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

早期的潜水喷发可能含有年轻的物质，这些物质提供了关于上升岩浆的信息，因此可以作为即将到来的岩浆喷发的前兆。1980年3月下旬在圣海伦斯火山喷发的早期特弗拉的样本是在5月18日的侧向爆炸和随后的普林尼安火山喷发的高潮前7周采集的。最近，在这些早期特弗拉样本中发现的稀疏、泡状的玻璃碎片被重新挖掘出来。在1980年3月28日喷发的火山灰中发现了一些新鲜的、缺乏微晶的泡状玻璃碎片，浓缩了几个早期火山灰样本的光密度组分。然后使用电子探针分析(EPMA)对玻璃碎片进行分析，以获得它们的化学成分，以便与1980年前的Tephra进行比较。此外，还利用显微傅里叶变换红外光谱仪(Micro-FTIR)测量了几种碎石在加热过程中的脱水过程，以评估其水化程度。这些实验的结果如下：(1)1980年初圣海伦斯火山喷发的火山灰不仅含有1980年前火山灰层中的物质(回收材料)，还含有幼年物质(尽管数量很少)。(2)SH0328的泡状玻璃碎屑中有一半是幼年物质，其余的碎屑来自1980年前的苔藓层(主要来自苔藓)。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。根据化学成分的不同，可以将幼年的泡状玻璃碎屑与1980年前的苔原岩层中的玻璃碎屑区分开来。(3)气泡玻璃碎片从山上喷发。圣海伦斯，通过脱水曲线可以区分年龄在几万岁以上的玻璃碎片和年龄在几千岁以下的玻璃碎片。因此，有一种可能性，即可以利用喷发活动早期阶段的特弗拉，用这项技术预测上升岩浆的直接参与。

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.