Investigation of volcanic ash monitoring system to predict the manner of magma

在火山活动初期喷发的火山灰有可能含有直接从潜伏岩浆中剥离出来的年轻物质。对提取的年轻物质的详细研究可能为预测岩浆成分和岩浆方式提供有效信息，包括喷发的时间和规模。因此，火山灰监测系统可能成为减轻火山灾害的潜在工具之一。基于这一概念，我们研究了1990-1991年在日本西南部九州岛中部的温州火山福根达克喷发的火山灰样品和1995年在库州火山喷发的硫山火山喷发的火山灰样品。为了评价近地表潜伏岩浆的存在，我们收集了1990年11月至1991年4月在温泉喷发的火山灰样品。在显微镜下，我们发现在1991年2月和4月，火山灰中有少量折射率极低的泡状火山玻璃喷发。在观测的基础上，我们预测了浅层潜伏岩浆的存在。为了确认泡化的火山玻璃是幼稚的物质，从每个抛光的切片上提取了不含杂质的泡化玻璃作为玻璃鳞片。这些玻璃薄片被分成两等份。一种是测量未经处理的折射率，另一种是测量退火脱水后的折射率。火山灰中的气泡状火山玻璃在这两种折射率上显示出几乎相同的值。因此，火山灰中的非水化气泡火山玻璃似乎来自新的岩浆。这意味着这些玻璃是新生产的，因为它们在形成后没有经历过水化。对熔岩穹隆出现三个多月时主要物质的检测表明，火山灰监测对于预测潜伏岩浆和岩浆喷发的方式是有效的。

Volcanic ash erupted in an initial stage of volcanic activity has possibility to contain juvenile material directly deprived from latent magma. Detailed studies of the extracted juvenile material may provide effective information to predict magma composition and the manner of magma involving timing and scale of eruption if it will occur. Therefore, monitoring system of volcanic ashes may be one of a potential tool to mitigate volcanic disasters. Based on this concept, we have examined a series of volcanic ash samples erupted from Fugen-dake in Unzen volcano during 1990-1991 and from Iwo-yama in Kuju volcano erupted in 1995, located in the central part of Kyushu island in SW Japan.In order to asses the existence of latent magma near the surface, we collected ash samples erupted from November 1990 to April 1991 in Unzen. Under a microscope, we found a small amount of vesiculated volcanic glasses with remarkably low refractive index in ash erupted on February and April, 1991. We predicted that the existence of latent magma in the shallow level on the basis of our observation. To confirm that the vesiculated volcanic glass is juvenile material, the vesiculated glass without impurities were extracted as glass flakes from each polished this sections. These glass flakes were sprited into two aliquots. One is for measurement of untreated refractive index and other is for measurement of refractive index after annealing to dehydrate. The vesiculated volcanic glass in ash indicated almost the same value in these two kinds of refractive index. Thus the non-hydrated vesiculated volcanic glass in ash seems to be from new magma. It means that these glasses are newly produced because these have not experienced hydration after the formation. The detection of essential material at the time processed more than three months of the lava dome appearance suggests that the monitoring of the volcanic ash is effective for prediction of the manner of latent magma and magmatic eruption.