The millennium eruption of Hekla in February 2000     

Ármann Höskuldsson  Níels Óskarsson  Rikke Pedersen  Karl Grönvold  Kristín Vogfjörð  Rósa Ólafsdóttir 《Bulletin of Volcanology》2007,70(2):169-182

The 18th historic eruption of Hekla started on 26 February, 2000. It was a short-lived but intense event, emitting basaltic andesitic (55.5 wt% SiO₂) pyroclastic fragments and lava. During the course of the eruption, monitoring was done by both instruments and direct observations, together providing unique insight into the current activity of Hekla. During the 12-day eruption, a total of 0.189 km³ DRE of magma was emitted. The eruptive fissure split into five segments. The segments at the highest altitude were active during the first hours, while the segments at lower altitude continued throughout the eruption. The eruption started in a highly explosive manner giving rise to a Subplinian eruptive column and consequent basaltic pyroclastic flows fed by column collapses. After the explosive phase reached its maximum, the eruption went through three more phases, namely fire-fountaining, Strombolian bursts and lava effusion. In this paper, we describe the course of events of the eruption of Hekla and the origin of its magma, and then show that the discharge rate can be linked to different style of eruptive activity, which are controlled by fissure geometry. We also show that the eruption phases observed at Hekla can be linked with inferred magma chamber overpressure prior to the eruption.  相似文献   

滇西腾冲中更新世英安质岩浆的爆发机制   总被引：2，自引：2，他引：0  

丁磊磊  刘嘉麒  郭正府  张磊 《岩石学报》2018,34(1):113-125

本文对腾冲马站钻孔的中酸性火山岩进行了岩相学、锆石SIMS U-Pb年代学及地球化学研究,确定其岩石类型、形成时代及岩浆喷发前的岩浆状态,从而揭示腾冲火山喷发机制。根据矿物组成将腾冲钻孔的中酸性火山岩分为两层:上层灰白色角闪熔岩(矿物组合:单斜辉石+斜方辉石+角闪石+黑云母+斜长石+钾长石+钛磁铁矿+磁铁矿+石英);下层黑色辉石熔岩(矿物组合:单斜辉石+斜方辉石+斜长石+钛磁铁矿+磁铁矿)。腾冲钻孔中酸性火山岩的锆石均呈半自形-自形,振荡环带明显,为岩浆成因。测年结果表明,灰白色角闪熔岩的结晶年龄为0.7Ma,黑色辉石熔岩中最年轻的锆石为0.6Ma;结合上覆中更新世粗面岩,我们推断其喷发时代为中更新世。地球化学显示角闪熔岩和辉石熔岩都为高钾钙碱性英安岩。相似的全岩和斑晶核部的地化特征,指示角闪英安岩和辉石英安岩来自同一个岩浆房。综合斑晶的地化特征及平衡结晶的温压条件,我们认为早期的岩浆房经历一次基性岩浆补给事件,导致火山爆发产生黑色辉石英安岩,后期岩浆房又经历一次酸性岩浆补给事件,导致火山爆发产生灰白色角闪英安岩。两次岩浆补给事件是导致火山爆发的直接原因。  相似文献   

Complex proximal deposition during the Plinian eruptions of 1912 at Novarupta,Alaska   总被引：2，自引：1，他引：1  

Bruce?F.?HoughtonEmail author  C.?J.?N.?Wilson  J.?Fierstein  W.?Hildreth 《Bulletin of Volcanology》2004,66(2):95-133

Proximal (<3 km) deposits from episodes II and III of the 60-h-long Novarupta 1912 eruption exhibit a very complex stratigraphy, the result of at least four transport regimes and diverse depositional mechanisms. They contrast with the relatively simple stratigraphy (and inferred emplacement mechanisms) for the previously documented, better known, medial–distal fall deposits and the Valley of Ten Thousand Smokes ignimbrite. The proximal products include alternations and mixtures of both locally and regionally dispersed fall ejecta, and numerous thin complex deposits of pyroclastic density currents (PDCs) with no regional analogs. The locally dispersed component of the fall deposits forms sector-confined wedges of material whose thicknesses halve radially from and concentrically about the vent over distances of 100–300 m (cf. several kilometers for the medial–distal fall deposits). This locally dispersed fall material (and many of the associated PDC deposits) is rich in andesitic and banded pumices and richer in shallow-derived wall-rock lithics in comparison with the coeval medial fall units of almost entirely dacitic composition. There are no marked contrasts in grain size in the near-vent deposits, however, between locally and widely dispersed beds, and all samples of the proximal fall deposits plot as a simple continuation of grain size trends for medial–distal samples. Associated PDC deposits form a spectrum of facies from fines-poor, avalanched beds through thin-bedded, landscape-mantling beds to channelized lobes of pumice-block-rich ignimbrite. The origins of the Novarupta near-vent deposits are considered within a spectrum of four transport regimes: (1) sustained buoyant plume, (2) fountaining with co-current flow, (3) fountaining with counter-current flow, and (4) direct lateral ejection. The Novarupta deposits suggest a model where buoyant, stable, regime-1 plumes characterized most of episodes II and III, but were accompanied by transient and variable partitioning of clasts into the other three regimes. Only one short period of vent blockage and cessation of the Plinian plume occurred, separating episodes II and III, which was followed by a single PDC interpreted as an overpressured "blast" involving direct lateral ejection. In contrast, regimes 2 and 3 were reflected by spasmodic sedimentation from the margins of the jet and perhaps lower plume, which were being strongly affected by short-lived instabilities. These instabilities in turn are inferred to be associated with heterogeneities in the mixture of gas and pyroclasts emerging from the vent. Of the parameters that control explosive eruptive behavior, only such sudden and asymmetrical changes in the particle concentration could operate on time scales sufficiently short to explain the rapid changes in the proximal 1912 products.Editorial responsibility: R. Cioni  相似文献   

Modelling of thermochemical plumes and implications for the origin of the Siberian traps     

Nickolay L. Dobretsov  Alexey A. Kirdyashkin  Anatoliy G. Kirdyashkin  Valery A. Vernikovsky  Igor N. Gladkov 《Lithos》2008,100(1-4):66-92

Thermochemical plumes form at the base of the lower mantle as a consequence of heat flow from the outer core and the presence of local chemical doping that decreases the melting temperature. Theoretical and experimental modelling of thermochemical plumes show that the diameter of a plume conduit remains practically constant during plume ascent. However, when the top of a plume reaches a refractory layer, whose melting temperature is higher than the melt temperature in the plume conduit, a mushroom-shaped plume head develops. Main parameters (melt viscosity, ascent time, ascent velocity, temperature differences in the plume conduit, and thermal power) are presented for a thermochemical plume ascending from the core–mantle boundary. In addition, the following relationships are developed: the pressure distribution in the plume conduit during the ascent of a plume, conditions for eruption-conduit formation, the effect of the P–T conditions and controls on the shape and size of a plume top, heat transfer between a thermochemical plume and the lithosphere (when the plume reaches the bottom of a refractory layer in the lithosphere), and eruption volume versus the time interval t₁ between plume formation and eruption. These relationships are used to determine thermal power and time t₁ for the Tunguska syneclise and the Siberian traps as a whole.

The Siberian and other trap provinces are characterized by giant volumes of lavas and sills formed a very short time period. Data permit a model for superplumes with three stages of formation: early (variable picrites and alkali basalts), main (tholeiite plateau basalts), and final (ultrabasic and alkaline lavas and intrusions). These stages reflect the evolution of a superplume from the ascent of one or several independent plumes, through the formation of thick lenses of mantle melts underplating the lithosphere and, finally, intrusion and extrusion of differentiated mantle melts. Synchronous syenite–granite intrusions and bimodal volcanism abundant in the margins of the Siberian traps are the result of melting of the lower crust at depths of 65–70 km under the effect of plume melts.  相似文献   

Tephra Fallout Models: The Effect of Different Source Shapes on Isomass Maps     

Leng L. Lim  Winston L. Sweatman  Robert McKibbin  Charles B. Connor 《Mathematical Geosciences》2008,40(2):147-157

Numerous tephra dispersion and sedimentation models rely on some abstraction of the volcanic plume to simplify forecasts of tephra accumulation as a function of the distance from the volcano. Here we present solutions to the commonly used advection–dispersion equation using a variety of source shapes: a point, horizontal and vertical lines, and a circular disk. These may be related to some volcanic plume structure, such as a strong plume (vertical line), umbrella cloud (circular disk), or co-ignimbrite plume (horizontal line), or can be used to build a more complex plume structure such as a series of circular disks to represent a buoyant weak plume. Basing parameters upon eruption data, we find that depositions for the horizontal source shapes are very similar but differ from the vertical line source deposition. We also compare the deposition from a series of stacked circular disk sources of increasing radius above the volcanic vent with that from a vertical line source.  相似文献   

长白山天池火山千年大喷发火山碎屑流堆积相特征     

衣健  王璞珺  单玄龙  CARNIEL Roberto  武成智  王寒非  孙松  郭剑南 《岩石学报》2020,36(11):3346-3362

发生于公元946年的长白山天池火山千年大喷发（Millennium Eruption,ME）形成的火山碎屑堆积物体积高达100~172km³,并可分为大规模的ME-Ⅰ和小规模的ME-Ⅱ两个喷发阶段。通过对围绕长白山天池火山53个典型露头剖面进行火山地质测量（单元构成、垂向堆积序列和堆积特征）,结合筛析法粒度分析、偏光显微镜成分分析,刻画了长白山千年大喷发火山碎屑流堆积物特征,探讨了相和亚相划分,并建立了火山碎屑流搬运和堆积模式。根据火山碎屑的堆积特征,将长白山千年大喷发火山碎屑流堆积分为峡谷充填火山碎屑流相（包括块状峡谷充填亚相和层状峡谷充填亚相）和火山碎屑流冲击扇相（包括扇头亚相和扇体亚相）等两相四亚相。峡谷充填火山碎屑流相主要发育在天池火山锥体周缘距离喷发中心8~23km左右范围内（坡度在15°~60°之间）的火山U型谷中;火山碎屑流冲积扇相主要发育在距离喷发中心23~45km左右范围内,地形相对平缓的熔岩台地处（坡度在5°~15°之间）,火山碎屑流的搬运不受地形限制,一般形成较大纵横比扇状堆积。块状峡谷充填亚相和扇体亚相以块状混杂堆积为主要特征,而层状峡谷充填亚相和扇头亚相则以多火山碎屑流单元垂向叠加为主要特征。多单元叠加现象是由搬运过程中火山碎屑流单元发生分离增生作用形成。根据火山碎屑流的最大分布范围和厚度,如果再次发生与长白山千年大喷发类似规模的普林尼式喷发,至少距长白山天池火山喷发中心45km范围内具有巨大的火山碎屑流灾害风险。该研究有利于进一步深入认识长白山千年大喷发火山碎屑流堆积物的空间分布特征和相变规律,对火山碎屑喷发灾害的预防具有指导作用。  相似文献   

Continuous monitoring of volcanic eruption dynamics: a review of various techniques and new results from a frequency-modulated radar Doppler system     

Ralf Seyfried  Matthias Hort 《Bulletin of Volcanology》1999,60(8):627-639

 In situ measurement of volcanic eruption velocities is one of the great challenges left in geophysical volcanology. In this paper we report on a new radar Doppler technique for monitoring volcanic eruption velocities. In comparison with techniques employed previously (e.g., photographic methods or acoustic Doppler measurements), this method allows continuous recordings of volcanic eruptions even during poor visibility. Also, radar Doppler instruments are usually light weight and energy efficient, which makes them superior to other Doppler techniques based on laser light or sound. The proposed new technique was successfully tested at Stromboli Volcano in late 1996 during a period of low activity. The recorded data allow a clear distinction between particles rising from the vent and particles falling back towards the vent. The mean eruption velocity was approximately 10 m/s. Most of the eruptions recorded by radar were correlated to seismic recordings. The correlation between the magnitude of the volcanic shocks and the eruption force index defined in the paper may provide new insights into magma transport in the conduit. Received: 15 May 1998 / Accepted: 15 December 1998  相似文献   

Physical volcanology and structural development of Cerro Azul Volcano, Isabela Island, Galápagos: implications for the development of Galápagos-type shield volcanoes     

Terry Naumann  Dennis Geist 《Bulletin of Volcanology》2000,61(8):497-514

3 ) erupted from circumferential vents near the summit. These flows are nearly an order of magnitude smaller in volume than the predominantly aa flows erupted from radial eruptive fissures near the break in slope (0.06–0.1 km³). The differences in volume and flow morphology with altitude are due to slower eruption rates from summit vents than from flank vents, which, in turn, are attributable to the different heights the magmas must ascend from shallow reservoirs. These observations support the contention that the steep upper flanks were constructed by the buildup of short lava flows rather than by the structural deformation of originally gently dipping flanks. In addition to the higher eruption rates, a subdued lower flank geometry is promoted by the deposition of lava deltas onto the shallow Galápagos platform on the western, northern, and eastern flanks of the volcano. ⁴⁰Ar/³⁹Ar geochronology and volume estimates show that, despite their morphologic differences, the growth of the western Galápagos shields has been nearly synchronous, precluding an evolutionary model for their development. The wide variations in elevation, volume, area, and the distribution of slope angles among the western volcanoes can be linked instead to different long-term eruption rates and, to a lesser degree, the position of each volcano relative to the edge of the Galápagos platform. Received: 24 September 1998 / Accepted: 7 August 1999  相似文献   

Reprint of Glass compositions and tempo of post-17 ka eruptions from the Afar Triangle recorded in sediments from lakes Ashenge and Hayk,Ethiopia     

《Quaternary Geochronology》2017

Numerous volcanoes in the Afar Triangle and adjacent Ethiopian Rift Valley have erupted during the Quaternary, depositing volcanic ash (tephra) horizons that have provided crucial chronology for archaeological sites in eastern Africa. However, late Pleistocene and Holocene tephras have hitherto been largely unstudied and the more recent volcanic history of Ethiopia remains poorly constrained. Here, we use sediments from lakes Ashenge and Hayk (Ethiopian Highlands) to construct the first <17 cal ka BP tephrostratigraphy for the Afar Triangle. The tephra record reveals 21 visible and crypto-tephra layers, and our new database of major and trace element glass compositions will aid the future identification of these tephra layers from proximal to distal locations. Tephra compositions include comendites, pantellerites and minor peraluminous and metaluminous rhyolites. Variable and distinct glass compositions of the tephra layers indicate they may have been erupted from as many as seven volcanoes, most likely located in the Afar Triangle. Between 15.3−1.6 cal. ka BP, explosive eruptions occurred at a return period of <1000 years. The majority of tephras are dated at 7.5−1.6 cal. ka BP, possibly reflecting a peak in regional volcanic activity. These findings demonstrate the potential and necessity for further study to construct a comprehensive tephra framework. Such tephrostratigraphic work will support the understanding of volcanic hazards in this rapidly developing region.  相似文献   

Vesiculation path of ascending magma in the 1983 and the 2000 eruptions of Miyakejima volcano, Japan   总被引：1，自引：0，他引：1  

T. Shimano  S. Nakada 《Bulletin of Volcanology》2006,68(6):549-566

The vesiculation of magma during the 1983 eruption of Miyakejima Volcano, Japan, is discussed based on systematic investigations of water content, vesicularity, and bubble size distribution for the products. The eruption is characterized by simultaneous lava effusion and explosive sub-plinian (‘dry’) eruptions with phreatomagmatic (‘wet’) explosions. The magmas are homogeneous in composition (basaltic andesite) and in initial water content (H₂O = 3.9±0.9 wt%), and residual groundmass water contents for all eruption styles are low (H₂O <0.4 wt%) suggestive of extensive dehydration of magma. For the scoria erupted during simultaneous ‘dry’ and ‘wet’ explosive eruptions, inverse correlation was observed between vesicularity and residual water content. This relation can be explained by equilibrium exsolution and expansion of ca. 0.3 wt% H₂O at shallow level with different times of quenching, and suggests that each scoria with different vesicularity, which was quenched at a different time, provides a snapshot of the vesiculation process near the point of fragmentation. The bubble size distribution (BSD) varies systematically with vesicularity, and total bubble number density reaches a maximum value at vesicularity Φ ∼ 0.5. At Φ  ∼ 0.5, a large number of bubbles are connected with each other, and the average thickness of bubble walls reaches the minimum value below which they would rupture. These facts suggest that vesiculation advanced by nucleation and growth of bubbles when Φ < 0.5, and then by expansion of large bubbles with coalescence of small ones for Φ > 0.5, when bubble connection becomes effective. Low vesicularity and low residual water content of lava and spatter (Φ  < 0.1, H₂O  < 0.1 wt%), and systematic decrease in bubble number density from scoria through spatter to lava with decrease in vesicularity suggest that effusive eruption is a consequence of complete degassing by bubble coalescence and separation from magma at shallow levels when magma ascent rate is slow.

T. ShimanoEmail:

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