Seismic assessment of phreatic-explosion hazard at ‘La Fossa’ Volcano (Island of Vulcano,Italy)

Active volcanic areas are sometimes affected by phreatic eruptions, which are explosions due to the outbreak of a confined pocket of steam and gas without the direct involvement of molten magma. Eruptive activity at La Fossa Volcano, Island of Vulcano (Italy), typically starts with a phreatic explosion, continues as phreatomagmatic and turns into a late magmatic stage, depending on the reduction in the efficiency of magma-water interaction. The present risk is mainly related to a village located at the very foot of the active volcanic cone, which in summer seasons is inhabited by more than 10,000 people. Because the last eruptive episode occurred in 1888–1890, when no local instrumental control was performed, the scientific community does not have any information about the seismic phenomena which could precede the initial phreatic blast. Inferences from similar monitored volcanoes, which awakened in the last few years after more or less long quiescence, are therefore made to tentatively depict possible pre-eruptive seismic scenarios and also to evaluate the most appropriate surveillance and alert criteria to adopt. Appreciable signs of volcanic unrest have been detected in recent years, mainly related to modifications in the physical and chemical features of fumarolic fluids discharged at the active crater. Following the classification of local seismic shocks, the recording oflong-period events (peak frequency 2 Hz) should be taken into account as a realistic seismic warning of volcanic hazard enhancement. A likely short-term seismic precursor may be finally given by the appearance of the so-calledbanded tremor, a seismic signal likely produced by intermittent hydrothermal boiling accompanying the hydraulic fracturing of rocks at the top of a pressurized aquifer. Real-time recognition of tremor can be therefore very useful to avoid, at least, injuries to volcanologists and visitors working or staying on the most dangerous sectors of the volcano.     

长白山火山活动历史、岩浆演化与喷发机制探讨

广义的长白山火山在我国境内包括著名的天池火山、望天鹅火山、图们江火山和龙岗火山,是我国最大的第四纪火山岩分布区。图们江火山和望天鹅火山活动都始于上新世,喷发活动分别介于上新世—中更新世（5.5~0.19 Ma）和上新世—早更新世（4.77 ~2.12 Ma）。天池火山和龙岗火山属于第四纪火山,喷发活动从早更新世（~2 Ma）持续到全新世。图们江火山岩为溢流式喷发的拉斑玄武岩,望天鹅火山、天池火山和龙岗火山母岩浆都是钾质粗面玄武岩,但经历了不同的演化过程。天池火山和望天鹅火山都经历了钾质粗面玄武岩造盾、粗面岩造锥和晚期碱性酸性岩浆（碱流岩和碱性流纹岩）的喷发;龙岗火山来自地幔的钾质粗面玄武岩浆则未经演化和混染直接喷出地表。图们江火山岩以溢流式喷发的拉斑玄武岩为主,少量玄武质粗安岩等。天池火山造盾之后,地壳岩浆房和地幔岩浆房具互动式喷发特点,来自地幔的钾质粗面玄武岩浆一方面在天池火山锥体内外形成诸多小火山渣锥,另一方面持续补给地壳岩浆房发生岩浆分离结晶作用和混合作用,分别导致双峰式火山岩分布特征和触发千年大喷发。火山岩微量元素和Sr-Nd-Pb同位素示踪揭示,长白山东（图们江火山、望天鹅火山和天池火山）、西（龙岗火山）两区显示地幔非均一性,东区岩浆源区具有软流圈地幔与富集岩石圈地幔混合特征,西区岩浆源区具有相对亏损的较原始地幔特征。西太平洋板块俯冲—东北亚大陆弧后引张是长白山火山活动的动力学机制。     

GIS and Volcanic Risk Management

Volcanic catastrophes constitute a majorproblem in many developing and developed countries. Inrecent years population growth and the expansion ofsettlements and basic supply lines (e.g., water, gas,etc.) have greatly increased the impact of volcanicdisasters. Correct land-use planning is fundamental inminimising both loss of life and damage to property.In this contribution Geographical Information Systems(GIS), linked with remote sensing technology andtelecommunications/warning systems, have emerged asone of the most promising tools to support thedecision-making process. Some GIS are presented fortwo volcanic areas in Italy, Mt. Etna and Vesuvius.GIS role in risk management is then discussed, keepingin mind the different volcanic scenarios of effusiveand explosive phenomena. Mt. Etna system covers alarge area (more than 1,000 km²) potentiallyaffected by effusive phenomena (lava flows) whichcause damage to both houses and properties in general.No risk to life is expected. The time-scales of lavaflows allow, at least in principle, modification ofthe lava path by the building of artificial barriers.Vesuvius shows typically an explosive behaviour. Inthe case of a medium size explosive eruption, 600,000people would potentially have to be evacuated from anarea of about 200 km² around the Volcano, sincethey are exposed to ruinous, very fast phenomena likepyroclastic surges and flows, lahars, ash fallout,etc. Ash fallout and floods/lahars are also expectedin distal areas, between Vesuvius and Avellino,downwind of the volcano. GIS include digital elevationmodels, satellite images, volcanic hazard maps andvector data on natural and artificial features (energysupply lines, strategic buildings, roads, railways,etc.). The nature and the level of detail in the twodata bases are different, on the basis of thedifferent expected volcanic phenomena. The GIS havebeen planned: (a) for volcanic risk mitigation (hazard,value, vulnerability and risk map assessing), (b) toprovide suitable tools during an impending crisis, (c)to provide a basis for emergency plans.     

火山喷爆与太湖成因

以详尽野外地质成果与实物证据首次发现湖内最新的地质事件——火山喷爆活动, 并划定该事件的时代、地点, 重新认识太湖地区、特别是自中生代以来的地质演变史、演变方式, 勾划出太湖湖盆形成的前提条件、内在因果原因.其后采用了本地区第四系变化、考古成果、古脊椎动物化石记录与古籍记载等丰富资料证明该区实际存在的沉降现象.沉降速率、成湖过程与成湖时代.最后以详实资料阐述了太湖及其周边未来演变规律以供地方规划与发展经济所利用.      

Magma Chamber Process of Post-collisional Magmatism: Insight from Textural and Elemental Characteristics of Plagioclase from the Tatun Volcanic Group, Northern Taiwan Volcanic Zone

The Taiwan mountain belt, one of the youngest orogenies in the world, is caused by the collision of the Luzon arc with the Eurasian margin, which leads to post-collisional extension and magmatism in the Northern Taiwan Volcanic Zone (NTVZ). The magma chamber process in this region has not previously been elucidated in detail. In this paper, the textural and compositional features of plagioclase phenocrysts in basalt from the Tatun Volcanic Group (TTVG) were studied to restrict the dynamics of magma system. Results show that the magma melts in TTVG are mainly sourced from the underlying MORB-like mantle wedge but influenced by incorporation of subduction components, causing the elevated Sr/Y and Ba/Y ratios in magma melts. The subduction components are mainly transported in the form of sediment melt. The plagioclase phenocrysts in the TTVG volcanic rocks are generally coarsely core-sieved with a clear rim. The An contents in the rims of plagioclase are much lower than those of cores, and elevated FeO concentrations are detected in the plagioclase rims. We propose there exists a double-layer magma chamber in this region. The core of the plagioclase was crystalized in the deeper quiescent magma chamber (~21 km), which was subsequently partially dissolved during the ascent of magma melt under H2O-undersaturated condition, forming the typical coarsely sieved texture and synneusis. When this crystal-rich melt migrates into the shallower chamber, water saturation is reached and more sodic plagioclase formed as the rim of phenocryst. Due to the considerably higher fO2 in the shallow chamber than in the deeper one, and the distribution of Fe between plagioclase and melt positively correlates with fO2, the FeO content in the plagioclase rim elevates in conjunction with increasing fO2.     

Volcanic Eruption Induced Floods. A Rainfall-Runoff Model Applied to the Vesuvian Region (Italy)

Explosive events are commonly accompanied or followed byheavy rains. These eruption-induced storms together with thedeposition of large amounts of ash contribute to destabilise thehydrological cycle in the areas affected by volcanic eruptions.Flooding of the region surrounding the active volcano can easilyfollow, increasing the complexity of the volcanic crisis and itsmanagement. This is particularly true in the case of Vesuvius,that is not only characterized by a dramatic volcanic hazard, butit is also located within an area that is normally prone to flood hazard. A complete assessment of the impact associated with explosive volcaniceruptions should involve a flood hazard assessment for the region.This work represents a first attempt to address the problem: atopographically based rainfall-runoff model was here applied to theVesuvian area where two main sub-basins were analysed. The modelwas applied to evaluate the role of selected parameters on the totaldischarge at the basins' outlet. These parameters were chosen amongthose likely to be affected by an explosive event and were variedthrough a reasonable range. Results confirm that the deposition oflarge amounts of ash can affect the temporal evolution of the dischargeand its maximum value, for a given precipitation event. The simulationspresented outline the need for a detailed flood forecasting study for theVesuvian area, that should be included within the hazard mitigation strategies.     

闽西南廖天山破火山活动过程与岩石成因：锆石U- Pb、Hf同位素及微量元素制约

火山是岩浆活动在地表的主要呈现形式,古老火山由于剥蚀作用出露多阶段火山喷发产物及岩浆通道、岩浆房等,从而为揭示岩浆房内部结构和成分演化提供重要窗口。锆石在长期结晶生长过程中,能够记录岩浆系统的结晶分异、晶体- 熔体分离和岩浆补给等过程。闽西南晚侏罗世廖天山破火山是保存最完整的中国东南部晚中生代早期代表性破火山之一,本文对其开展锆石U- Pb年代学、Lu- Hf同位素组成和微量元素成分分析,以期揭示其火山活动时序、岩浆来源和演化过程。廖天山火山活动具有阶段性和多期次性,一段火山活动开始于约161. 5±0. 7 Ma,喷发断续且规模较为有限;二段火山岩形成于159. 9±0. 9~156. 9±0. 8 Ma,该时段喷发产物规模巨大,构成破火山主体;最后岩浆在153. 2±0. 7 Ma沿火山通道侵出形成流纹斑岩岩穹,标志着火山活动的结束。锆石Lu- Hf同位素组成显示,廖天山破火山岩浆主要源于古元古代地壳基底的部分重熔,但有不同程度的亏损幔源物质贡献,且在三个火山活动阶段中幔源物质参与比例不同,二段火山岩中相对较低。不同批次岩浆可能从源区分别上升,在深部岩浆房发生岩浆混合,而后在浅部岩浆房短暂留存发生分离结晶作用。根据岩浆来源的变化和火山岩相组合,我们认为廖天山火山活动形成于相对挤压转向伸展的构造环境,受制于晚侏罗世古太平洋板块俯冲以及板片俯冲角度变化的地球动力学背景。     

Volcanic natural dams: identification,stability, and secondary effects

Volcanic activity can enhance several secondary effects, including the formation of one or more natural dams. A common example is from volcanic collapse, where huge mass volumes are rapidly emplaced, obstructing the drainage around a volcano. Their duration depends on the volume of the obstructing mass, inflow rate, and on its textural characteristics. A block facies of a debris avalanche produces durable and permeable dams that consist of decimeter to meter-sized blocks without matrix, whereas a mixed facies is easily eroded after overflowing. Analysis of the sedimentological characteristics of different volcaniclastic deposits that formed natural dams indicate that a mean grain size (Md) equal to −1 phi divides the field of debris avalanche dams (Md < −1 phi) from that formed from other types of volcanic deposits. In addition, the matrix proportion of dams formed by debris avalanches are less than the 50% and the percentage of mud fraction is highly variable, up to 30%. Combining the granulometric textures with duration time of the dam shows no clear relation. Dam durability is probably more dependent on the volume of the lake and the inflow rate. Only in some cases, as mud fraction increases is the blockage also less durable because the lower permeability favors rapid infilling. The texture of the dam also determines the types of secondary flows that originate by their breakdown. These vary from cohesive debris flow to hyperconcentrated flow, representing different hazards due to their magnitude and their different behavior downstream.     

北秦岭火山岩地球化学特征,成因和构造环境

本文研究了北秦岭及华北地台南缘火山岩约500件岩石化学、120件微量元素和135件稀土元素样品数据。熊耳群、大红口组火山岩属B类的地壳混染型,为大陆裂谷环境;宽坪群、秦岭群变拉斑玄武岩建造属A类的幔源型,为海槽环境;二郎坪群、丹凤群细碧角斑岩建造属C+A、C类的壳幔混合型,为海盆环境。     

The Petrology of the Rotoiti Eruption Sequence, Taupo Volcanic Zone: an Example of Fractionation and Mixing in a Rhyolitic System

The Rotoiti eruption from the Taupo Volcanic Zone (TVZ) in northernNew Zealand produced voluminous pyroclastic deposits. The ferromagnesianmineral assemblage in these dominantly consists of cummingtonite+ hornblende + orthopyroxene with uniform magnesium/iron ratios;a second assemblage of biotite + hornblende + orthopyroxene,also with uniform Fe/Mg ratios, appears midway through the eruptionsequence and, thereafter, increases in abundance. These contrastingmineral assemblages, together with pumice clast and groundmassglass compositions, provide evidence for mingling of two discretemagmas. Similarities in the chemical characteristics of thetwo magmas suggest that they developed from a similar source.The eruption initially tapped relatively homogeneous magma thatwas erupted throughout most of this phase of activity. The middlestages of the eruption included some mixed magma. The finalstages of the eruption were dominated by a second magma composition,which was probably injected into the bottom of the main magmabody as the eruption proceeded. The source that fed the eruptionwas complex, and discrete magma bodies existed and evolved separatelyprior to the eruption. We conclude that eruptions in the TVZare fed from a diffuse upper-crustal zone of partially interconnected,and at times physically separate, magma bodies rather than fromcentralized and necessarily large long-lived magma chambers. KEY WORDS: Taupo Volcanic Zone; Okataina Volcanic Centre; Rotoiti eruption; rhyolite system; magma mixing     

五大连池钾质火山岩中的壳源捕虏体特征及地质意义

黑龙江省五大连池钾质火山岩石中普遍含有壳源捕虏体和捕虏晶,并多赋存于火山集块、浮岩砾、火山渣等各级火山碎屑物及盾状台地的表皮相到内部相火山熔岩中。花岗岩质捕虏体一般大小为5～50cm,捕虏晶一般为0.2～5mm,成分以二长花岗岩为主,少见花岗闪长岩及黑云母英云闪长岩和少量砂岩捕虏体。石英、斜长石、少数钾长石捕虏晶见于各期火山碎屑(物)和熔岩中,其中以石英捕虏晶最为常见。壳源捕虏体中的代表性矿物及浅色玻璃电子探针分析结果表明,火山岩SiO2偏高是岩浆上涌喷发过程中捕获了大量壳源物质的结果。五大连池火山群富钾火山岩岩石化学、地球化学长期保持均一,说明其岩浆直接来自EMⅠ型钾质地幔,岩浆在上升过程中遭受了地壳物质的明显混染。     

广东二叠纪火山事件地层学研究

广东二叠纪曾发生两期规模较大的火山喷发事件。第一期发生在茅口期与吴家坪期之间,影响范围涉及粤中、粤西等地;第二期主要发生于长兴期,可细分为３次喷发,其中最后一次于二叠纪未、三叠纪纪实发生,在粤东、粤北、粤西北均有影响,规模最大。伴随两期火山喷发,生物发生显著的更替和绝灭,并且形成的岩石等时性强,对比精度高。系统研究火山事件的岩石学特征、生物绝灭效应、火山喷发成因,对建立广东二岩石等年代地层单位,厘     

Volcanic Risk Assessment and Mapping in the Vesuvian Area Using GIS

This paper assesses the risk to people and property from lava flow hazard in the Vesuvian area of Italy using a Geographical Information System (GIS). The intense urbanisation and dense population near Mt. Vesuvius make the area very hazardous. Due to the large amount of available data, GIS is an essential tool to facilitate risk evaluation and constant monitoring of the zone. This analysis is based mainly on a lava flow hazard map of Mt. Vesuvius, determined from volcanic activity between 1631 and 1944. A land-use zonation map of the area was created in order to show areal distribution of the resources, built-up centres and population. For each of the 17 municipalities in the area, demographic and urban data were entered into the GIS database and linked to each appropriate geographic unit in order to create a set of reference maps at the 1:50 000 scale. The lava flow hazard map was overlain on the land use map, and spatial and numerical information of risk were extracted from the resulting maps.     

五大连池第四纪火山岩层序划分及其构造与生态意义

大量前人成果和1:5万区调钻孔资料证实,五大连池第四纪火山地层属于水平岩层.因此,该地区第四纪火山地层的划分,是在地表岩石层序与钻孔岩石层序充分划分与对比的基础上进行的.从沉积角度看,五大连池火山地层属于松嫩盆地连续沉积过程中的第四纪短暂幕式火山喷发所形成的水平岩层.本文结合K-Ar同位素测年新资料,将该区第四纪岩石地层重新划分为11个组级地层单位.其中的火山岩石地层可以自下而上划分为:下更新统焦得布玄武岩(1.214—1.113 Ma);中更新统尾山玄武岩(0.62—0.285 Ma);中更新统笔架山玄武岩(0.24—0.132 Ma)和全新统老黑山玄武岩(距今290～288 a),对夹于其间的正常沉积地层也进行了相应的划分.对层状火山岩层序的层位划分和空间分布研究对于理解五大连池火山群的构造背景和生态环境具有特别重要的意义,五大连池火山群处于中国大陆内部的大同—大兴安岭火山岩带的最北东端,是地幔流体向北东方向流动的最前缘;这类富钾的碱性玄武岩的火山喷发活动对东北富饶的黑土地的形成具有重要贡献,火山岩在嫩江平原上塑造的台地和火山锥地形地貌对生态多样性和优质地下水的生态要素具有重要影响.     

辽西彰武大四家子地区中生代火山岩锆石U-Pb年代学及地球化学特征

通过对辽西彰武县以东大四家子乡高城窝堡村义县组标准剖面中生代火山岩锆石U-Pb年代学研究表明, 其火山岩年龄为122.4±0.4Ma, 属早白垩世.对该区域23件典型火山岩样品的地球化学研究表明, 除3件流纹岩样品外, 其余样品具有高镁埃达克岩地球化学特征(SiO₂=56.46%~65.14%、Al₂O₃=14.60%~17.19%、Mg#=50~59、Sr=501~700μg/g、Yb=1.04~1.54μg/g、Y=12.0~17.5μg/g、Eu/Eu*=0.85~0.97、Sr/Y=29~46、La_N/Yb_N=13~28), 同位素上具有高的初始87Sr/86Sr(0.705464~0.705812)比值, 低的ε_Nd(122Ma)(-6.12~-12.80)值特征, 同时样品中存在具有反环带的辉石斑晶, 辉石中稀土元素含量分布存在着从核部到边部逐渐降低的趋势, 且存在负铕异常(Eu/Eu*=0.64~0.76).结合前人对彰武义县组下部火山岩的研究, 笔者倾向认为该套火山岩的成因是拆沉作用与岩浆混合作用共同作用的结果, 即拆沉作用导致软流圈地幔物质上涌加热下地壳形成的长英质岩浆, 与来自地幔由拆沉作用形成的埃达克质高镁安山岩浆混合形成.      

Volcanic Hazard in New Zealand: Scaling and Attenuation Relations for Tephra fall deposits from Taupo Volcano

This paper is a first step in developing a probabilistic hazard model for tephra fall deposits in New Zealand. The database consists of measurements of tephra thickness and eruptive volumefrom 32 past eruptive events at Taupo Volcano. From these are derivedrelations for the mean and maximum tephra thickness as a function ofevent volume and distance from the vent, and for the area enclosed byan isopach as a function of tephra thickness and volume. Thedirectional effects due to wind do not vary appreciably over thevolcanic region of New Zealand. The main feature of wind velocity is aflow to the east that becomes more consistent at higher altitudes. Thelarger the eruption, the more the location of the deposit of maximumthickness (the centre of deposit) tends to be displaced to the east ofvent. A directional attenuation relation about the centre of deposit isderived from the Taupo data. This, in combination with arelation for the position of the centre of deposit relative to thevent, provides a means of estimating the probability of a giventhickness of tephra fall deposit being exceeded at any distance anddirection from the vent in an eruption of given volume.     

碧口群不同岩片火山岩微量元素组成差异与古构造意义

对碧口群内部大茅坪和筏子坝2个岩片变火山岩微量元素的研究表明,二者在岩石组合、源区组成及产出的构造环境等方面均存在系统差异。大茅坪岩片主要由偏碱性与非碱性的玄武质岩石组成,二者分别具有类似OIB和MORB的微量元素组成,起源于相对富集和相对亏损的地幔,并受到陆源物质不同程度的混染。偏碱性与非碱性的玄武质岩石分别显示板内火山岩（WPB）与洋脊玄武岩（MORB）的属性,表明大茅坪岩片火山岩主要形成于裂谷－有限洋盆环境。筏子坝岩片由玄武质与中酸性火山岩组成,岩石化学显示明显的“双峰式”组成特征,二者分别具有类似MORB和壳源岩石的微量元素组成,为亏损地幔与壳源岩石部分熔融的产物,形成于大陆裂谷环境。根据不同火山岩构造属性及组合特征,推断碧口群主要形成于陆内裂谷－有限洋盆构造环境。     

福建浦城—三都澳火山喷发带早白垩世火山地层划分对比

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