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161.
Petrologic and experimental evidence for the movement and heating of the pre-eruptive Minoan rhyodacite (Santorini, Greece) 总被引:1,自引:1,他引:0
Elizabeth Cottrell James E. Gardner Malcolm J. Rutherford 《Contributions to Mineralogy and Petrology》1999,135(4):315-331
Hydrothermal experiments combined with petrologic observations form the basis for a new two-stage model for the evolution
of the pre-eruption Minoan magma chamber at Santorini, Greece. Ninety-nine percent of the erupted volume is two-pyroxene,
rhyodacitic magma that had been stored at a temperature of ∼885 °C, based on magnetite-ilmenite and QUILF geothermometry.
The rest of the volume is basaltic to andesitic magma, which occurs as <10 cm scoria clasts and as small inclusions in rhyodacite
pumices. Petrologic observations show that these magmas mixed at different scales and at different times (i.e., multiple batches
of mafic magma). Hydrothermal experiments were carried out on samples of rhyodacite and a mafic scoria in order to determine
magma storage conditions and the mixing history of the two magmas. At 885 °C, the rhyodacite must have been stored at water-saturated
pressures of ∼50 MPa, based on its phase assemblage, matrix-glass composition, and crystal content. However, glass inclusions
inside rhyodacitic plagioclase phenocrysts contain more than 6 wt% H2O, indicating they formed at pressures >200 MPa. In addition, the composition of the plagioclase hosts (An56 ± 6) of the inclusions require temperatures of 825 ± 25 °C at pressures >200 MPa. This demonstrates that the Minoan rhyodacitic
magma underwent a two-stage evolution, first crystallizing at ∼825∘C and >200 MPa, and then rinsing to a shallow ∼50 MPa storage region with a concomitant rise in temperature to ∼885 °C. We
suggest that the episodic intrusion of mafic magmas provided the necessary heat and perhaps contributed to the ascent of the
magma to shallow crustal depths where it reequilibrated before the cataclysmic eruption. Phase equilibria suggest that much
of the heating of the rhyodacite occurred in the shallow storage region. Thermal budget calculations suggest that the rhyodacite
magma could have been heated by intrusions of basalt rising at reasonable upwelling rates and injected into the storage zone
over several hundred years. Preservation of amphibole in the mafic scoria indicate that injection of mafic magma continued
up until days before the cataclysmic eruption, perhaps triggering the event.
Received: 30 September 1997 / Accepted: 5 October 1998 相似文献
162.
Previous analyses have identified the active width of braided rivers, the bed area over which bed load flux and short‐term morphological change occurs, as an important element of braiding dynamics and predictions of bed load flux. Here we compare theoretical predictions of active width in gravel‐bed braided rivers with observations from Sunwapta River, and from a generic physical model of gravel braided rivers, to provide general observations of the variation in active width, and to develop an understanding of the causes of variation. Bed topography was surveyed daily along a 150 m reach of the pro‐glacial Sunwapta River for a total of four weeks during summer when flow was above threshold for morphological activity. In the laboratory, detailed digital elevation models (DEMs) were derived from photogrammetric survey at regular intervals during a constant discharge run. From the field and flume observations there is considerable local and circumstantial variation in active width, but also a general trend in average active width with increasing discharge. There is also a clear relationship of active width with active braiding index (number of active branches in the braided channel network), and with dimensionless stream power, which appears to be consistent across the range of data from field and physical models. Thus there is a link between active width and the river morphology and dynamics, and the possibility of a general relationship for estimating active width from channel pattern properties or reach‐scale stream power values, from which approximate bedload flux calculations may be made. The analysis also raises questions about differences between hydraulically‐based numerical model computations of instantaneous active width and observation of time‐integrated morphological active width. Understanding these differences can give insight into the nature of bedload transport in braided rivers and the relationship to morphological processes of braiding. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
163.
Kristin Stock Vera Karasova Anne Robertson Guillaume Roger Mark Small Mohamed Bishr Jens Ortmann Tim Stojanovic Femke Reitsma Lukasz Korczynski Boyan Brodaric Zoe Gardner 《Transactions in GIS》2013,17(4):612-639
Current approaches to the discovery of scientific resources (publications, data sets and web services) are dominated by keyword search. These approaches do not allow scientists to search on the deeper semantics of scientific resources, or to discover resources on the basis of the scientific approaches taken. This article evaluates a user interface that allows users to discover scientific resources through structured knowledge in the form of ontologies describing the domain and the scientific knowledge inherent within the scientific resource, and also through informal user tags. These combined capabilities provide scientists with new and powerful options for resource discovery. A qualitative user evaluation explored how scientists felt about the approach for resource discovery in the context of their scientific work. The study showed that marine scientists were enthusiastic about the capabilities of such an approach and appreciated the ability to browse the visual structure of the knowledge and query on scientific method but, overall, preferred the use of tags over ontologies. The exploratory nature of the user study was used to identify future directions for such improvements. 相似文献
164.
This paper reports soil losses from 15 erosion plots in the Middle Hills, Nepal, for the 1992 and 1993 monsoon and pre-monsoon seasons. In total, 912 rainfall events were monitored. Land cover varied from grassland and relatively undisturbed mixed broadleaf forest, to degraded Sal forest and bare ground. Soil losses ranged from less than 0.1 t ha-1 yr-1 for grassland and undisturbed forest plots, to 3–10 t ha-1 yr-1 for Sal forest in various states of degradation, and over 15 t ha-1 yr-1 for the bare sites. These results are broadly consistent with those reported in other parts of the Himalayan Middle Hills. Soil loss values could be explained by variations in runoff amounts and rainfall intensity, as well as by the nature of the land cover. Ground and low shrub cover was more important than canopy cover in protecting the forest soils. Human activity has unquestionably led to accelerated rates of soil loss but the degree of acceleration depends on the nature of the human activity and especially the care with which the land is managed. [Key words: soil loss, land use, Nepal, land degradation.] 相似文献
165.
166.
Biver Nicolas Bockelée-Morvan Dominique Colom Pierre Crovisier Jacques Henry Florence Lellouch Emmanuel Winnberg Anders Johansson Lars E.B. Gunnarsson Marcus Rickman Hans Rantakyrö Fredrik Davies John K. Dent William R.F. Paubert Gabriel Moreno Raphaël Wink Jörn Despois Didier Benford Dominic J. Gardner Matt Lis Dariusz C. Mehringer David Phillips Thomas G. Rauer Heike 《Earth, Moon, and Planets》2002,90(1-4):5-14
The bright comet Hale–Bopp provided the first opportunity to follow the outgassing rates of a number of molecular species over a large range of heliocentric distances. We present the results of our observing campaign at radio wavelengths which began in August 1995 and ended in January 2002. The observations were carried out with the telescopes of Nançay, IRAM, JCMT, CSO and, since September 1997, SEST. The lines of nine molecules (OH, CO, HCN, CH3OH, H2CO, H2S, CS, CH3CN and HNC) were monitored. CS, H2S, H2CO, CH3CN were detected up to rh= 3–4 AU from the Sun, while HCN and CH3OH were detected up to 6 AU. CO, which is the main driver of cometary activity at heliocentric distances larger than 3–4 AU, was last detected in August 2001, at rh= 14 AU. The gas production rates obtained from this programme contain important information on the nature of cometary ices, their thermal properties and sublimation mechanisms.Line shapes allow to measure gas expansion velocities, which, at large heliocentric distances, might be directly connected to the temperature of the nucleus surface. Inferred expansion velocity of the gas varied as rh -0.4 within 7 AU from the Sun, but remained close to 0.4 km s-1 further away. The CO spectra obtained at large rhare strongly blueshifted and indicative of an important day-to-night asymmetry in outgassing and expansion velocity. The kinetic temperature of the coma, estimated from the relative intensities of the CH3OH and CO lines, increased with decreasing rh, from about 10 K at 7 AU to 110 K around perihelion. 相似文献
167.
168.
Fangqiang Wei Kechang Gao Kaiheng Hu Yong Li James Smith Gardner 《Environmental Geology》2008,55(3):619-627
Southwest China, including the Provinces of Yunnan, Guizhou, Sichuan and Chongqing, is a region with serious debris flow hazards,
where 7,561 debris flow sites have been identified. Based on the data from these sites, the distribution regularity of debris
flows was analyzed. Earth surface factors that may influence the formation of debris flows were analyzed from the viewpoints
of energy and material conditions. Four major earth surface factors were selected: relative relief, stratigraphy, fault density
and land-use conditions. With the support of GIS, the research region was divided into 125,177 grid cells and for each cell
data for the four factors were collected. Based on this information, the distribution of quantity and the occurrence probability
of debris flows and the role of each factor were statistically analyzed. The results should be helpful for the assessment
of debris flow hazards and debris flow forecasting in the research region. 相似文献
169.
170.