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

白须是日本九州南部未焊接流纹质火山碎屑流存款的总称。白须的陡坡有时会在大雨中坍塌。弄清白须坡面的风化过程对预测白须崩塌具有重要意义。我们研究了火山玻璃碎片的水化程度，使用的折射率在白须斜坡恶化的顺序。在研究露头处，Shirasu斜坡表面由原始火山碎屑流存款（第I层）、岩屑沉积和上覆的沉降火山灰组成。根据岩相和颜色，岩屑堆积物可分为三层（第二层至第四层，从上到下）。火山玻璃碎片（第一层至第四层）的折射率范围为1.4960至1.5000。层II至IV倾向于显示高折射率（1.4978 - 1.4982）。另一方面，层I具有比前面的层显著低的折射率（1.4976）。各层脱水样品的折射率比水化玻璃的折射率低0.002-0.001。第I层有许多未完全水化的厚玻璃碎片。另一方面，第二至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白须坡面恶化程度的指标。

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.