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Despite the recent recognition of Mount Etna as a periodically violently explosive volcano, the hazards from various types of pyroclastic density currents (PDCs) have until now received virtually no attention at this volcano. Large-scale pyroclastic flows last occurred during the caldera-forming Ellittico eruptions, 15–16 ka ago, and the risk of them occurring in the near future is negligible. However, minor PDCs can affect much of the summit area and portions of the upper flanks of the volcano. During the past ~ 20 years, small pyroclastic flows or base-surge-like vapor and ash clouds have occurred in at least 8 cases during summit eruptions of Etna. Four different mechanisms of PDC generation have been identified during these events: (1) collapse of pyroclastic fountains (as in 2000 and possibly in 1986); (2) phreatomagmatic explosions resulting from mixing of lava with wet rock (2006); (3) phreatomagmatic explosions resulting from mixing of lava with thick snow (2007); (4) disintegration of the unstable flanks of a lava dome-like structure growing over the rim of one of the summit craters (1999). All of these recent PDCs were of a rather minor extent (maximum runout lengths were about 1.5 km in November 2006 and March 2007) and thus they represented no threat for populated areas and human property around the volcano. Yet, events of this type pose a significant threat to the lives of people visiting the summit area of Etna, and areas in a radius of 2 km from the summit craters should be off-limits anytime an event capable of producing similar PDCs occurs. The most likely source of further PDCs in the near future is the Southeast Crater, the youngest, most active and most unstable of the four summit craters of Etna, where 6 of the 8 documented recent PDCs originated. It is likely that similar hazards exist in a number of volcanic settings elsewhere, especially at snow- or glacier-covered volcanoes and on volcano slopes strongly affected by hydrothermal alteration. 相似文献
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六盘山地区多处有臭水泉群出露。臭水泉就是含有Na2SO4、NaCl等盐类矿物离子，并排放硫化氢(H2S)气体的高矿化度咸苦水。在系统分析以往地质资料的基础上，借助ETM卫星图像，提取臭水泉群的分布信息，通过野外实地调查、访问、取样化验，初步查明臭水泉的位置、流量、水质及其对环境的危害，结合地层岩性、地质构造特点，分析臭水泉的地质成因。分析结果表明，六盘山地区的臭水泉主要分布于马东山复背斜的南北两端及东西两翼，以上升泉的形式出露于沟底，中、新生代高含盐量地层是臭水泉群中盐分的物质来源，北东向、北东东向、近南北向断裂和构造裂隙是臭水溢出的通道。臭水泉对环境的危害主要表现在：造成局部大气质量下降，污染地表水体和河谷潜水，造成土壤盐 相似文献
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在系统分析以往地质地质资料的基础上，借助ETM卫星图像，提取臭水泉群的分布信息，通过野外实地调查、访问、取样化验，初步查明六盘山地区的臭水泉主要分布于马东山复背斜的南北两端及东西两翼，以上升泉的形式出露于沟底，中、新生代高含盐量地层是臭水泉群中盐分的物质来源，北东向、北东东向、近南北向断裂和构造裂隙是臭水溢出的通道。臭水泉流出的高矿化度咸苦水及H2S气体，已造成的局部大气质量下降，地表水体和河谷潜水矿化度升高，土壤盐渍化，严重影响了当地人民的生活。 相似文献
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Ulrich Klein 《地球物理与天体物理流体动力学》2013,107(1-3):27-34
Abstract The observational facts about magnetic fields in galactic halos are reviewed. The existence, origin and significance of poloidal field components are described. Observational evidence that magnetic fields channel winds from active galactic centres is discussed. Field strengths adduced from the radio polarizations of edge-on galaxies are given. Priorities for future research are suggested. 相似文献
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Factors influencing the height of Hawaiian lava fountains: implications for the use of fountain height as an indicator of magma gas content 总被引：1，自引：1，他引：0
The heights of lava fountains formed in Hawaiian-style eruptions are controlled by magma gas content, volume flux and the amounts of lava re-entrainment and gas bubble coalescence. Theoretical models of lava fountaining are used to analyse data on lava fountain height variations collected during the 1983–1986 Pu'u 'O'o vent of Kilauea volcano, Hawaii. The results show that the variable fountain heights can be largely explained by the impact of variations in volume flux and amount of lava re-entrainment on erupting magmas with a constant gas content of 0.32 wt.% H2O. However, the gas content of the magma apparently declined by 0.05 wt.% during the last 10 episodes of the eruption series and this decline is attributed to more extensive pre-eruption degassing due to a shallowing of the sub-vent feeder dike. It is concluded that variations in lava fountain height cannot be simply interpreted as variations in gas content, as has previously been suggested, but that fountain height can still be a useful guide to minimum gas contents. Where sufficient data are available on eruptive volume fluxes and extent of lava entrainment, greatly improved estimates can be made of magma gas content from lava fountain height. 相似文献
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