A study on the weathering process of the rhyolitic pyroclastic flow deposits at steep slopes using the refractive index of volcanic glass shards

Shirasu是日本九州南部未焊接的流纹岩火山碎屑流沉积的总称。雪拉苏陡坡在下雨时有时会坍塌。弄清石拉苏坡面的风化过程对预测石拉苏崩塌具有重要意义。我们用折射率来检测火山玻璃碎屑的水化程度，按石拉苏斜坡退化的顺序排列。在研究露头时，石拉苏斜坡面由原始火山碎屑流沉积(I层)、滑石沉积和上覆的尘埃特弗拉组成。按相和颜色将距骨矿细分为三层(按升序分为II层至IV层)。火山玻璃碎屑(I-IV层)的折射率在1.4960-1.5000之间。第II~IV层有较高的指数(1.4978~1.4982)。另一方面，第I层的指数(1.4976)明显低于前几层。各层脱水样品的折射率比水合玻璃的折射率低0.002-0.001。第一层有许多不完全水化的厚玻璃碎片，而第二层到第四层主要由完全水合的火山玻璃组成。可以说，火山玻璃碎片的折射率是石拉苏坡面退化程度的指标。

Shirasu is a general name for unwelded rhyolitic pyroclastic flow deposit in southern Kyushu, Japan. Shirasu steep slopes are sometimes collapsed during eavy rains. To clarify the weathering process of Shirasu slope surface would be important to predict the collapse of Shirasu. We examined degree of hydration of volcanic glass shards using the refractive index in the order of deterioration of Shirasu slope. At study outcrop, the Shirasu slope surface composed of original pyroclastic flow deposit (layer I), talus deposits and overlying fallout tephra. Talus deposits are subdivided into three layers (layer II to IV in ascending order) by the facies and color. The refractive index of volcanic glass shards (layer I to IV) ranges from 1.4960 to 1.5000. Layer II to IV tend to show high index (1.4978 - 1.4982). On the other hand, layer I have a significantly low index (1.4976) than those of the former layers. Refractive index of dehydrated samples from each layer are 0.002-0.001 lower than those of hydrated glass. There are many incompletely hydrated thick glass shards in layer I. On the other hand, layer II to IV are mainly composed of completely hydrated volcanic glass. We may say that the refractive index of volcanic glass shards is an indicator of the degree of deteriorated Shirasu slope surface.