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

通过对2000年三宅岛火山喷发产物的地质检测，结合地球物理监测资料和现场录像资料，对三宅岛火山喷发过程中发生的火山喷发机理进行了研究。还对2003-2005年Anatahan（北马里亚纳群岛）和1996年Akademy Nauk湖（堪察加半岛）的火山爆发进行了比较研究。在三宅岛，许多地方都发现了每一次爆炸事件的火源，并利用地层maker确定了沉积时代。每次爆炸事件的火山喷发量给出了精确的喷发速率。通过颗粒分析估计了每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爆炸事件的贡献比例。这些数据是模拟水火山爆发的非常有价值的参数，没有人能在这项研究中提出。岩浆喷发速率与灰状喷发柱高度呈正相关，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。如此高的破碎率可能是由于：(1)陨石坑内的粗颗粒反复破碎，由于缺乏热能而未能上升；(2)超临界水的作用使硅酸盐立即破碎成碎片。Wahletz提出的从热到碎裂最有效的能量转换在岩浆-水比为0.3时的证据线在这次喷发中没有得到证实。地下水对2000年宫崎岛火山喷发的巨大贡献也通过喷发产物的外观和附着在产物上的物质的同位素比率得到证实。需要进一步的调查，包括对Anatahan和Akadamy Nauk的产品进行质地比较，以澄清呼吸岩浆爆炸的情况。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