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

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幼年物质。对提取的幼期物质进行详细的研究，可以为预测岩浆成分、岩浆方式、喷发时间和喷发规模提供有效的信息。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，我们对位于日本西南部九州岛中部的云仙火山1990-1991年的fugendake和1995年爆发的久州火山Iwo-yama喷发的一系列火山灰样本进行了研究。为了评估近地表潜伏岩浆的存在，我们收集了1990年11月至1991年4月在云仙火山喷发的火山灰样本。在显微镜下，我们在1991年2月和4月喷发的火山灰中发现了少量具有极低折射率的水泡状火山玻璃。根据观测结果，我们预测了浅层潜伏岩浆的存在。为了确认空泡火山玻璃是幼年材料，从每个抛光的剖面中提取不含杂质的空泡玻璃作为玻璃薄片。这些玻璃薄片被喷成两个等分瓶。一种用于测量未处理的折射率，另一种用于测量退火脱水后的折射率。火山灰中泡状火山玻璃的这两种折射率值基本相同。因此，火山灰中不水化的水泡状火山玻璃似乎来自新岩浆。这意味着这些玻璃是新生产的，因为这些玻璃在形成后没有经历水合作用。在处理时间超过3个月的熔岩穹丘出现时所检测到的必需物质表明，对火山灰的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.