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1.
松辽盆地营城组火山机构类型划分为碎屑岩火山机构、熔岩火山机构和复合火山机构3种。碎屑岩火山机构中火山通道相占9.3%,爆发相占55.3%,喷溢相占29.2%,侵出相占1.4%,火山沉积相占4.8%;火山机构半径为0.5～1.5 km,高度为50～220 m。熔岩火山机构中火山通道相占8.1%,爆发相占23.4%,喷溢相占62.6%,侵出相占5.1%,火山沉积相占0.8%;火山机构半径为1～2 km,高度为200～300 m。复合火山机构中火山通道相占4.9%,爆发相占34.4%,喷溢相占56.8%,侵出相占2.6%,火山沉积相占1.3%;火山机构半径为2～4 km,厚度为300～450 m。亚相控制储层流动单元的规模,碎屑岩火山机构的储层流动单元范围为0.3～1.5 km,厚度小于60 m。熔岩火山机构的储层流动单元范围为1～2 km,厚度小于80 m。复合火山机构的储层流动单元范围为1.5～3.0 km,厚度小于100 m。各类火山机构气藏开发时的井距和压裂方案均存在差异。  相似文献   

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3.
松辽盆地改造残留的古火山机构与现代火山机构的类比分析   总被引：20，自引：3，他引：17  
现代火山机构形态有盾状、锥状和穹状,可按喷发样式进一步划分为7种类型。据此分类,在松辽盆地周缘剖面及其北部徐家围子断陷区可识别出4类火山机构：盾状火山机构,由喷溢相熔岩组成,可夹有薄层爆发相火山碎屑岩;层火山机构,由互层的熔岩与火山碎屑岩组成,喷溢相与爆发相交替的序列明显;火山碎屑锥,几乎全部由火山碎屑（熔）岩组成,爆发相为主;熔岩穹丘由高粘度的流纹质、英安质熔岩堵塞火山口后缓慢挤出形成,喷溢相和侵出相发育,兼有火山通道相。盆地内埋藏火山机构最小坡度为3°,最大坡度为25°,底部直径为2～14 km,分布面积为4～50 km2,火山岩厚度为100～600 m;总体上呈现出数目多、个体规模小、受区域大断裂控制、具裂隙式-多中心喷发、彼此相互叠置的特征。火山岩岩性和岩相是控制松辽盆地古火山机构类型及形态的主要因素。  相似文献   

4.
三台地区下白垩统营城组一段岩石类型有隐爆角砾岩、角砾熔岩、熔结角砾岩、珍珠岩、流纹岩和凝灰岩,所属岩相为火山通道相、侵出相、喷溢相、爆发相。厘定古火山口位置的依据主要是通过分析隐爆角砾岩和珍珠岩的岩性岩相及其分布。整个火山机构划分为中心相组、近源相组和远源相组。中心相组即火山口相,主要包括火山通道相和侵出相,近源相组主要包括喷溢相,远源相组主要包括爆发相。相序（以火山口为基点）为火山通道相隐爆角砾岩亚相-侵出相中带亚相-侵出相内带亚相-喷溢相下部亚相-喷溢相中部亚相-爆发相热碎屑流亚相。近源相组较中心相组储层物性优越。其中喷溢相上部亚相是储层物性最好的岩相带。  相似文献   

5.
内蒙锡林浩特鸽子山火山地质研究   总被引：4，自引：3，他引：1  
鸽子山火山位于内蒙古自治区锡林浩特市东南,处于大兴安岭-大同新生代火山喷发带中段,是锡林浩特-阿巴嘎火山群中保存最为完好的一座玄武质火山。火山喷发物的分布面积约55km2,主要为降落火山渣、溅落熔结火山碎屑岩和熔岩流,成分主要为碧玄岩,晚期有少量的橄榄拉斑玄武岩,碧玄岩中含有较多二辉橄榄岩包体和辉石及歪长石巨晶。火山由锥体、熔岩流和火山碎屑席组成,锥体由早期的降落锥和晚期溅落锥复合而成。火山口经历多次塌陷而成为破火口。锥体西侧及北东侧出露两个仍保留了原始形态的熔岩溢出口,熔岩流类型为结壳熔岩,由多个岩流单元组成,局部地区的熔岩流中发育较多保存完好的喷气锥、喷气碟或喷气塔。火山碎屑席主要分布在锥体的东侧,厚度由锥体向外逐渐减薄。火山活动可分为早、晚两个阶段,早期为爆破式喷发,形成火山渣锥和碎屑席,属亚布里尼型喷发,晚期主要为溢流式喷发,形成溅落锥和大规模熔岩流,其活动时代为晚更新世末-全新世。  相似文献   

6.
云南腾冲大六冲火山机构的发现及意义   总被引：1，自引：0，他引：1  
腾冲火山群位于我国云南省西部和缅甸交界处的腾冲县境内,是我国著名的第四纪火山群,既有黑空山、打鹰山、马鞍山等一系列晚第四纪新期火山,也有早更新世以来有过喷发活动的大六冲、余家大山、来凤山等老火山。其中,位于腾冲火山区中东部的大六冲山势高峻,其顶峰是本区内的最高峰。野外地质调查发现,大六冲山体由一系列巨厚层爆发相火山碎屑堆积物和少量溢流相熔岩类岩石构成,喷发物类型极其丰富。在大六冲最高峰以南约100m处,首次发现存在着一个直径超百米的火山通道,可能是区内早期火山喷发的主通道,火山颈、熔岩穹丘、岩墙、爆发相与溢出相堆积物构成了大六冲完整的火山机构,在其周边多处地方还发现了因山体岩石破碎后形成的垮塌和滑坡堆积物。大六冲火山机构及其滑塌物的发现,不仅可以解释腾冲火山区大范围分布的火山碎屑岩的来源,也为防治以后类似大规模喷发可能造成的次生地质灾害提供了理想的研究样本和未来灾害预警。  相似文献   

7.
Bontâu is a major eroded composite volcano filling the Miocene Zârand extensional basin, near the junction between the Codru-Moma and Highi?-Drocea Mountains, at the tectonic boundary between the South and North Apuseni Mountains. It is a quasi-symmetric structure (16–18 km in diameter) centered on an eroded vent area (9×4 km), buttressed to the south against Mesozoic ophiolites and sedimentary deposits of the South Apuseni Mountains. The volcano was built up in two sub-aerial phases (14–12.5 Ma and 11–10 Ma) from successive eruptions of andesite lava and pyroclastic rocks with a time-increasing volatile budget. The initial phase was dominated by emplacement of pyroxene andesite and resulted in scattered individual volcanic lava domes associated marginally with lava flows and/or pyroclastic block-and-ash flows. The second phase is characterized by amphibole-pyroxene andesite as a succession of pyroclastic eruptions (varying from strombolian to subplinian type) and extrusion of volcanic domes that resulted in the formation of a central vent area. Numerous debris flow deposits accumulated at the periphery of primary pyroclastic deposits. Several intrusive andesitic-dioritic bodies and associated hydrothermal and mineralization processes are known in the volcano vent complex area. Distal epiclastic deposits initially as gravity mass flows and then as alluvial volcaniclastic and terrestrial detritic and coal filled the basin around the volcano in its western and eastern part. Chemical analyses show that lavas are calc-alkaline andesites with SiO2 ranging from 56–61%. The petrographical differences between the two stages are an increase in amphibole content at the expense of two pyroxenes (augite and hypersthene) in the second stage of eruption; CaO and MgO contents decrease with increasing SiO2. In spite of a ～4 Ma evolution, the compositions of calc-alkaline lavas suggest similar fractionation processes. The extensional setting favored two pulses of short-lived magma chamber processes.  相似文献   

8.
The San Ignacio Fm, a late Palaeozoic foreland basin succession that crops out in the Frontal Cordillera (Argentinean Andes), contains lacustrine microbial carbonates and volcanic rocks. Modification by extensive pedogenic processes contributed to the massive aspect of the calcareous beds. Most of the volcanic deposits in the San Ignacio Fm consist of pyroclastic rocks and resedimented volcaniclastic deposits. Less frequent lava flows produced during effusive eruptions led to the generation of tabular layers of fine-grained, greenish or grey andesites, trachytes and dacites. Pyroclastic flow deposits correspond mainly to welded ignimbrites made up of former glassy pyroclasts devitrified to microcrystalline groundmass, scarce crystals of euhedral plagioclase, quartz and K-feldspar, opaque minerals, aggregates of fine-grained phyllosilicates and fiammes defining a bedding-parallel foliation generated by welding or diagenetic compaction. Widespread silicified and silica-permineralized plant remains and carbonate mud clasts are found, usually embedded within the ignimbrites. The carbonate sequences are underlain and overlain by volcanic rocks. The carbonate sequence bottoms are mostly gradational, while their tops are usually sharp. The lower part of the carbonate sequences is made up of mud which appear progressively, filling interstices in the top of the underlying volcanic rocks. They gradually become more abundant until they form the whole of the rock fabric. Carbonate on volcanic sandstones and pyroclastic deposits occur, with the nucleation of micritic carbonate and associated production of pyrite. Cyanobacteria, which formed the locus of mineral precipitation, were related with this nucleation. The growth of some of the algal mounds was halted by the progressive accumulation of volcanic ash particles, but in most cases the upper boundary is sharp and suddenly truncated by pyroclastic flows or volcanic avalanches. These pyroclastic flows partially destroyed the carbonate beds and palaeosols. Microbial carbonate clasts, silicified and silica-permineralized tree trunks, log stumps and other plant remains such as small branches and small roots inside pieces of wood (interpreted as fragments of nurse logs) are commonly found embedded within the ignimbrites. The study of the carbonate and volcanic rocks of the San Ignacio Fm allows the authors to propose a facies model that increases our understanding of lacustrine environments that developed in volcanic settings.  相似文献   

9.
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10.
The uplifted and deeply eroded volcanic succession of Porto Santo (central East-Atlantic) is the product of a wide spectrum of dynamic processes that are active in shoaling to emergent seamounts. Two superimposed lapilli cones marking the base of the exposed section are interpreted as having formed from numerous submarine to subaerial phreatomagmatic explosions, pyroclastic fragmentation being subordinate. The lower basaltic and the upper mugearitic to trachytic sections are dominated by redeposited tephra and are called 'lapilli cone aprons'. Vertical growth due to accumulation of tephra, voluminous intrusions, and minor pillowed lava flows produced ephemeral islands which were subsequently leveled by wave erosion, as shown by conglomerate beds. Periods of volcanic quiescence are represented by abundant biocalcarenite lenses at several stratigraphic levels. The loose tephra piles became stabilized by widespread syn-volcanic intrusions such as dikes and trachytic to rhyolitic domes welding the volcanic and volcaniclastic ensemble into a solid edifice. Shattering of a submarine extrusive trachytic dome by pyroclastic and phreatomagmatic explosions, accentuated by quench fragmentation, resulted in pumice- and crystal-rich deposits emplaced in a prominent submarine erosional channel. The dome must have produced an island as indicated by a collapse breccia comprising surf-rounded boulders of dome material. Subaerial explosive activity is represented by scoria cones and tuff cones. Basaltic lava flows built a resistant cap that protected the island from wave erosion. Some lava flows entered the sea and formed two distinct types of lava delta: 1. closely-packed pillow lava and massive tabular lava flows along the southwestern coast of Porto Santo, and 2. a steeply inclined pillow-hyaloclastite breccia prism composed of foreset-bedded hydroclastic breccia, variably-shaped pillows, and thin sheet flows capped by subhorizontal submarine to subaerial lava flows along the eastern coast of Porto Santo.The facies architectures indicate emplacement: 1. on a gently sloping platform in southwestern Porto Santo, and 2. on steep offshore slopes along high energy shorelines in eastern Porto Santo.Growth of the pillow-hyaloclastite breccia prism is dominated by the formation of foreset beds but various types of syn-volcanic intrusions contributed significantly. Submarine flank eruptions occurred in very shallow water on the flanks of the hyaloclastite prism in eastern Porto Santo. The island became consolidated by intrusion of numerous dikes and by emplacement of prominent intrusions that penetrate the entire volcanic succession. Volcanic sedimentation ended with the emplacement of a debris avalanche that postdates the last subaerial volcanic activity.  相似文献   

11.
Volcán Citlaltépetl (Pico de Orizaba) with an elevation of 5,675 m is the highest volcano in North America. Its most recent catastrophic events involved the production of pyroclastic flows that erupted approximately 4,000, 8,500, and 13,000 years ago. The distribution of mapped deposits from these eruptions gives an approximate guide to the extent of products from potential future eruptions. Because the topography of this volcano is constantly changing computer simulations were made on the present topography using three computer algorithms: energy cone, FLOW2D, and FLOW3D. The Heim Coefficient (), used as a code parameter for frictional sliding in all our algorithms, is the ratio of the assumed drop in elevation (H) divided by the lateral extent of the mapped deposits (L). The viscosity parameter for the FLOW2D and FLOW3D codes was adjusted so that the paths of the flows mimicked those inferred from the mapped deposits. We modeled two categories of pyroclastic flows modeled for the level I and level II events. Level I pyroclastic flows correspond to small but more frequent block-and-ash flows that remain on the main cone. Level II flows correspond to more widespread flows from catastrophic eruptions with an approximate 4,000-year repose period. We developed hazard maps from simulations based on a National Imagery and Mapping Agency (NIMA) DTED-1 DEM with a 90 m grid and a vertical accuracy of ±30 m. Because realistic visualization is an important aid to understanding the risks related to volcanic hazards we present the DEM as modeled by FLOW3D. The model shows that the pyroclastic flows extend for much greater distances to the east of the volcano summit where the topographic relief is nearly 4,300 m. This study was used to plot hazard zones for pyroclastic flows in the official hazard map that was published recently.  相似文献   

12.
In the Central American Volcanic Arc, adakite-like volcanism has often been described as volumetrically insignificant. However, extensive silicic adakitic volcanism does occur in the Panamanian arc and provides an opportunity to evaluate the origin of this magma-type as well as to contrast its origin with other Central American silicic magmas. The Quaternary volcanic deposits of El Valle volcano are characterized by pronounced depletions in the heavy rare earth elements, low Y, high Sr, high Sr/Y, relatively high MgO, and low K2O/Na2O, when compared with other Quaternary Central American volcanics at similar SiO2. These chemical features are also diagnostic of adakitic signatures. Our new 40Ar/39Ar ages of lava flows and ash flows that compose the volcanic edifice of El Valle volcano illustrate that the eruptive volume of adakitic-like volcanism is substantial during the Quaternary (~120 km3). Adakitic-like magmas dominate the stratigraphic record. Common to all models for the origin of an adakite geochemical signature is the involvement of garnet, as a residual or fractionating phase. The stability of garnet in hydrous magmas has been recently reevaluated with important consequences; garnet is a stable primary igneous phase at pressure and temperature conditions expected for magma differentiation at the roots of a mature island arc. Moreover, adakite-like volcanism erupted at El Valle volcano displays the middle rare earth element depletion observed in other Panamanian volcanic centers that has been attributed to significant amphibole fractionation. Extensive amphibole fractionation may have occurred in two stages. The first stage of fractionation, garnet + amphibole fractionation, occurs from hydrous basaltic–andesitic parental magmas that have ponded at the base of an overthickened crust. The second stage occurs at mid-lower crustal levels where abundant amphibole + plagioclase and minor sphene crystallized from water-rich magmas. These two stages combined may have resulted in an amphibole-rich cumulate layer. This amphibole layer is likely the source of the abundant amphibole-rich cumulate enclaves and blobs found in volcanic products across the Panamanian arc. Stalling of water-rich magmas during this two-stage fractionation process could drive the interstitial liquids to the evolved compositions typical of continental crust, while leaving behind amphibole-rich cumulate rocks that may eventually be returned to the asthenosphere. Differentiation of H2O-rich magmas under the conditions appropriate for the roots of island arcs may therefore be a key process in developing a better understanding of the generation of continental crust in island arc environments.  相似文献   

13.
松辽盆地断陷层火山机构类型及其气藏特征   总被引：2，自引：0，他引：2  
松辽盆地断陷层(火石岭组和营城组)火山机构类型见3类6型。营城组以酸性火山机构为主,其中:盆地北部酸性熔岩火山机构占33%,南部占38%,北部的中基性火山机构比例小于南部。火山岩气藏类型主要为构造-岩性气藏,气水界面是一个起伏变化的界面。气藏内部特征受火山机构类型控制：酸性碎屑岩火山机构的气层厚度变化较小,其形态为板状或席状;酸性复合和熔岩火山机构气藏的气层厚度变化小,其形态为丘状和席状;中基性熔岩火山机构的厚度变化较大,其形态为丘状或楔状。松辽盆地火山岩气藏成藏主控因素为有效烃源岩、通源断层和储层物性。从松辽盆地发现的火山岩工业气藏来看:火山岩气藏主要集中在酸性熔岩火山机构中,北部贡献率为50%,南部贡献率为38%;盆地北部中基性火山机构的成藏效率高于南部。火山岩勘探方向应该聚焦在具有烃源岩和通源断层的区带,首先针对酸性火山机构,其次是中基性火山机构。  相似文献   

14.
The pyroclastic deposits of Etna have been correlated over the whole volcanic edifice for the first time, allowing the construction of a continuous record of tephra-producing events, which extends from approximately 100 ka to the Present. In this interval, five main periods of explosive activity have been identified: (a) ~100-ka strombolian to subplinian activity; (b) 80- to 100-ka plinian benmoreitic activity; (c) 16- to 80-ka strombolian to subplinian from basaltic to mugearitic activity; (d) 15.5- to 15-ka plinian benmoreitic activity accompanying the caldera-forming eruptions of the Ellittico Volcano; and (e) the most recent 13-ka basaltic explosive activity of strombolian and subplinian type of the present edifice that also includes the 122-B.C. plinian eruption. This study results in a semi-quantitative and in some cases quantitative definition of the intensity and chronology of the explosive activity at Etna. Moreover, this work gives a new significance to the volcanic hazards of Etna, a volcano generally considered to be the site of gentle effusive eruptions.  相似文献   

15.
The late-orogenic Archaean Duparquet, Kirkland and Stormy basins of the Canadian Superior Province are characterized by bounding crustal-scale faults and abundant porphyry stock emplacement. Lava flows and pyroclastic deposits are restricted to the Kirkland and Stormy basins, and coarse clastic detritus characterizes the Duparquet basin. Seven distinct lithofacies are identified: (1) mafic volcanic, (2) felsic volcanic, (3) pyroclastic, (4) volcaniclastic, (5) conglomerate-sandstone, (6) sandstone-argillite  conglomerate), and (7) argillite-sandstone  tuffaceous sandstone). The mafic and felsic volcanic lithofacies represent effusive lava flows, the pyroclastic lithofacies is formed of subaerial surge and airfall deposits and the volcaniclastic lithofacies is composed of reworked volcanic debris. The conglomerate-sandstone lithofacies is interpreted as alluvial fan, fan delta or proximal braided stream deposits, whereas the sandstone-argillite lithofacies is consistent with sandy-dominated flood- or braidplain deposits. A dominantly shallow-water lacustrine setting is inferred for the argillite-sandstone lithofacies. These different lithofacies record the basin history and can be used to identify basin-forming processes. Lithofacies stacking and rapid lateral changes of lithological units in conjunction with interformational unconformities and basin margin faults suggest tectonically induced sedimentation. Volcanism can also influence basin evolution and the delicate balance between erosion, sedimentation, and prevalent transport processes is affected by volcanic input. Catastrophic influx of pyroclastic material facilitated mass-wasting processes and formation of non-confined hyperconcentrated flood flow deposits account for local congestion of alluvial or fluvial dispersal patterns. Confined stream flow processes govern sedimentation during intravolcanic phases or prominent tectonic uplift. In addition, climate which controls the weathering processes, and vegetation which stabilizes unconsolidated material, affects the transport and depositional process. A CO2-rich aggressive weathering, humid Archaean atmosphere favours traction current deposits and an absence of vegetation promotes rapid denudation. Although tectonism is the prevalent long-term controlling factor in restricted basins, the effects of volcanism, climate and lack of vegetation can also be detected.  相似文献   

16.
基于不同岩性、岩相条件从根本上决定了储集空间的发育程度与规模,所以找火山岩的储层集中表现为火山岩相、亚相的识别。文中以松辽盆地庆深气田为例,重点研究了深层火山碎屑熔岩形成机理及其在火山岩地质相和测井相识别中的意义。研究表明,火山碎屑熔岩类火山岩常见火山碎屑流、泡沫熔岩流、岩流自碎作用、近地表隐爆作用、再熔结(胶结)型5种成因类型,且不同成因火山碎屑熔岩具有明显不同的矿物岩石学特征和测井响应特征。根据其形成机理、矿物岩石学特征和FMI成像特征,认为上述5种成因类型的火山碎屑熔岩分别发育于爆发相热碎屑流亚相、介于爆发相和喷溢相之间的爆溢相、喷溢相、火山通道相隐爆角砾岩亚相、火山通道相火山颈亚相或近火山口相。该研究成果对于促进火山岩相、亚相的准确识别和优质储层的有效预测具有重要的指导意义。  相似文献   

17.
乔乐  陈剑  凌宗成 《地质学报》2021,95(9):2678-2691
火山活动是月球最主要的内动力地质作用之一,是研究月球地质历史和热演化的重要窗口,也是月球科学及探测的重点目标.本文概要总结了月球火山作用的基本原理,并重点介绍了"岩墙扩展"模型.基于此模型,列举了由于岩墙在月壳内部上升程度的不同,导致的不同形式的喷发活动,并在月表产生了一系列火山地貌特征:① 当岩墙仅扩展到浅月表、未能穿透月壳并引起喷发活动时,可能会在月表产生坑链构造、地堑或底部断裂型撞击坑;② 当岩墙穿透了整个月壳并引起爆裂式喷发活动时,会在月表产生小型火山锥、区域性火山碎屑堆积物、全月分布的微小火山玻璃、暗晕凹陷构造及环形火山碎屑堆积物;③ 当岩墙穿透了整个月壳并引起溢流式喷发活动时,随着岩浆喷发通量的逐步增高,会在月表产生小型熔岩流、月海穹窿、复合熔岩流、蜿蜒型月溪、巨型熔岩流及火山高原复合体.本文也简要介绍了在月表观测到的若干非典型火山地貌特征,包括不规则月海斑块、环形凹陷穹丘及非月海富硅质穹窿.近年来新的探月数据加深了对这些特殊火山地貌特征的认识,但是更多的地质特征及成因模型细节仍有待未来月球研究及探测去解决.  相似文献   

18.
Cerro Machín is a dacitic tuff ring located in the central part of the Colombian Andes. It lies at the southern end of the Cerro Bravo–Cerro Machín volcanic belt. This volcano has experienced at least six major explosive eruptions during the last 5000 years. These eruptions have generated pyroclastic flows associated with Plinian activity that have traveled up to 8 km from the crater, and pyroclastic flows associated with Vulcanian activity with shorter runouts of 5 km from the source. Today, some 21,000 people live within a 8 km radius of Cerro Machín. The volcano is active with fumaroles and has shown increasing seismic activity since 2004, and therefore represents a potentially increasing threat to the local population. To evaluate the possible effects of future eruptions that may generate pyroclastic density currents controlled by granular flow dynamics we performed flow simulations with the TITAN2D code. These simulations were run in all directions around the volcano, using the input parameters of the largest eruption reported. The results show that an eruption of 0.3 km3 of pyroclastic flows from a collapsing Plinian column would travel up to 9 km from the vent, emplacing a deposit thicker than 60 m within the Toche River valley. Deposits >45 m thick can be expected in the valleys of San Juan, Santa Marta, and Azufral creeks, while 30 m thick deposits could accumulate within the drainages of the Tochecito, Bermellón, and Coello Rivers. A minimum area of 56 km2 could be affected directly by this kind of eruption. In comparison, Vulcanian column-collapse pyroclastic flows of 0.1 km3 would travel up to 6 km from the vent depositing >45 m thick debris inside the Toche River valley and more than 30 m inside the valleys of San Juan, Santa Marta, and Azufral creeks. The minimum area that could be affected directly by this kind of eruption is 33 km2. The distribution and thickness of the deposits obtained by these simulations are consistent with the hazard map presented by INGEOMINAS (Geological Survey of Colombia) in 2002. The composite map of the simulated flow deposits suggests that after major explosive events such as these, the generation of lahars is probable.  相似文献   

19.
Eruption records in the terrestrial stratigraphy are often incomplete due to erosion after tephra deposition, limited exposure and lack of precise dating owing to discontinuity of strata. A lake system and sequence adjacent to active volcanoes can record various volcanic events such as explosive eruptions and subaqueous density flows being extensions of eruption triggered and secondary triggered lahars. A lacustrine environment can constrain precise ages of such events because of constant and continuous background sedimentation. A total of 71 subaqueous density flow deposits in a 28 m long core from Lake Inawashiro‐ko reveals missing terrestrial volcanic activity at Adatara and Bandai volcanoes during the past 50 kyr. Sedimentary facies, colour, grain size, petrography, clay mineralogy, micro X‐ray fluorescence analysis and chemistry of included glass shards characterize the flow event deposits and clarify their origin: (i) clay‐rich grey hyperpycnites, extended from subaerial cohesive lahars at Adatara volcano, with sulphide/sulphate minerals and high sulphur content which point to a source from hydrothermally altered material ejected by phreatic eruptions; and (ii) clay‐rich brown density flow deposits, induced by magmatic hydrothermal eruptions and associated edifice collapse at Bandai volcano, with the common presence of fresh juvenile glass shards and low‐grade hydrothermally altered minerals; whereas (iii) non‐volcanic turbidites are limited to the oldest large slope failure and the 2011 Tohoku‐oki earthquake events. The high‐resolution chronology of volcanic activity during the last 50 kyr expressed by lacustrine event deposits shows that phreatic eruption frequency at Adatara has roughly tripled and explosive eruptions at Bandai have increased by ca 50%. These results challenge hikers, ski‐fields and downstream communities to re‐evaluate the increased volcanic risks from more frequent eruptions and far‐reaching lahars, and demonstrate the utility of lahar and lacustrine volcanic density flow deposits to unravel missing terrestrial eruption records, otherwise the recurrence rate may be underestimated at many volcanoes.  相似文献   

20.
中亚造山带北山南部的火山—沉积地层对探讨古地理演化有重要意义。本文通过北山南部双鹰山地块5条剖面实测，将该区火山—沉积地层分为4种相组合19种岩相，并通过LA- ICP- MS锆石U- Pb测年获得酸性火山岩夹层年龄(418. 9±1. 2 Ma、417. 8±1. 2 Ma、417. 5±1. 4 Ma)，证明各剖面均形成于早泥盆世洛赫科夫期(Lochkovian)。双鹰山地块南缘的下泥盆统由共生火山岩相(coherent facies)和原生火山岩相组合(primary volcanic facies)构成，是熔岩溢流、爆发坠落、火山碎屑流和岩浆侵出的产物，代表陆上隆升的普林尼型(Plinian)古火山作用。双鹰山地块中部墩墩山盆地充填序列自下而上由原生火山岩相组合、含火山碎屑沉积岩相组合(volcanogenic sedimentary facies)和正常沉积的河流相组合构成，代表火山活动逐步减弱和水体深度逐步加深的沉积过程。墩墩山盆地火山—沉积层序指示伸展构造背景，与同造山期磨拉石建造有显著区别。  相似文献   

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