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

根据对产物的地质考察、地球物理监测数据和录像资料，研究了2000年宫克岛火山喷发期间发生的云雾岩浆爆炸的机理。还对2003-2005年期间在Anatahan(北马里亚纳群岛)喷发和1996年在Akademy Nauk湖(堪察加)喷发进行了比较研究。在宫城岛，许多地方发现了每一次爆炸事件的盖弗拉，并利用地层学标志确定了沉积时代。每个爆炸事件的火山灰喷发量给出了准确的喷发率。通过颗粒分析，估算了各事件的岩浆破碎程度和整个产物的破碎程度。成分分析给出了岩浆对每一次爆炸事件的贡献比例。这些数据对模拟火山喷发的爆发是非常有价值的参数，这是本研究无法提供的。TH…之间存在正相关岩浆喷发速率与灰柱高度呈正相关，碎屑程度与岩浆喷发速率呈负相关。后者意味着非岩浆爆炸的碎裂程度最高。这种高度破碎的原因可能是：(1)火山口内粗颗粒的反复破碎，由于缺乏热能而未能上升；(2)超临界水的影响，立即将硅酸盐破碎成碎片。Wahletz提出的在镁-水比为0.3时从热到碎裂的最有效能量转换的证据在这次喷发中没有得到证实。2000年宫城岛喷发中地下水的巨大贡献也被喷发产物的出现和附着在产物上的物质的同位素比率所证实。还需要进一步调查，包括对Anatahan和Akadamy Nauk的产品进行质地比较，以澄清雾状爆炸的情况。较少

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