黑龙江省科洛火山群火山地质研究

科洛火山群的新生代火山共有23座,坐落于科洛河两岸,火山岩面积约为350km2,岩性主要为碱性玄武岩.由于地处NE向断陷盆地这一特殊的构造位置,科洛地区的火山活动及展布主要受到区域基底断裂的制约.火山喷发形式总体为中心式,属斯通博利式火山.火山活动可划分为上新世、更新世和全新世3期.上新世在断陷盆地边缘形成了一系列NE向线性展布的中心式溢出型火山,其中部分火山因风化剥蚀而失去了原有的火山地貌特征,仅保留盾形熔岩台地.早更新世火山活动相对平静.中-晚更新世火山活动仍受到NE向基底断裂的控制,但喷发中心、喷发方式及喷发强度均发生改变,火山由碱玄质火山渣锥和熔岩流组成.进入全新世以后南山喷发,其火山结构保存完好,裸露的熔岩台地保留了较好的微地貌特征.该期火山亦由碱玄质火山渣锥和熔岩流构成.在科洛火山群的火山活动过程中,其熔岩流覆盖了早期沉积地层,并对盆地中的河流进行了改造,最终导致该区断陷盆地初始地貌的改变.     

大兴安岭焰山、高山火山——一种新的火山喷发型式

在野外地质资料基础上,利用火山形态学方法,探讨了大兴安岭焰山、高山火山的喷发型式。结果表明,大兴安岭哈拉哈河-绰尔河火山群中的焰山和高山火山不同于斯通博利式喷发形成的火山,其早期爆破喷发的火山碎屑形成火山渣锥、空降火山碎屑席和小型火山碎屑流,晚期溢出大量熔岩。两火山具有较高大的锥体(标高200～300m以上),在结构上,松散火山砾、火山弹等构成下部的降落锥,熔结集块岩构成上部的溅落锥。由火山砾和火山灰组成的空降火山碎屑席分布在火山锥体周围。两火山溢出的熔岩经历了从结壳熔岩→翻花石→渣状熔岩的演变。根据喷发产物可推断焰山和高山火山具有以下喷发特征:爆破喷发形成持续的喷发柱→斯通博利式喷发→熔岩喷泉喷溢,其中以持续时间较长的喷发柱区别于典型的斯通博利式喷发。类似焰山、高山火山的喷发特征,在龙岗第四纪火山群、镜泊湖全新世火山群中也都有个例,这是中国大陆火山作用中一种新的喷发型式。     

Scoria cone formation through a violent Strombolian eruption: Irao Volcano, SW Japan

Scoria cones are common volcanic features and are thought to most commonly develop through the deposition of ballistics produced by gentle Strombolian eruptions and the outward sliding of talus. However, some historic scoria cones have been observed to form with phases of more energetic violent Strombolian eruptions (e.g., the 1943–1952 eruption of Parícutin, central Mexico; the 1975 eruption of Tolbachik, Kamchatka), maintaining volcanic plumes several kilometers in height, sometimes simultaneous with active effusive lava flows. Geologic evidence shows that violent Strombolian eruptions during cone formation may be more common than is generally perceived, and therefore it is important to obtain additional insights about such eruptions to better assess volcanic hazards. We studied Irao Volcano, the largest basaltic monogenetic volcano in the Abu Monogenetic Volcano Group, SW Japan. The geologic features of this volcano are consistent with a violent Strombolian eruption, including voluminous ash and fine lapilli beds (on order of 10^?1 km³ DRE) with simultaneous scoria cone formation and lava effusion from the base of the cone. The characteristics of the volcanic products suggest that the rate of magma ascent decreased gradually throughout the eruption and that less explosive Strombolian eruptions increased in frequency during the later stages of activity. During the eruption sequence, the chemical composition of the magma became more differentiated. A new K–Ar age determination for phlogopite crystallized within basalt dates the formation of Irao Volcano at 0.4?±?0.05 Ma.     

Shallow-seated controls on styles of explosive basaltic volcanism: a case study from New Zealand

The pyroclastic deposits of many basaltic volcanic centres show abrupt transitions between contrasting eruptive styles, e.g., Hawaiian versus Strombolian, or `dry' magmatic versus `wet' phreatomagmatic. These transitions are controlled dominantly by variations in degassing patterns, magma ascent rates and degrees of interaction with external water. We use Crater Hill, a 29 ka explosive/effusive monogenetic centre in the Auckland volcanic field, New Zealand, as a case study of the transitions between these end-member eruptive styles. The Crater Hill eruption took place from at least 4 vents spaced along a NNE-trending, 600-m-long fissure that is contained entirely within a tuff ring generated during the earliest eruption phases. Early explosive phases at Crater Hill were characterised by eruption from multiple unstable and short-lived vents; later, dominantly extrusive, volcanism took place from a more stable point source. Most of the Crater Hill pyroclastic deposits were formed in 3 phreatomagmatic (P) and 4 `dry' magmatic (M) episodes, forming in turn the outer tuff ring and maar crater (P1, M1, P2) and scoria cone 1 (M2–M4). This activity was followed by formation of a lava shield and scoria cone 2. Purely `wet' activity is represented by the bulk of P1 and P2, and purely `dry' activity by much of M2–M4. However, M1 and parts of M2 and M4 show evidence for simultaneous eruptions of differing style from adjacent vents and rapid variations in the extent and timing of magma:water interaction at each vent. The nature of the wall-rock lithics, and these rapid variations in inferred water/magma ratios imply interaction was occurring mostly at depths of ≤80 m, and the vesicularity patterns in juvenile clasts from these and the P beds imply that rapid degassing occurred at these shallow levels. We suggest that abrupt transitions between eruptive styles, in time and space, at Crater Hill were linked to changes in the local magma supply rate and patterns and vigour of degassing during the final metres of ascent.     

THE CHARACTERISTICS OF VOLCANIC ROCKS STRUCTURE AND LITHOFACIES OF DALIUCHONG VOLCANO IN THE TENGCHONG VOLCANIC FIELD,YUNNAN PROVINCE

A set of grey-purple layered volcanic rocks are found widely distributed from the mountain flank to the main peak of Daliuchong volcano, but it's difficult to identify whether they are volcaniclastic rock or lava rock just by field investigation and the crystal structure observation under microscope. The study of matrix microstructure of the volcanic rocks can help to identify the volcanic facies. We recognize the eruptive facies rocks through observation of the matrix microstructure and pore shape with comparison to those of the volcanic vent facies, extrusive facies and effusive facies rocks under microscope, thus the mentioned layered volcanic rocks could be named as dacitic crystal fragment tuff. Combining the joint work of field investigation, systematic sampling, chemical analyzing and microscopic observation, we summary the Daliuchong volcanic facies as follows:1. The effusive facies lava constitutes the base of Daliuchong volcano and was produced by early eruption.2. The explosive facies is composed of dacite crystal fragment welded tuff and volcanic breccia and mainly distributes on the W, S and NE flank of the volcanic cone.3. The volcanic conduit with its diameter more than one hundred meters is located about 100 meters south of the main peak of the Daliuchong volcano.4. The extrusive facies rock is only exposed near the peak of Daliuchong volcano.Therefore, the volcanism of Daliuchong volcano can be speculated as:Large-scale lava overflowing occurred in the early eruption period; then explosive eruptions happened; at last, the volcanisms ceased marked with magma extrusion as lava dome and plug.     

Rothenberg scoria cone,East Eifel: a complex Strombolian and phreatomagmatic volcano

Rothenberg scoria cone Eifel formed by an alternation of three Strombolian and three phreatomagmatic eruptive phases. Eruptions took place from up to six vents on a 600 m-long fissure, building an early tuff ring and then two coalescing scoria cones. Strombolian volcanism dominated volumetrically, as the supply of external water was severely limited. Magma/water interaction only occurred during the opening stages of eruption at any vent, when discharge rates were low and the fragmentation surface was below the water table. The phreatomagmatic deposits consist of relatively well-sorted fall beds and only minor surge deposits. They contain juvenile clasts with a wide range of vesicularity and grain size, implying considerable heterogeneity in the assemblage of material ejected by the phreatomagmatic explosions. the transition from phreatomagmatic to Strombolian eruption at any vent was rapid and irreversible, and Strombolian volcanism persisted even when eruption rates are inferred to have waned at the close of each eruptive phase as, by then, the fragmentation surfaces were high in the growing cones and water was denied access to the magma. The Strombolian deposits are relatively homogenous, consisting of alternating coarser- and finer-grained, well-sorted fall beds erupted during periods of open-vent eruption and partial blockage of the vent respectively. The intervals of Strombolian eruption were always a delicate balance between discharge of freely degassing magma and processes such as ponding of degassed magma in the vent, collapse of the growing cones, and repeated recycling of clasts through the vent. Clear evidence of the instability of the Rothenberg cones is preserved in numerous unconformities within deposits of the inner walls of the cones. The close of Strombolian phases was probably marked by a decreasing supply of magma to the vents accompanied by ponding and stagnation of lava in the craters.     

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.     

Eruptive style of the young high-Mg basaltic-andesite Pelagatos scoria cone,southeast of México City

The eruption of the Pelagatos scoria cone in the Sierra Chichinautzin monogenetic field near the southern suburbs of Mexico City occurred less than 14,000 years ago. The eruption initiated at a fissure with an effusive phase that formed a 7-km-long lava flow, and continued with a phase of alternating and/or simultaneous explosive and effusive activity that built a 50-m-high scoria cone on the western end of the fissure and formed a compound lava flow-field near the vent. The eruption ended with the emplacement of a short lava flow that breached the cone and was accompanied by weak explosions at the crater. Products consist of a microlite-rich high-Mg basaltic andesite. Samples were analyzed to determine the magma’s initial properties as well as the effects of degassing-induced crystallization on eruptive style. Although distal ash fallout deposits from this eruption are not preserved, a recent quarry exposes a large section of the scoria cone. Detailed study of exposed layers allows us to elucidate the mode of cone-building activity. Petrological and textural data, combined with models calibrated by experimental work and melt-inclusion analyses of similar magmas elsewhere, indicate that the magma was initially hot (>1,200°C), gas-rich (up to 5 wt.% H₂O), crystal-poor (~10 vol.% Fo₉₀ olivine phenocrysts) and thus poorly viscous (40–80 Pa s). During the early phase, low magma ascent velocity at the fissure vent allowed low-viscosity magma to degas and crystallize during ascent, producing lava flows with elevated crystal contents at T < 1,100°C, and blocky surfaces. Later, the closure of the fissure by cooling dikes focused the magma flow at a narrow section of the fissure. This led to an increased magma ascent velocity. Rapid and shallow degassing (<3 km deep) triggered ~40 vol.% microlite crystallization. Limited times for gas-escape and higher magma viscosity (6 × 10⁵–4 × 10⁶ Pa s) drove strong explosions of highly (60–80 vol.%) and finely vesicular magma. Coarse clasts broke on landing, which implies brittle behavior due to complete solidification. This requires sufficient time to cool and in turn implies ejection heights of over 1 km, which is much higher than “normal” Strombolian activity. Hence, magma viscosity significantly impacts eruption style at monogenetic volcanoes because it affects the kinetics of shallow degassing. The long-lasting eruptions of Jorullo and Paricutin, which produced similar magmas in western México, were more explosive. This can be related to higher magma fluxes and total erupted volumes. Implications of this study are important because basaltic andesites are commonly erupted to form monogenetic scoria cones of the Trans-Mexican Volcanic Belt.     

Volcanism in Iceland in historical time: Volcano types,eruption styles and eruptive history

The large-scale volcanic lineaments in Iceland are an axial zone, which is delineated by the Reykjanes, West and North Volcanic Zones (RVZ, WVZ, NVZ) and the East Volcanic Zone (EVZ), which is growing in length by propagation to the southwest through pre-existing crust. These zones are connected across central Iceland by the Mid-Iceland Belt (MIB). Other volcanically active areas are the two intraplate belts of Öræfajökull (ÖVB) and Snæfellsnes (SVB). The principal structure of the volcanic zones are the 30 volcanic systems, where 12 are comprised of a fissure swarm and a central volcano, 7 of a central volcano, 9 of a fissure swarm and a central domain, and 2 are typified by a central domain alone.Volcanism in Iceland is unusually diverse for an oceanic island because of special geological and climatological circumstances. It features nearly all volcano types and eruption styles known on Earth. The first order grouping of volcanoes is in accordance with recurrence of eruptions on the same vent system and is divided into central volcanoes (polygenetic) and basalt volcanoes (monogenetic). The basalt volcanoes are categorized further in accordance with vent geometry (circular or linear), type of vent accumulation, characteristic style of eruption and volcanic environment (i.e. subaerial, subglacial, submarine).Eruptions are broadly grouped into effusive eruptions where >95% of the erupted magma is lava, explosive eruptions if >95% of the erupted magma is tephra (volume calculated as dense rock equivalent, DRE), and mixed eruptions if the ratio of lava to tephra occupy the range in between these two end-members. Although basaltic volcanism dominates, the activity in historical time (i.e. last 11 centuries) features expulsion of basalt, andesite, dacite and rhyolite magmas that have produced effusive eruptions of Hawaiian and flood lava magnitudes, mixed eruptions featuring phases of Strombolian to Plinian intensities, and explosive phreatomagmatic and magmatic eruptions spanning almost the entire intensity scale; from Surtseyan to Phreatoplinian in case of “wet” eruptions and Strombolian to Plinian in terms of “dry” eruptions. In historical time the magma volume extruded by individual eruptions ranges from ∼1 m³ to ∼20 km³ DRE, reflecting variable magma compositions, effusion rates and eruption durations.All together 205 eruptive events have been identified in historical time by detailed mapping and dating of events along with extensive research on documentation of eruptions in historical chronicles. Of these 205 events, 192 represent individual eruptions and 13 are classified as “Fires”, which include two or more eruptions defining an episode of volcanic activity that lasts for months to years. Of the 159 eruptions verified by identification of their products 124 are explosive, effusive eruptions are 14 and mixed eruptions are 21. Eruptions listed as reported-only are 33. Eight of the Fires are predominantly effusive and the remaining five include explosive activity that produced extensive tephra layers. The record indicates an average of 20–25 eruptions per century in Iceland, but eruption frequency has varied on time scale of decades. An apparent stepwise increase in eruption frequency is observed over the last 1100 years that reflects improved documentation of eruptive events with time. About 80% of the verified eruptions took place on the EVZ where the four most active volcanic systems (Grímsvötn, Bárdarbunga–Veidivötn, Hekla and Katla) are located and 9%, 5%, 1% and 0.5% on the RVZ–WVZ, NVZ, ÖVB, and SVB, respectively. Source volcano for ∼4.5% of the eruptions is not known.Magma productivity over 1100 years equals about 87 km³ DRE with basaltic magma accounting for about 79% and intermediate and acid magma accounting for 16% and 5%, respectively. Productivity is by far highest on the EVZ where 71 km³ (∼82%) were erupted, with three flood lava eruptions accounting for more than one half of that volume. RVZ–WVZ accounts for 13% of the magma and the NWZ and the intraplate belts for 2.5% each. Collectively the axial zone (RVZ, WVZ, NVZ) has only erupted 15–16% of total magma volume in the last 1130 years.     

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.     

Monogenetic vulcanism in sierra Chichinautzin,Mexico

The variation in the activity patterns of the Chichinautzin volcanic rocks is discussed. This sequence of lavas and pyroclastic deposits is located in the central part of the Mexican Volcanic Belt, directly south of Mexico City, and is typical of its Quaternary monogenetic vulcanism. One-hundred and fourty-six volcanoes and their deposits covering 952 km² were mapped. Cone density is 0.15 km² with heights ranging from to 315 m and crater diameters from 50 to 750 m. Ratios of cone height/diameter decreased from 0.20 to 0.12 with age. Basal diameters varied from 0.1 km to 2 km. Lavas are mainly blocky andesites but some dacites and basalts were found. Lengths of flows range from 1.0 to 21.5 km with heights of 0.5 to 300 m and aspect rations of 21.4 to 350. Three types of volcanic structures are found in the area: scoria cones, lavas cones and thick flows lacking a cone. Pyroclastic deposits are basically Strombolian although some deposits were produced by more violent activity and lava cones seem to have formed by activity transitional to Hawaiian-type vulcanism. Therre is a dominant E-W trend shown mainly by the orientation of cone clusters. The Chichinautzin volcanic centers are compared to the monogenetic volcanoes of the Toluca and Paricutin areas which are similar.     

Changing eruptive styles in basaltic explosive volcanism: Examples from Croscat complex scoria cone, Garrotxa Volcanic Field (NE Iberian Peninsula)

The Croscat pyroclastic succession has been analysed to investigate the transition between different eruptive styles in basaltic monogenetic volcanoes, with particular emphasis on the role of phreatomagmatism in triggering Violent Strombolian eruptions. Croscat volcano, an 11 ka basaltic complex scoria cone in the Quaternary Garrotxa Volcanic Field (GVF) shows pyroclastic deposits related both to magmatic and phreatomagmatic explosions.Lithofacies analysis, grain size distribution, chemical composition, glass shard morphologies, vesicularity, bubble-number density and crystallinity of the Croscat pyroclastic succession have been used to characterize the different eruptive styles. Eruptions at Croscat began with fissural Hawaiian-type fountaining that rapidly changed to eruption types transitional between Hawaiian and Strombolian from a central vent. A first phreatomagmatic phase occurred by the interaction between magma and water from a shallow aquifer system at the waning of the Hawaiian- and Strombolian-types stage. A Violent Strombolian explosion then occurred, producing a widespread (8 km²), voluminous tephra blanket. The related deposits are characterized by the presence of wood-shaped, highly vesicular scoriae. Glass-bearing xenoliths (buchites) are also present within the deposit. At the waning of the Violent Strombolian phase a second phreatomagmatic phase occurred, producing a second voluminous deposit dispersed over 8.4 km². The eruption ended with a lava flow emission and consequent breaching of the western-side of the volcano. Our data suggest that the Croscat Violent Strombolian phase was related to the ascent of deeper, crystal-poor, highly vesicular magma under fast decompression rate. Particles and vesicles elongation and brittle failure observed in the wood-shaped clasts indicate that fragmentation during Violent Strombolian phase was enhanced by high strain-rate of the magma within the conduit.     

Volcanic Tremor at Mt. Etna,Italy, Preceding and Accompanying the Eruption of July – August, 2001

The July 17 – August 9, 2001 flank eruption of Mt. Etna was preceded and accompanied by remarkable changes in volcanic tremor. Based on the records of stations belonging to the permanent seismic network deployed on the volcano, we analyze amplitude and frequency content of the seismic signal. We find considerable changes in the volcanic tremor which mark the transition to different styles of eruptive activity, e.g., lava fountains, phreatomagmatic activity, Strombolian explosions. In particular, the frequency content of the signal decreases from 5 Hz to 3 Hz at our reference station ETF during episodes of lava fountains, and further decreases at about 2 Hz throughout phases of intense lava emission. The frequency content and the ratios of the signal amplitude allow us to distinguish three seismic sources, i.e., the peripheral dike which fed the eruption, the reservoir which fed the lava fountains, and the central conduit. Based on the analysis of the amplitude decay of the signal, we highlight the migration of the dike from a depth of ca. 5 km to about 1 km between July 10 and 12. After the onset of the effusive phase, the distribution of the amplitude decay at our stations can be interpreted as the overall result of sources located within the first half kilometer from the surface. Although on a qualitative basis, our findings shed some light on the complex feeding system of Mt. Etna, and integrate other volcanological and geophysical studies which tackle the problem of magma replenishment for the July–August, 2001 flank eruption. We conclude that volcanic tremor is fundamental in monitoring Mt. Etna, not only as a marker of the different sources which act within the volcano edifice, but also of the diverse styles of eruptive activity. An erratum to this article is available at .     

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.     

petrological and geochemical characteristics of quarternary volcanic rocks in haixing area,eastern north china

Field investigation and lab analysis on samples were carried out for Quaternary volcanoes, including Xiaoshan volcano, Dashan volcano and Bianzhuang hidden volcano, in Haixing area, east of North China. Results show that Xiaoshan volcano with the eruptive material of volcanic scoria, crystal fragments and volcanic ash is a maar volcano, the eruptive pattern is pheatomagmatic eruption, and the influence scope is near the crater. Dashan volcano exploded in the early stage, and then the magma intruded, forming the volcanic neck. The eruption strength and scale are limited, and the eruptive materials are scoria, volcanic agglomerate and dense lava neck. The volcanic rocks in Bianzhuang are porosity and dense volcanic rocks and volcanic breccia, reflecting the pattern of weak explosive eruption and lava flow, and the K-Ar age dating on volcanic rocks indicates that the eruption happened in early Pleistocene. Xiaoshan volcanic scoria and Bianzhuang hidden volcanic rocks are mainly basaltic, Dashan volcanic rocks with lower SiO₂ content are nephelinite in composition. Their oxide contents have no linear relationship, indicating that there is no magma evolution relationship between these magmas from the three places. Three volcanic rocks all have enrichment of light rare earth. The Bianzhuang volcanic rocks are rich in large ion lithophile elements, and have no high field strength elements Zr and Hf, Ti losses. The volcanic materials from Xiaoshan and Dashan are intensively rich in Th, U, Nb and Ta, and significantly poor in K and Ti. Although the magmas from these three places in Haixing area may all come from asthenosphere, the volcanic materials have different petrological and geochemical features, and relatively independent volcanic structures, therefore, they experienced different magma processes.     

五大连池火山群最新喷发时间的热释光(TL)年代学证据

首次利用ＴＬ测年技术,测定了五大连池火山群中两座最新喷发的火山熔岩中的烘烤捕虏体的年代。测定结果（（２６４±１９）ａＢ．Ｐ．,（２７３±１９）ａＢ．Ｐ．）在误差范围内与喷发的历史记载（１７１９～１７２１年）一致,表明这两座火山是同期喷发的产物,同时也表明利用ＴＬ测年技术能比较准确地测定年轻火山活动的年代,它为恢复火山最新喷发历史的研究提供了一种比较可靠的年代测定途径     

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.     

Volcanic tremor at Mt. Etna: Inferences on magma dynamics during effusive and explosive activity

During 1999, the volcanic activity at Mt. Etna was both explosive and effusive at the summit craters: Strombolian activity, lava fountains and lava flows affected different areas of the volcano, involving three of the four summit craters. Results from analysis of the 1999 volcanic tremor features are shown at two different time scales. First, the long-term time variation of the features of the volcanic tremor (including spectral and polarization parameters), during the entire year, was compared with the evolution of the eruptive activity. This approach demonstrated the good agreement between tremor data and observed eruptive activity; the activation of different tremor sources was suggested. Then, a more refined analysis of the volcanic tremor, recorded during 14 lava fountain eruptions, was performed. In particular, a shift of the dominant frequencies towards lower values was noted which corresponds with increasing explosive activity. Similar behaviour in the frequency content has already been observed in other explosive eruptions at Mt. Etna as well as on other volcanoes. This behaviour has been explained in terms of either an increase in the tremor source dimension or a decrease in the sound speed in the magma within the conduit. These results confirm that the volcanic tremor is a powerful tool for better understanding the physical processes controlling explosive eruptions at Mt. Etna volcano.     

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.     

Intraglacial volcanism in the Western Volcanic Zone, Iceland

The Western Volcanic Zone in Iceland (64.19° to 65.22° N) has the morphological characteristics of a distinct Mid-Atlantic ridge segment. This volcanic zone was mapped at a scale of 1:36.000, and 258 intraglacial monogenetic volcanoes from the Late Pleistocene (0.01–0.78?Ma) were identified and investigated. The zone is characterized by infrequent comparatively large volcanic eruptions and the overall volcanic activity appears to have been low throughout the Late Pleistocene. Tholeiitic basaltic rocks dominate in the Western Volcanic Zone with about 0.5?vol.?% of intermediate and silicic rocks. The basalts divide into picrites, olivine tholeiites, and tholeiites. Three main eruptive phases can be distinguished in the intraglacial volcanoes: an effusive deep-water lava phase producing basal pillow lavas, an explosive shallow-water phase producing hyaloclastites and an effusive subaerial capping lava phase. Three evolutionary stages therefore charcterize these volcanoes; late dykes and irregular minor intrusions could be added as the fourth main stage. These intrusions are potential heat sources for short-lived hydrothermal systems and may play an important role in the final shaping of the volcanoes. Substantial parts of the hyaloclastites of each unit are proximal sedimentary deposits. The intraglacial volcanoes divide into two main morphological groups, ridge-shaped volcanoes, i.e., tindars (including pillow lava ridges) and subrectangular volcanoes, i.e., tuyas and hyaloclastite or pillow lava mounds. The volume of the tuyas is generally much larger than that of the tindars. The largest tuya, Eiríksj?kull, is about 48?km³ and therefore the largest known monogenetic volcano in Iceland. Many of the large volcanoes, both tuyas and tindars, show a similar, systematic range in geochemistry. The most primitive compositions were erupted first and the magmas then changed to more differentiated compositions. The ridge-shaped tindars clearly erupted from volcanic fissures and the more equi-dimensional tuyas mainly from a single crater. It is suggested that the morphology and structure of the intraglacial volcanos mainly depends on two factors, (a) tectonic control and (b) availability of magma at the time of eruption.