Prediction of Magma Processes in the Early Eruptive Stage and Mitigation of Volcanic Hazards

早期的潜水喷发可能包含幼年物质，提供有关上升的岩浆的信息，从而作为即将发生的岩浆喷发的前兆。1980年3月下旬从圣海伦斯火山喷发的早期火山碎屑的样本是在5月18日随后的侧向爆炸和高潮普林尼火山喷发前7周收集的。最近，在这些早期火山灰样品中发现的稀疏的囊状玻璃碎片被重新检查。从早期火山灰的几个样品的光密度馏分集中与一些新鲜的，微晶穷人的泡状玻璃碎片检测火山灰爆发于1980年3月28日。然后使用电子探针分析（EPMA）分析玻璃碎片，以获得它们的化学成分，并与1980年前的火山灰进行比较。此外，利用显微傅立叶变换红外光谱仪（Micro-FTIR）对几种碎片在加热过程中的脱水过程进行了测量，以估计其水化程度。这些实验的结果如下：（1）1980年从圣海伦斯火山喷发的早期火山灰不仅含有1980年前火山灰层的物质（回收物质），而且还含有幼年物质（尽管数量很小）。(2)A SH 0328中一半的泡状玻璃碎片被鉴定为幼年物质，其余碎片来自1980年以前的火山灰层（主要来自P火山灰）。根据化学成分的不同，可以将两种不同来源的碎片区分开来。幼年泡状玻璃碎片可以通过化学成分的差异与来自1980年前火山灰层的玻璃碎片区分开来。(3)Of泡状的玻璃碎片从火山喷发出来。St.Helens，根据脱水曲线可以区分数万年以上和数千年以下的玻璃碎片。因此，有可能直接参与上升的岩浆可以预测与这种技术使用火山灰从早期阶段的喷发活动。

Early phreatic eruptions may contain juvenile material that provide information about the ascending magma, and thus serve as precursors of impending magmatic eruptions. Samples of early tephra erupted in late March, 1980, from Mount St.Helens, were collected seven weeks before the lateral blast and climactic Plinian eruption that followed on May 18. Recently, sparse, vesicular glass shards that were found in these early tephra samples were reexawmined. Light density fractions from several samples of early tephra were concentrated with some fresh, microlite-poor vesicular glass shards detected in the tephra erupted in March 28 1980. The glass shards were then analyzed using electron microprobe analysis (EPMA) to obtain their chemical compositions for comparison to those of the pre-1980 tephra. Furthermore dehydration process of several shards during heating was measured using micro fourier transform infrared spectrophotometery (Micro-FTIR) to estimate the extent of hydration. Results of these experimentas follows;(1)The 1980 early tephra erupted from Mount St.Helens contained not only the material from the pre-1980 tephra layers (recycled material) but also juvenile material (although the amount was very small). (2)A half of vesicular glass shards in SH0328 are identified juvenile material with the remaining shards from pre 1980 tephra layers (mainly from P tephra). The shards from two different origins can be divided by difference of chemical composition. Juvenile vesicular glass shards could be distinguished from those derived from the pre-1980 tephra layers by difference of chemical composition. (3)Of the vesicular glass shards erupted from Mount. St.Helens, glass shards aged more than several ten thousands can be distinguished from those aged less than several thousands by dehydration curve. Thus, there is a possibility that direct participation of ascending magma could be predicted with this technique using tephra from earlier stages of eruptive activity.