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21.
Two adjacent beaver-pond sequences of seven ponds each were examined along the eastern boundary of Glacier National Park, Montana, to determine spatial patterns and amounts of sedimentation, both within individual ponds and along pond sequences. Dams and their associated ponds were categorized into two age groups, young versus old, based on dam morphology and surface vegetation. Old ponds contained significantly more sediment than did young ponds. However, hypothesized patterns of downstream reductions in coarse-grained sediment, both within a pond and along a sequence, were not observed, nor were there any discernible patterns in sediment texture across ponds. [Key words: beaver pond, sedimentation, Rocky Mountains, Montana.] 相似文献
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Anatoly V. Lozhkin Patricia Anderson Wendy R. Eisner Tatiana B. Solomatkina 《Quaternary Research》2011,76(3):383-392
New palynological and sedimentological data from St. Lawrence Island present a rare view into late-glacial and Holocene environments of the central Bering Land Bridge. The late glaciation was a time of dynamic landscape changes in south-central Beringia, with active thermokarst processes, including the formation and drainage of thaw lakes. The presence of such a wet, unstable substrate, if widespread, probably would have had an adverse impact on food sources and mobility for many of the large mammal populations. The establishment of Betula shrub tundra on the island suggests late-glacial summers that were warmer than present, consistent with regional paleoclimatic interpretations. However, the increasing proximity to the Bering Sea, as postglacial sea levels rose, modified the intensity of warming and prevented the establishment of deciduous forest as found in other areas of Beringia at this time. The mid- to late Holocene is marked by more stable land surfaces and development of Sphagnum and Cyperaceae peat deposits. The accumulation of organic deposits, decline of shrub Betula, and decrease in thermokarst disturbance suggest that conditions were cooler than the previous. A recent decline in peat accumulation at the study sites may relate to local geomorphology, but similar decreases have been noted for other arctic regions. 相似文献
24.
Black African refugee communities are a relatively recent addition to the Australia landscape. Between 2001 and 2007, over 5000 refugees from various African nations resettled in Queensland. There are very few data about African refugee settlement geography in Australia and no previous study delineating African refugee settlement within the State of Queensland. This research addresses the knowledge gap by examining the geographic distribution and secondary migration patterns of eight African refugee communities in Southeast Queensland. The research adopted a multi-method approach, mapping quantitative data from an existing secondary database and comparing these to qualitative primary data to determine geographical distribution. Additional qualitative data were used to establish secondary migration patterns of the case study communities. Results show noticeable discrepancies between existing secondary datasets and primary data collected from the communities. These inconsistencies are significant because settlement service providers who use the secondary data to budget, plan and deliver essential settlement services might be underestimating the size of the African communities and missing some settlement locations altogether. The results also reveal a tension between the main socio-cultural forces of concentration and housing forces of dispersion that are driving secondary migration in the communities. A policy recommendation to mitigate the potentially negative effects of residential dispersion on settlement outcomes includes the strategic location of service hubs in key suburbs, such as Moorooka, to which the communities are already drawn. 相似文献
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Wendy R. Nelson Michael J. Dorais Eric H. Christiansen Garret L. Hart 《Contributions to Mineralogy and Petrology》2013,165(2):397-417
Mafic complexes in the central Sierra Nevada batholith record valuable geochemical information regarding the role mafic magmas play in arc magmatism and the generation of continental crust. In the intrusive suite of Yosemite Valley, major and trace element compositions of the hornblende-bearing gabbroic rocks from the Rockslides mafic complex and of the mafic dikes in the North America Wall are compositionally similar to high-alumina basalt. Of these rocks, two samples have higher Ni and Cr abundances as well as higher εNd values than previously recognized for the intrusive suite. Plagioclase crystals in rocks from the North America Wall and the Rockslides have prominent calcic cores and sharply defined sodic rims, a texture commonly associated with mixing of mafic and felsic magmas. In situ analyses of 87Sr/86Sr in plagioclase show no significant isotopic difference from the cores to the rims of these grains. We propose that the high 87Sr/86Sr (~0.7067) and low εNd (~?3.4) of bulk rocks, the homogeneity of 87Sr/86Sr in plagioclase, and the high δ18O values of bulk rocks (6.6–7.3 ‰) and zircon (Lackey et al. in J Petrol 49:1397–1426, 2008) demonstrate that continental crust was assimilated into the sublithospheric mantle-derived basaltic precursors of the mafic rocks in Yosemite Valley. Contamination (20–40 %) likely occurred in the lower crust as the magma differentiated to high-alumina basalt prior to plagioclase (and zircon) crystallization. As a consequence, the isotopic signatures recorded by whole rocks, plagioclase, and zircon do not represent the composition of the underlying lithospheric mantle. We conclude that the mafic and associated felsic members of the intrusive suite of Yosemite Valley represent 60–80 % new additions to the crust and include significant quantities of recycled ancient crust. 相似文献
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Hannah L. Hilbert‐Wolf Edward L. Simpson Wendy S. Simpson Sarah E. Tindall Michael C. Wizevich 《Basin Research》2009,21(6):856-871
The Upper Cretaceous Wahweap Formation accumulated in the active Cordilleran foreland basin of Utah. Soft‐sediment deformation structures are abundant in the capping sandstone member of the Wahweap Formation. By comparing with well‐established criteria, a seismogenic origin was determined for the majority of structures, which places these soft‐sediment deformation features in a class of sedimentary features referred to as seismites. A systematic study of the seismite trends included their vertical and horizontal distribution and a semi‐quantitative intensity analysis using a scale from 1 to 5 that is based on magnitude, sedimentary structure type, and the predominance of inferred process of hydroplastic deformation, liquefaction or fluidization. In addition, orientations of soft‐sediment fold axes were recorded. Construction of a northwest‐to‐southeast stratigraphic and seismite intensity cross‐section demonstrates: (1) reduction in stratigraphic thickness and percentage of conglomerates to the southeast, (2) the presence of lower seismite, middle nonseismite, and upper seismite zones within the capping sandstone (permitting subdivision of the capping sandstone member), and (3) elimination of the nonseismite zone and amalgamation of the lower and upper seismite zones to the southeast. Regional isoseismal contour maps generated from the semi‐quantitative analysis indicate a decrease in overall intensity from northwest to southeast in the upper and lower seismic zones and in sandstone within 5 m stratigraphically of the contact between the upper and capping sandstone members. In addition, cumulative seismite fold orientations support a west–northwest direction towards regional epicentres. Isoseismal maps are used to distinguish the effects of intrabasinal normal faulting from those of regional orogenic thrusting. Thus, this study demonstrates the utility of mapping seismites to separate the importance of regional vs. local tectonic activity influencing foreland basin sedimentation by identifying patterns that delineate palaeoepicentres associated with specific local intrabasinal normal faults vs. regional trends in soft‐sediment deformation related to Sevier belt earthquakes. 相似文献
27.
Hans-Ulrich Schmincke Steffen Kutterolf Wendy Perez Juanita Rausch Armin Freundt Wilfried Strauch 《Bulletin of Volcanology》2009,71(5):479-493
We present the stratigraphy, lithology, volcanology, and age of the Acahualinca section in Managua, including a famous footprint
layer exposed in two museum pits. The ca. 4-m-high walls of the main northern pit (Pit I) expose excellent cross sections
of Late Holocene volcaniclastic deposits in northern Managua. We have subdivided the section into six lithostratigraphic units,
some of which we correlate to Late Holocene eruptions. Unit I (1.2 m thick), chiefly of hydroclastic origin, begins with the
footprint layer. The bulk is dominated by mostly massive basaltic-andesitic tephra layers, interpreted to represent separate
pulses of a basically phreatomagmatic eruptive episode. We correlate these deposits based on compositional and stratigraphic
evidence to the Masaya Triple Layer erupted at Masaya volcano ca. 2,120 ± 120 a B.P.. The eruption occurred during the dry
season. A major erosional channel unconformity up to 1 m deep in the western half of Pit I separates Units II and I. Unit
II begins with basal dacitic pumice lapilli up to 10 cm thick overlain by a massive to bedded fine-grained dacitic tuff including
a layer of accretionary lapilli and pockets of well-rounded pumice lapilli. Angular nonvesicular glass shards are interpreted
to represent hydroclastic fragmentation. The dacitic tephra is correlated unequivocally with the ca. 1.9-ka-Plinian dacitic
Chiltepe eruption. Unit III, a lithified basaltic-andesitic deposit up to 50 cm thick and extremely rich in branch molds and
excellent leaf impressions, is correlated with the Masaya Tuff erupted ca. 1.8 ka ago. Unit IV, a reworked massive basaltic-andesitic
deposit, rich in brown tuff clasts and well bedded and cross bedded in the northwestern corner of Pit I, cuts erosionally
down as far as Unit I. A poorly defined, pale brown mass flow deposit up to 1 m thick (Unit V) is overlain by 1–1.5 m of dominantly
reworked, chiefly basaltic tephra topped by soil (Unit VI). A major erosional channel carved chiefly between deposition of
Units II and I may have existed as a shallow drainage channel even prior to deposition of the footprint layer. The swath of
the footprints is oriented NNW, roughly parallel to, and just east of, the axis of the channel. The interpretation of the
footprint layer as the initial product of a powerful eruption at Masaya volcano followed without erosional breaks by additional
layers of the same eruptive phase is strong evidence that the group of 15 or 16 people tried to escape from an eruption. 相似文献
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Mahdi Zoorabadi Serkan Saydam Wendy Timms Bruce Hebblewhite 《Geomechanics and Geoengineering》2016,11(3):189-200
Roughness on rock joints produces a variable aperture across the joints and increases the flow path length. These conditions should be taken into account for a good approximation from cubic law. In this paper, the concept of local true aperture and tortuosity is applied to assumed joints where surfaces are matched to each other and correspond with standard Joint Roughness Coefficient (JRC) profiles. Furthermore, the hydraulic behaviour of JRC profiles is studied by a new laboratory experiment setup. The analytical approach provides new insights into the effects of roughness on hydraulic properties of rock joints. The results indicate that for a constant mechanical aperture, both the minimum local aperture and hydraulic aperture decrease with increasing JRC. Furthermore, tortuosity and standard deviation of local true aperture increase with JRC increment. The trend obtained between different parameters and JRC shows an obvious fluctuation for JRC lower than 10. On one hand, the results of this study along with a critical review of previous studies demonstrate that JRC profiles cannot present a precise roughness increment when JRC is less than 10. A new laboratory setup was designed to study the flow behaviour of JRC profiles. The results obtained from laboratory experiments under linear flow conditions validate the accuracy of the applied analytical method. 相似文献