首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   82篇
  免费   5篇
  国内免费   1篇
测绘学   1篇
大气科学   9篇
地球物理   5篇
地质学   38篇
海洋学   9篇
天文学   5篇
自然地理   21篇
  2018年   1篇
  2014年   3篇
  2013年   9篇
  2011年   2篇
  2010年   2篇
  2008年   1篇
  2006年   3篇
  2005年   6篇
  2004年   2篇
  2003年   2篇
  2002年   5篇
  2001年   4篇
  2000年   3篇
  1999年   3篇
  1998年   4篇
  1997年   4篇
  1996年   9篇
  1995年   1篇
  1994年   3篇
  1993年   2篇
  1992年   1篇
  1991年   2篇
  1990年   2篇
  1989年   1篇
  1986年   1篇
  1985年   2篇
  1983年   1篇
  1980年   1篇
  1979年   2篇
  1978年   2篇
  1977年   1篇
  1969年   2篇
  1963年   1篇
排序方式: 共有88条查询结果,搜索用时 15 毫秒
1.
Abstract— A widely held view of nebular evolution is that during the ~0.5 Ma while interstellar material was collapsing onto the disk, the latter grew in mass to the point of gravitational instability. It responded to this by losing axial symmetry, growing spiral arms that had the capacity to tidally redistribute disk mass (inward) and angular momentum (outward) and prevent further increase in the disk/protosun mass ratio. The spiral arms (density waves) rotated differently than the substance of the nebula, and in some parts of the disk, nebular material may have encountered the arms at supersonic velocities. The disk gas, and solid particles entrained in it, would have been heated to some degree when they passed through shock fronts at the leading edges of the spiral arms. The present paper proposes this was the energetic nebular setting or environment that has long been sought, in which the material now in the planets and chondritic meteorites was thermally processed.  相似文献   
2.
Quaternary lavas and pyroclastic rocks of Mount Mazama, CraterLake caldera, and the surrounding area have variable Sr, Nd,and Pb isotopic compositions. High-alumina olivine tholeiites(HAOT) have 87Sr/86Sr ratios of 0.70346–0.70364; basalticandesite, 0–70349–0.70372; shoshonitic basalticandesite, 0.70374–0.70388; and andesite, 0.70324–0.70383.Dacites of Mount Mazama have 87Sr/86Sr ratios of 0.70348–0.70373.Most rhyodacites converge on 0.7037. However, rhyodacite ofthe caldera-forming, climactic eruption has 87Sr/86Sr=0.70354because of an admixed low-87Sr/86Sr component. Andesitic tomafic-cumulate scoriae of the climactic eruption, and enclavesin preclimactic rhyodacites, cluster in two groups but shownearly the entire 87Sr/86Sr range of the data set, confirmingpreviously suggested introduction of diverse parental magmasinto the growing climactic chamber. Pb and Nd isotope ratiosdisplay less variation (206Pb/204Pb= 18.838–18.967, 207Pb/204Pb=15.556–15.616,208Pb/204Pb=38.405–38.619; Nd= +3.9 to +6.1) and generallycovary with 87Sr/86 Sr ratios. Radiogenic isotope data fromCrater Lake plot with published data for other Cascade volcanoeson isotope ratio correlation diagrams. The isotopic data for the Crater Lake area require sources ofprimitive magmas to consist of depleted mantle and a subductioncomponent, introduced in variable quantity to the depleted mantlewedge. Variable degrees of melting of this heterogeneous mantle,possibly at different depths, produced the diversity of isotopiccompositions and large-ion lithophile element (LILE) abundancesin primitive magmas. Trace element ratios do not indicate presenceof an ocean island basalt (OIB) source component that has beenreported in lavas of some other Cascade volcanoes. Crustal contamination may have affected isotope ratios and LILEconcentrations in evolved HAOT, where initial LILE concentrationswere low. Contamination is more difficult to detect in the calcalkalinelavas because of their higher LILE concentrations and the smallisotopic contrast with likely contaminants, such as mid- tolower-crustal rocks thought to be equivalents of igneous rocksof the Klamath Mountains and associated lower crust. Crustalassimilation appears to be required for calcalkaline rocks onlyby 18O values, which vary from lows of +5.6 to + 6.0% in HAOTand primitive basaltic andesites to a high of +7.0% in dacite,a range that is too high to be explained by plagioclase-dominatedclosed-system fractional crystallization. Elevated 18O valuesof differentiated lavas may be attributed to interaction withrelatively 18O-rich, 87Sr-poor crustal rocks. Variably fused granitoid blocks ejected in the climactic eruption,and rarely in late Pleistocene eruptive units, have 18Opl of–3.4 to +6.5% and 18Oqz of –2.2 to +8.0% but haveSr, Nd, and Pb isotope ratios similar to volcanic rocks (e.g.87Sr/86Sr0.7037). Rb and Sr data for glass separates from granodioritessuggest that the source pluton is Miocene. Glass from granodioritehas 87Sr/86Sr ratios as high as 0.70617. Oxygen isotope fractionationbetween quartz, plagioclase, and glass indicates requilibrationof O isotopes at magmatic temperatures, after 18O/16O had beenlowered by exchange with meteoric hydrothermal fluids. Unmeltedgranodiorite xenoliths from pre-climactic eruptive units have18O values that are consistent with onset of hydrothermal exchangeearly during growth of the climactic magma chamber. Assimilationof such upper-crustal granodiorite apparently lowered 18O valuesof rhyodacites without significantly affecting their magmaticcompositions in other ways.  相似文献   
3.
ABSTRACT. The spatial world of the child comprises two great realms, “in’ and “out,’ more or less corresponding to the great behavioral domains of “got to’ and “doing nothing.’ Within the “got‐to’ domain–and thus “in’–are home, school, and other loci of secure, stable authority. Included in “doing nothing’–and so “out’–are street and yard, field and stream, pool hall and movie theater, the spaces of games, things to do, and really doing nothing. This study looks at the shifting relationships of these realms across the life cycle of kids in the highlands of Puerto Rico in 1969–1970.  相似文献   
4.
ABSTRACT. Vietnamese Americans have made places for themselves in Northern Virginia by reconfiguring the geography of the suburban places they inherited, including former high-order central-place nodes. Vietnamese American residences, churches, cemetery plots, and other distinctive ethnic markers are by and large dispersed and rarely noticeable. Their retail districts, however, serve them in multiple material and symbolic ways, not unlike suburban Chinatowns.  相似文献   
5.
6.
Rocks of the Proterozoic gabbro—diorite—tonalite—trondhjemitesuite of southwest Finland were analyzed for major elements,REE, Rb, Sr, Ba, U, Th, and isotopic composition of Sr and O.Petrographic continuity from hornblendite through hornblendegabbro, hornblende—biotite diorite, hornblende—biotitetonalite and trondhjemite is reflected in regular variationof major and trace elements in samples ranging from 42 to 74per cent of SiO2. The suite is calc alkaline—trondhjemiticand is distinguished from ‘normal’ calc-alkalinesuites by the increase in Na2O and decrease in K2O and REE concentrationsfrom intermediate to silicic rocks. Prior to solidification, the magmas may have undergone additionof water from metamorphic country rock producing a variationof whole-rock O18 values of from 6.0 to 11.3 per mil. This processmay have been accompanied by introduction of Rb and Sr and removalof Ba from intermediate to trondhjemitic magmas. A linear whole-rockisochron age of 1.9 x 109 years may only be approximate owingto the possibility of Rb and Sr exchange. REE concentrationsshow regular variation through the suite and were not noticeablyaffected by exchange processes. Samples ranging from 50 to 60per cent SiO2 show chondrite-normalized light REE contents rangingfrom 40 to 160 whereas heavy REE contents are constant at 10,and there are no Eu anomalies. Samples ranging from 60 to 74per cent SiO2 have REE patterns which decrease in both lightand heavy REE with increasing SiO2 to values of 10 and 1 timeschondritic values respectively, and show increasingly positiveEu anomalies. Three possible models for magma genesis are consistent withthe petrographic, major and trace element variations, isotopicdata, and experimental petrologic studies: (1) fractional crystallizationof a gabbroic liquid involving hornblende, plagioclase, andbiotite as the major precipitating phases; (2) partial meltingof amphibolite leaving a hornblende-rich residue; (3) partialmelting of amphibolite or eclogite leaving an eclogitic residue.Model (1) is preferred because of the presence of hornblende-cumulaterocks, and because there is a continuum of compositions fromgabbro to trondhjemite.  相似文献   
7.
P. A. WOOD 《Sedimentology》1977,24(3):437-445
Suspended sediment concentrations were determined for samples collected from the River Rother, West Sussex, and rating loops constructed for several hydrographs. The rating loops often exhibit hysteresis with a greater suspended sediment concentration for a given discharge occurring on the rising limb than on the falling limb. A comparison of these loops indicates that the hysteresis, and the suspended sediment concentration (for a given discharge) become progressively reduced when storm events occur in rapid succession. Various types of rating curves can be identified, and a model of suspended sediment concentration and discharge constructed. Controlling factors of suspended sediment concentration are river discharge, duration and frequency of the storm event, the length of time between successive events, and the time of year.  相似文献   
8.
9.
10.
Zircons from the Devils Kitchen rhyolite in the PleistoceneCoso Volcanic field, California have been analyzed by in situPb/U ion microprobe (SHRIMP-RG) and by detailed cathodoluminescenceimaging. The zircons yield common-Pb-corrected and disequilibrium-corrected206Pb/238U ages that predate a previously reported K–Arsanidine age by up to 200 kyr, and the range of ages exhibitedby the zircons is also approximately 200 kyr. Cathodoluminescenceimaging indicates that zircons formed in contrasting environments.Most zircons are euhedral, and a majority of the zircons areweakly zoned, but many also have anhedral, embayed cores, witheuhedral overgrowths and multiple internal surfaces that aretruncated by later crystal zones. Concentrations of U and Thvary by two orders of magnitude within the zircon population,and by 10–20 times between zones within some zircon crystals,indicating that zircons were transferred between contrastingchemical environments. A zircon saturation temperature of 750°Coverlaps within error a previously reported phenocryst equilibrationtemperature of 740 ± 25°C. Textures in zircons indicativeof repeated dissolution and subsequent regrowth are probablycaused by punctuated heating by mafic magma input into rhyolite.The overall span of ages and large variation in U and Th concentrations,combined with calculated zircon saturation temperatures andresorption times, are most compatible with crystallization inmagma bodies that were emplaced piecemeal in the crust at Cosoover 200 kyr prior to eruption, and that were periodically rejuvenatedor melted by subsequent basaltic injections. KEY WORDS: zircon geochronology; residence time; rhyolite; ion microprobe; California  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号