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81.
Thin, clay-rich beds form a key component of the lithostratigraphic scheme established for Middle and Upper Turonian sediments in northern Germany. Previously, using limited petrographic evidence, clay-rich beds across much of this region have been classified as either containing altered volcanic ash (bentonites) or detrital clays. This paper demonstrates that the use of rare-earth element (REE) data enables a rapid and reliable subdivision of clay-rich beds into those composed of bentonitic clays and those composed of detrital clays. Application of this method to the Lower Saxony region of northern Germany demonstrates that four bentonites (TC Tdi , Te and Tf ) and a number of detrital beds can be reliably identified and correlated. Three beds previously proposed to be bentonites are reinterpreted as being composed of detrital clays (To , TD2 and TG ) and a revision of the stratigraphic nomenclature is proposed. Analysis of clay-rich beds from the Munster Basin demonstrates that it is possible to correlate individual bentonites and detrital beds between Lower Saxony and the Miinster Basin, and between shallow and deep water facies. 相似文献
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The study regards an evaluation of site effects on and near the Monte Po hill, located in the north-eastern part of the city of Catania (Italy), an area at high seismic risk. At the beginning of 2007 a seismic station was located in a school building, situated at the slope toe, but no seismic events have been recorded as yet. Therefore, synthetic seismograms have been used to evaluate the ground response analysis at the surface. Because the average slope is moderate (less than 15°), 1-D computer codes have been used to model the equivalent-linear earthquake site response analyses of layered hill deposits, as generally performed by professionals. However, the slope response has also been analysed in greater detail, using a 2-D computer code and the soil characterisation has been evaluated accurately by means of borings, Down-Hole tests, SDMT tests and laboratory tests. Comparing 1-D with 2-D results the stratigraphic site amplification and the Topographic Aggravation Factor (TAF) have also been computed. The aim of the study is that it will form a basis for the design of works to remediate the damage caused by a landslide reactivated by the earthquake in Eastern Sicily on December 13, 1990 (ML=5.4). 相似文献
86.
Limestone pavement is a unique part of Britain's physical landscape. Along with other forms of naturally weathered rock, it is under threat from exploitation for the horticultural market. Valued for its delicate and sculptured form, limestone pavement has been frequently used as decorative stone in rockeries and other landscaped ground. If this unique landscape is to be protected, its exploitation must stop. Yet it must be remembered that the use of rock in gardens and landscaped ground has an important role in raising the public profile of geology within urban areas. We have a simple message: the use of limestone pavement is wrong and must stop, but the use of other forms of stone in urban gardens is beneficial for the Earth sciences. 相似文献
87.
The northern lobe of the Bushveld Complex is currently a highly active area for platinum-group element (PGE) exploration.
This lobe hosts the Platreef, a 10–300-m thick package of PGE-rich pyroxenites and gabbros, that crops out along the base
of the lobe to the north of Mokopane (formerly Potgietersrus) and is amenable to large-scale open pit mining along some portions
of its strike. An early account of the geology of the deposit was produced by Percy Wagner where he suggested that the Platreef
was an equivalent PGE-rich layer to the Merensky Reef that had already been traced throughout the eastern and western lobes
of the Bushveld Complex. Wagner’s opinion remains widely held and is central to current orthodoxy on the stratigraphy of the
northern lobe. This correlates the Platreef and an associated cumulate sequence that includes a chromitite layer—known as
the Grasvally norite-pyroxenite-anorthosite (GNPA) member—directly with the sequence between the UG2 chromitite and the Merensky
Reef as it is developed in the Upper Critical Zone of the eastern and western Bushveld. Implicit in this view of the magmatic
stratigraphy is that similar Critical Zone magma was present in all three lobes prior to the development of the Merensky Reef
and the Platreef. However, when this assumed correlation is examined in detail, it is obvious that there are significant differences
in lithologies, mineral textures and chemistries (Mg# of orthopyroxene and olivine) and the geochemistry of both rare earth
elements (REE) and PGE between the two sequences. This suggests that the prevailing interpretation of the stratigraphy of
the northern lobe is not correct. The “Critical Zone” of the northern lobe cannot be correlated with the Critical Zone in
the rest of the complex and the simplest explanation is that the GNPA-Platreef sequence formed from a separate magma, or mixture
of magmas. Chilled margins of the GNPA member match the estimated initial composition of tholeiitic (Main Zone-type) magma
rather than a Critical Zone magma composition. Where the GNPA member is developed over the ultramafic Lower Zone, hybrid rocks
preserve evidence for mixing between new tholeiitic magma and existing ultramafic liquid. This style of interaction and the
resulting rock sequences are unique to the northern lobe. The GNPA member contains at least seven sulphide-rich horizons with
elevated PGE concentrations. Some of these are hosted by pyroxenites with similar mineralogy, crystallisation sequences and
Pd-rich PGE signatures to the Platreef. Chill zones are preserved in the lowest Main Zone rocks above the GNPA member and
the Platreef and this suggests that both units were terminated by a new influx of Main Zone magma. This opens the possibility
that the Platreef and GNPA member merge laterally into one another and that both formed in a series of mixing/quenching events
involving tholeiitic and ultramafic magmas, prior to the main influx of tholeiitic magma that formed the Main Zone. 相似文献
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Many of the large pterodactyloid pterosaurs come from marine sediments or inland freshwater deposits (Lawson, 1975; Bennett, 1994; Wang et al., 2005; Unwin, 2006). 相似文献