Magma plumbing system of the 2000 eruption of Miyakejima Volcano, Japan   总被引：1，自引：0，他引：1  

Mizuho Amma-Miyasaka  Mitsuhiro Nakagawa  Setsuya Nakada 《Bulletin of Volcanology》2005,67(3):254-267

During the 2000 eruption at Miyakejima Volcano, two magmas with different compositions erupted successively from different craters. Magma erupted as spatter from the submarine craters on 27 June is aphyric basaltic andesite (<5 vol% phenocrysts, 51.4–52.2 wt% SiO₂), whereas magma issued as volcanic bombs from the summit caldera on 18 August is plagioclase-phyric basalt (20 vol% phenocrysts, 50.8–51.3 wt% SiO₂). The submarine spatter contains two types of crystal-clots, A-type and A-type (andesitic type). The phenocryst assemblages (plagioclase, pyroxenes and magnetite) and compositions of clinopyroxene in these clots are nearly the same, but only A-type clots contain Ca-poor plagioclase (An < 70). We consider that the A-type clots could have crystallized from a more differentiated andesitic magma than the A-type clots, because FeO*/MgO is not strongly influenced during shallow andesitic differentiation. The summit bombs contain only B-type (basaltic type) crystal-clots of Ca-rich plagioclase, olivine and clinopyroxene. The A-type and B-type clots have often coexisted in Miyakejima lavas of the period 1469–1983, suggesting that the magma storage system consists of independent batches of andesitic and basaltic magmas. According to the temporal variations of mineral compositions in crystal-clots, the andesitic magma became less evolved, and the basaltic magma more evolved, over the past 500 years. We conclude that gradually differentiating basaltic magma has been repeatedly injected into the shallower andesitic magma over this period, causing the andesitic magma to become less evolved with time. The mineral chemistries in crystal-clots of the submarine spatter and 18 August summit bombs of the 2000 eruption fall on the evolution trends of the A-type and B-type clots respectively, suggesting that the shallow andesitic and deeper basaltic magmas existing since 1469 had successively erupted from different craters. The 2000 summit collapse occurred due to drainage of the andesitic magma from the shallower chamber; as the collapse occurred, it may have caused disruption of crustal cumulates which then contaminated the ascending, deeper basalt. Thus, porphyritic basaltic magma could erupt alone without mixing with the andesitic magma from the summit caldera. The historical magma plumbing system of Miyakejima was probably destroyed during the 2000 eruption, and a new one may now form.Editorial responsibility: S Nakada, T Druitt  相似文献   

An Upper Pliocene coarse pumice breccia generated by a shallow submarine explosive eruption,Milos, Greece     

Andrew?L.?StewartEmail author  Jocelyn?McPhie 《Bulletin of Volcanology》2004,66(1):15-28

The Filakopi Pumice Breccia (FPB) is a very well exposed, Pliocene volcaniclastic unit on Milos, Greece, and has a minimum bulk volume of 1 km³. It consists of three main units: (A) basal lithic breccia (4–8 m) mainly composed of angular to subangular, andesitic and dacitic clasts up to 2.6 m in diameter; (B) very thickly bedded, poorly sorted pumice breccia (16–17 m); and (C) very thick, reversely graded, grain-supported, coarse pumice breccia (6.5–20 m), at the top. The depositional setting is well constrained as shallow marine (up to a few hundred metres) by overlying fossiliferous and bioturbated mudstone. This large volume of fine pumice clasts is interpreted to be the product of an explosive eruption from a submarine vent because: (1) pumice clasts are the dominant component; (2) the coarse pumice clasts (>64 mm) have complete quenched margins; (3) very large (>1 m) pumice clasts are common; (4) overall, the formation shows good hydraulic sorting; and (5) a significant volume of ash was deposited together with the coarsest pyroclasts.The bed forms in units A and B suggest deposition from lithic-rich and pumiceous, respectively, submarine gravity currents. In unit C, the coarse (up to 6.5 m) pumice clasts are set in matrix that grades upwards from diffusely stratified, fine (1–2 cm) pumice clasts at the base to laminated shard rich mud at the top. The coarse pumice clasts in unit C were settled from suspension and the framework was progressively infilled by fine pumice clasts from waning traction currents and then by water-settled ash. The FPB displays important features of the products of submarine explosive eruptions that result from the ambient fluid being seawater, rather than volcanic gas or air. In particular, submarine pyroclastic deposits are characterised by the presence of very coarse juvenile pumice clasts, pumice clasts with complete quenched rims, and good hydraulic sorting.Electronic Supplementary Material Supplementary material is available for this article if you access the article at . A link in the frame on the left on that page takes you directly to the supplementary material.Editorial responsibility: J. Donelly-Nolan  相似文献   

Emplacement mechanisms of the South Kona slide complex,Hawaii Island: sampling and observations by remotely operated vehicle Kaiko     

Hisayoshi?YokoseEmail author  Peter?W.?Lipman 《Bulletin of Volcanology》2004,66(7):569-584

Emplacement of a giant submarine slide complex, offshore of South Kona, Hawaii Island, was investigated in 2001 by visual observation and in-situ sampling on the bench scarp and a megablock, during two dives utilizing the Remotely Operated Vehicle (ROV) Kaiko and its mother ship R/V Kairei. Topography of the bench scarp and megablocks were defined in 3-D perspective, using high-resolution digital bathymetric data acquired during the cruise. Compositions of 34 rock samples provide constraints on the landslide source regions and emplacement mechanisms. The bench scarp consists mainly of highly fractured, vesiculated, and oxidized aa lavas that slumped from the subaerial flank of ancestral Mauna Loa. The megablock contains three units: block facies, matrix facies, and draped sediment. The block facies contains hyaloclastite interbedded with massive lava, which slid from the shallow submarine flank of ancestral Mauna Loa, as indicated by glassy groundmass of the hyaloclastite, low oxidation state, and low sulfur content. The matrix facies, which directly overlies the block facies and is similar to a lahar deposit, is thought to have been deposited from the water column immediately after the South Kona slide event. The draped sediment is a thin high-density turbidite layer that may be a distal facies of the Alika-2 debris-avalanche deposit; its composition overlaps with rocks from subaerial Mauna Loa. The deposits generated by the South Kona slide vary from debris avalanche deposit to turbidite. Spatial distribution of the deposits is consistent with deposits related to large landslides adjacent to other Hawaiian volcanoes and the Canary Islands.  相似文献   

风沙流中沙粒浓度分布的实验研究   总被引：9，自引：0，他引：9  

王洪涛  董治宝  张晓航 《地球科学进展》2004,19(5):732-735

利用粒子图像测速技术（PIV）,在风洞内对与天然沙接近的石英沙风沙流沙颗粒浓度沿高度分布进行了研究。对 3种不同轴线风速研究结果表明,风沙流中沙粒浓度沿水平方向基本不变,而随高度呈指数衰减,而且其衰减速率与风速密切相关。风速越大,衰减速率越慢;风速越小,衰减速率越快。风沙运动一旦发生时,靠近沙床表面的沙粒浓度随风速变化很小,可以认为趋于稳定,达到最大值。  相似文献   

The aeolian sedimentation record of the Thar desert   总被引：1，自引：0，他引：1  

A. K. Singhvi  Amal Kar 《Journal of Earth System Science》2004,113(3):371-401

A review of the aeolian sedimentary record of the Thar desert is presented. This includes a regional survey of the major dune forms, their genesis and their relationship to climate and other regional landforms. A key aspect of this work is the chronometry of the dunes using luminescence methods. Luminescence dating of sand has enabled quantification of the duration of the phases of sand aggradation and quiescence, time scales of dune migration and the dating of pedogenic carbonates. We demonstrate that the conventional wisdom of synchronicity of dune aggradation with glacial epoch is not true in the context of Thar sands and here only a short durationwindow of opportunity existed for dune aggradation. Luminescence ages further suggest that this window occurred during a transitional climatic regime from glacial to interglacial about 4–10 ka after the glacial epoch. Other inferences included are that:

–	• the aeolian activity in the Thar began over > 150 ka, resolving that Thar is not of anthropogenic origin as suggested previously;
–	• the present spatial extent of the aeolian activity in the Thar is in a contracted stage compared to that in the geological past, which refutes the arguments on its rapid north-eastward expansion;
–	• the current dune migration rates in areas of significant human-induced disturbances are much higher than during the geological past;
–	• the monsoon activity in the Thar varied significantly, from being minimal during the isotopic marine stages 4 and 2 to being close to the present during stage 3;
–	• on shorter time scales the dune activities correlated with the lacustrine records of the region with a phase difference of a few centuries and a periodicity of ∼ 1500 years;
–	• the sand aggradation climate in the southern margin in Gujarat gradually shrank northwards such that in general dunes older than 10 ka are seen in the extreme southern margin and dunes younger than 2ka ages occur mostly in the western part of Rajasthan.

  相似文献   

陕北风沙区大保当煤矿开采的生态影响分析     

钱者东  秦卫华  李道进 《西部资源》2012,(1):84-86

陕北风沙区位于毛乌素沙地东南缘,是毛乌素沙地向黄土高原的过渡地带,自然环境十分脆弱。本文通过对陕北风沙区大保当井田进行实地调查研究,结合遥感影像,运用图形叠置法、类比分析法和生态机理法分析了在该地区进行煤矿开采对当地自然植被、野生动物以及神木臭柏自然保护区的影响,并提出相应的生态保护措施。  相似文献   

A study on the limit void ratio characteristics of medium to fine mixed graded sands     

Y. Yilmaz   《Engineering Geology》2009,104(3-4):290-294

The minimum and maximum void ratios, corresponding to states of densest and loosest packing, of 111 systematically prepared mixed graded sand samples are determined with Method 2A and Method B according to ASTM D 4253 and ASTM D 4254 standards, respectively. Although, those standards are applicable to soils that may contain up to at most 15% fines content, for the harmony of the results even for samples having fines content greater 15%, no other methods are used throughout the experimental program. The test results show that 3rd degree polynomial equations derived from existing experimental data, as a function of packing material percentage, are quite reasonable (R² ≈ 1.0) to estimate the variation of minimum and maximum void ratios for the sands considered. Furthermore, error analysis of the limit void ratios is also carried out. It is found that predictability of maximum void ratio for the soil types and gradations considered is more accurate than that of minimum void ratio.  相似文献   

Physical modelling of sand injectites     

N. Rodrigues  P.R. Cobbold  H. Lseth 《Tectonophysics》2009,474(3-4):610-632

Sand injectites are structures that result from intrusion of fluidized sand into fractures. We have studied them in the Tampen Spur area of the North Sea, and have reproduced them experimentally, by driving compressed air through layers of sand, glass microspheres, and silica powder. The silica powder was cohesive and capable of hydraulic fracturing, whereas the sand and glass microspheres were almost non-cohesive and therefore able to fluidize. The models were dynamically similar to their natural counterparts, for as long as equilibrium was static. When the processes became dynamic, so that inertial forces were significant, the scaling was approximate and the corresponding Reynolds numbers differed. The experimental apparatus was a square box, 1 m × 1 m wide, resting on a grid of fluid diffusers. During the experiments, the fluid pressure increased, until it attained and surpassed the weight of overburden. Flat-lying hydraulic fractures, containing air, formed within cohesive and least permeable layers. Heterogeneities in material properties and layer thicknesses were responsible for localizing fracture networks. When any one network broke through to the surface, rapid flow of air through the fractures fluidized the underlying mobile materials and even depleted some of the layers. Some of the fluidized material extruded at the surface through vents, forming volcanoes and sheets. The remainder lodged at depth, forming sand injectites or laccoliths. Conical sand injectites formed preferentially, where layers had high resistance to bending. Laccoliths formed nearer the surface, where overlying layers had low resistance to bending. The experimental sand injectites were broadly similar to those in the Tampen Spur area of the North Sea, as well as other areas.  相似文献   

Interfacial instability of sand patterns induced by turbulent shear flow   总被引：1，自引：0，他引：1  

Sk Zeeshan Ali  Subhasish Dey 《国际泥沙研究》2021,36(4):449-456

When a turbulent shear flow above a plane sand surface entrains sand grains,it generates a variety of sand patterns.Fluvial sand forms two major interfacial patterns:meso-scale dunes and antidunes,and large-scale bars.Measurements have evidenced that under erosive conditions,meso-scale patterns either change to or coexist with large-scale patterns.However,it remains elusive what exactly drives the switching of interfacial patterns and how the switching occurs.Here,we showdcombing a flow model with a grain transport model,allowing for both the surface and suspended sand fluxes dthat the switching of patterns emerges from the shear-driven complex feedback between grain transport and topographic perturbations.The switching predominantly depends on the magnitudes of the Rouse number and the grain size to undisturbed flow depth ratio.The model offers quantitative predictions of the maximum amplification of sand patterns and unveils a new attraction erepulsion phenomenon.  相似文献   

碎石桩复合地基的抗液化特性探讨   总被引：8，自引：2，他引：6  

黄春霞张鸿儒  隋志龙 《工程地质学报》2004,12(2):148-154

饱和砂土地震液化问题是岩土地震工程中一个重要的研究课题。在多种可行的防治液化措施中 ,最普遍的方法是采用碎石桩复合地基。本文结合目前国内外碎石桩复合地基的抗液化研究的最新进展 ,对碎石桩的密实、排水减压和减震作用做了较详细的评述 ,最后还提出碎石桩复合地基抗液化特性需要进一步研究的问题  相似文献