[H-GM02_30PM1] Geomorphology

[HGM02-04] Volumetric changes of various rocks during weathering and their geologic significance

*Masahiro CHIGIRA¹, Eiji NAKATA², Takahiro OOYAMA² (1.Disaster Prevention Research Institute, Kyoto University, 2.Central Research Institute of Electric Power Industry)

Keywords:weathering, volumetric change, active fault

A rock volume may change during weathering, which would have various importance in earth surface processes. However, little is known on the volumetric change. We summarize our research results of various rock types and refer to it's geomorphological importance. Basic idea is so-called isocon concept, which assumes immobile chemical elements during weathering and calculate a volume change from density and chemistry of rock before and after weathering (Grant, 1986). We assume TiO2 is the immobile element.Granitic rocks: Volumetric change may be different between granite and granodiorite/quartz diorite. White et al. (2002) reported that granodiorite and quartz diorite are isovolumetric during weathering. Chigira (2002) reported that granite expands 50% during weathering, which is consistent with Folk and Patton (1982) who estimated the volume change from the inflection of a pegmatite vein in a weathering zone. These expansions lead to spheroidal weathering or micro-sheeting.Sandstone: Matsuzawa (2008) studied the weathering of sandstone of the Cretaceous Izumi Formation in Ehime and estimated 20-30% expansion during weathering, which closed joint openings.Mudstone: Calculation from the data of Chigira (1988) for the Quaternary Haizume Formation in Niigata suggested that mudstone expanded 10-30% during weathering in the upper part of the dissolved zone. Such expansion may facilitate downslope soil creep.Vapor-phase crystallized tuff: Calculation from the data of Chigira et al. (2002) for vapor-phase crystallized tuff of the Shirakawa ignimbrite in Fukushima suggested it's weathering was isovolumetric except for the topmost part, in which fabric collapsed and the rock shrank.Tuff breccia: We studied the weathering of tuff breccia of the Miocene Tomari Formation in Aomori, which suggested that it occurred with 20-50% expansion. So-called active faults in the Higashidori nuclear power plant site are apparent active faults made by rock expansion during weathering. Chigira , M. 1988. J. Japan Geol. Soc. 94, 419-431. Chigira, M., Nakamoto, M., and Nakata, E., 2002. Engineering Geology 66, 111-125. Folk, R.L., and Patton, E.B., 1982. Zeitschrift fur Geomorphologie N. F. Bd 26, 17-32. Matsuzawa, M. 2008, Master thesis of Kyoto University. Grant, J.A., 1986. Economic Geology, 81, 1976-1982. White, A.F., 2002. Chemical Geology 190, 69-89.