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61.
Meteoroids that orbit the Sun encounter the Earth with speeds between 11 and 74 km/sec. However, the distribution of the velocities of meteoroids between these limits is not well known. The uncertainty is caused by the difficulty in measuring the true flux of meteors at the extrema of the velocity distribution. Whilst the most comprehensive measurements of meteor flux are those obtained using radio techniques, meteors with speeds > 50 km/sec occur at heights where the effects of initial radius of the trail and diffusion significantly reduce the radio reflection from the trails; on the other hand the high dependence of the collisional ionization probability on velocity (to the power 3.5) significantly inhibits the detection of meteors with speeds < 20 km/sec. Recent developments in meteor radar systems are now making it possible to measure the velocity of meteors at the extrema of the distribution. For meteoroids ablating at heights between 100 and 120 km the speed of entry can be measured at 2 and 6 MHz using a radar with a 1 km diameter array located near Adelaide; these observations will commence early in 1995. In the meantime a 54 MHz MST radar is being operated at a pulse repetition frequency of 1024 Hz to search for the presence of interstellar (speed > 74 km/sec) meteors. Both these radars exploit the phase information available prior to the closest-approach (to) point. 相似文献
62.
63.
Ron J. Steel 《Tectonophysics》1976,36(1-3)
The Devonian basins of western Norway are notable for their small size (<2000 km) and for the great stratigraphie thickness (up to 25 km) and the cyclic nature of their coarse-grained, alluvial infilling. These general features alone strongly suggest that tectonism was the dominant control on sedimentation but it is argued in particular that a coarsening-upwards motif, which permeates the three main basins usually on a scale of less than 200 m, was the direct sedimentary response to basin-floor subsidence. The coarsening-upwards is interpreted in terms of progradation of alluvial plain and alluvial fan facies in response to rapid lowering of the basin floor, while the overall cyclicity of the basin fills reflects repetition of such tectonic events. Detailed study within Hornelen Basin shows that the repeated coarsening-upwards theme overprints different sedimentary facies, and, in particular, the clastic wedges on opposite sides of the basin have contrasting thicknesses, radii, grain-size gradients and conglomerate types. These features suggest differing rates or degree of continuity of fault movement on opposite margins of the basin. Hornelen and Solund Basins, the two largest, are strongly contrasted in their differing mode of filling and sediment grade. It is suggested that this may reflect their development under strike-slip and dip-slip dominated regimes, respectively. 相似文献
64.
Earth, Moon, and Planets - 相似文献
65.
D. I. Steel 《Earth, Moon, and Planets》1996,72(1-3):279-292
The geological and biological sciences have gradually dispensed with the nineteenth-century concept of substantive uniformitarianism - or gradualism - whereby the physical and biological features of our planet are assumed to have been brought about by the long-term accumulation of small changes. The catastrophist alternative sees the changes as being wrought largely by discrete, exceptional events; one such type of event is an impact by a substantial asteroid or comet. It is argued here that scientists working on small solar system bodies generally still labour under a form of this gradualism, in that a conventional starting point is to presume a steady-state, and what is seen now is assumed to be diagnostic of the long-term average conditions. This is here termed NEO-uniformitarianism, the NEO referring to Near-Earth Objects. It is maintained herein that this area of science needs to revise its philosophical basis by allowing catastrophist principles to be entertained; that is, the presumption of a steady-state needs to be rejected until such time as evidence to support it is revealed. It is argued that the weight of evidence favours the contrary. For example, evidence is outlined for (a) Variations in the terrestrial cratering rate, disallowing any equating of the crater record with the presently-observed large impactor population; (b) The presence of significant NEO complexes which may be due to giant comet disintegrations within the last 20 kyr, hence solving the problem of the supply of short-period comets; (c) A misbalance between the present supply of meteoroids, there being too many to be supplied by presently-observed comets and also a surplus above the population needed to maintain the interplanetary dust complex; and (d) A substantial variation in the interplanetary dust flux in the past 20 kyr, as might be expected from (b and c). 相似文献
66.
Glen S. Mattioli Pamela E. Jansma Lorna Jaramillo Alan L. Smith 《Bulletin of Volcanology》1996,58(5):401-410
A desktop image processing and photogrammetric method was developed for digitizing black-and-white aerial photographs. The
technique was applied to airborne optical images of Mt. Pelée, Martinique, a historically active volcano in the tropical Lesser
Antilles island arc, to evaluate its utility for rapid geologic mapping and hazard assessment in vegetated areas. The digital
approach provides several advantages over traditional air-photo interpretation by allowing for change detection in time-series
images, morphologic characterization, development of digital elevation models from stereopairs, and geo-referencing with other
digital data sets. A digital mosaic of Mt. Pelée was created from air photos acquired in 1951, which covered the region affected
by the 1902 eruption. Severe mismatches occurred along edges of adjacent photographs prior to correction, which precluded
quantitative morphologic analysis of the volcanic edifice. Geometric corrections and histogram equalization of digitized air
photos allowed creation of a continuous mosaic. Comparison of the mosaic and a map based on differences in gray scale and
texture to a volcanostratigraphic map revealed that not only the various deposits produced during the 1902 event were easily
differentiated, but that older eruptive products were identified, suggesting that this approach may be used for rapid hazard
evaluation of historically active tropical volcanoes.
Received: 22 January 1996 / Accepted: 26 July 1996 相似文献
67.
Lorna STEEL 《地球学报》2010,31(Z1):59-61
Although the Natural History Museum’s pterosaur collection is not large (approximately 1060 specimens), it is of great historical and scientific importance. Most of the material is figured, and several holotypes are housed here. Pterosaurs from UK localities are well represented, but foreign material is also present as original specimens and as high quality casts. 相似文献
68.
Sebastian Cardona Lesli J. Wood Brandon Dugan Zane Jobe Lorna J. Strachan 《Sedimentology》2020,67(4):2111-2148
Submarine mass-transport deposits are important in many ancient and modern basins. Mass-transport deposits can play a significant role in exploration as reservoir, seal or source units. Although seismic data has advanced the knowledge about these deposits, more outcrop studies are needed to better understand gravity mass flows and predict the properties of their resultant deposits. It is proposed that sufficiently well-exposed outcrops of mass-transport deposits can be divided into three strain-dominant morphodomains: headwall, translational and toe. The outcrops of the Rapanui mass-transport deposit, part of the Lower Mount Messenger Formation in the Taranaki Basin, New Zealand, are exposed along a ca 4 km transect in coastal cliffs that enable the identification of the three morphodomains. The aim of this study is to characterize the stratigraphic and sedimentological nature of the Miocene-age Rapanui mass-transport deposit outcrops and the evolution of its basal shear zone. The basal shear zone of a mass-transport deposit is defined as the stratal zone formed in the interface between the overriding mass flow and the underlying in situ deposits or sea floor. Accordingly, the deformation structures in the Rapanui mass-transport deposit and the basal shear zone were documented in an established spatial framework. Traditional methodologies were used to characterize the sedimentology of the Rapanui mass-transport deposit. Data collected from intrafolial folds, rafted blocks and samples from the Rapanui mass-transport deposit were used to investigate strain and matrix texture evolution, estimate palaeoflow direction, and calculate yield strength and overpressure at time of deposition. Additionally, a one-dimensional numerical model was used to test sedimentation-driven overpressure as probable trigger. This work demonstrates that the basal shear zone, as well as the matrix texture of a mass-transport deposit, can vary spatially as sediments from underlying deposits are entrained during shear-derived mixing. This phenomenon can impact the seal potential of mass-transport deposits and their interaction with fluids in the subsurface. 相似文献