全文获取类型
收费全文 | 425篇 |
免费 | 4篇 |
国内免费 | 2篇 |
专业分类
测绘学 | 3篇 |
大气科学 | 47篇 |
地球物理 | 26篇 |
地质学 | 240篇 |
海洋学 | 27篇 |
天文学 | 12篇 |
自然地理 | 76篇 |
出版年
2014年 | 4篇 |
2013年 | 26篇 |
2012年 | 6篇 |
2011年 | 9篇 |
2010年 | 10篇 |
2009年 | 25篇 |
2008年 | 11篇 |
2007年 | 8篇 |
2006年 | 13篇 |
2005年 | 18篇 |
2004年 | 9篇 |
2003年 | 11篇 |
2002年 | 11篇 |
2001年 | 7篇 |
2000年 | 13篇 |
1999年 | 6篇 |
1998年 | 6篇 |
1997年 | 24篇 |
1996年 | 23篇 |
1995年 | 12篇 |
1994年 | 11篇 |
1993年 | 17篇 |
1992年 | 17篇 |
1991年 | 15篇 |
1990年 | 10篇 |
1989年 | 16篇 |
1988年 | 11篇 |
1987年 | 9篇 |
1986年 | 10篇 |
1985年 | 4篇 |
1984年 | 5篇 |
1983年 | 6篇 |
1982年 | 5篇 |
1981年 | 2篇 |
1980年 | 3篇 |
1979年 | 1篇 |
1978年 | 6篇 |
1977年 | 2篇 |
1976年 | 2篇 |
1975年 | 4篇 |
1974年 | 2篇 |
1973年 | 2篇 |
1972年 | 3篇 |
1971年 | 1篇 |
1969年 | 3篇 |
1967年 | 4篇 |
1966年 | 3篇 |
1965年 | 1篇 |
1964年 | 1篇 |
1960年 | 1篇 |
排序方式: 共有431条查询结果,搜索用时 15 毫秒
231.
232.
233.
234.
235.
PETER M. SADLER 《Sedimentology》1982,29(1):37-51
In measured sequences of limestone- and greywacke-turbidites the bed-thickness is found to vary proportionally with the fall velocity of the maximum grain size, found at the base of the bed. A simple theoretical model, based on the decay of isotropic turbulence, suggests that bed-thickness should be a function not only of this fall velocity, but also of bottom slope, flow depth and the concentration and grain-size distribution of the sediment in the turbidity current. The field data do show some influence of these additional factors. Nevertheless, for many natural sequences of turbidites the flows must have carried very poorly sorted sediments and the inferred flow volumes and densities must cluster tightly about modal values. Thus, grain size remains the primary variable and the modal regression curve of bed-thickness on maximum grain size is well defined and resembles a fall-velocity curve. Relatively steep basin floors near to source can, theoretically, cause these modal regressions for distal and very proximal parts of a turbidite to diverge, introducing a crudely parabolic appearance in the form of the total regression curve. The form of this parabolic curve predicts the deposition of thin but relatively coarse proximal beds. Such beds do occur. They are different from the thin, but relatively fine, proximal beds that have been interpreted as the result of a fractionation of a turbidity current during levee-forming processes. 相似文献
236.
PETER N. SOUTHGATE 《Sedimentology》1982,29(3):391-407
A suite of unusual highly skeletal halite pseudomorphs is described from the lower Middle Cambrian rocks that crop out at Ardmore in the Georgina Basin, northern Australia. The pseudomorphs are preserved as both moulds and casts within a dark and light colour-banded chert. They are orientated parallel to bedding and represent halite crystal growth within a brine pool. Laterally equivalent rocks are characterized by sedimentary textures and structures indicative of periodic emergence and desiccation. Laboratory grown halite crystals are also described and compared with the pseudomorphs from Ardmore. Experimentally produced halite crystals formed from either: (1) brine solutions obtained as residues after organic matter extraction from phosphate rocks, or (2) solutions of NaCI and distilled water. Both pyramidal rafted hoppers and floor nucleated cuboids formed in solutions containing only NaCI and distilled water; whereas in the residue solutions, that contained humic acids, rafted pyramidal forms were absent and crystal nucleation was restricted to the floor of the evaporative dish. As brine depth decreased the halite precipitation rate increased and resulted in a suite of excrescent, highly skeletal crystals that formed as a result of brine evaporation to dryness. The variable crystal morphology depended upon both the brine depth and slope of the evaporative dish floor. Horizontally orientated chevron halite crystals formed where the evaporative dish was inclined and precipitation preferentially occurred on cube faces. Pagoda, reticulate ridge and dendritic forms represent an increasingly skeletal crystal suite characterized by the preferential precipitation of NaCI on cube edges and corners rather than faces. Using the experimentally grown crystals as analogues the pseudomorphs at Ardmore are interpreted as forming in very shallow brine pools that evaporated to dryness. 相似文献
237.
PETER NEUMANN-MAHLKAU 《Sedimentology》1967,9(3):245-261
Granulometric analysis of rudites on outcrop-photographs As it is impossible to determine the grain-size distribution of well-consolidated rudites by sieving, it has been attempted to use outcrop-photographs for their granulometric analysis. In order to assess the accuracy of this photo-method, nineteen samples of poorly consolidated rudites were subjected to analysis by both methods. It resulted that the two sets of data obtained could be correlated rather well, using a relatively simple formula. Although some broader experimental work would still seem advisable, it became quite clear that the photo-method is simpler as well as more accurate than the sieving method. It is therefore suggested to develop it as the standard method for granulometric studies of all rudites. 相似文献
238.
Conditions of Formation and Crystallization Kinetics of Highly Oxidized Pseudotachylytes from the High Tatras (Slovakia) 总被引:2,自引:0,他引:2
PETRIK IGOR; NABELEK PETER I.; JANAK MARIAN; PLASIENKA DUSAN 《Journal of Petrology》2003,44(5):901-927
Tectonic activity during the Miocene exhumation of the Tatragranitoid basement resulted in frictional melting of granite.The activity marks the early stages of the faulting that isresponsible for uplift of the High Tatras. As indicated by pre-existingcataclasite metamorphic mineral assemblages, the ambient pressurewas about 250300 MPa, corresponding to depths between10 and 12 km. The pseudotachylytes are Fe rich, highly oxidizedand crystalline. The matrix composition suggests disequilibriumpartial melting of a biotite-dominated assemblage. Oxygen isotopiccompositions of a pseudotachylyte sample and its constituentminerals show equilibration with the host granodiorite and allowfor the introduction of oxidizing external water rather thanoxidation as a result of the dissociation of free water liberatedduring melting. The kinetic information extracted from hematitecrystal-size distributions (CSDs) that are preserved in feldspathicmatrices shows that crystals in most places were accumulatedwhile the system was open. The melt was highly mobile and proneto strong differentiation. The hematite crystals reach a maximumof 26 vol. % (45 wt % Fe2O3). In rare places where the flowceased, the system became closed and produced distinct CSDs.The longest apparent crystallization times (90 s) are recordedmostly in pools in the central parts of the pseudotachylyteswhereas the shortest times (10 s) come from rims and tips offractures. The estimated hematite growth rate was about fiveorders of magnitude higher than that of ilmenite in lava lakes.Such extreme crystallization rates result from high undercoolingsassociated with high cooling rates. Very high cooling ratesare promoted by the extremely high surface/volume ratios ofthe pseudotachylyte sheets. KEY WORDS: pseudotachylyte; hematite; kinetics; High Tatra; Slovakia 相似文献
239.
What status for the Quaternary? 总被引:3,自引:0,他引:3
PHILIP L. GIBBARD ALAN G. SMITH JAN A. ZALASIEWICZ TIFFANY L. BARRY DAVID CANTRILL ANGELA L. COE JOHN C. W. COPE REW S. GALE F. JOHN GREGORY JOHN H. POWELL PETER F. RAWSON PHILIP STONE COLIN N. WATERS 《Boreas: An International Journal of Quaternary Research》2005,34(1):1-6
The status of the Quaternary, long regarded as a geological period effectively coincident with the main climatic deterioration of the current Ice Age, has recently been questioned as a formal stratigraphic unit. We argue here that it should be retained as a formal period of geological time. Furthermore, we consider that its beginning should be placed at the Gauss-Matuyama magnetic chron boundary at about 2.6 Ma, rather than at its current position at about 1.8 Ma. The Quaternary would be formally subdivided into the Pleistocene and Holocene epochs. The global chronostratigraphical correlation table proposed is enclosed at the back of this issue. 相似文献
240.
Contemporaneous Trachyandesitic and Calc-alkaline Volcanism of the Huerto Andesite, San Juan Volcanic Field, Colorado, USA 总被引:2,自引:0,他引:2
Locally, voluminous andesitic volcanism both preceded and followedlarge eruptions of silicic ash-flow tuff from many calderasin the San Juan volcanic field. The most voluminous post-collapselava suite of the central San Juan caldera cluster is the 28Ma Huerto Andesite, a diverse assemblage erupted from at least56 volcanic centres that were active around the southernmargins of the La Garita caldera shortly after eruption of theFish Canyon Tuff. These andesitic centres are inferred, in part,to represent eruptions of magma that ponded and differentiatedwithin the crust below the La Garita caldera, thereby providingthe thermal energy necessary for rejuvenation and remobilizationof the Fish Canyon magma body. The multiple Huerto eruptivecentres produced two magmatic series that differ in phenocrystmineralogy (hydrous vs anhydrous assemblages), whole-rock majorand trace element chemistry and isotopic compositions. Hornblende-bearinglavas from three volcanic centres located close to the southeasternmargin of the La Garita caldera (Eagle MountainFourmileCreek, West Fork of the San Juan River, Table Mountain) definea high-K calc-alkaline series (5765 wt % SiO2) that isoxidized, hydrous and sulphur rich. Trachyandesitic lavas fromwidely separated centres at Baldy MountainRed Lake (westernmargin), Sugarloaf Mountain (southern margin) and Ribbon Mesa(20 km east of the La Garita caldera) are mutually indistinguishable(5561 wt % SiO2); they are characterized by higher andmore variable concentrations of alkalis and many incompatibletrace elements (e.g. Zr, Nb, heavy rare earth elements), andthey contain anhydrous phenocryst assemblages (including olivine).These mildly alkaline magmas were less water rich and oxidizedthan the hornblende-bearing calc-alkaline suite. The same distinctionscharacterize the voluminous precaldera andesitic lavas of theConejos Formation, indicating that these contrasting suitesare long-term manifestations of San Juan volcanism. The favouredmodel for their origin involves contrasting ascent paths anddifferentiation histories through crustal columns with differentthermal and density gradients. Magmas ascending into the mainfocus of the La Garita caldera were impeded, and they evolvedat greater depths, retaining more of their primary volatileload. This model is supported by systematic differences in isotopiccompositions suggestive of crustmagma interactions withcontrasting lithologies. KEY WORDS: alkaline; calc-alkaline; petrogenesis; episodic magmatism; Fish Canyon system 相似文献