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.     

A 780-year record of explosive volcanism from DT263 ice core in east Antarctica

Ice cores recovered from polar ice sheet received and preserved sulfuric acid fallout from explosive volcanic eruptions. DT263 ice core was retrieved from an east Antarctic location. The ice core is dated using a combination of annual layer counting and volcanic time stratigraphic horizon as 780 years (1215-1996 A.D.). The ice core record demonstrates that during the period of approximately 1460-1800 A.D., the accumulation is sharply lower than the levels prior to and after this period. This period coincides with the most recent neoglacial climatic episode, the "Little Ice Age (LIA)", that has been found in numerous Northern Hemisphere proxy and historic records. The non-sea-salt SO42- concentrations indicate seventeen volcanic events in DT263 ice core. Compared with those from previous Antarctic ice cores, significant discrepancies are found between these records in relative volcanic flux of several well-known events. The discrepancies among these records may be explained by the differences in surface topography, accumulation rate, snow drift and distribution which highlight the potential impact of local glaci-ology on ice core volcanic records, analytical techniques used for sulfate measurement, etc. Volcanic eruptions in middle and high southern latitudes affect volcanic records in Antarctic snow more intensively than those in the low latitudes.     

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.     

甲烷-空气预混气体泄爆作用下容器振动响应特性

利用自主搭建的气体泄爆容器动态力学响应测试系统,研究甲烷-空气预混气体泄爆过程中实验舱的振动响应特性,并结合舱内部超压、火焰演化和实验舱固有频率等特征,探讨前述实验舱动态力学的响应机制.研究发现:1)泄爆过程中实验舱动态力学响应存在明显的双峰现象,且两者的幅值、频谱均存在较大差异; 2)低幅值振动与舱内气体冲击波特征密切相关,其主要由初始火焰传播、外部爆炸、亥姆霍兹振荡和泰勒不稳定性等因素综合影响导致; 3) 480和980 Hz的高幅值振动主要为舱内声波和火焰耦合作用触发高频振荡导致;而1 100 Hz左右的高幅值振动主要为受冲击后实验舱的自由振动,由舱固有频率决定.     

Microstructure and penetration behavior of electroformed copper liners of shaped charges during explosive detonation deformation

The microstructure in the electroformed copper liners of shaped charges prepared with different electrolytes was studied by Scanning Electron Microscopy (SEM) and Electron Backscattering Kikuchi Pattern (EBSP) methods. SEM observations revealed the existence of columnar grains in electroformed copper liners of shaped charges formed by electrolyte without any additive and the average grain size is about 3 μm. When an additive is introduced to the electrolyte, the grains formed in the copper liners become equiaxed and finer. EBSP results show that the columnar grain grown during electroformation has the most preferential growth direction, whereas a micro-texture does not exit in the specimen prepared by electrolyte with the additive. Further, explosive detonation deformation experiments show that penetration depth is dramatically improved when the electroformed copper liners of shaped charges exhibit equiaxed grains.     

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.     

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.     

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.     

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.     

核四极矩共振炸药探测原理与试验

为了解决探雷虚警率，实现探雷的有效性，介绍了核四极矩共振（Nuclear quadrupole resonance简称NQR）基本原理及其用于炸药探测的依据；分析了黑索金（RDX）、梯恩梯（TNT）分子中氮原子（N^14）的共振频率及其探测原理；介绍了一种可行的炸药探测研究试验系统，重点研究分析了黑索金（RDX）自由感应衰减（FID）信号、FFT变换、频率响应以及不同质量的RDX对探测信号的影响。实现了RDX的有效探测。     

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。