Explosive volcanism may not be an inevitable consequence of magma fragmentation

The fragmentation of magma, containing abundant gas bubbles, is thought to be the defining characteristic of explosive eruptions. When viscous stresses associated with the growth of bubbles and the flow of the ascending magma exceed the strength of the melt, the magma breaks into disconnected fragments suspended within an expanding gas phase. Although repeated effusive and explosive eruptions for individual volcanoes are common, the dynamics governing the transition between explosive and effusive eruptions remain unclear. Magmas for both types of eruptions originate from sources with similar volatile content, yet effusive lavas erupt considerably more degassed than their explosive counterparts. One mechanism for degassing during magma ascent, consistent with observations, is the generation of intermittent permeable fracture networks generated by non-explosive fragmentation near the conduit walls. Here we show that such fragmentation can occur by viscous shear in both effusive and explosive eruptions. Moreover, we suggest that such fragmentation may be important for magma degassing and the inhibition of explosive behaviour. This implies that, contrary to conventional views, explosive volcanism is not an inevitable consequence of magma fragmentation.     

爆发沸腾形核及压力模拟

对爆发沸腾形核阶段的微观过程进行了分析与描述,运用分子势能模型并结合经典形核功理论计算得到临界气泡半径;针对形核与气泡生长阶段功量的重要影响因素--压力,采用Ls-Dyna显示动力学软件对爆发沸腾过程的压力分布进行了模拟分析.     

爆炸焊接用半球结构体的降压实验

如何减小爆炸焊接噪声影响的问题成为目前迫切需要解决的问题.为解决冲击波和噪声扰民问题,因为药量不能做到微差起爆降低声源,这里着重从传播途径上考虑,提出了在爆破场地建设一种专用的半球阻波结构.依据内爆、裸爆超压测试,比较了裸爆、内爆超压降低趋势.在半球阻波体开口方向,消压效果可达50%,在开口侧向90°方向,消压效果可达90%.     

The formation of plagioclase chains during convective transfer in basaltic magma

The basaltic rock in the lower part of the thick Holyoke lava flow in Connecticut and Massachusetts has been shown to have a remarkable texture, with crystals of feldspar linked together in a continuous three-dimensional network of chains. Heating experiments have revealed that this network persists to temperatures where the rock is 75% liquid, and therefore the network was interpreted to have formed at an early stage of crystallization and to have played an important role in the compaction of crystal mush in the lower part of the flow. Despite the texture's importance to our understanding of how such basalt flows form, the origin of the texture has remained uncertain. Here we show that, although the network is present in the lower third of the flow, it was actually formed in the upper solidification front and was transported down in plumes of dense crystal mush. Convection of this type has been postulated for intrusive magma chambers, but corroborative field evidence has been equivocal, especially in lava lakes and flows. Preservation of the roof-generated texture in the lower part of a thick flood-basalt flow therefore constitutes important evidence for the role of convection in the solidification and differentiation of a simple magma sheet.     

Non-equilibrium degassing and a primordial source for helium in ocean-island volcanism

Radioactive decay of uranium and thorium produces 4He, whereas 3He in the Earth's mantle is not produced by radioactive decay and was only incorporated during accretion-that is, it is primordial. 3He/4He ratios in many ocean-island basalts (OIBs) that erupt at hotspot volcanoes, such as Hawaii and Iceland, can be up to sixfold higher than in mid-ocean ridge basalts (MORBs). This is inferred to be the result of outgassing by melt production at mid-ocean ridges in conjunction with radiogenic ingrowth of 4He, which has led to a volatile-depleted upper mantle (MORB source) with low 3He concentrations and low 3He/4He ratios. Consequently, high 3He/4He ratios in OIBs are conventionally viewed as evidence for an undegassed, primitive mantle source, which is sampled by hot, buoyantly upwelling deep-mantle plumes. However, this conventional model provides no viable explanation of why helium concentrations and elemental ratios of He/Ne and He/Ar in OIBs are an order of magnitude lower than in MORBs. This has been described as the 'helium concentration paradox' and has contributed to a long-standing controversy about the structure and dynamics of the Earth's mantle. Here we show that the helium concentration paradox, as well as the full range of noble-gas concentrations observed in MORB and OIB glasses, can self-consistently be explained by disequilibrium open-system degassing of the erupting magma. We show that a higher CO2 content in OIBs than in MORBs leads to more extensive degassing of helium in OIB magmas and that noble gases in OIB lavas can be derived from a largely undegassed primitive mantle source.     

Uplift, thermal unrest and magma intrusion at Yellowstone caldera

The Yellowstone caldera, in the western United States, formed approximately 640,000 years ago when an explosive eruption ejected approximately 1,000 km3 of material. It is the youngest of a series of large calderas that formed during sequential cataclysmic eruptions that began approximately 16 million years ago in eastern Oregon and northern Nevada. The Yellowstone caldera was largely buried by rhyolite lava flows during eruptions that occurred from approximately 150,000 to approximately 70,000 years ago. Since the last eruption, Yellowstone has remained restless, with high seismicity, continuing uplift/subsidence episodes with movements of approximately 70 cm historically to several metres since the Pleistocene epoch, and intense hydrothermal activity. Here we present observations of a new mode of surface deformation in Yellowstone, based on radar interferometry observations from the European Space Agency ERS-2 satellite. We infer that the observed pattern of uplift and subsidence results from variations in the movement of molten basalt into and out of the Yellowstone volcanic system.     

A rapid mechanism to remobilize and homogenize highly crystalline magma bodies

The largest products of magmatic activity on Earth, the great bodies of granite and their corresponding large eruptions, have a dual nature: homogeneity at the large scale and spatial and temporal heterogeneity at the small scale. This duality calls for a mechanism that selectively removes the large-scale heterogeneities associated with the incremental assembly of these magmatic systems and yet occurs rapidly despite crystal-rich, viscous conditions seemingly resistant to mixing. Here we show that a simple dynamic template can unify a wide range of apparently contradictory observations from both large plutonic bodies and volcanic systems by a mechanism of rapid remobilization (unzipping) of highly viscous crystal-rich mushes. We demonstrate that this remobilization can lead to rapid overturn and produce the observed juxtaposition of magmatic materials with very disparate ages and complex chemical zoning. What distinguishes our model is the recognition that the process has two stages. Initially, a stiff mushy magma is reheated from below, producing a reduction in crystallinity that leads to the growth of a subjacent buoyant mobile layer. When the thickening mobile layer becomes sufficiently buoyant, it penetrates the overlying viscous mushy magma. This second stage rapidly exports homogenized material from the lower mobile layer to the top of the system, and leads to partial overturn within the viscous mush itself as an additional mechanism of mixing. Model outputs illustrate that unzipping can rapidly produce large amounts of mobile magma available for eruption. The agreement between calculated and observed unzipping rates for historical eruptions at Pinatubo and at Montserrat demonstrates the general applicability of the model. This mechanism furthers our understanding of both the formation of periodically homogenized plutons (crust building) and of ignimbrites by large eruptions.     

Granite magma formation, transport and emplacement in the Earth's crust

The origin of granites was once a question solely for petrologists and geochemists. But in recent years a consensus has emerged that recognizes the essential role of deformation in the segregation, transport and emplacement of silica-rich melts in the continental crust. Accepted petrological models are being questioned, either because they require unrealistic rheological behaviours of rocks and magmas, or because they do not satisfactorily explain the available structural or geophysical data. Provided flow is continuous, mechanical considerations suggest that--far from being geologically sluggish--granite magmatism is a rapid, dynamic process operating at timescales of < or = 100,000 years, irrespective of tectonic setting.     

抚顺煤田矿震现状及其城市地质环境效应初探

抚顺煤田矿震的平均发震频度达4000次／年，强度急剧增大，已发生ML＝3．7级矿震，研究预测表明最大震级可达4．2级，矿震是由煤田开采引起的，是矿山开采伴生的动力现象，矿震分为断层活动型，顶板活动型和工作面冲击型三种。矿震不会造成地面灾害，但是，矿震与其他潜在地质灾害相互作用具有不溶忽视的危害性。讨论矿震与城市防震，滑坡，区域地质构造活动之间的相互作用。     

Increased subaerial volcanism and the rise of atmospheric oxygen 2.5 billion years ago

The hypothesis that the establishment of a permanently oxygenated atmosphere at the Archaean-Proterozoic transition (approximately 2.5 billion years ago) occurred when oxygen-producing cyanobacteria evolved is contradicted by biomarker evidence for their presence in rocks 200 million years older. To sustain vanishingly low oxygen levels despite near-modern rates of oxygen production from approximately 2.7-2.5 billion years ago thus requires that oxygen sinks must have been much larger than they are now. Here we propose that the rise of atmospheric oxygen occurred because the predominant sink for oxygen in the Archaean era-enhanced submarine volcanism-was abruptly and permanently diminished during the Archaean-Proterozoic transition. Observations are consistent with the corollary that subaerial volcanism only became widespread after a major tectonic episode of continental stabilization at the beginning of the Proterozoic. Submarine volcanoes are more reducing than subaerial volcanoes, so a shift from predominantly submarine to a mix of subaerial and submarine volcanism more similar to that observed today would have reduced the overall sink for oxygen and led to the rise of atmospheric oxygen.     

建筑物的加固与抗震

近年来建筑物的加固改造发展迅速,在实践中形成了多种成熟的加固方法。文章着重介绍了几种常用的结构加固方法,并对各方法的特点进行了比较,阐述了各方法的使用条件及工艺流程;另外在加固方法的基础上介绍了国家对抗震加固构造要求及应用方法。     

轴对称聚能药管及其聚能效应

煤尘与瓦斯突出一直是威胁煤炭矿井安全生产最严重的自然灾害之一,而不合理的炸药装药及爆破将会引爆瓦斯与煤层而造成严重的生产事故。通过理论计算,确定了轴对称聚能药管的参数及煤的裂隙长度,并选择水胶炸药在钢管内进行模拟实验,得出较好的聚能爆破效果,对聚能爆破机理进行了模拟分析并得出:轴对称聚能爆破药管具有较好的聚能效应,有利于实现定向预裂爆破;药管可用PVC材料加。结论为轴对称聚能药管的设计提供理论和实验支持。     

阜新王营井田浅成气的成藏与岩浆活动

王营井田是产气率较低,裂隙又很不发育的长焰煤井田,但该井田却很有成为很具有开发价值的煤成气田。本文从该井田的地质特征－成煤期后有剧烈的岩浆活动这一现象出发,依据井下瓦斯突出及矿井抽放瓦斯的实际资料,同时运用煤成气的生成、储藏、运移理论,科学分析了该井田能产出大量煤成气的原因－即岩墙、岩床不但提高了长焰煤的产气率,而且也造就了煤成气的储运场所和运移通道。从而得出了岩浆活动是促成浅产质煤能成为可资开发的煤成气田的重要因素这一结论。     

Discovery of a magma chamber and faults beneath a Mid-Atlantic Ridge hydrothermal field

Crust at slow-spreading ridges is formed by a combination of magmatic and tectonic processes, with magmatic accretion possibly involving short-lived crustal magma chambers. The reflections of seismic waves from crustal magma chambers have been observed beneath intermediate and fast-spreading centres, but it has been difficult to image such magma chambers beneath slow-spreading centres, owing to rough seafloor topography and associated seafloor scattering. In the absence of any images of magma chambers or of subsurface near-axis faults, it has been difficult to characterize the interplay of magmatic and tectonic processes in crustal accretion and hydrothermal circulation at slow-spreading ridges. Here we report the presence of a crustal magma chamber beneath the slow-spreading Lucky Strike segment of the Mid-Atlantic Ridge. The reflection from the top of the magma chamber, centred beneath the Lucky Strike volcano and hydrothermal field, is approximately 3 km beneath the sea floor, 3-4 km wide and extends up to 7 km along-axis. We suggest that this magma chamber provides the heat for the active hydrothermal vent field above it. We also observe axial valley bounding faults that seem to penetrate down to the magma chamber depth as well as a set of inward-dipping faults cutting through the volcanic edifice, suggesting continuous interactions between tectonic and magmatic processes.     

岩溶地震解释技术及其应用

提出了岩溶地震解释技术 ,并将其应用到轮南西部地区奥陶系的地震解释中 ,取得了较好效果 ,为碳酸盐岩储层的预测提供了一种新的思路和方法。     

中国大陆东部地区及朝鲜半岛上地幔深部间断面和各向异性

使用中国国家地震台网、区域内美国IRIS台站和日本OHP台站,收集到超过三年的震级大于6级的全球范围内的宽频带地震资料,采用接收函数和SKS分裂分析技术,研究了中国大陆东部地区及朝鲜半岛上地幔深部间断面和各向异性。在中国东北地区、华北地区及朝鲜半岛下方,存在大范围的更浅的410km间断面和更深的660km间断面,该特征与太平洋滞留板块的作用密切相关。区域内的上地幔各向异性主要由上地幔流引起,该地幔流与欧亚板块和太平洋板块相对运动有关,而SKS快轴方向的空间分布揭示了这种复杂的运动模式.     

模拟爆炸地震波及其对框架结构的影响

基于能够描述爆炸地震波的非平稳随机过程模型进行爆炸地震波的模拟研究,给出能够考虑装药量和爆心距影响的单点爆炸地震波实用模拟方法.以15层框架结构为例,运用有限元分析软件建立空间板梁结构的有限元模型,计算结构的固有频率和固有振型,分析高层框架结构在模拟爆炸地震波作用下的动力响应.根据结构动力响应结果,判断结构的薄弱层,为工程抗震设计提供参考依据.     

Decadal to monthly timescales of magma transfer and reservoir growth at a caldera volcano

Caldera-forming volcanic eruptions are low-frequency, high-impact events capable of discharging tens to thousands of cubic kilometres of magma explosively on timescales of hours to days, with devastating effects on local and global scales. Because no such eruption has been monitored during its long build-up phase, the precursor phenomena are not well understood. Geophysical signals obtained during recent episodes of unrest at calderas such as Yellowstone, USA, and Campi Flegrei, Italy, are difficult to interpret, and the conditions necessary for large eruptions are poorly constrained. Here we present a study of pre-eruptive magmatic processes and their timescales using chemically zoned crystals from the 'Minoan' caldera-forming eruption of Santorini volcano, Greece, which occurred in the late 1600s BC. The results provide insights into how rapidly large silicic systems may pass from a quiescent state to one on the edge of eruption. Despite the large volume of erupted magma (40-60 cubic kilometres), and the 18,000-year gestation period between the Minoan eruption and the previous major eruption, most crystals in the Minoan magma record processes that occurred less than about 100 years before the eruption. Recharge of the magma reservoir by large volumes of silicic magma (and some mafic magma) occurred during the century before eruption, and mixing between different silicic magma batches was still taking place during the final months. Final assembly of large silicic magma reservoirs may occur on timescales that are geologically very short by comparison with the preceding repose period, with major growth phases immediately before eruption. These observations have implications for the monitoring of long-dormant, but potentially active, caldera systems.