全文获取类型
收费全文 | 691篇 |
免费 | 92篇 |
国内免费 | 103篇 |
专业分类
测绘学 | 5篇 |
大气科学 | 6篇 |
地球物理 | 439篇 |
地质学 | 330篇 |
海洋学 | 45篇 |
天文学 | 2篇 |
综合类 | 20篇 |
自然地理 | 39篇 |
出版年
2024年 | 2篇 |
2023年 | 5篇 |
2022年 | 19篇 |
2021年 | 17篇 |
2020年 | 24篇 |
2019年 | 13篇 |
2018年 | 16篇 |
2017年 | 17篇 |
2016年 | 13篇 |
2015年 | 14篇 |
2014年 | 32篇 |
2013年 | 24篇 |
2012年 | 16篇 |
2011年 | 28篇 |
2010年 | 11篇 |
2009年 | 63篇 |
2008年 | 95篇 |
2007年 | 51篇 |
2006年 | 56篇 |
2005年 | 57篇 |
2004年 | 40篇 |
2003年 | 42篇 |
2002年 | 18篇 |
2001年 | 15篇 |
2000年 | 48篇 |
1999年 | 38篇 |
1998年 | 32篇 |
1997年 | 17篇 |
1996年 | 19篇 |
1995年 | 8篇 |
1994年 | 6篇 |
1993年 | 8篇 |
1992年 | 6篇 |
1991年 | 4篇 |
1990年 | 4篇 |
1989年 | 4篇 |
1987年 | 1篇 |
1984年 | 1篇 |
1983年 | 1篇 |
1954年 | 1篇 |
排序方式: 共有886条查询结果,搜索用时 15 毫秒
51.
Llullaillaco is one of a chain of Quaternary stratovolcanoes that defines the present Andean Central Volcanic Zone (CVZ), and marks the border between Chile and Argentina/Bolivia. The current edifice is constructed from a series of thick dacitic lava flows, forming the second tallest active volcano in the world (6739 m). K–Ar and new biotite laser 40Ar/39Ar step-heating dates indicate that the volcano was constructed during the Pleistocene (≤1.5 Ma), with a youngest date of 0.048±0.012 Ma being recorded for a fresh dacite flow that descends the southern flank. Additional 40Ar/39Ar measurements for andesitic and dacitic lava flows from the surrounding volcanic terrain yield dates of between 11.94±0.13 Ma and 5.48±0.07 Ma, corresponding to an extended period of Miocene volcanism which defines much of the landscape in this region. Major- and trace-element compositions of lavas from Llullaillaco are typical of Miocene–Pleistocene volcanic rocks from the western margin of the CVZ, and are related to relatively shallow-dipping subduction of the Nazca plate beneath northern Chile and Argentina.Oversteepening of the edifice by stacking of thick, viscous, dacitic lava flows resulted in collapse of its southeastern flank to form a large volcanic debris avalanche. Biotite 40Ar/39Ar dating of lava blocks from the avalanche deposit indicate that collapse occurred at or after 0.15 Ma, and may have been triggered by extrusion of a dacitic flow similar to the one dated at 0.048±0.012 Ma. The avalanche deposits are exceptionally well preserved due to the arid climate, and prominent levées, longitudinal ridges, and megablocks up to 20-m diameter are observed.The avalanche descended 2.8 km vertically, and bifurcated around an older volcano, Cerro Rosado, before debouching onto the salt flats of Salina de Llullaillaco. The north and south limbs of the avalanche traveled 25 and 23 km, respectively, and together cover an area of approximately 165 km2. Estimates of deposit volume are hampered by a lack of thickness information except at the edges, but it is likely to be between 1 and 2 km3. Equivalent coefficients of friction of 0.11 and 0.12, and excess travel distances of 20.5 and 18.5 km, are calculated for the north and south limbs, respectively. The avalanche ascended 400 m where it broke against the western flank of Cerro Rosado, and a minimum flow velocity of 90 m s−1 can be calculated at this point; lower velocities of 45 m s−1 are calculated where distal toes ascend 200 m slopes.It is suggested that the remaining precipitous edifice has a high probability for further avalanche collapse in the event of renewed volcanism. 相似文献
52.
Guido Giordano Massimiliano Porreca Pietro Musacchio Massimo Mattei 《Bulletin of Volcanology》2008,70(10):1221-1236
The edifice of Stromboli volcano gravitationally collapsed several times during its volcanic history (>100 ka–present). The
largest Holocene event occurred during the final stage of the Neostromboli activity (∼13–5 ka), and was accompanied by the
emplacement of phreatomagmatic and lahar deposits, known as the Secche di Lazzaro succession. A stratigraphic and paleomagnetic
study of the Secche di Lazzaro deposits allows the interpretation of the emplacement and the eruptive processes. We identify
three main units within the succession that correspond to changing eruption conditions. The lower unit (UA) consists of accretionary
lapilli-rich, thinly bedded, parallel- to cross-stratified ash deposits, interpreted to indicate the early stages of the eruption
and emplacement of dilute pyroclastic density currents. Upward, the second unit (UB) of the deposit is more massive and the
beds thicker, indicating an increase in the sedimentation rate from pyroclastic density currents. The upper unit (UC) caps
the succession with thick, immediately post-eruptive lahars, which reworked ash deposited on the volcano’s slope. Flow directions
obtained by Anisotropy of Magnetic Susceptibility (AMS) analysis of the basal bed of UA at the type locality suggest a provenance
of pyroclastic currents from the sea. This is interpreted to be related to the initial base-surges associated with water–magma
interaction that occurred immediately after the lateral collapse, which wrapped around the shoulder of the sector collapse
scar. Upward in the stratigraphy (upper beds of UA and UB) paleoflow directions change and show a provenance from the summit
vent, probably related to the multiple collapses of a vertical, pulsatory eruptive column. 相似文献
53.
简单介绍了SAR图像的纹理特征以及正交小波变换纹理提取方法。论述了SAR图像的纹理特征参与分类的重要性。以长白山天池火山为例,通过对ERS2SAR图像进行纹理分析,提取了SAR图像两个层次的尺度变化、时频局部化和方向性纹理特征。并将SAR纹理特征与TM图像及DEM进行复合,利用多源信息各自的优势,进行了BP神经元网络分类,从较大范围对长白山天池火山735±15aB.P.大喷发的喷发物空间分布进行评价。获取了长白山天池火山近代喷发物的空间分布及规模。这对长白山天池火山未来喷发危险性初步评价、火山地质制图及火山灾害预测有重要意义。 相似文献
54.
中国及邻区分布着众多的全新世陆相活火山,并留下了大量的喷发历史记录。对这些活火山开展监测和研究工作是预测火山喷发和火山灾害减灾防灾工作的重要途径。高度剥蚀的晚中生代火山区的大量地质填图,以及在活火山区的地球物理、地球化学等近实时的探测工作,极大地深化了我国东部中生代—新生代陆相火山岩的成分特征、岩浆演化规律及喷发方式等方面的认识,是开展火山监测的理想研究对象。近年来,火山灾害日益引起关注,全球火山监测研究进展迅速。本文系统梳理了目前国内外主要的火山监测方法及取得的重要进展,并分析了未来火山灾害减灾防灾工作的发展趋势及我国火山监测工作的发展方向,旨在通过介绍国际火山监测工作,为我国将来开展火山监测、预测与火山灾害相关的减灾防灾等方面的研究和国际合作提供借鉴。 相似文献
55.
利用蒙古中南部台阵记录的连续背景噪声数据、天然地震面波数据和远震P波波形数据, 开展瑞利波相速度、椭圆率和远震P波叠加波形的联合反演, 建立了蒙古中南部地壳三维S波速度模型.结果显示, 蒙古中南部的瑞利波相速度及椭圆率、沉积基底深度、莫霍面深度、地壳S波速度在蒙古—鄂霍茨克缝合线和蒙古主构造线南北两侧分布均有明显差异, 暗示了这两条缝合线至少为地壳级别的分界.中戈壁火山和Bus-Obo火山在地壳内相互连通, 在下地壳存在大面积低速层.我们推测杭盖穹隆上地幔热物质上涌在中戈壁带地壳底部形成岩浆囊, 为这两座板内火山活动提供了岩浆来源.
相似文献56.
Nicole C. Lautze Bruce F. Houghton 《Journal of Volcanology and Geothermal Research》2008,170(3-4):262-268
A combination of fortuitous eruptive and wind conditions at Stromboli volcano in September 2002 enabled the collection of samples of multiple lapilli from individual observed explosions. These ejecta present the first opportunity to analyze the vesicularity of material ejected in a single Strombolian explosion. Samples of between 40 and 92 lapilli were collected from each of six sequential explosions on 30 September 2002, and 28 lapilli were obtained from a single explosion 24 h later (1 October). Density measurements and microtextural observations show that considerable heterogeneity existed within each of the seven samples. Centimeter to millimeter size bubble-rich and bubble-poor zones are present and are, in places, mingled together. These data confirm that the shallow conduit at Stromboli is a texturally diverse environment at the instant of a single explosion, and that a similar range of heterogeneity can persist through closely-spaced sequences of explosions on a timescale of hours and, probably, days, despite the dynamic processes of ascent of melt and decoupled gas phases. 相似文献
57.
Katla volcano,Iceland: magma composition,dynamics and eruption frequency as recorded by Holocene tephra layers 总被引:1,自引:0,他引:1
Bergrún Arna Óladóttir Olgeir Sigmarsson Gudrun Larsen Thor Thordarson 《Bulletin of Volcanology》2008,70(4):475-493
The Katla volcano in Iceland is characterized by subglacial explosive eruptions of Fe–Ti basalt composition. Although the
nature and products of historical Katla eruptions (i.e. over the last 1,100 years) at the volcano is well-documented, the
long term evolution of Katla’s volcanic activity and magma production is less well known. A study of the tephra stratigraphy
from a composite soil section to the east of the volcano has been undertaken with emphasis on the prehistoric deposits. The
section records ∼8,400 years of explosive activity at Katla volcano and includes 208 tephra layers of which 126 samples were
analysed for major-element composition. The age of individual Katla layers was calculated using soil accumulation rates (SAR)
derived from soil thicknesses between 14C-dated marker tephra layers. Temporal variations in major-element compositions of the basaltic tephra divide the ∼8,400-year
record into eight intervals with durations of 510–1,750 years. Concentrations of incompatible elements (e.g. K2O) in individual intervals reveal changes that are characterized as constant, irregular, and increasing. These variations
in incompatible elements correlate with changes in other major-element concentrations and suggest that the magmatic evolution
of the basalts beneath Katla is primarily controlled by fractional crystallisation. In addition, binary mixing between a basaltic
component and a silicic melt is inferred for several tephra layers of intermediate composition. Small to moderate eruptions
of silicic tephra (SILK) occur throughout the Holocene. However, these events do not appear to exhibit strong influence on
the magmatic evolution of the basalts. Nevertheless, peaks in the frequency of basaltic and silicic eruptions are contemporaneous.
The observed pattern of change in tephra composition within individual time intervals suggests different conditions in the
plumbing system beneath Katla volcano. At present, the cause of change of the magma plumbing system is not clear, but might
be related to eruptions of eight known Holocene lavas around the volcano. Two cycles are observed throughout the Holocene,
each involving three stages of plumbing system evolution. A cycle begins with an interval characterized by simple plumbing
system, as indicated by uniform major element compositions. This is followed by an interval of sill and dyke system, as depicted
by irregular temporal variations in major element compositions. This stage eventually leads to a formation of a magma chamber,
represented by an interval with increasing concentrations of incompatible elements with time. The eruption frequency within
the cycle increases from the stage of a simple plumbing system to the sill and dyke complex stage and then drops again during
magma chamber stage. In accordance with this model, Katla volcano is at present in the first interval (i.e. simple plumbing
system) of the third cycle because the activity in historical time has been characterized by uniform magma composition and
relatively low eruption frequency. 相似文献
58.
59.
The magma evolution of Tianchi volcano, Changbaishan 总被引:4,自引:0,他引:4
The Changbaishan Tianchi volcano is composed of the basaltic rocks at the shield-forming stage, the trachyte and pantellerite
at the cone-forming stage and modern eruption. Studies on their REE, incompatible elements and Sr, Nd, Pb isotopes suggest
that rocks at different stages have a common magma genesis and close evolution relationship with differentiation crystallization
playing the key role. The co-eruption of basaltic trachyandesite magma and pantellerite magma indicates that there exist both
crustal magma chamber and mantle magma reservoir beneath the Tianchi volcano.
Project supported by the National Natural Science Foundation of China (Grant No. 49672109). 相似文献
60.
琼北火山区是中国的主要火山群之一。笔者应用HDS - 1型快速数字闪烁测氡仪对琼北火山群马鞍岭和雷虎岭段的火山及断裂活动性进行了勘测和研究 ,对荣山 -岭南断裂进行了重点测量 ,查明了该断裂的具体位置 ,研究了火山及断裂的气体释放特征。发现Rn(氡 ) ,Th(钍 )的变化与断裂构造有密切关系 ,在Ⅰ ,Ⅱ测线上Th ,Rn异常对断裂的反映相当好。根据Th ,Rn的展布特征和浓度变化比较深入地分析了断裂的活动性。在雷虎岭西南 ,荣山 -岭南断裂活动性最强 ,而且 ,自西北向东南断裂规模及活动性均有增强的趋势。此外 ,还对火山的活动性进行了讨论和分析 相似文献