Clay Minerals at the Geto Landslide Area, Southwest of Iwate Prefecture

Abstract

The Geto landslide area, being dormant at present, is divided into three groups of the landslide cluster based on their elevation. Five units of landslide topography are recognized at the places of around 1, 050 meters in altitude, six are around 900 meters, and three are around 800 meters, respectively. Of these, those at the lowest place seem to be formed most recently, since they still leave their head scarp and toe ridge. In order to estimate the capability of recurrence of the Geto landslides, mineral contents in the landslide clay found over the area were investigated. As the clay minerals, montmorillonite, chlorite and sericite are abundantly found, and clinoptilolite, heulandite etc. are found as the zeolite minerals. According to Shuzui (1987) chlorite changes into smectite through the mixed mineral of chlorite-smectite. And it is inferred that clinoptilolite changes into smectite (1984). These suggest that smectite and montmorillonite may increase, but cannot decrease, in content with time. The montmorillonite, when it absorbs water, swells to bring decreasing in its shear strength that acts a role of the primary factor of landslide. The Geto landslide site is considered to have high potential of future activity, since there is abundant in montmorillonite. It is feared, therefore, that the landslide may recur when an artificial change of landform is carried out.