Types of eruptions of Etna volcano AD 1670–2003: implications for short-term eruptive behaviour     

Stefano Branca  Paola Del Carlo 《Bulletin of Volcanology》2005,67(8):732-742

Analysis of the historical records of Etnas eruptive activity for the past three centuries shows that, after the large 1669 eruption, a period of about 60 years of low-level activity followed. Starting from 1727, explosive activity (strombolian, lava fountaining and subplinian) at the summit crater increased exponentially to the present day. Since 1763, the frequency of flank eruptions also increased and this value remained high until 1960; afterward it further increased sharply. In fact, the number of summit and flank eruptions between 1961 and 2003 was four times greater than that of the pre-1960 period. This long-term trend of escalating activity rules out a pattern of cyclic behaviour of the volcano. We propose instead that the 1670–2003 period most likely characterises a single eruptive cycle which began after the large 1669 eruption and which is still continuing.On the basis of the eruptive style, two distinct types of flank eruptions are recognised: Class A and Class B. Class A eruptions are mostly effusive with associated weak strombolian activity; Class B eruptions are characterised by effusive activity accompanied by intense, long-lasting, strombolian and lava fountaining activity that produces copious tephra fallouts, as during the 2001 and 2002–2003 eruptions. Over the past three centuries, seven Class B eruptions have taken place with vents located mainly on the south-eastern flank, indicating that this sector of the volcano is a preferential zone for the intrusion of volatile-rich magma rising from the deeper region of the Etna plumbing system.Electronic Supplementary Material Supplementary material is available for this article at Editorial responsibility: M. Carroll  相似文献   

Significance of strain localization and fracturing in relation to hydrothermal mineralization at Mount Isa, Australia   总被引：1，自引：0，他引：1  

Klaus Gessner  Peter A. Jones  Andy R. Wilde  Michael Kühn   《Journal of Geochemical Exploration》2006,89(1-3):129

The rheology of layered meta-sedimentary rocks, and their orientation and position relative to major fault systems were the key controls on Proterozoic hydrothermal copper mineralization at Mount Isa, Australia. Compositional layering in the host rock partitioned mechanical behavior and strain, leading to selective permeability generation and the focusing of fluid flow. Shale layers preferentially failed by plastic shearing, whereas meta-siltstones remained elastic or failed in tension depending on magnitude of deformation and fluid pressure. Numerical simulations support the hypothesis that the orientation of layering and the proximity to major fault systems controlled fracturing and permeability increase in the Urquhart shale. The dilating shale provided a pathway for an upward-flowing, reduced basement fluid, from which quartz was precipitated during cooling. During a later event, the reactivation of steep structures provided access to surface derived oxidized metal-bearing brine, causing the precipitation of dolomite followed by chalcopyrite ore in the brecciated silicified shale.  相似文献   

Evolution of a Holocene delta driven by episodic sediment delivery and coseismic deformation, Puget Sound, Washington, USA     

WALTER A. BARNHARDT  BRIAN L. SHERROD† 《Sedimentology》2006,53(6):1211-1228

Episodic, large‐volume pulses of volcaniclastic sediment and coseismic subsidence of the coast have influenced the development of a late Holocene delta at southern Puget Sound. Multibeam bathymetry, ground‐penetrating radar (GPR) and vibracores were used to investigate the morphologic and stratigraphic evolution of the Nisqually River delta. Two fluvial–deltaic facies are recognized on the basis of GPR data and sedimentary characteristics in cores, which suggest partial emplacement from sediment‐rich floods that originated on Mount Rainier. Facies S consists of stacked, sheet‐like deposits of andesitic sand up to 4 m thick that are continuous across the entire width of the delta. Flat‐lying, highly reflective surfaces separate the sand sheets and comprise important facies boundaries. Beds of massive, pumice‐ and charcoal‐rich sand overlie one of the buried surfaces. Organic‐rich material from that surface, beneath the massive sand, yielded a radiocarbon age that is time‐correlative with a series of known eruptive events that generated lahars in the upper Nisqually River valley. Facies CF consists of linear sandbodies or palaeochannels incised into facies S on the lower delta plain. Radiocarbon ages of wood fragments in the sandy channel‐fill deposits also correlate in time to lahar deposits in upstream areas. Intrusive, sand‐filled dikes and sills indicate liquefaction caused by post‐depositional ground shaking related to earthquakes. Continued progradation of the delta into Puget Sound is currently balanced by tidal‐current reworking, which redistributes sediment into large fields of ebb‐ and flood‐oriented bedforms.  相似文献   

New formulae for estimating lava flow volumes at Mt. Etna Volcano, Sicily     

J. B. Murray  N. F. Stevens 《Bulletin of Volcanology》2000,61(8):515-526

 A new data set of Etna lava flows erupted since 1868 has been compiled from eight topographic maps of the volcano published at intervals since then. Volumes of 59 flows or groups of flows were measured from topographic difference maps. Most of these volumes are likely to be considerably more accurate than those published previously. We cut the number of flow volumes down to 25 by selecting those examples for which the volume of an individual eruption could be derived with the highest accuracy. This refined data set was searched for high correlations between flow volume and more directly measurable parameters. Only two parameters showed a correlation coefficient of 70% or greater: planimetric flow area A (70%) and duration of the eruption D (79%). If only short duration (<18 days) flows were used, flow length cubed, L³, had a correlation coefficient of 98%. Using combinations of measured parameters, much more significant correlations with volume were found. Dh had a correlation coefficient of 90% (h is the hydrostatic head of magma above the vent), and  , 92% (where W is mean width and E is the degree of topographic enclosure), and a combination of the two , 97%. These latter formulae were used to derive volumes of all eruptions back to 1868 to compare with those from the complete data set. Values determined from the formulae were, on average, lower by 16% (Dh), 7% (, and 19% . Received: 30 November 1998 / Accepted: 20 June 1999  相似文献   

A model for the magmatic–hydrothermal system at Mount Rainier,Washington, from seismic and geochemical observations     

S. C. Moran  D. R. Zimbelman  S. D. Malone 《Bulletin of Volcanology》2000,61(7):425-436

 Mount Rainier is one of the most seismically active volcanoes in the Cascade Range, with an average of one to two high-frequency volcano-tectonic (or VT) earthquakes occurring directly beneath the summit in a given month. Despite this level of seismicity, little is known about its cause. The VT earthquakes occur at a steady rate in several clusters below the inferred base of the Quaternary volcanic edifice. More than half of 18 focal mechanisms determined for these events are normal, and most stress axes deviate significantly from the regional stress field. We argue that these characteristics are most consistent with earthquakes in response to processes associated with circulation of fluids and magmatic gases within and below the base of the edifice.Circulation of these fluids and gases has weakened rock and reduced effective stress to the point that gravity-induced brittle fracture, due to the weight of the overlying edifice, can occur. Results from seismic tomography and rock, water, and gas geochemistry studies support this interpretation. We combine constraints from these studies into a model for the magmatic system that includes a large volume of hot rock (temperatures greater than the brittle–ductile transition) with small pockets of melt and/or hot fluids at depths of 8–18 km below the summit. We infer that fluids and heat from this volume reach the edifice via a narrow conduit, resulting in fumarolic activity at the summit, hydrothermal alteration of the edifice, and seismicity. Received: 10 February 1999 / Accepted: 26 June 1999  相似文献   

夏季念青唐古拉峰地区云结构分析          下载免费PDF全文

张晓  段克勤  刘焕才 《水科学进展》2015,26(2):196-200

利用2006—2010年夏季6~8月CloudSat资料对念青唐古拉峰地区云水分布和云类型特征进行分析,从而为研究高原天气过程与其水循环过程的相互作用提供理论依据。结果表明,云水含量垂直分布结构与云类型有关,而冰川区和非冰川区云类型差异主要为降水云类型不同,其中有冰川覆盖的高山上空降水云以深厚对流云为主,无冰川覆盖的高山降水云类型以雨层云为主。念青唐古拉峰南坡冰川区云水平均含量为0.14 g/m3,非冰川区云水平均含量为0.18 g/m3,一定程度说明来自孟加拉湾的水汽在经过冰川附近时,多会产生降水,反映了冰川对水汽传输的阻碍作用。  相似文献   

泰山南坡第四纪沉积物石英砂表面特征及沉积环境指示     

张建伟  王锡魁  袁西龙  郭慧玲  申卫星  陈岩  周长忠 《世界地质》2011,30(3):463-468

为考察泰山地区第四纪沉积环境,采用石英砂表面特征分析法,采集泰山南坡第四纪沉积物样品,经观察研究后,随机挑选W₁～W₆共6颗石英砂颗粒进行扫描电镜观察。除W₄外,其余颗粒表面均表现出形态不规则、边缘棱角清晰、具有多样的贝壳状断口和解理片特征,该特征所指示的沉积环境为冰川沉积环境条件。沉积物样品热释光测得年龄为30.54±2.59 ka B．P．与末次冰期主冰期时段具有时间上的吻合性。  相似文献   

旅游活动对九华山风景区大型土壤动物群落影响   总被引：1，自引：0，他引：1  

晋秀龙  陆林  巩劼  王立龙 《地理研究》2011,30(1):103-114

土壤动物是旅游活动对风景区生态系统影响最敏感的环境因素之一,也是旅游生态环境影响研究的重点。在九华山风景区6条主要游道附近选取了8条取样样带,样带设置与游道相垂直。每条样带按照距游道距离的不同设置4个取样样方,每个样方取枯枝落叶层并以5cm为一个层垂直向下取4层土样,用手拣法和网筛法拾取大型土壤动物。共捕获大型土壤动物2076头,隶属于3门8纲27个类群。运用多样性H′指数、集聚度等指标对不同样带和样方的不同土层中捕获的大型土壤动物类群和数量进行对比分析,结果表明:(1)九华山风景区大型土壤动物受到旅游活动的影响明显,旅游活动量越大土壤动物捕获量越少;(2)由近游道向远离游道的水平方向上大型土壤动物的类群和数量呈现递增规律;(3)在垂直分布上土壤动物向表层集聚明显,游憩活动的影响导致大型土壤动物类群和数量的表层聚集度由近游道样方向远离游道样方逐渐增大;(4)旅游活动量与各游道不同样方的捕获量呈现出不同的相关性。  相似文献   

Tree population dynamics of three altitudinal vegetation communities on Mount Cameroon (1989�C2004)     

Philip F.FORBOSEH  Terry C.H.SUNDERLAND  James A.COMISKEY  Michael BALINGA 《山地科学学报》2011,8(4):495-504

Changes in permanent sample plots in the lowland,submontane and montane forests on Mount Cameroon(4,095 m above sea level),an active volcano,are described for 15 years from 1989 to 2004.Throughout the study period,the stocking level of trees with a diameter at breast height(DBH) ≥ 10 cm in the three forests were lower than in pan-tropical stands suggesting a significant impact of volcanic and human-related activities on the vegetation communities on the mountain.Annual mortality rates in the submontane and montane forests were consistent with those reported for comparable altitudinal ranges in the Blue Mountains of Jamaica.The annual mortality rate was higher in the lowland forest than other lowland sites included.Divergence between recruitment and mortality rates was large suggesting that the three vegetation communities have not reached their climax.The seven-year difference in half-life of large trees(with a DBH ≥ 50 cm) in the submontane and montane forests suggests an altitudinal effect on turnover of larger trees that in turn contributes to the frequent small stature of high altitude forests.There was little evidence of an altitudinal effect on species turnover and growth rate.This finding supports generalizations about the zero effect of growth on the stature of high altitude trees.Understanding forest dynamics is crucially important in the management of tropical montane environmentsand in this instance particularly so given the recent creation of the Mount Cameroon National Park.  相似文献   

Cordierite-anthophyllite rocks from north-west Queensland, Australia: metamorphosed magnesian pelites     

J. REINHARDT 《Journal of Metamorphic Geology》1987,5(4):451-472

Abstract Cordierite-anthophyllite rocks and related cordierite-rich, talc-rich and chlorite-rich rocks occur in the Rosebud Syncline, north-west Queensland, Australia, as part of a Proterozoic metasedimentary sequence. Field relations and rock compositions attest the sedimentary origin of these rather unusual metamorphic rocks. Their chemical composition is comparable to that of unmetamorphosed, alkali- and Ca-poor pelites, which are associated with some evaporite deposits. Other occurrences of cordierite-anthophyllite rocks have commonly been interpreted as metamorphosed chloritic alteration products derived from mafic or felsic volcanics. A comparative chemical study, using analyses of cordierite-anthophyllite rocks from such alteration zones and analyses of unmetamorphosed magnesian pelites, demonstrates the general chemical similarity between these two rock groups of entirely different origin. However, distinct differences in major element relations help to distinguish these two genetic groups. Particularly useful are Al₂O₃–FeO–MgO plots, in which evaporitic pelites occupy the Fe-poor side. The highly magnesian metamorphic rocks from the Rosebud Syncline fall entirely into the compositional field of evaporitic clays and shales. Furthermore, analyses of relatively immobile trace elements give supporting evidence for the sedimentary origin of these cordierite-anthophyllite rocks. The correlation with trace element ranges of clays and shales is very good. However, the correlation with trace element ranges of mafic and felsic volcanics is poor, and major discrepancies occur with Cr, Ni, Co, Nb, Sc, Th and Ti. Thus, the magnesian metamorphics of the Rosebud Syncline appear to be derived from evaporitic clays rich in magnesian clay minerals, such as palygorskite, sepiolite, chlorite or corrensite. The complete metamorphic rock assemblage of interlayered calcareous, aluminous and magnesian rocks is interpreted as a metamorphosed carbonate-evaporite-pelite sequence.  相似文献