Geological and petrological study on phreatomagmatic explosions of the Miyakejima eruption in 2000

根据对产物的地质检查以及地球物理监测数据和录像，对2000年三宅岛火山喷发期间发生的蒸汽岩浆爆炸的机理进行了研究。还对2003-2005年期间在Anatahan（北方马里亚纳群岛）和1996年在Akademy Nauk湖（堪察加）的火山爆发进行了比较研究。在三宅岛，每次爆发事件的火山灰在许多地方被识别，并使用地层标记确定沉积时间。每一次火山喷发事件的火山灰喷发量给出了精确的喷发速率。通过粒度分析，估算了各次事件的岩浆和整体产物的破碎程度。成分分析给出了岩浆对各次爆发事件的贡献比例。这些数据是模拟水火山爆发的非常有价值的参数，这是本研究无法提出的。 关于我们 岩浆喷发速率与火山灰柱高度有关，破碎程度与岩浆喷发速率呈负相关。后者意味着在非岩浆爆炸中破碎程度最高。这种高破碎可能是由于（1）由于缺乏热能而未能上升的火山口内的粗颗粒的重复破碎和（2）将硅酸盐立即破碎成碎片的超临界水的作用。Wahletz提出的在mgama-water比为0.3时，最有效的能量转化为碎片的证据在本次喷发中没有得到证实，地下水对2000年三宅岛喷发的贡献也被喷发产物的出现和产物中物质的同位素比值所证实。需要进一步的研究，包括Anatahan和Akadamy Nauk产品的结构比较，以澄清蒸汽岩浆爆炸的情况。少

The mechanisms of phreatomagmatic explosions that occurred during the eruption at Miyakejima in 2000 were investigated based on geological inspection of the products together with the geophysical monitoring data and video footage. Comparative study was also carried out for the eruptions in Anatahan (Northern Mariana Islands) during 2003-2005 and in the Akademy Nauk lake (Kamchatka) in 1996. In Miyakejima, tephra of every explosion events were identified in many places and the depositional times were determined using stratigraphical makers. Eruption volume of tephra of each explosion event gave the precise eruption rates. The fragmentation extents of magma and whole products of each event were estimated by grain analysis. Component analysis gave the proportion of contribution of magma to each explosion event. These data are very valuable parameters to model the explosions of hydrovolcanic eruptions, which nobody could have presented by this study.There is positive correlation between th … More e magma eruption rate and the height of ashy eruption column, and negative correlation between the extent of fragmentation and magma eruption rate. The latter means the highest extent of fragmentation in non-magmatic explosions. Such the high fragmentation may result from (1) repeated fragmentation of coarse grains within the crater, which failed to rise due to the lack of thermal energy and (2) the effect of supercritical water that breaks silicate into pieces immediately. The lines of evidence showing the most effective energy conversion from heat to fragmentation at the mgama-water ration of 0.3, proposed by Wahletz was not confirmed in the present eruption.Large contribution of groundwater into the Miyakejima eruption in 2000 is also verfied by the appearance of erupted products and the isotopic ratios of materials attached to the products. Further investigation including textural comparison of products from the Anatahan and Akadamy Nauk is needed to clarify the circumstance of phreatomagmatic explosions. Less