Volcanic activity at Santiaguito volcano,Guatemala June 1968 – August 1969

Information regarding activity of Santiaguito dome, in western Guatemala, for the period June 1968–August 1969, has been derived from phots taken repeatedly at strategic locations, weekly geologic observations from a nearby lookout and ten expeditions to the mountain. Activity continues at the principal Caliente vent. Ash, bombs and gas eruptions have been interspersed with repeated extrusion and blasting away of small plug domes. At the subsidiary vent, El Brujo, dome extrusion and accompanying avalanching, so prominent in 1967–68, has declined since late 1968. The minimum volume of the El Brujo dome (formed since 1966) is estimated to be 8.5 × 10⁶ m³.     

The volcanic and magmatic evolution of Volcán Ollagüe, a high-K, late quaternary stratovolcano in the Andean Central Volcanic Zone

Volcán Ollagüe is a high-K, calc-alkaline composite volcano constructed upon extremely thick crust in the Andean Central Volcanic Zone. Volcanic activity commenced with the construction of an andesitic to dacitic composite cone composed of numerous lava flows and pyroclastic deposits of the Vinta Loma series and an overlying coalescing dome and coulée sequence of the Chasca Orkho series. Following cone construction, the upper western flank of Ollagüe collapsed toward the west leaving a collapse-amphitheater about 3.5 km in diameter and a debris avalanche deposit on the lower western flank of the volcano. The deposit is similar to the debris avalanche deposit produced during the May 18, 1980 eruption of Mount St. Helens, U.S.A., and was probably formed in a similar manner. It presently covers an area of 100 km² and extends 16 km from the summit. Subsequent to the collapse event, the upper western flank was reformed via eruption of several small andesitic lava flows from vents located near the western summit and growth of an andesitic dome within the collapse-amphitheater. Additional post-collapse activity included construction of a dacitic dome and coulée of the La Celosa series on the northwest flank. Field relations indicate that vents for the Vinta Loma and post-collapse series were located at or near the summit of the cone. The Vinta Loma series is characterized by an anhydrous, two-pyroxene assemblage. Vents for the La Celosa and Chasca Orkho series are located on the flanks and strike N55 W, radial to the volcano. The pattern of flank eruptions coincides with the distribution in the abundance of amphibole and biotite as the main mafic phenocryst phases in the rocks. A possible explanation for this coincidence is that an unexposed fracture or fault beneath the volcano served as a conduit for both magma ascent and groundwater circulation. In addition to the lava flows at Ollagüe, magmas are also present as blobs of vesiculated basaltic andesite and mafic andesite that occur as inclusions in nearly all of the lavas. All eruptive activity at Ollagüe predates the last glacial episode ( 11.000 a B.P.), because post-collapse lava flows are overlain by moraine and are incised by glacial valleys. Present activity is restricted to emission of a persistent, 100-m-high fumarolic steam plume from a vent located within the summit andesite dome.Sr and Nd isotope ratios for the basaltic andesite and mafic andesite inclusions and lavas suggest that they have assimilated large amounts of crust during crystal fractionation. In contrast, narrow ranges in ¹⁴³Nd/¹⁴⁴Nd and ⁸⁷Sr/⁸⁶Sr in the andesitic and dacitic lavas are enigmatic with respect to crustal contamination.     

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).     

1998 Eruption at Volcán Cerro Azul, Galápagos Islands: I. Syn-Eruptive Petrogenesis

The 1998 eruption of Volcán Cerro Azul in the Galápagos Islands produced two intra-caldera vents and a flank vent that erupted more than 1.0×10⁸ m³ of lava. Lava compositions changed notably during the 5-week eruption, and contemporaneous eruptions in the caldera and on the flank produced different compositions. Lavas erupted from the flank vent range from 6.3 to 14.1% MgO, nearly the entire range of MgO contents previously reported from the volcano. On-site monitoring of eruptive activity is linked with petrogenetic processes such that geochemical variations are evaluated in a temporal context. Lavas from the 1998 eruption record two petrogenetic stages characterized by progressively more mafic lavas as the eruption proceeded. Crystal compositions, whole rock major and trace element compositions, and isotope ratios indicate that early lavas are the product of mixing between 1998 magma and remnant magma of the 1979 eruption. Intra-caldera lavas and later lavas have no 1979 signature, but were produced by the 1998 magma incorporating olivine and clinopyroxene xenocrysts. Thus, early magma petrogenesis is characterized by mixing with the 1979 magma, followed by the magma progressively entraining wehrlite cumulate mush.Editorial Responsibility: M.R. Carroll     

Mapping complex vent eruptive activity at Santiaguito,Guatemala using network infrasound semblance

Pyroclastic-laden explosive eruptions from Santiaguito Volcano (Guatemala) are vented from the 200-m diameter Caliente Dome summit and result in a superposition of spatially extensive and temporally sustained (tens of seconds to minutes) acoustic sources. A network of infrasonic microphones distributed on various sides of the volcano record distinct waveforms, which are poorly correlated across the network and suggestive of acoustic interference from multiple sources. Presuming the infrasound wavefield is a linear superposition of spatially and temporally distinct sub-events, we introduce a semblance mapping technique to recover the time history of the spatially evolving sources during successive time windows. Coincident high-resolution video footage corroborates that both rapid dome uplift and individual explosive pulses are likely sources of high semblance infrasound that are identifiable during short (2 s) time windows. This study suggests that complex and network-variable infrasound waveforms are produced whenever a volcanic vent source dimension is large compared to the wavelength of the sound being produced. Non-compact infrasound radiators are probably commonplace at silicic volcanic systems, where venting often occurs across a dome surface.     

Tungurahua Volcano,Ecuador: structure,eruptive history and hazards

Tungurahua, one of Ecuador's most active volcanoes, is made up of three volcanic edifices. Tungurahua I was a 14-km-wide andesitic stratocone which experienced at least one sector collapse followed by the extrusion of a dacite lava series. Tungurahua II, mainly composed of acid andesite lava flows younger than 14,000 years BP, was partly destroyed by the last collapse event, 2955±90 years ago, which left a large amphitheater and produced a ∼8-km³ debris deposit. The avalanche collided with the high ridge immediately to the west of the cone and was diverted to the northwest and southwest for ∼15 km. A large lahar formed during this event, which was followed in turn by dacite extrusion. Southwestward, the damming of the Chambo valley by the avalanche deposit resulted in a ∼10-km-long lake, which was subsequently breached, generating another catastrophic debris flow. The eruptive activity of the present volcano (Tungurahua III) has rebuilt the cone to about 50% of its pre-collapse size by the emission of ∼3 km³ of volcanic products. Two periods of construction are recognized in Tungurahua's III history. From ∼2300 to ∼1400 years BP, high rates of lava extrusion and pyroclastic flows occurred. During this period, the magma composition did not evolve significantly, remaining essentially basic andesite. During the last ∼1300 years, eruptive episodes take place roughly once per century and generally begin with lapilli fall and pyroclastic flow activity of varied composition (andesite+dacite), and end with more basic andesite lava flows or crater plugs. This pattern is observed in the three historic eruptions of 1773, 1886 and 1916–1918. Given good age control and volumetric considerations, Tungurahua III growth's rate is estimated at ∼1.5×10⁶ m³/year over the last 2300 years. Although an infrequent event, a sector collapse and associated lahars constitute a strong hazard of this volcano. Given the ∼3000 m relief and steep slopes of the present cone, a future collapse, even of small volume, could cover an area similar to that affected by the ∼3000-year-old avalanche. The more frequent eruptive episodes of each century, characterized by pyroclastic flows, lavas, lahars, as well as tephra falls, directly threaten 25,000 people and the Agoyan hydroelectric dam located at the foot of the volcano.     

Volcanic hazards at Mount Semeru,East Java (Indonesia), with emphasis on lahars

Mt. Semeru, the highest mountain in Java (3,676 m), is one of the few persistently active composite volcanoes on Earth, with a plain supporting about 1 million people. We present the geology of the edifice, review its historical eruptive activity, and assess hazards posed by the current activity, highlighting the lahar threat. The composite andesite cone of Semeru results from the growth of two edifices: the Mahameru ‘old’ Semeru and the Seloko ‘young’ Semeru. On the SE flank of the summit cone, a N130-trending scar, branched on the active Jonggring-Seloko vent, is the current pathway for rockslides and pyroclastic flows produced by dome growth. The eruptive activity, recorded since 1818, shows three styles: (1) The persistent vulcanian and phreatomagmatic regime consists of short-lived eruption columns several times a day; (2) increase in activity every 5 to 7 years produces several kilometer-high eruption columns, ballistic bombs and thick tephra fall around the vent, and ash fall 40 km downwind. Dome extrusion in the vent and subsequent collapses produce block-and-ash flows that travel toward the SE as far as 11 km from the summit; and (3) flank lava flows erupted on the lower SE and E flanks in 1895 and in 1941–1942. Pyroclastic flows recur every 5 years on average while large-scale lahars exceeding 5 million m³ each have occurred at least five times since 1884. Lumajang, a city home to 85,000 people located 35 km E of the summit, was devastated by lahars in 1909. In 2000, the catchment of the Curah Lengkong River on the ESE flank shows an annual sediment yield of 2.7 × 10⁵ m³ km⁻² and a denudation rate of 4 10⁵ t km⁻² yr⁻¹, comparable with values reported at other active composite cones in wet environment. Unlike catchments affected by high magnitude eruptions, sediment yield at Mt. Semeru, however, does not decline drastically within the first post-eruption years. This is due to the daily supply of pyroclastic debris shed over the summit cone, which is remobilised by runoff during the rainy season. Three hazard-prone areas are delineated at Mt. Semeru: (1) a triangle-shaped area open toward the SE has been frequently swept by dome-collapse avalanches and pyroclastic flows; (2) the S and SE valleys convey tens of rain-triggered lahars each year within a distance of 20 km toward the ring plain; (3) valleys 25 km S, SE, and the ring plain 35 km E toward Lumajang can be affected by debris avalanches and debris flows if the steep-sided summit cone fails.     

Reconstruction of the eruptive history of Usu volcano,Hokkaido, Japan,inferred from petrological correlation between tephras and dome lavas

Usu volcano has erupted nine times since 1663. Most eruptive events started with an explosive eruption, which was followed by the formation of lava domes. However, the ages of several summit lava domes and craters remain uncertain. The petrological features of tephra deposits erupted from 1663 to 1853 are known to change systematically. In this study, we correlated lavas with tephras under the assumption that lava and tephra samples from the same event would have similar petrological features. Although the initial explosive eruption in 1663 was not accompanied by lava effusion, lava dome or cryptodome formation was associated with subsequent explosive eruptions. We inferred the location of the vent associated with each event from the location of the associated lava dome and the pyroclastic flow deposit distribution and found that the position of the active vent within the summit caldera differed for each eruption from the late 17th through the 19th century. Moreover, we identified a previously unrecognized lava dome produced by a late 17th century eruption; this dome was largely destroyed by an explosive eruption in 1822 and was replaced by a new lava dome during a later stage of the 1822 event at nearly the same place as the destroyed dome. This new interpretation of the sequence of events is consistent with historical sketches and documents. Our results show that petrological correlation, together with geological evidence, is useful not only for reconstructing volcanic eruption sequences but also for gaining insight into future potential disasters.     

Geology and eruptive history of Unzen volcano, Shimabara Peninsula, Kyushu, SW Japan

During the past 500 thousand years, Unzen volcano, an active composite volcano in the Southwest Japan Arc, has erupted lavas and pyroclastic materials of andesite to dacite composition and has developed a volcanotectonic graben. The volcano can be divided into the Older and the Younger Unzen volcanoes. The exposed rocks of the Older Unzen volcano are composed of thick lava flows and pyroclastic deposits dated around 200–300 ka. Drill cores recovered from the basal part of the Older Unzen volcano are dated at 400–500 ka. The volcanic rocks of the Older Unzen exceed 120 km³ in volume. The Younger Unzen volcano is composed of lava domes and pyroclastic deposits, mostly younger than 100 ka. This younger volcanic edifice comprises Nodake, Myokendake, Fugendake, and Mayuyama volcanoes. Nodake, Myokendake and Fugendake volcanoes are 100–70 ka, 30–20 ka, and <20 ka, respectively. Mayuyama volcano formed huge lava domes on the eastern flank of the Unzen composite volcano about 4000 years ago. Total eruptive volume of the Younger Unzen volcano is about 8 km³, and the eruptive production rate is one order of magnitude smaller than that of the Older Unzen volcano.     

Co-ignimbrite ash-fall deposits of the 1991 eruptions of Fugen-dake, Unzen Volcano, Japan

Fugen-dake, the main peak of Unzen Volcano, began a new eruption sequence on November 17, 1990. On May 20, 1991, a new lava dome appeared near the eastern edge of the Fugen-dake summit. Small-scale, 10⁴–10⁶ m³ in volume, Merapi-type block and ash flows were frequently generated from the growing lava dome during May–June, 1991. These pyroclastic flows were accompanied by co-ignimbrite ash plumes that deposited ash-fall deposits downwind of the volcano. Three examples of co-ignimbrite ash-fall deposits from Unzen pyroclastic flows are described. The volume of fall deposits was estimated to be about 30% by volume of the collapsed portions of the dome that formed pyroclastic flows. This proportion is smaller than that described for other larger co-ignimbrite ash-fall deposits from other volcanoes. Grain size distributions of the Unzen co-ignimbrite ash-fall deposits are bi-modal or tri-modal. Most ashes are finer than 4 phi and two modes were observed at around 4–7 phi and 9 phi. They are composed mainly of groundmass fragments. Fractions of another mode at around 2 phi are rich in crystals derived from dome lava. Some of the fine ash component fell as accretionary lapilli from the co-ignimbrite ash cloud indicating either moisture or electrostatic aggregation. We believe that the co-ignimbrite ash of Unzen block and ash flows were formed by the mechanical fracturing of the cooling lava blocks as they collapsed and moved down the slope. These ashes were entrained into the convective plumes generated off the tops of the moving flows.     

Rheology of arc dacite lavas: experimental determination at low strain rates

Andesitic–dacitic volcanoes exhibit a large variety of eruption styles, including explosive eruptions, endogenous and exogenous dome growth, and kilometer-long lava flows. The rheology of these lavas can be investigated through field observations of flow and dome morphology, but this approach integrates the properties of lava over a wide range of temperatures. Another approach is through laboratory experiments; however, previous studies have used higher shear stresses and strain rates than are appropriate to lava flows. We measured the apparent viscosity of several lavas from Santiaguito and Bezymianny volcanoes by uniaxial compression, between 1,109 and 1,315?K, at low shear stress (0.085 to 0.42?MPa), low strain rate (between 1.1?×?10^?8 and 1.9?×?10^?5?s^?1), and up to 43.7 % total deformation. The results show a strong variability of the apparent viscosity between different samples, which can be ascribed to differences in initial porosity and crystallinity. Deformation occurs primarily by compaction, with some cracking and/or vesicle coalescence. Our experiments yield apparent viscosities more than 1 order of magnitude lower than predicted by models based on experiments at higher strain rates. At lava flow conditions, no evidence of a yield strength is observed, and the apparent viscosity is best approached by a strain rate- and temperature-dependent power law equation. The best fit for Santiaguito lava, for temperatures between 1,164 and 1,226?K and strain rates lower than 1.8?×?10^?4?s^?1, is $ \log {\eta_{\text{app}}} = - 0.738 + 9.24 \times {10^3}{/}T(K) - 0.654 \cdot \log \dot{\varepsilon } $ where η _app is apparent viscosity and $ \dot{\varepsilon } $ is strain rate. This equation also reproduced 45 data for a sample from Bezymianny with a root mean square deviation of 0.19 log unit Pa?s. Applying the rheological model to lava flow conditions at Santiaguito yields calculated apparent viscosities that are in reasonable agreement with field observations and suggests that internal shear heating may be significant ongoing heat source within these flows, enabling highly viscous lava to travel long distances.     

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.     

Preliminary report on the activity at Unzen Volcano (Japan), November 1990-November 1991: Dacite lava domes and pyroclastic flows

The eruption of Unzen Volcano commenced on 17 November 1990. Phreatic and phreatomagmatic eruptions occurred by early May 1991. No large-scale explosive eruptions preceded the extrusion of lava domes. Lava domes appeared in a summit crater on 20 May 1991, and they grew on the steep slope of Mt. Fugen at Unzen Volcano. Rockfalls from the margins of the domes frequently generated pyroclastic flows. Major pyroclastic flows occurred on 3 June, 8 June, and 15 September 1991. The 3 June pyroclastic flow killed forty-three persons. Many of the pyroclastic flows seem to have resulted from the simple rockfalls, except one flow on 8 June, which was accompanied by an explosion from the crater. Many of the rockfalls that generated pyroclastic flows were witnessed. As of November 1991. Unzen Volcano was still active with a nearly constant magma-supply rate of about 0.3 × 10⁶ m³/d. The total magma output exceeded 45 × 10⁶ m³ by the beginning of November 1991. The volume of the lava domes is more than 23 × 10⁶ m³.     

Overview of the 1990–1995 eruption at Unzen Volcano

Following 198 years of dormancy, a small phreatic eruption started at the summit of Unzen Volcano (Mt. Fugen) in November 1990. A swarm of volcano-tectonic (VT) earthquakes had begun below the western flank of the volcano a year before this eruption, and isolated tremor occurred below the summit shortly before it. The focus of VT events had migrated eastward to the summit and became shallower. Following a period of phreatic activity, phreatomagmatic eruptions began in February 1991, became larger with time, and developed into a dacite dome eruption in May 1991 that lasted approximately 4 years. The emergence of the dome followed inflation, demagnetization and a swarm of high-frequency (HF) earthquakes in the crater area. After the dome appeared, activity of the VT earthquakes and the summit HF events was replaced largely by low-frequency (LF) earthquakes. Magma was discharged nearly continuously through the period of dome growth, and the rate decreased roughly with time. The lava dome grew in an unstable form on the shoulder of Mt. Fugen, with repeating partial collapses. The growth was exogenous when the lava effusion rate was high, and endogenous when low. A total of 13 lobes grew as a result of exogenous growth. Vigorous swarms of LF earthquakes occurred just prior to each lobe extrusion. Endogenous growth was accompanied by strong deformation of the crater floor and HF and LF earthquakes. By repeated exogenous and endogenous growth, a large dome was formed over the crater. Pyroclastic flows frequently descended to the northeast, east, and southeast, and their deposits extensively covered the eastern slope and flank of Mt. Fugen. Major pyroclastic flows took place when the lava effusion rate was high. Small vulcanian explosions were limited in the initial stage of dome growth. One of them occurred following collapse of the dome. The total volume of magma erupted was 2.1×10⁸ m³ (dense-rock-equivalent); about a half of this volume remained as a lava dome at the summit (1.2 km long, 0.8 km wide and 230–540 m high). The eruption finished with extrusion of a spine at the endogenous dome top. Several monitoring results convinced us that the eruption had come to an end: the minimal levels of both seismicity and rockfalls, no discharge of magma, the minimal SO₂ flux, and cessation of subsidence of the western flank of the volcano. The dome started slow deformation and cooling after the halt of magma effusion in February 1995.     

The rhyolitic–andesitic eruptive history of Cotopaxi volcano,Ecuador

At Cotopaxi volcano, Ecuador, rhyolitic and andesitic bimodal magmatism has occurred periodically during the past 0.5 Ma. The sequential eruption of rhyolitic (70–75% SiO₂) and andesitic (56–62% SiO₂) magmas from the same volcanic vent over short time spans and without significant intermingling is characteristic of Cotopaxi’s Holocene behavior. This study documents the eruptive history of Cotopaxi volcano, presenting its stratigraphy and geologic field relations, along with the relevant mineralogical and chemical nature of the eruptive products, in order to determine the temporal and spatial relations of this bimodal alternation. Cotopaxi’s history begins with the Barrancas rhyolite series, dominated by pumiceous ash flows and regional ash falls between 0.4 and 0.5 Ma, which was followed by occasional andesitic activity, the most important being the ample andesitic lava flows (∼4.1 km³) that descended the N and NW sides of the edifice. Following a ∼400 ka long repose without silicic activity, Cotopaxi began a new eruptive phase about 13 ka ago that consisted of seven rhyolitic episodes belonging to the Holocene F and Colorado Canyon series; the onset of each episode occurred at intervals of 300–3,600 years and each produced ash flows and regional tephra falls with DRE volumes of 0.2–3.6 km³. Andesitic tephras and lavas are interbedded in the rhyolite sequence. The Colorado Canyon episode (4,500 years BP) also witnessed dome and sector collapses on Cotopaxi’s NE flank which, with associated ash flows, generated one of the largest cohesive debris flows on record, the Chillos Valley lahar. A thin pumice lapilli fall represents the final rhyolitic outburst which occurred at 2,100 years BP. The pumices of these Holocene rhyolitic eruptions are chemically similar to those of older rhyolites of the Barrancas series, with the exception of the initial eruptive products of the Colorado Canyon series whose chemistry is similar to that of the 211 ka ignimbrite of neighboring Chalupas volcano. Since the Colorado Canyon episode, andesitic magmatism has dominated Cotopaxi’s last 4,400 years, characterized by scoria bomb and lithic-rich pyroclastic flows, infrequent lava flows that reached the base of the cone, andesitic lapilli and ash falls that were carried chiefly to the W, and large debris flows. Andesitic magma emission rates are estimated at 1.65 km³ (DRE)/ka for the period from 4,200 to 2,100 years BP and 1.85 km³ (DRE)/ka for the past 2,100 years, resulting in the present large stratocone.     

Sequence and eruptive style of the 1783 eruption of Asama Volcano, central Japan: a case study of an andesitic explosive eruption generating fountain-fed lava flow, pumice fall, scoria flow and forming a cone

The 3-month long eruption of Asama volcano in 1783 produced andesitic pumice falls, pyroclastic flows, lava flows, and constructed a cone. It is divided into six episodes on the basis of waxing and waning inferred from records made during the eruption. Episodes 1 to 4 were intermittent Vulcanian or Plinian eruptions, which generated several pumice fall deposits. The frequency and intensity of the eruption increased dramatically in episode 5, which started on 2 August, and culminated in a final phase that began on the night of 4 August, lasting for 15 h. This climactic phase is further divided into two subphases. The first subphase is characterized by generation of a pumice fall, whereas the second one is characterized by abundant pyroclastic flows. Stratigraphic relationships suggest that rapid growth of a cone and the generation of lava flows occurred simultaneously with the generation of both pumice falls and pyroclastic flows. The volumes of the ejecta during the first and second subphases are 0.21 km³ (DRE) and 0.27 km³ (DRE), respectively. The proportions of the different eruptive products are lava: cone: pumice fall=84:11:5 in the first subphase and lava: cone: pyroclastic flow=42:2:56 in the second subphase. The lava flows in this eruption consist of three flow units (L1, L2, and L3) and they characteristically possess abundant broken phenocrysts, and show extensive "welding" texture. These features, as well as ghost pyroclastic textures on the surface, indicate that the lava was a fountain-fed clastogenic lava. A high discharge rate for the lava flow (up to 10⁶ kg/s) may also suggest that the lava was initially explosively ejected from the conduit. The petrology of the juvenile materials indicates binary mixing of an andesitic magma and a crystal-rich dacitic magma. The mixing ratio changed with time; the dacitic component is dominant in the pyroclasts of the first subphase of the climactic phase, while the proportion of the andesitic component increases in the pyroclasts of the second subphase. The compositions of the lava flows vary from one flow unit to another; L1 and L3 have almost identical compositions to those of pyroclasts of the first and second subphases, respectively, while L2 has an intermediate composition, suggesting that the pyroclasts of the first and second subphases were the source of the lava flows, and were partly homogenized during flow. The complex features of this eruption can be explained by rapid deposition of coarse pyroclasts near the vent and the subsequent flowage of clastogenic lavas which were accompanied by a high eruption plume generating pumice falls and/or pyroclastic flows.Editorial responsibility: T. Druitt     

Notes on the 1902 eruption of Santa María volcano,Guatemala

The first historic activity of Santa María volcano, Guatemala constituted one of the ten largest historic eruptions in the world, producing 5.5 km³ of debris. In hindsight, the six-month period before the October 1902 eruption was one of extremely abnormal seismicity in all of western Guatemala. The pyroclastis from the eruption were scattered widely over Western Guatemala and Southern Mexico and also caused world-wide atmospheric effects. The volcanics produced were of andesitic-dacitic composition, but there was wide variation from place to place in the sampled material — a fact apparently chiefly attributable to atmospheric-fractionation. There was apparently a change in chemistry of ash during the two-day eruption as well, the first, most voluminous ash was pumicious and white; later ash was finer, denser, darker, and slightly less silicious. The kinetic energy/thermal energy partition is determined to be similar to the value derived for Krakatoa,E _k/E _th ? 5.0 %. The thermal energy of the eruption was estimated at 4.2×10²⁵ ergs. The explosion crater left on Santa María’s southwest flank after the eruption had a volume equal to less than 0.5 km³, a small fraction of the volume of material erupted. The two-day 1902 blast has greatly overshadowed subsequent activity; extrusion of the Santiaguito dome, which has occurred since 1922 in the explosion crater, has produced about 0.7 km³ of dacite lava and 1.6×10²⁵ ergs of thermal energy in 48 years of activity.     

The eruptive rate and history of Kuju volcano in Japan during the past 15,000 years

The eruptive history of Kuju volcano on Kyushu, Japan, during the past 15,000 years has been determined by tephrochronology and ¹⁴C dating. Kuju volcano comprises isolated lava domes and cones of hornblende andesite together with aprons of pyroclastic-flow deposits on its flanks. Kuju volcano produced tephras at roughly 1000-yr intervals during the past 5000 years and 70% of the domes and cones have formed during the past 15,000 years. The youngest magmatic activity of Kuju volcano was the 1.6 km³ andesite eruption about 1600 years ago which emplaced a lava dome and block-and-ash flow. Kuju volcano shows a nearly constant long-term eruption rate (0.7–0.4 km³ for 1000 years) during the past 15,000 years. This rate is within the range of estimated average eruption rates of late Quaternary volcanoes in the Japanese Arc, but is about one order of magnitude higher than the eruption rate of Unzen volcano. Kuju volcano has been in phreatic eruption since October 1995. The late Quaternary history of Kuju indicates that it poses a significant volcanic hazard, primarily due to block-and-ash flows from collapsing lava domes.     

Eruptive history of Mount Mazama and Crater Lake Caldera, Cascade Range, U.S.A.

New investigations of the geology of Crater Lake National Park necessitate a reinterpretation of the eruptive history of Mount Mazama and of the formation of Crater Lake caldera. Mount Mazama consisted of a glaciated complex of overlapping shields and stratovolcanoes, each of which was probably active for a comparatively short interval. All the Mazama magmas apparently evolved within thermally and compositionally zoned crustal magma reservoirs, which reached their maximum volume and degree of differentiation in the climactic magma chamber 7000 yr B.P.The history displayed in the caldera walls begins with construction of the andesitic Phantom Cone 400,000 yr B.P. Subsequently, at least 6 major centers erupted combinations of mafic andesite, andesite, or dacite before initiation of the Wisconsin Glaciation 75,000 yr B.P. Eruption of andesitic and dacitic lavas from 5 or more discrete centers, as well as an episode of dacitic pyroclastic activity, occurred until 50,000 yr B.P.; by that time, intermediate lava had been erupted at several short-lived vents. Concurrently, and probably during much of the Pleistocene, basaltic to mafic andesitic monogenetic vents built cinder cones and erupted local lava flows low on the flanks of Mount Mazama. Basaltic magma from one of these vents, Forgotten Crater, intercepted the margin of the zoned intermediate to silicic magmatic system and caused eruption of commingled andesitic and dacitic lava along a radial trend sometime between 22,000 and 30,000 yr B.P. Dacitic deposits between 22,000 and 50,000 yr old appear to record emplacement of domes high on the south slope. A line of silicic domes that may be between 22,000 and 30,000 yr old, northeast of and radial to the caldera, and a single dome on the north wall were probably fed by the same developing magma chamber as the dacitic lavas of the Forgotten Crater complex. The dacitic Palisade flow on the northeast wall is 25,000 yr old. These relatively silicic lavas commonly contain traces of hornblende and record early stages in the development of the climatic magma chamber.Some 15,000 to 40,000 yr were apparently needed for development of the climactic magma chamber, which had begun to leak rhyodacitic magma by 7015 ± 45 yr B.P. Four rhyodacitic lava flows and associated tephras were emplaced from an arcuate array of vents north of the summit of Mount Mazama, during a period of 200 yr before the climactic eruption. The climactic eruption began 6845 ± 50 yr B.P. with voluminous airfall deposition from a high column, perhaps because ejection of 4−12 km³ of magma to form the lava flows and tephras depressurized the top of the system to the point where vesiculation at depth could sustain a Plinian column. Ejecta of this phase issued from a single vent north of the main Mazama edifice but within the area in which the caldera later formed. The Wineglass Welded Tuff of Williams (1942) is the proximal featheredge of thicker ash-flow deposits downslope to the north, northeast, and east of Mount Mazama and was deposited during the single-vent phase, after collapse of the high column, by ash flows that followed topographic depressions. Approximately 30 km³ of rhyodacitic magma were expelled before collapse of the roof of the magma chamber and inception of caldera formation ended the single-vent phase. Ash flows of the ensuing ring-vent phase erupted from multiple vents as the caldera collapsed. These ash flows surmounted virtually all topographic barriers, caused significant erosion, and produced voluminous deposits zoned from rhyodacite to mafic andesite. The entire climactic eruption and caldera formation were over before the youngest rhyodacitic lava flow had cooled completely, because all the climactic deposits are cut by fumaroles that originated within the underlying lava, and part of the flow oozed down the caldera wall.A total of 51−59 km³ of magma was ejected in the precursory and climactic eruptions, and 40−52 km³ of Mount Mazama was lost by caldera formation. The spectacular compositional zonation shown by the climactic ejecta — rhyodacite followed by subordinate andesite and mafic andesite — reflects partial emptying of a zoned system, halted when the crystal-rich magma became too viscous for explosive fragmentation. This zonation was probably brought about by convective separation of low-density, evolved magma from underlying mafic magma. Confinement of postclimactic eruptive activity to the caldera attests to continuing existence of the Mazama magmatic system.