PRELIMINARY VOLCANIC HAZARD ZONATION IN JINLONGDINGZI VOLCANO,LONGANG VOLCANO AREA,JILIN PROVINCE,CHINA

Longgang volcano cluster is 150km away from the Tianchi volcano, located in Jingyu and Huinan Counties, Jilin Province, China. It had a long active history and produced hundreds of volcanoes. The latest and largest eruption occurred between 1 500 and 1 600 years ago by Jinlongdingzi(JLDZ)volcano which had several eruptions in the history. This paper discusses the volcanic hazard types, and using the numerical simulations of lava flow obtained with the Volcflow model, proposes the hazard zonation of JLDZ volcano area. JLDZ volcano eruption type is sub-plinian, which produced a great mass of tephra fallout, covering an area of 260km². The major types of volcanic hazards in JLDZ area are lava flow, tephra fallout and spatter deposits. Volcflow is developed by Kelfoun for the simulation of volcanic flows. The result of Volcflow shows that the flows are on the both sides of the previous lava flows which are low-lying areas now. According to the physical parameters of historical eruption and Volcflow, we propose the preliminary volcanic hazard zonation in JLDZ area. The air fall deposits are the most dangerous product in JLDZ. The highly dangerous region of spatter deposits is limited to a radius of about 2km around the volcano. The high risk area of tephra fallout is between 2km to 9km around the volcano, and between 9km to 14km is the moderate risk area. Out of 14km, it is the low risk area. Lava flow is controlled by topography. From Jinchuan Town to Houhe Village near the volcano is the low-lying area. If the volcano erupts, these areas will be in danger.     

Strombolian and phreatomagmatic deposits of Ohakune craters, Ruapehu, New Zealand: A complex interaction between external water and rising basaltic magma

The Ohakune Craters form one of several parasitic centres surrounding Ruapehu volcano, at the southern end of the Taupo Volcanic Zone. An inner scoria cone and an outer, probably older, tuff ring are the principal structures in a nested cluster of four vents.The scoria cone consists of alternating lava flows and coarse, welded and unwelded, strombolian block and bomb beds. The strombolian beds consist of principally two discrete types of essential clast, vesicular bombs and dense angular blocks. Rare finer-grained beds are unusually block-rich. The tuff ring consists of alternating strombolian and phreatomagmatic units. Strombolian beds have similar grain size characteristics to scoria cone units, but contain more highly vesicular unoxidised bombs and few blocks. Phreatomagmatic deposits, which contain clasts with variable degrees of palagonitisation, consist of less well-sorted airfall deposits and very poorly sorted, crystal-rich pyroclastic surge deposits.Disruption by expanding magmatic gas bubbles was a major but relatively constant influence on both strombolian and phreatomagmatic eruptions at Ohakune. Instead, the nature of deposits was principally controlled by two other variables, vent geometry and the relative influence of external water during volcanism. During tuff-ring construction, magma is considered to have risen rapidly to the surface, and to have been ejected without sufficient residence time in the vent for non-explosive degassing. Availability of external water principally governed the eruption mechanism and hence the nature of the deposits. Essentials clasts of the scoria cone are, by comparison, dense, degassed and oxidised. It is suggested that a change in vent geometry, possibly the construction of the tuff ring itself, permitted lava ponding and degassing during scoria cone growth. During strombolian eruptions, magma remaining in the vent probably became depleted in gas, leading to the formation of an inert zone, or crust, above actively degassing magma. Subsequent explosions had therefore to disrupt both this passive crust and underlying, vesiculating magma “driving” the eruption. Cycles of strombolian eruption are thought to have stopped when the thickness of the inert crust precluded explosive eruption and only recommenced when some of this material was removed, either as a lava flow or during phreatomagmatic explosions when external water entered the vent. Such explosions probably formed the unusually fine-grained and block-rich beds in the strombolian sequence.The Ohakune deposits are an excellent example of the products of explosive eruption of fluid, gas-rich basic magma vesiculating under very near-surface conditions. A complex interplay of rate of magma rise, rate and depth of formation of gas bubbles, vent geometry, abundance of shallow external water, wind velocity and accumulation rate of ejecta determines the nature of deposits of such eruptions.     

Early Proterozoic,basaltic andesite tuff-breaccia: downslope,subaqueous mass transport of phreatomagmatically-generated tephra

At Bear Lake, in the Flin Flon-Snow Lake greenstone belt of Manitoba, 400⁺ m of thick-to very thick-bedded, generally ungraded, basaltic andesite tuff-breccia, breccia, and lapilli-tuff are intercalated with pillowed lava flows in the upper part of an early Proterozoic submarine basaltic andesite shield volcano. The fragmental rocks comprise angular, amygdaloidal blocks and lapilli, many with partial chilled selvages, in a matrix of blocky, non-amygdaloidal to highly amygdaloidal vitric basaltic andesite ash and small lapilli. Minor thin-to medium-bedded, commonly normally graded tuff occurs in the upper part of the sequence. Clasts in fragmental beds consistently have higher amygdule contents than intercalated lava flows. Although similar to pillow-fragment breccias, the Bear Lake fragmental rocks were produced by extended surtseyan-type, phreatomagmatic eruptions, with associated fire fountain activity, at a progressively subsiding, shallow water vent. Periodic tephra slumping generated debris flows that transported particles down the uppe, gentle slope of the volcano to a depositional site at a water depth of less than 1 km. Turbidity currents probably carried much fine tephra to deeper water; tuff was deposited in the preserved section only after explosive volcanism ceased.     

Eruption processes and deposit characteristics at the monogenetic Mt. Gambier Volcanic Complex, SE Australia: implications for alternating magmatic and phreatomagmatic activity

The ～5 ka Mt. Gambier Volcanic Complex in the Newer Volcanics Province, Australia is an extremely complex monogenetic, volcanic system that preserves at least 14 eruption points aligned along a fissure system. The complex stratigraphy can be subdivided into six main facies that record alternations between magmatic and phreatomagmatic eruption styles in a random manner. The facies are (1) coherent to vesicular fragmental alkali basalt (effusive/Hawaiian spatter and lava flows); (2) massive scoriaceous fine lapilli with coarse ash (Strombolian fallout); (3) bedded scoriaceous fine lapilli tuff (violent Strombolian fallout); (4) thin–medium bedded, undulating very fine lapilli in coarse ash (dry phreatomagmatic surge-modified fallout); (5) palagonite-altered, cross-bedded, medium lapilli to fine ash (wet phreatomagmatic base surges); and (6) massive, palagonite-altered, very poorly sorted tuff breccia and lapilli tuff (phreato-Vulcanian pyroclastic flows). Since most deposits are lithified, to quantify the grain size distributions (GSDs), image analysis was performed. The facies are distinct based on their GSDs and the fine ash to coarse+fine ash ratios. These provide insights into the fragmentation intensities and water–magma interaction efficiencies for each facies. The eruption chronology indicates a random spatial and temporal sequence of occurrence of eruption styles, except for a “magmatic horizon” of effusive activity occurring at both ends of the volcanic complex simultaneously. The eruption foci are located along NW–SE trending lineaments, indicating that the complex was fed by multiple dykes following the subsurface structures related to the Tartwaup Fault System. Possible factors causing vent migration along these dykes and changes in eruption styles include differences in magma ascent rates, viscosity, crystallinity, degassing and magma discharge rate, as well as hydrological parameters.     

Volcanology and eruptive styles of Barren Island: an active mafic stratovolcano in the Andaman Sea,NE Indian Ocean

Barren Island (India) is a relatively little studied, little known active volcano in the Andaman Sea, and the northernmost active volcano of the great Indonesian arc. The volcano is built of prehistoric (possibly late Pleistocene) lava flows (dominantly basalt and basaltic andesite, with minor andesite) intercalated with volcaniclastic deposits (tuff breccias, and ash beds deposited by pyroclastic falls and surges), which are exposed along a roughly circular caldera wall. There are indications of a complete phreatomagmatic tephra ring around the exposed base of the volcano. A polygenetic cinder cone has existed at the centre of the caldera and produced basalt-basaltic andesite aa and blocky aa lava flows, as well as tephra, during historic eruptions (1787–1832) and three recent eruptions (1991, 1994–95, 2005–06). The recent aa flows include a toothpaste aa flow, with tilted and overturned crustal slabs carried atop an aa core, as well as locally developed tumuli-like elliptical uplifts having corrugated crusts. Based on various evidence we infer that it belongs to either the 1991 or the 1994–95 eruptions. The volcano has recently (2008) begun yet another eruption, so far only of tephra. We make significantly different interpretations of several features of the volcano than previous workers. This study of the volcanology and eruptive styles of the Barren Island volcano lays the ground for detailed geochemical-isotopic and petrogenetic work, and provides clues to what the volcano can be expected to do in the future.     

A transition from strombolian to phreatomagmatic activity induced by a lava flow damming water in a valley

The Golan Heights is a Plio-Pleistocene volcanic plateau. Cinder cones of Late Pleistocene age are very common in the eastern and northern Golan, while phreatomagmatic deposits are relatively rare and occur just in two structures — the maar of Birket Ram and the tuff ring of Mt. Avital. The complex of Mt. Avital includes two large cinder cones, a tuff ring with an elongated central depression and several basaltic flows, some of them breach the cinder cones. The (exposed) eruptive history of the complex includes (1) an early stage of basaltic lava flows, (2) strombolian activity and the buildup of the southern cinder cone, (3) a second stage of basaltic flows and the buildup of the northern cinder cone, and then a transition to (4) phreatomagmatic explosions. The phreatomagmatic deposits include surges, lapilli fallout deposits and coarse-grained lithic tuff breccias, which were found up to 200 m above the central depression. Basaltic and scoriaceous clasts are the main component of all deposits, while juvenile material is usually a minor component, almost absent in the lapilli deposits.It is suggested that the phreatomagmatic events in Mt. Avital were induced by the infiltration of water from a lake that existed in a nearby topographic low (Quneitra Valley). The lake was formed or significantly expanded at about 300 ka due to a lava flow that blocked the drainage of the valley to the west. The interlayering of tuff and scoria at the top of the northern cinder cone and the good preservation of a lava flow top breccia under the surges imply that the phreatomagmatic activity immediately followed and even coincided with the last stages of strombolian activity. It is suggested that the dry–wet transition was triggered by the effusion of the second stage lavas and the buildup of the northern cinder cone, which probably caused a reduction of pressure in the magmatic system and allowed the lake water an access to the magmatic system. The minimum age of the phreatomagmatic events is determined by a 54 ka Musterian site which lies directly on top of the tuff in the Quneitra Valley.     

The geology of split butte — A maar of the south-central snake river plain,Idaho

Split Butte is a volcanic crater of Quaternary age consisting of a tephra ring which at one time retained a lava lake. The tephra is thinly bedded and is composed of partially palagonitized sideromelane clasts and subordinate lithic fragments. The beds typically dip radially away from the center of the crater, but locally dip toward the crater center. The tephra ring resulted from phreatomagmatic eruptions as a result of interaction of groundwater with rising basaltic magma, evidenced by glassy and granulated pyroclastic debris, the presence of abundant palagonite and other secondary minerals, numerous armored lapilli, and plastically deformed ash layers below ejecta blocks. Statistical analysis of the grain size distribution of the ash also indicates a phreatomagmatic origin of Split Butte tephra. In addition, the analysis reveals that the stratigraphically lowest tephra was deposited primarily by pyroclastic flow mechanisms while the upper tephra layers, comprising the bulk of the deposits, were deposited dominantly by airfall and pyroclastic surge. The lava lake and four en echelon basalt dikes were emplaced when phreatomagmatic activity at the vent ceased. Subsequent collapse caused a broad, shallow pit crater to form in the laval lake, and minor spattering occurred at one point along the pit crater scarp. Partial erosion of the tephra, deposition of aeolian sediments and encroachment of the Butte by later lava flows completed the development of Split Butte.     

Products and processes in Pliocene–Recent, subaqueous to emergent volcanism in the Antarctic Peninsula: examples of englacial Surtseyan volcano construction

 Pliocene–Recent volcanic outcrops at Seal Nunataks and Beethoven Peninsula (Antarctic Peninsula) are remnants of several monogenetic volcanoes formed by eruption of vesiculating basaltic magma into shallow water, in an englacial environment. The diversity of sedimentary and volcanic lithofacies present in the Antarctic Peninsula outcrops provides a clear illustration of the wide range of eruptive, transportational and depositional processes which are associated with englacial Surtseyan volcanism. Early-formed pillow lava and glassy breccia, representing a pillow volcano stage of construction, are draped by tephra erupted explosively during a tuff cone stage. The tephra was resedimented around the volcano flanks, mainly by coarse-grained sediment gravity flows. Fine-grained lithofacies are rare, and fine material probably bypassed the main volcanic edifice, accumulating in the surrounding englacial basin. The pattern of sedimentation records variations in eruption dynamics. Products of continuous-uprush eruptions are thought to be represented by stacks of poorly bedded gravelly sandstone, whereas better bedded, lithologically more diverse sequences accumulated during periods of quiescence or effusive activity. Evidence for volcano flank failure is common. In Seal Nunataks, subaerial lithofacies (mainly lavas and cinder cone deposits) are volumetrically minor and occur at a similar stratigraphical position to pillow lava, suggesting that glacial lake drainage may have occurred prior to or during deposition of the subaerial lithofacies. By contrast, voluminous subaerial effusion in Beethoven Peninsula led to the development of laterally extensive stratified glassy breccias representing progradation of hyaloclastite deltas. Received: 5 February 1996 / Accepted: 17 January 1997     

Mixed deposits of simultaneous strombolian and phreatomagmatic volcanism: Rothenberg volcano, east Eifel volcanic field

The Pleistocene basanite-tephrite Rothenberg cone complex in the East Eifel was constructed by alternating dominantly Strombolian (S1–3) and dominantly phreatomagmatic (P1–3) phases of volcanism along a NNE-SSW linear vent system. Strombolian eruptions, from the central vent of the S1 scoria cone, and phreatomagmatic eruptions, from a vent on the southern margin of the cone, occurred simultaneously during the second phreatomagmatic phase (P2). The P2 deposits are a complex sequence in which Strombolian fallout ejecta is intimately admixed with phreatomagmatic fallout and pyroclastic surge material. Every bed contains at least trace amounts of ejecta from both sources but, at every site, an alternation of Strombolian-dominant and phreatomagmatic-dominant units is recorded. Each bed also shows marked lateral changes with a progressive northward increase in the proportion of Strombolian material. The two eruptive styles produced morphologically distinct clast populations often with widely separated (5–7 φ) grain size modes. The phreatomagmatic component of the P2 deposits is inferred to be the result of shallow interaction of external water and cool, partially degassed magma which reached the surface at a time when the magma column was retreating from the northern Strombolian central vent.The Rothenberg deposits illustrate the complexity and sensitivity of controls on Strombolian and associated phreatomagmatic volcanism, and the shallow depth of fragmentation during such eruptions. During such shallow eruptions minor, ephemeral and localised variations in the rate of rise and discharge of magma, and vent geometry and hydrology significantly influence the magma:water ratio and hence eruptive style.     

Ignimbrites of basaltic andesite and andesite compositions from Tanna,New Hebrides Arc

Tanna island is part of a large volcanic complex mainly subsided below sea-level. On-land, two series of hydroclastic deposits and ignimbrites overlie the subaerial remains of a basal, mainly effusive volcano. The ‘Older’ Tanna Ignimbrite series (OTI), Late Pliocene or Pleistocene in age, consists of ash flows and ash- and scoria-flow deposits associated with fallout tephra layers, overlain by indurated pumice-flow deposits. Phreatomagmatic features are a constant characteristic of these tuffs. The ‘younger’ Late Pleistocene pyroclastics, the Siwi sequence, show basal phreatomagmatic deposits overlain by two successive flow units, each comprising a densely welded layer and a nonwelded ash-flow deposit. Whole-rock analyses of 17 juvenile clasts from the two sequences (vitric blocks from the phreatomagmatic deposits, welded blocks, scoriaceous bombs and pumices from the ignimbrites) show basaltic andesite and andesite compositions (SiO₂=53–60%). In addition, 296 microprobe analyses of glasses in these clasts show a wide compositional range from 51 to 69% SiO₂. Dominant compositions at ∼54, 56, 58.5 and 61–62% SiO₂ characterize the glass from the OTI. Glass compositions in the lower – phreatomagmatic – deposits from the Siwi sequence also show multimodal distribution, with peaks at SiO₂=55, 57.5, 61–62 and 64% whereas the upper ignimbrite has a predominant composition at 61–62% SiO₂. In both cases, mineralogical data and crystal fractionation models suggest that these compositions represent the magmatic signature of a voluminous layered chamber, the compositional gradient of which is the result of fractional crystallization. During two major eruptive stages, probably related to two caldera collapses, the OTI and Siwi ignimbrites represent large outpourings from these magmatic reservoirs. The successive eruptive dynamics, from phreatomagmatic to Plinian, emphasize the role of water in initiating the eruptions, without which the mafic and intermediate magmas probably would not have erupted. Received: February 19, 1993/Accepted October 10, 1993     

The last 5000 years of activity at Sete Cidades volcano (São Miguel Island,Azores): Implications for hazard assessment

Sete Cidades is a central volcano with a summit caldera at the western end of São Miguel Island, Azores. Its stratigraphy comprises two main geological groups: the Inferior Group, the units of which date from more than 200 000 years ago through to 36 000 years before present, consisting of thick lava flows and subaerial volcaniclastic deposits that built the base of the central volcano; and the Superior Group which comprises all the activity from the last 36 000 years, including pumice and scoria fallout and PDC deposits with minor lava flows. The volcanostratigraphy is divided into six main formations — Risco, Ajuda, Bretanha, Lombas, Santa Bárbara and Lagoas, each defined by different activity phases in the volcano's evolution.     

Miocene lamproite volcanoes in south-eastern Spain—an association of phreatomagmatic and magmatic products

A series of small Miocene (8.3–6.7 Ma) lamproite rock occurrences (as monogenetic volcanoes and/or dykes) cover a large area in southeastern Spain. These rocks are associated with extensional basins filled by Neogene deposits in the Betic and Subbetic structural units. At Cancarix (Sierra de las Cabras), Calasparra, Barqueros, Cerro de Monagrillo, Jumilla, and Vera, eruptions occurred, whereas at Fortuna, Mula and Zeneta there were only small-scale intrusions (mainly dykes). This paper describes volcanic centers at Cancarix, Calasparra and Barqueros, which show initial phreatomagmatic eruptions driven by interaction of rising lamproite magma with groundwater. Tuff ring formed during this volcanic activity. Subsequent activity consisted of dome extrusion in the vent areas of Cancarix and Calasparra and by explosive to effusive magmatic activity accompanied by extensive lava flows at Barqueros.Calasparra and Cancarix are relatively symmetric monogenetic tuff rings filled by late stage massive vertical plug, extruded as degassed crystalline high-viscosity magma along the volcanic conduit. Barqueros was initially a tuff ring, whose late stage Hawaiian-style fountaining generated spatter and clastogenic lavas that built the intra-tuff ring cone of Cabezo del Morron. Finally, extensive lava flows spread from the base of the cone toward the northern part of the edifice. Variations in the tectonic (extensional regime) and local hydrogeologic conditions (shallow aquifers) influenced the occurrence of these lamproite volcanoes. Late stage magma rise was dependent on the magmatic volatile regime, being already degassed at Calasparra and Cancarix, by showing higher viscosity (high crystallization rate) of intra-tuff ring dome extrusions, or still rich in volatiles at Barqueros, displaying lower viscosity lava fountaining and then lava flows.     

40Ar/39Ar Ages and stratigraphy of the Latera caldera,Italy

The Latera caldera is a well-exposed volcano where more than 8 km³ of mafic silica-undersaturated potassic lavas, scoria and felsic ignimbrites were emplaced between 380 and 150 ka. Isotopic ages obtained by ⁴⁰Ar/³⁹Ar analysis of single sanidine crystals indicate at least four periods of explosive eruptions from the caldera. The initial period of caldera eruptions began at 232 ka with emplacement of trachytic pumice fallout and ignimbrite. They were closely followed by eruption of evolved phonolitic magma. After roughly 25 ky, several phonolitic ignimbrites were deposited, and they were followed by phreatomagmatic eruptions that produced trachytic ignimbrites and several smaller ash-flow units at 191 ka. Compositionally zoned magma then erupted from the northern caldera rim to produce widespread phonolitic tuffs, tephriphonolitic spatter, and scoria-bearing ignimbrites. After 40 ky of mafic surge deposit and scoria cone development around the caldera rim, a compositionally zoned pumice sequence was emplaced around a vent immediately northwest of the Latera caldera. This activity marks the end of large-scale explosive eruptions from the Latera volcano at 156 ka.     

Structure and evolution of the Rockeskyllerkopf Volcanic Complex, West Eifel Volcanic Field, Germany

The Rockeskyllerkopf Volcanic Complex (RVC) comprises three overlapping monogenetic volcanic centers: Southeast Lammersdorf (SEL), Mäuseberg (M) and Rockeskyllerkopf (RKK). Each volcanic center comprises proximal wall deposits with a well defined crater wall unconformity and crater fill deposits that partially to completely cover the outer crater wall. The SEL Center is a phreatomagmatic tuff ring composed of lithic rich tephra deposited by pyroclastic falls and surges. The second center, Mäuseberg, with its crater to the northwest of the SEL Center is predominantly magmatic. Topographic and outcrop patterns suggest that this center may have formed a series of overlapping scoria cones along a N–S trending fissure. The youngest center, RKK, which lies on a poorly developed palaeosol within the earlier Mäuseberg deposits, comprises a well developed proximal crater wall sequence. This sequence of magmatic, likely Strombolian, fall and grain avalanche deposits passes upward into a crater fill sequence that comprises variably welded bombs. The final eruptions in the center were massive lava flows that were ponded within the RKK crater. Ar–Ar age dating of reequilibrated fragments of phlogopite megacrysts in the SEL lavas indicates volcanic activity began at 474?±?39 ka. Literature K–Ar dates for the youngest lava flows in the RKK Center give ages of 360?±?60 to 470 ka. Our interpretation of the age data and the presence of the poorly developed palaeosol between the Mäuseberg and RKK centers indicates that volcanism in the RVC began around 470 ka with the eruption of the SEL and Mäuseberg centers followed a few thousand years later by the eruption of the RKK Center.     

Volcanic hazard assessment for Ruapehu composite volcano,taupo volcanic zone,New Zealand

Ruapehu is a very active andesitic composite volcano which has erupted five times in the past 10 years. Historical events have included phreatomagmatic eruptions through a hot crater lake and two dome-building episodes. Ski-field facilities, road and rail bridges, alpine huts and portions of a major hydroelectrical power scheme have been damaged or destroyed by these eruptions. Destruction of a rail bridge by a lahar in 1953 caused the loss of 151 lives. Other potential hazards, with Holocene analogues, include Strombolian and sub-Plinian explosive eruptions, lava extrusion from summit or flank vents and collapse of portions of the volcano. The greatest hazards would result from renewed phreatomagmatic activity in Crater Lake or collapse of its weak southeastern wall. Three types of hazard zones can be defined for the phreatomagmatic events: inner zones of extreme risk from ballistic blocks and surges, outer zones of disruption to services from fall deposits and zones of risk from lahars, which consist of tongues down major river valleys. Ruapehu is prone to destructive lahars because of the presence of 10⁷ m³ of hot acid water in Crater Lake and because of the surrounding summit glaciers and ice fields. The greatest risks at Ruapehu are to thousands of skiers on the ski field which crosses a northern lahar path. Three early warning schemes have been established to deal with the lahar problems. Collapse of the southeastern confining wall would release much of the lake into an eastern lahar path causing widespread damage. This is a long-term risk which could only be mitigated by drainage of the lake.     

The Taurewa Eruptive Episode: evidence for climactic eruptions at Ruapehu volcano, New Zealand

 The ca. 10,500 years B.P. eruptions at Ruapehu volcano deposited 0.2–0.3 km³ of tephra on the flanks of Ruapehu and the surrounding ring plain and generated the only known pyroclastic flows from this volcano in the late Quaternary. Evidence of the eruptions is recorded in the stratigraphy of the volcanic ring plain and cone, where pyroclastic flow deposits and several lithologically similar tephra deposits are identified. These deposits are grouped into the newly defined Taurewa Formation and two members, Okupata Member (tephra-fall deposits) and Pourahu Member (pyroclastic flow deposits). These eruptions identify a brief (<ca. 2000-year) but explosive period of volcanism at Ruapehu, which we define as the Taurewa Eruptive Episode. This Episode represents the largest event within Ruapehu's ca. 22,500-year eruptive history and also marks its culmination in activity ca. 10,000 years B.P. Following this episode, Ruapehu volcano entered a ca. 8000-year period of relative quiescence. We propose that the episode began with the eruption of small-volume pyroclastic flows triggered by a magma-mingling event. Flows from this event travelled down valleys east and west of Ruapehu onto the upper volcanic ring plain, where their distal remnants are preserved. The genesis of these deposits is inferred from the remanent magnetisation of pumice and lithic clasts. We envisage contemporaneous eruption and emplacement of distal pumice-rich tephras and proximal welded tuff deposits. The potential for generation of pyroclastic flows during plinian eruptions at Ruapehu has not been previously considered in hazard assessments at this volcano. Recognition of these events in the volcanological record is thus an important new factor in future risk assessments and mitigation of volcanic risk at Tongariro Volcanic Centre. Received: 5 July 1998 / Accepted: 12 March 1999     

Tyatya Volcano, southwestern Kuril arc: Recent eruptive activity inferred from widespread tephra

Abstract Tyatya Volcano, situated in Kunashir Island at the southwestern end of Kuril Islands, is a large composite stratovolcano and one of the most active volcanoes in the Kuril arc. The volcanic edifice can be divided into the old and the young ones, which are composed of rocks of distinct magma types, low‐ and medium‐K series, respectively. The young volcano has a summit caldera with a central cone. Recent eruptions have occurred at the central cone and at the flank vents of the young volcano. We found several distal ash layers at the volcano and identified their ages and sources, that is, tephras of ad 1856, ad 1739, ad 1694 and ca 1 Ka derived from three volcanoes of Hokkaido, Japan, and caad 969 from Baitoushan Volcano of China/North Korea. These could provide good time markers to reveal the eruptive history of the central cone, which had continued intermittently with Strombolian eruptions and lava flow effusions since before 1 Ka. Relatively explosive eruptions have occurred three times at the cone during the past 1000 years. We revealed that, topographically, the youngest lava flows from the cone are covered not by the tephra of ad 1739 but by that of ad 1856. This evidence, together with a report of dense smoke rising from the summit in ad 1812, suggests that the latest major eruption with lava effusion from the central cone occurred in this year. In 1973, after a long period of dormancy, short‐lived phreatomagmatic eruptions began to occur from fissure vents at the northern flank of the young volcano. This was followed by large eruptions of Strombolian to sub‐Plinian types occurring from several craters at the southern flank. The 1973 activity is evaluated as Volcanic Explosivity Index = 4 (approximately 0.2 km³), the largest eruption during the 20th century in the southwestern Kuril arc. The rocks of the central cone are strongly porphyritic basalt and basaltic andesite, whereas the 1973 scoria is aphyric basalt, suggesting that magma feeding systems are definitely different between the summit and flank eruptions.     

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.     

The architecture, eruptive history, and evolution of the Table Rock Complex, Oregon: From a Surtseyan to an energetic maar eruption

The Table Rock Complex (TRC; Pliocene–Pleistocene), first documented and described by Heiken [Heiken, G.H., 1971. Tuff rings; examples from the Fort Rock-Christmas Lake valley basin, south-central Oregon. J. Geophy. Res. 76, 5615-5626.], is a large and well-exposed mafic phreatomagmatic complex in the Fort Rock–Christmas Lake Valley Basin, south-central Oregon. It spans an area of approximately 40 km², and consists of a large tuff cone in the south (TRC1), and a large tuff ring in the northeast (TRC2). At least seven additional, smaller explosion craters were formed along the flanks of the complex in the time between the two main eruptions. The first period of activity, TRC1, initiated with a Surtseyan-style eruption through a 60–70 m deep lake. The TRC1 deposits are dominated by multiple, 1-2 m thick, fining upward sequences of massive to diffusely-stratified lapilli tuff with intermittent zones of reverse grading, followed by a finely-laminated cap of fine-grained sediment. The massive deposits are interpreted as the result of eruption-fed, subaqueous turbidity current deposits; whereas, the finely laminated cap likely resulted from fallout of suspended fine-grained material through a water column. Other common features are erosive channel scour-and-fill deposits, massive tuff breccias, and abundant soft sediment deformation due to rapid sediment loading. Subaerial TRC1 deposits are exposed only proximal to the edifice, and consist of cross-stratified base-surge deposits. The eruption built a large tuff cone above the lake surface ending with an effusive stage, which produced a lava lake in the crater (365 m above the lake floor). A significant repose period occurred between the TRC1 and TRC2 eruptions, evidenced by up to 50 cm of diatomitic lake sediments at the contact between the two tuff sequences. The TRC2 eruption was the last and most energetic in the complex. General edifice morphology and a high percentage of accidental material suggest eruption through saturated TRC1 deposits and/or playa lake sediments. TRC2 deposits are dominated by three-dimensional dune features with wavelengths 200–500 m perpendicular to the flow, and 20–200 m parallel to the direction of flow depending on distance from source. Large U-shaped channels (10–32 m deep), run-up features over obstacles tens of meters high, and a large (13 m) chute-and-pool feature are also identified. The TRC2 deposits are interpreted as the products of multiple, erosive, highly-inflated pyroclastic surges resulting from collapse of an unusually high eruption column relative to previously documented mafic phreatomagmatic eruptions.