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991.
A. Carlos P. Borrero Jose Luis H. Naranjo 《Journal of Volcanology and Geothermal Research》1990,41(1-4)
The Upper Pliocene to Pleistocene Casabianca Formation is an assemblage of coarse-grained volcanogenic sediments derived from the Ruiz-Cerro Bravo volcanic axis, which were deposited on the west and east flanks of the middle Colombian Central Cordillera (5°–5°30′ N Lat.; 74°30′–76° W Long.).Facies assemblages, paleocurrent data, and geomorphic expression define four depositional settings: (1) an alluvial fan with debris-flow lobes represented by the Manizales fan in the western sector and the Fresno fan in the eastern sector, characterized by the facies assemblage of Gms, Gp and Gt; (2) valley fill deposits represented by the Arauca section at the west sector, characterized by the facies assemblage of Gms and Gi; (3) deposits produced by the diversion of the debris-flow and hyperconcentrated flood-flow deposits from the main channels into narrow effluent channels; represented by the Delgaditas and Manzanares-Marquetalia sections, in the eastern sector and characterized by the facies assemblage Gms and Gm(a); and (4) lateral accretion in gravelly, medium to high-sinuosity rivers, represented by the Casabianca-Villa Hermosa, Palo Cabildo-Falan, Lagunillas and Guali sections of the eastern sector, characterized by the facies assemblage Gms, Gp and Gt.Casabianca Formation deposition records the response of a semi-arid to tropical fluvial system to large, volcanism-induced sediment loads. 相似文献
992.
Grant Heiken Napoleon Ramos Wendell Duffield John Musgrave Kenneth Wohletz Sue Priest James Aldrich Wilmer Flores Alexander Ritchie Fraser Goff Dean Eppler Carlos Escobar 《Journal of Volcanology and Geothermal Research》1991,45(1-2)
The Platanares geothermal area, Departamento de Copán, Honduras, is located within a graben that is complexly faulted. The graben is bounded on the north by a highland composed of Paleozoic (?) metamorphic rocks in contact with Cretaceous - Tertiary redbeds of unknown thickness. These are unconformably overlain by Tertiary andesitic lavas, rhyolitic ignimbrites, and associated sedimentary rocks. The volcanic rocks are mostly older than 14 Ma, and thus are too old to represent the surface expression of an active crustal magma body. Thermal fluids that discharge in the area are heated during deep circulation of meteoric water along faults in a region of somewhat elevated heat flow. Geothermometry based upon the chemical composition of thermal fluids from hot springs and from geothermal gradient coreholes suggests that the reservoir equilibrated at temperatures as high as 225 to 240°C, within the Cretaceous redbed sequence. Three continuously cored geothermal gradient holes have been drilled; fluids of about 165°C have been produced from two drilled along a NW-trending fault zone, from depths of 250 to 680 m. A conductive thermal gradient of 139°C/km, at a depth of 400 m, was determined from the third well, drilled 0.6 km west of that fault zone. These data indicate that the Platanares geothermal area holds considerable promise for electrical generation by moderate- to hightemperature geothermal fluids. 相似文献
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996.
Amphiboles are the main mafic minerals in most metaluminous to peralkaline alkali-feldspar granites and syenites, and they usually preserve an important record of the compositional evolution of the melts from which they crystallize. In the alkaline association of the Serra da Graciosa A-type Granites and Syenites (southern Brazil), amphibole compositions span a large range, including calcic, sodic–calcic, and sodic amphiboles. Calcic amphiboles are typically observed in the metaluminous rocks, while sodic amphiboles are characteristic of the more strongly peralkaline rocks; sodic–calcic amphiboles are found in intermediate varieties. Compositional variations record the differentiation trends within two petrographic series of the alkaline association. The overall evolution of amphibole compositions is similar in both: they reveal a progressive increase in Na and Fe3+ with differentiation (increase in alkalinity of the magmas), a characteristic shared by undersaturated peralkaline (or agpaitic) differentiation trends. In detail, however, the evolutions of the amphibole compositions in the two series are distinct. In Alkaline series 1, the cores of the crystals form a continuum from calcic to sodic compositions, with the exception of a small compositional gap within the sodic–calcic amphiboles. The rims, however, show compositions that diverge from this main trend; this divergence results from increasing amounts of the oxy-amphibole component, and reflects more oxidizing conditions at the final stages of magmatic crystallization. In Alkaline series 2, these oxidation trends are much more subtle and a reverse trend is observed in the sodic amphiboles. Sodic–calcic amphiboles are in several cases replaced by intergrowths of post-magmatic sodic amphibole and Al-poor (“tetrasilicic”) biotite. 相似文献
997.
The discrimination between distinct remote compressions and multiple local stress deviations within a single compressive stress
field has been carried out in the central-eastern Iberian Chain, by using structural criteria, computing palaeostress directions,
identifying and ‘filtering’ stress deviations, and analysing time relationships. A probabilistic analysis based on a systematic
comparison of real and expected frequencies of coexistence of two compressions is applied by means of the χ
2 Test. This allows us to identify those tectonic compressions that behave as independent events from the probabilistic point
of view. The results suggest that among five initially defined compression directions only three can be considered as representative
of distinct (although partially superposed) externally applied intraplate stress fields: Iberian s.l. (NE-SW), Betic s.l.
(NW-SE), and Pyrenean (N-S to NNE-SSW). 相似文献
998.
Today, in the early 21st century, goods, information, services, financial capital and human beings are flowing across national
borders at an ever-accelerating rate. In this context, transnationalism has become a key paradigm in the study of international
migration and urbanism. This theme issue on “Immigrants and transnational experiences in world cities” explores these new
trends in contemporary international migration, with respect to transnational communities and geographies, in articles grouped
according to four themes: international migration and world cities; highly-skilled and low-skilled immigrants; economic impacts;
and immigrant experiences in world cities. 相似文献
999.
Atacamite, a copper hydroxychloride, is an important constituent of supergene oxide zones of copper deposits in northern Chile,
whereas in similar deposits elsewhere, it is rare. In Chile, it has generally been assumed to be a primary constituent of
the supergene zones. There are two difficulties with this supposition. The first is that atacamite requires saline water for
its formation, whereas supergene oxidation was caused by percolating, oxygenated meteoric water, mainly rainwater. The second
is that atacamite dissolves rapidly or undergoes phase change when exposed to fresh water. Supergene enrichment of copper
deposits in northern Chile extended over a long period, 44 to 9 Ma, being terminated by the onset of hyperaridity. During
this period, there was at least intermittent rainfall, exposing previously formed atacamite to dissolution or phase change.
Furthermore, atacamite-bearing oxide zones in several deposits are directly overlain by thick Miocene alluvial gravels; the
stream waters that transported these gravels would have permeated the oxide zones. In some deposits, atacamite-bearing assemblages
occur both in the oxide zones and in contiguous gravels. We suggest that atacamite-bearing oxide assemblages are more likely
to have been a replacement of preexisting oxide phases after the onset of hyperaridity at about 9 Ma. A hyperarid climate
made possible evaporation and concentration of chloride in meteoric waters. In this paper, we discuss another source of saline
waters to modify oxide zones. Dewatering of the Domeyko Basin expelled brines along faults, some of which had earlier guided
the location of porphyry deposits. At the Spence porphyry copper deposit, saline waters, which δD vs δ
18O isotope analyses identify as basinal brines, are presently rising through the deposit, then flowing away along the base
of the covering gravels. Compositions of these waters lie within the stability fields of atacamite and brochantite, the two
minerals that comprise the oxide zone. Evidence is presented for other porphyry deposits, Radomiro Tomic and Gaby Sur, that
basinal brines may have been involved in the late formation of atacamite. 相似文献
1000.
Carlos Palacios Luis E. Ramírez Brian Townley Marcelo Solari Nelson Guerra 《Mineralium Deposita》2007,42(3):301-308
During the Late Jurassic–Early Oligocene interval, widespread hydrothermal copper mineralization events occurred in association
with the geological evolution of the southern segment of the central Andes, giving rise to four NS-trending metallogenic belts
of eastward-decreasing age: Late Jurassic, Early Cretaceous, Late Paleocene–Early Eocene, and Late Eocene–Early Oligocene.
The Antofagasta–Calama Lineament (ACL) consists of an important dextral strike-slip NE-trending fault system. Deformation
along the ACL system is evidenced by a right-lateral displacement of the Late Paleocene–Early Eocene metallogenic belts. Furthermore,
clockwise rotation of the Early Cretaceous Mantos Blancos copper deposit and the Late Paleocene Lomas Bayas porphyry copper
occurred. In the Late Eocene–Early Oligocene metallogenic belt, a sigmoidal deflection and a clockwise rotation is observed
in the ACL. The ACL is thought to have controlled the emplacement of Early Oligocene porphyry copper deposits (34–37 Ma; Toki,
Genoveva, Quetena, and Opache), whereas it deflected the Late Eocene porphyry copper belt (41–44 Ma; Esperanza, Telégrafo,
Centinela, and Polo Sur ore deposits). These observations suggest that right-lateral displacement of the ACL was active during
the Early Oligocene. We propose that the described structural features need to be considered in future exploration programs
within this extensively gravel-covered region of northern Chile. 相似文献