全文获取类型
收费全文 | 2141篇 |
免费 | 381篇 |
国内免费 | 880篇 |
专业分类
测绘学 | 2篇 |
大气科学 | 25篇 |
地球物理 | 465篇 |
地质学 | 2569篇 |
海洋学 | 81篇 |
天文学 | 6篇 |
综合类 | 110篇 |
自然地理 | 144篇 |
出版年
2024年 | 8篇 |
2023年 | 24篇 |
2022年 | 51篇 |
2021年 | 52篇 |
2020年 | 68篇 |
2019年 | 87篇 |
2018年 | 75篇 |
2017年 | 103篇 |
2016年 | 95篇 |
2015年 | 86篇 |
2014年 | 125篇 |
2013年 | 128篇 |
2012年 | 178篇 |
2011年 | 136篇 |
2010年 | 106篇 |
2009年 | 129篇 |
2008年 | 195篇 |
2007年 | 175篇 |
2006年 | 149篇 |
2005年 | 132篇 |
2004年 | 118篇 |
2003年 | 151篇 |
2002年 | 107篇 |
2001年 | 107篇 |
2000年 | 114篇 |
1999年 | 143篇 |
1998年 | 99篇 |
1997年 | 77篇 |
1996年 | 82篇 |
1995年 | 65篇 |
1994年 | 69篇 |
1993年 | 39篇 |
1992年 | 34篇 |
1991年 | 26篇 |
1990年 | 16篇 |
1989年 | 13篇 |
1988年 | 17篇 |
1987年 | 10篇 |
1986年 | 6篇 |
1985年 | 2篇 |
1984年 | 1篇 |
1983年 | 1篇 |
1982年 | 1篇 |
1978年 | 1篇 |
1954年 | 1篇 |
排序方式: 共有3402条查询结果,搜索用时 31 毫秒
781.
大兴安岭南段东麓中生代O型埃达克质火山岩及其成因、古构造环境和找矿意义 总被引:7,自引:1,他引:7
蒙东西拉禾伦河北侧查干木伦河流域,属中生代大兴安岭火山-侵入杂岩带南段。最近区内晚侏罗世满克头鄂博组陆相火山杂岩火山盾中,发现有埃达克质火山岩,与少量钠质流纹岩和玄武岩共生。埃达克岩 Al_2O_3>15%,MgO<3.4%,K_2O 平均=1.77%,Na_2O/K_2O>1,LREE 富集,HREE 及 LIRE 强烈亏损,无负铕异常,高 Sr(497.91~896.40×10~(-6)),低 Y(4.33~15.77<18×10~(-6)),低 Yb(0.35~1.54<1.9×10~(-6)),I_(Sr)=0.70374~0.70594,ε_(Nd)(t)为-0.45~ 6.58,在 K-Na-Ca、Rb-Sr-Ba、Sr/Y-Y 和 Sr/Yb-Yb 等图中属埃达克岩,具奥长花岗岩演化趋势,是典型的 O 型埃达克岩。其与共生岩石ε_(Nd)(t)、I_(Sr)与晚古生代玄武岩的接近,部分岩石 t_(DM)(677~845Ma)与晚古生代俯冲洋壳(t_(DM)=627~998Ma)相同,Sr-Nd 同位素及Zr/Hf-Nb/Ta 比值具有亏损地幔与下地壳混合特征,另有岩石(t_(DM)=343~453Ma)源于古生代年轻地幔。据此推论其源岩是以晚元古代俯冲洋壳为主,多源物质混合的增生地体下地壳。La_N/Yb_N-Yb_N 及 La/Sm-La 图证明埃达质岩浆是榴辉岩化源岩部分熔融的原始岩浆,钠质流纹岩是该原始岩浆分离结晶产物,玄武岩浆则可能是源岩深度熔融的结果。岩石成分及微量元素判别该火山活动环境是大陆边缘弧。大兴安岭地区在中生代出现埃达克质火山岩及该类火山弧,未必与环太平洋或者鄂霍茨克海俯冲构造直接相关,而是晚古生代古亚洲洋残存洋壳在中生代沿续存俯冲断裂继续向增生地体下地壳俯冲并发生部分熔融结果。区内与满克头鄂博组有关的铁、铜、金矿化,应有大矿前景。 相似文献
782.
西藏改则县托和平错地区展金组中发育一套早-中二叠世以玄武岩类为主、玄武安山岩次之、安山岩少量的海相火山岩组合.该火山岩中玄武岩、玄武安山岩的SiO2含量为44.43%~54.87%,TiO2含量多在4%以上,K2O+Na2O平均为4.57%,玄武岩类富K,玄武安山岩富Na.区内火山岩富集K、Ba、Th、Ta、Ce、P、Zr、Sm、Ti、Cr,亏损Rb、Nb、Hf、Sc,Y/Nb在0.322~0.499之间.综合常量元素和微量元素特征,可将该火山岩归入碱性玄武岩系列.区内火山岩从基性到中性稀土元素总量基本没有变化,REE平均为402.31×10-6,LREE/HREE为8.17~12.69,为轻稀土强烈富集型,δEu为0.93~1.04,平均为0.99,无Eu异常,玄武岩的Th/Ta值多数大于1.6.火山岩的地质特征和地球化学特征表明,本区火山岩产出的大地构造环境为冈瓦纳大陆北缘裂谷环境. 相似文献
783.
Phil Shane 《Bulletin of Volcanology》1998,60(3):224-238
Grain-specific analyses of Fe–Ti oxides and estimates of eruption temperature (T) and oxygen fugacity (fO2) have been used to fingerprint rhyolitic fall and flow deposits that are important for tephrostratigraphic studies in and
around the Taupo volcanic zone of North Island, New Zealand. The analysed Fe–Ti oxides commonly occur in the rims of orthopyroxene
crystals and appear to reflect equilibrium immediately prior to eruption because of geochemical correlation with the co-existing
glass phase. The composition of the spinel phase is particularly diagnostic of eruptive centre for post-65 ka events and can
be used to distinguish many tephra beds from the same volcano. The 29 different units examined were erupted over a wide range
in T (690–990°C) and Δ log fO2 (–0.1 to 2.0). These parameters are closely related to the mafic mineral assemblage, with hydrous mineral-bearing units displaying
higher fO2. Such trends are superimposed on larger differences in fO2 that are related to eruptive centre. At any given temperature, all post-65 ka Okataina centre tephra have higher fO2 values than post-65 ka Taupo centre tephra. This provides a useful criterion for identifying the volcanic source. There are
no temporal T and fO2 trends in the tephra record; over intervals >20 ka, however, tephra sequences from Taupo centre form characteristic T-fO2 buffer trends mirroring the glass chemistry. Individual eruptive events display uniform spinel and rhombohedral phase compositions
and thus narrow ranges in T (± <20°C) and log fO2 (± <0.5), allowing these features to identify individual magma batches. These criteria can help distinguish tephra deposits
of similar bulk or glass composition that originated from the same volcano. Distal fall deposits record the same T-fO2 conditions as the proximal ignimbrite and enable distal–proximal correlation. Lateral and vertical compositional and T-fO2 variability displayed in large volume (>100 km3) ignimbrites, such as the Oruanui, Rotoiti and Ongatiti, is similar to that found in a single pumice clast and thus mainly
reflects analytical error; however, thermal gradients of ca. 50°C may occur in some units.
Received: 6 April 1998 / Accepted: 16 June 1998 相似文献
784.
Hüseyin Yaln M.Niyazi Gündodu Alain Gourgaud Philippe Vidal Ali Uurum 《Journal of Volcanology and Geothermal Research》1998,85(1-4)
Neogene Yamadağı volcanic rocks consist of basaltic trachyandesite, trachyandesite, andesite, and dacite. The major- and trace-element chemistry indicates that the lavas are dominantly calc-alkaline and mildly alkaline in character, sodic in nature, and intermediate to acidic in composition. REE and trace-element patterns of volcanic rocks are similar to those typical of within plate magmatics. Volcanic rocks have low 87Sr/86Sr (0.70389–0.70633) and high 143Nd/144Nd ratios (0.51267–0.51276) and mostly plot within the mantle array of the isotope ratio diagram. The linear correlations among 87Sr/86Sr−143Nd/144Nd, SiO2−87Sr/86Sr and SiO2−143Nd/144Nd isotope ratios in the volcanics suggest that fractional crystallization combined with minor assimilation was an important process within the collision zone. 相似文献
785.
Computer-simulation models of scoria cone degradation 总被引:2,自引:0,他引:2
Long-term erosional modifications of the relatively simple morphology of scoria (‘cinder') cones are ideally suited for study by field and computer-simulation methods. A series of temporally-distinct cones in the San Francisco and Springerville volcanic fields of Arizona provides the foundation for documenting the degradational evolution of scoria cones in a semi-arid climate. Progressive changes due to erosion are illustrated by the systematic decrease with increasing age of various morphometric parameters, including scoria cone height, cone height/width ratio (Hco/Wco), crater depth/width ratio, and slope angle. For example, Holocene–latest Pleistocene cones in the San Francisco field have a mean Hco/Wco value of 0.178±0.041, a mean maximum slope angle of 29.7±4.2°, and a mean average slope angle of 26.4±7.3°, whereas the group of Pliocene cones have values of 0.077±0.024, 20.5±5.8°, and 8.7±2.7°, respectively. Comparative morphology of scoria cones is a potentially useful dating tool for mapping volcanic fields.In order to better understand the degradational modifications of these volcanic landforms, we have developed a numerical approach to simulate the surficial processes responsible for the erosion of a typical scoria cone. The simulation algorithm can apply either a linear diffusion-equation model or a model with a nonlinear transport law. Using a finite-difference formulation, the simulation operates upon a three-dimensional scoria cone input as a matrix of elevation values. Utilizing both field and model results, the correlation between changing Hco/Wco value, cone age, and computer time step was expressed graphically to derive comprehensive values of the transport or diffusion coefficient (Df) for both volcanic fields. For the San Francisco volcanic field, Df had a calculated value of 21.4 m2/kyr for the linear model and 5.3 m/kyr for the nonlinear model, while for the Springerville volcanic field Df had a calculated value of 24.4 m2/kyr for the linear model and 6.3 m/kyr for the nonlinear model. 相似文献
786.
Ignacio Galindo Lev S. Ivlev Arturo Gonzlez Roberto Ayala 《Journal of Volcanology and Geothermal Research》1998,83(3-4)
Airborne and ground-based (correlation spectrometer, cascade impactor, and photoelectric counter together with intake filter probes) measurements are described for the volcanic emissions from Popocatépetl volcano (Mexico) from December 23, 1994 to January 28, 1995. Measurements of SO2 restarted 48 h after the eruption onset of December 21, 1994. Maximum sulfur dioxide (4560 t d−1) plus 3.8×104 t d−1 of particulate matter were ejected on December 24, 1994. The maximum rate of ejection occurred coincidentally with the maximum amplitude of harmonic tremor and the maximum number of seismic type B events. Sulfur dioxide emission rates ranged from 1790 to 2070 t d−1 (December 23–24, 1994). Afterwards, sulfur dioxide emission rates clearly indicated a consistent decline. However, frequent gas and ash emission puffs exhibited SO2 fluxes reaching values as high as 3060 t d−1. The emission SO2 baseline for the period of study (February 1994–January 1995) was about 1000 t d−1. Ejection velocity of particulate matter was approximately 270 m s−1 reaching a height of about 2.5 km over the summit. The immediate aerosol dispersion area was estimated at 6.0×104 km2 maximum. The microscopic structure of particles (aerosol and tephra) showed a fragile material, probably coming from weathered crustal layers. X-ray fluorescence and neutron-activation analysis from the impactor samples found the following elements: Si, Al, Ca, S, P, Cl, K, Ni, Fe, Ti, Sc, Cu, Zn, Mn, Sr, Cr, Co, Y, Br, Se, Ga, Rb, Hg and Pb. Morphological analysis shows that ash samples might be from pulverized basaltic rock indicating that the Popocatépetl eruption of December 21, 1994 was at low temperature. The microscopic structure of puff material showed substance aggregates consisted of fragile rock, water and adsorbed SO2. These aggregates were observed within water droplets of approximately 1 mm and even larger. Sulfur transformations in the droplets occurred intensively. Volcanic ash contained 5–6% of sulfur during the first expulsion hours. Elemental relative concentrations with respect to Al show that both Si and S have relative concentrations >1, i.e., 13.73 and 2.17, respectively in agreement with the photoelectric counter and COSPEC measurements. 相似文献
787.
788.
甘肃公婆泉铜矿田中—晚志留世浅海相火山喷发旋回及火山作用演化 总被引:9,自引:2,他引:7
根据甘肃公婆泉铜矿田内4条典型地层剖面,确立了甘肃公婆泉群中-上志留统浅海相火山喷发沉积岩系的标准层和层序,并将其划分为14个韵律和5个旋回。火山喷发旋回由早→晚,火山岩岩性由安山质凝灰岩、安山岩→英安岩、钠流纹岩→石英粗面岩→安山岩,火山岩中铁族元素的丰度由高→低→最低(在石英粗面岩中)→高,形成一个较完整的演化周期。中晚期火山岩所表现出的岩性(出现碱性岩-石英粗面岩)和铁族元素丰度(出现最低值)的突变与区域北西走向断裂的深切活动有关,为铜、银等金属成矿提供主要的地球化学条件。铜、银等成矿元素主要富集在火山喷发旋回中期的英安岩中,表明火山活动中期是主要的成矿期,英安岩可作为重要的找矿标志。此外,早中期火山旋回主要发育于矿田的中西部,而晚期旋回则主要分布在矿田的东部,并且火山喷发的强度和频率增高,表明晚期火山活动中心已明显向东部迁移,因此矿田的东部仍有较大的找矿潜力 相似文献
789.
Plinian式火山喷发柱最大高度定量计算模式探讨 总被引:1,自引:0,他引:1
Plinian式火山喷发柱最大高度是研究火山喷发对环境、气候影响时空范围的重要物理学参数,在分析了影响Plinian式火山喷发柱最大高度诸因素的基础上认为,火山喷发的质量通量是反映火山喷发柱最大高度最有效并且在实际中可以进行测量的参数之一,以此为基础,利用目前实测的火山喷发柱最大高度的数值与火山喷发质量通量的结果,建立两者的相关方程为:H=0.2×M0.28。 相似文献
790.