The Rokko Mountains are mainly composed of three rock units : Nunobiki granodiorite, Rokko granite, and Dobashi quartz diorite. Alteration of these rocks is examined by XRD analysis and magnetic susceptibility meter. All granite rocks are divided into four categories of alteration : unaltered (A), unaltered (B), weathered, and hydrothermally altered. Unaltered (A) rocks do not contain any secondary minerals and indicate high magnetic susceptibilities (MS), which are probably near the original ones (n × 10-3 SI unit). Unaltered (B) rocks also do not contain secondary minerals, but their MS is clearly low. These rocks are typically distributed in crushed zones along the main faults that enclose and cut the Rokko Mountains. Weathered rocks are defined by the presence of kaolin. Their MS are very low (n × 10-5 SI unit). Highly weathered rocks are preserved at the northern part of the backbone of mountains, while they are heavily eroded at the southern part. Six groups of hydrothermally altered rock are defined by the presence of characteristic minerals such as mordenite, calcium zeolites, sericite, chlorite-smectite series, kaolinite, and calcite. Hydrothermally altered rocks indicate comparatively low MS (n × 10-4-10-5 SI unit). Most hydrothermal alteration occurred along or parallel to main faults, especially Shabayama, Yusodani, and Arima faults at northern part and Nunobiki and Suwayama faults at southern part.
