Pit crater structure and processes governing persistent activity at Masaya Volcano, Nicaragua   总被引：1，自引：1，他引：0  

Hazel Rymer  Benjamin van Wyk de Vries  John Stix  Glyn Williams-Jones 《Bulletin of Volcanology》1998,59(5):345-355

 Persistent activity at Masaya Volcano, Nicaragua, is characterised by cycles of intense degassing, lava lake development and pit crater formation. It provides a useful site to study the processes which govern such activity, because of its easy accessibility and relatively short cycles (years to decades). An understanding of the present activity is important because Masaya is visited by large numbers of tourists, is located close to major cities and has produced voluminous lavas, plinian eruptions and ignimbrites in the recent past. We provide structural and geophysical data that characterise the "normal" present state of activity. These indicate that the ongoing degassing phase (1993 to present) was not caused by fresh magma intrusion. It was associated with shallow density changes within the active Santiago pit crater. The activity appears to be associated predominantly with shallow changes in the pit crater structure. More hazardous activity will occur only if there are significant departures from the present gravity, deformation and seismic signatures. Received: 16 May 1997 / Accepted: 29 October 1997  相似文献   

四川石棉田湾矿区晚震旦世露头层序地层研究     

杨逢清  杨铸生 《地质科技情报》1998,17(3):47-50

应用露头层序地层的原理和方法,对四川石棉田湾矿区以碳酸盐岩为主的上震旦统进行了研究,识别出４个３级层序,描述了各层序的特征,探讨了该区晚震旦世海平面的变化规律和层序地层中体系域与金等贵金属矿床的关系。  相似文献   

地史重大转折期的环境变化学术讨论会综述   总被引：7，自引：0，他引：7  

龚一鸣  殷鸿福 《地质科技情报》1998,17(4):101-106

概述了地史重大转折期的环境变化学术讨论会４个方面的主要科学内容：古生代与中生代之交的全球环境变化,泥贫纪弗拉期与法门期之交的全球环境变化,元古宙与显生宙之交扔全球环境变化和地史重大转折期环境变化的研究方法。围绕上述内容,评述了发言人和专题发言人的主要科学资料和科学思想以及本次会议形成的共识。  相似文献   

Inversion, error analysis, and validation of GPS/MET occultation data     

A. K. Steiner  G. Kirchengast  H. P. Ladreiter 《Annales Geophysicae》1998,17(1):122-138

The global positioning system meteorology (GPS/MET) experiment was the first practical demonstration of global navigation satellite system (GNSS)-based active limb sounding employing the radio occultation technique. This method measures, as principal observable and with millimetric accuracy, the excess phase path (relative to propagation in vacuum) of GNSS-transmitted radio waves caused by refraction during passage through the Earth’s neutral atmosphere and ionosphere in limb geometry. It shows great potential utility for weather and climate system studies in providing an unique combination of global coverage, high vertical resolution and accuracy, long-term stability, and all-weather capability. We first describe our GPS/MET data processing scheme from excess phases via bending angles to the neutral atmospheric parameters refractivity, density, pressure and temperature. Special emphasis is given to ionospheric correction methodology and the inversion of bending angles to refractivities, where we introduce a matrix inversion technique (instead of the usual integral inversion). The matrix technique is shown to lead to identical results as integral inversion but is more directly extendable to inversion by optimal estimation. The quality of GPS/MET-derived profiles is analyzed with an error estimation analysis employing a Monte Carlo technique. We consider statistical errors together with systematic errors due to upper-boundary initialization of the retrieval by a priori bending angles. Perfect initialization and properly smoothed statistical errors allow for better than 1 K temperature retrieval accuracy up to the stratopause. No initialization and statistical errors yield better than 1 K accuracy up to 30 km but less than 3 K accuracy above 40 km. Given imperfect initialization, biases ≫ 2 K propagate down to below 30 km height in unfavorable realistic cases. Furthermore, results of a statistical validation of GPS/MET profiles through comparison with atmospheric analyses of the European Centre for Medium-range Weather Forecasts (ECMWF) are presented. The comparisons indicate the high utility of the occultation data in that very good agreement of upper troposphere/lower stratosphere temperature (better than 1.5 K rms, ≪ 0.5 K bias) is found for a region (Europe+USA) where the ECMWF analyses are known to be good, but poorer agreement for a region (Southern Pacific) where the analyses are known to be degraded.  相似文献   

CUTLASS Finland radar observations of the ionospheric signatures of flux transfer events and the resulting plasma flows   总被引：1，自引：0，他引：1  

G. Provan  T. K. Yeoman  S. E. Milan 《Annales Geophysicae》1998,16(11):1411-1422

The CUTLASS Finland radar has been run in a two-beam special scan mode, which offered excellent temporal and spatial information on the flows in the high-latitude ionosphere. A detailed study of one day of this data revealed a convection reversal boundary (CRB) in the CUTLASS field of view (f.o.v) on the dayside, the direction of plasma flow either side of the boundary being typical of a dawn-cell convection pattern. Poleward of the CRB a number of pulsed transients are observed, seemingly moving away from the radar. These transients are identified here as the ionospheric signature of flux transfer events (FTEs). Equatorward of the CRB continuous backscatter was observed, believed to be due to the return flow on closed field lines. The two-beam scan offered a new and innovative opportunity to determine the size and velocity of the ionospheric signatures associated with flux transfer events and the related plasma flow pattern. The transient signature was found to have an azimuthal extent of 1900 ± 900 km and an poleward extent of 250 km. The motion of the transient features was in a predominantly westward azimuthal direction, at a velocity of 7.5 ± 3 km.  相似文献   

渤海湾盆地早第三纪水平面升降和全球海平面升降的不一致性   总被引：4，自引：0，他引：4  

李守军 《地质论评》1998,44(1):31-34

本文通过对渤海湾盆地下第三系中半咸水化石纵向分布特点研究,并根据新的地层划分方案,利用古生物－岩相综合分析方法绘制了该地区早第三纪水平面升降曲线,发现半咸水生物出现在水进初期,在最大水进期消失,水平面升降曲线一全球海平面升降曲线无直接关系,即水平面的上升示受海平面上升的影响,从而证明此阶段无海侵发生,非海侵观点对于总结我国陆相地层的油气勘探理论及实践具有重要意义。  相似文献   

华北地台中寒武统张夏组上部高频层序研究   总被引：8，自引：1，他引：7  

陈建强  史晓颖 《地层学杂志》1998,22(2):109-115

根据华北地台东部寒武系露头层序地层研究,将张夏组划分为两个正层序,其中上部正层序（５３０－５２５ＭａＢ．Ｐ．,三叶虫带Ａｍｐｈｏｔｏｎ－Ｔａｉｔｚｕｉａ带至Ｄａｍｅｓｅｌａ带）由４个亚层序和１７－２３个小层序组成,其中正层序和亚层序具有高分辨率区域地层对比意义。每一级层序均表现为向上变浅和变粗进积型序列,底部常以间断－加深面或洪泛面为界。该时期的海平面变化具有高频振荡特点,构成一个三级复合海平面变化旋回。由不同级别层序所反映的海平面变化旋回可以分别与天文周期的奥特周期（２—５Ｍａ）、米兰科维奇长周期（０．１—０．４Ｍａ）和米兰科维奇短周期（０．０２—０．０４Ｍａ）相对比。据层序地层和相序及相模式的研究,识别出６种类型的小层序,陆棚盆地至上缓坡显示出由Ａ—Ｆ类型的规律性变化。  相似文献   

Average Annual Temperature Changes in the Holocene in China     

BI Fuzhi  YUAN Youshen Peking University  Cheng Ze Yuan  Beijing 《《地质学报》英文版》1998,72(3):321-328

In 1876 Blytt proposed a post-glacial climatic classification, maintaining that the then temperature fluctuated 1- 2℃ higher or lower than that today. Lamb (1969) held that in Europe " the axis of the subtropical high pressure belt was generally displaced north by about 10° latitudes" during the Hypsithermal and that the temperature was three to six times higher than that in the postglacial period.In China, there appear relict beachrocks (living fossils) in tropical tidal zones from the Bohai Bay (40° N) to coastal areas in South China. Many relict tropical marine fossils were discovered, such as Ostrea gigas in Tianjin (which is larger than that found today in Hainan), Placuna placenta in Zhejiang and Spondylus sp. in Fujian. In Hebei, Henan and Zhejiang provinces and Guilin of Guangxi, such tropical fossil animals as Elephas maximus and Rhinoceros sp, were discovered, and in Hebei and Jiangsu provinces such fresh water fossils as Unio douglasiae were found. Their living species are still seen in So  相似文献   

宝石颜色的命名与判定方法评述     

张庆麟 《上海国土资源》1998,(3):24-29

较系统评述了现有的各种命名与判定宝石颜色的方法,如命名法中的二名法、类似法、色度法和三要素法;判定法中的目测法、仪器测量法和对比法。指出它们均存在有不同程度的缺点,但相对比较起来,用色谱图系的对比法是目前较好的方法。  相似文献   

A new population of brown dwarfs     

M. R. S. Hawkins  C. Ducourant  H. R. A. Jones  & M. Rapaport 《Monthly notices of the Royal Astronomical Society》1998,294(3):505-512

In this paper we report the first results from a survey for low-mass stars and brown dwarfs, based on a photographic stack of around 100 Schmidt plates. This survey extends photographic searches by about 2 mag, and covers an area of 25 deg². Some 30 faint objects with large R − I colours were selected for further study, and were found to have very strong molecular absorption in their spectra, but only moderately red infrared colours. Five of these stars were selected for a parallax programme; three of these were found to be at a distance of around 45 pc, implying a very low luminosity. On the basis of their luminosity alone it is clear that these stars are field brown dwarfs, and we discuss their likely evolutionary status in the context of current models of low-mass stellar evolution.  相似文献