flows.Field observations and two new ⁴⁰Ar–³⁹Ar incremental heating dates suggest the succession of lavas and tephra deposits exposed in this part of the Snake River canyon may all have been erupted from a closely related complex of vents at Sinker Butte. We propose that initial eruptions of lava flows built a small shield edifice that dammed or disrupted the flow of the ancestral Snake River. The shift from effusive to explosive eruptions occurred when the surface water or rising ground water gained access to the vent. As the river cut a new channel around the lava dam, water levels dropped and the volcano returned to an effusive style of eruption.     

A gigantic Bezymianny-type event at the beginning of modern volcan Popocatepetl

The history of volcan Popocatepetl can be divided into two main periods: the formation of a large primitive volcano — approximatively 30 km wide — on which is superimposed a modern cone (6–8 km in diameter and 1700m high). A major event of Bezymianny type marks the transition between these two dissimilar periods.The activity of the primitive volcano was essentially effusive and lasted several hundred thousands of years. The total volume of products ejected by the volcano is of the order of 500–600 km³. Its last differentiated magmas are dacitic.A gigantic debris flow (D.F.) spread on the southern side is related to the Bezymianny-type event which destroyed the summit area of the ancient edifice. An elliptical caldera ( 6.5 × 11 km wide) was formed by the landslide. Its deposits, with a typical hummocky surface, cover 300 km² for a volume of 28–30 km³. Numerous outcrops belonging to this debris flow show “slabs” of more or less fractured and dislocated rocks that come from the primitive volcano. These deposits are compared to two studied debris flows of similar extent and volume: the Mount Shasta and Colima's D.F.This eruption takes a major place in the volcanologic and magmatic history of Popocatepetl: pyroclastic products of surge-type with “laminites” and crude layers, ashflows, and pumiceous airfall layers are directly related to this event and begin the history of the modern volcano probably less than 50,000 years ago. In addition, a second andesitic and dacitic phase rose both from the central vent — forming the basis of modern Popo — and from lateral vents.The terminal cone is characterized by long periods of construction by lava flows alternating with phases of destruction, the duration of these episodes being 1000 to 2000 years. The cone is composed of two edifices: the first, volcan El Fraile, began with effusive activity and was partly destroyed by three periods of intense explosive activity. The first period occurred prior to 10.000 years B.P., the second from 10.000 to 8000 years B.P. and the third from 5000 to 3800 years B.P. Each period of destruction shows cycles producing collapsing pyroclastic flows or nuées of the St Vincent-type related to the opening of large craters, plinian air-fall deposits and minor lava flows. The second edifice, the summit Popo, produced lava flows until 1200 years B.P. and since that time, entered into an explosive period. Two cataclysmic episodes, each including major pyroclastic eruptions, occurred 1200 and 900–1000 years ago. During the Pre-Hispanic and historic times effusive activity was restricted entirely to the summit area alternating with plinian eruptions. Nevertheless, despite the quiet appearance of the volcano, the last period of pyroclastic activity which started 1200 years ago may not have ended and can be very dangerous for the nearby populations.