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91.
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Most, if not all forests in the Caribbean are subject to occasional disturbances from hurricanes. If current general circulation model (GCM) predictions are correct, with doubled atmospheric CO2 (2 × CO2), the tropical Atlantic will be between 1 °C and 4 °C warmer than it is today. With such a warming, more than twice as many hurricanes per year could be expected in the Caribbean. Furthermore, Emanuael (1987) indicates that in a warmed world the destructive potential of Atlantic hurricanes could be increased by 40% to 60%. While speculative, these increases would dramatically change the disturbance regimes affecting tropical forests in the region and might alter forest structure and composition. Global warming impacts through increased hurricane damage on Caribbean forests are presented.An individual tree, gap dynamics forest ecosystem model was used to simulate the range of possible hurricane disturbance regimes which could affect the Luquillo Experimental Forest in Puerto Rico. Model storm frequency ranged from no storms at all up to one storm per year; model storm intensity varied from no damage up to 100% mortality of trees. The model does not consider the effects of changing temperature and rainfall patterns on the forest. Simulation results indicate that with the different hurricane regimes a range of forest types are possible, ranging from mature forest with large trees, to an area in which forest trees are never allowed to reach maturity. 相似文献
93.
Random vibration analyses of structural systems subjected to seismic loading are dependent upon the characterization of earthquake ground motion as a stochastic process. The response of structural systems to earthquakes is dependent strongly on the local geological conditions, which should be incorporated into seismological models of ground motion. In the study presented herein, three previously developed ground-motion models are adapted to incorporate site-dependent characteristics. Records obtained from two recording stations in California are used as a basis for the ground-motion models. Single-degree-of-freedom (SDOF) oscillators are subjected to ensembles of accelerograms generated from these models, and both elastic and inelastic response are considered. Response statistics are compared to those generated by the analysis of structural response to ensembles of recorded motion from the two sites. The important features of the ground motion for effective reproduction of response statistics are identified, and observations are made on the sensitivity of specific response parameters to site-dependent characteristics of the ground motion. 相似文献
94.
A genetic model proposed for granitic rocks dredged from the Aves Rise involves a sodium-enriched hydrothermal fluid phase, shallow emplacement, and significant derivation of components from the upper mantle. Megascopic observations of serially slabbed sections of the rock samples show that aggregates of plagioclase phenocrysts concentrate into channel-like mats. The presence of a late-stage, hydrothermal fluid is suggested by crystal-lined cavities in the plagioclase aggregates. 相似文献
95.
Bruce E. Hobbs David M. Ransom Ron H. Vernon Paul F. Williams 《Mineralium Deposita》1968,3(4):293-316
The paper reviews recent works on the Broken Hill ore body to see what data is available on the origin and history of the ore. A high grade metamorphism is recognizable (the Willyama Metamorphism) and this was followed by a number of retrograde metamorphic events. The characteristics of these events is described and this is followed by an analysis of the hypothesis that there is a stratigraphic control of Broken Hill type mineralization. It is concluded that any control that does exist is of a regional nature and that the evidence is equally as good for a restriction of mineralization to areas of high grade metamorphism. Many of the isotopic and geochronological results for Broken Hill are not diagnostic as far as ore genesis is concerned. They do present an internally consistent set of data that records three major events, the Willyama Metamorphism and introduction of the Broken Hill type mineralization at 1700 m. y., the intrusion of the Mundi Mundi Granite and cross cutting pegmatites at 1560 m. y., and retrograde metamorphism and introduction of the Thackaringa Mineralization at or prior to 500 m.y. The sulphides have been deformed in the retrograde schist zones and the ore therefore predates this event. But there is no evidence at the moment that unambiguously establishes that the sulphides were present prior to the Willyama Metamorphism. If it was present prior to or during the Willyama Metamorphism and there is no period of deformation prior to the first recognizable folds at Broken Hill, then the sulphide mass was initially grossly discordant with bedding.
Zusammenfassung Die Arbeit bringt Ergebnisse der neuesten Untersuchungen über die Broken-Hill-Lagerstätte anschließend an einen Überblick früherer Arbeiten. Der intensiven Willyama-Metamorphose folgen verschiedene retrograde Vorgänge, über die Einzelheiten mitgeteilt werden. Ferner wird die Hypothese einer stratigraphischen Kontrolle der Vererzung einer Prüfung unterzogen und festgestellt, daß die maßgeblichen Frakturen regionaler Natur sein müssen und daß die Beobachtungen somit eine Annahme der Vererzung der hochmetamorphen Gebiete ebenso stark stützt. Viele Resultate von Isotopenzusammensetzungen und geochronologischen Untersuchungen sind für Broken Hill nicht diagnostisch, wenigstens was die Erzgenese betrifft. Sie belegen 3 Hauptereignisse: die Willyama-Metamorphose und den Beginn der Vererzung um 1700.106a, die Intrusion des Mundi Mundi Granits und querschlägiger Pegmatite um 1560.106a und eine retrograde Metamorphose und den Eintritt der Thackaringa-Vererzung um oder vor 500.106a. Die Sulfide sind in der Zone der retrograden Schiefer deformiert worden, was beweist, daß die Vererzung älter ist als dieses Ereignis. Es gibt aber z. Z. keinen Beweis, daß die Sulfide vor der Willyama Metamorphose vorhanden gewesen sind. Wenn sie vor oder während dieses Ereignisses vorhanden gewesen sind und wenn vor der Zeit der ersten deutlichen Falten in Broken Hill keine Deformation stattgefunden hat, dann war die Sulfidmasse ursprünglich in der Schichtung stark diskordant.相似文献
96.
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Hydrogen ion irradiation of powdered igneous rocks, including Apollo rocks, has been observed in the laboratory to darken the powders and to make their optical properties similar to the Moon's. An extensive series of investigations shows that this darkening is not spurious. These results are consistent with those of other investigators, including Nash (1967). Darkening of lunar igneous rock powders by the formation of solar wind-sputtered glass films is a real process which occurs on the Moon. The time scale for darkening of undisturbed lunar soil is of the order of 50000–100000 yr. Comparison of the rates of the formation of glasses on the lunar surface by solar wind sputter-deposition, meteorite impact melting and impact vaporization-deposition indicates that these processes are of comparable importance under the present flux of meteorites. Thus the formation of glass by sputter-deposition must be regarded as a major process on the lunar surface.Paper dedicated to Professor Harold C. Urey on the occasion of his 80th birthday on 29 April 1973. 相似文献
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