古期深層風化殻の形成と後期第三紀以降の地形化作用 ＩＩ 風化殻の形成と地形

Abstract

Wide and flat planes found on top of mountains are often related to deep weathering crusts of the Miocene and Pliocene Ages. Because surfaces of the crusts are porous and mechanically weak, they are easy to suffer from aeolian and frost actions, and flattened during glacial ages.Process of weathering is significant to the morphogeny and therefore to the recent morphology in Japan. In this respect the mechanism of deep weathering is discussed. In aquifer or waterholding parts of the crust, rocks as well as water change chemical properties in course of weathering reactions from surface to bottom. Near the surface rocks are leached under acidic conditions with CO2 and water. Beside silica, alkali and alkali-earth are romoved in ground water to react and replace mother rocks beneath. Further removed materials are deposited as interstitial clay or opal under nutral or slightly alkaline conditions. Therefore by removal of weathering products the reaction continues in the same direction to change chemical conditions, and form an almost closed system between rocks and ground water in the aquifer. The weathering crust includes a whole pack of reacted, replaced and deposited materials in the aquifer.Taking granitic rocks as typical components of the earth's crust, layered structure of weathering crusts are described. They are made up of 3 layers of horizontal zones as follows : In the top of the granitic crust completely leached rocks are found as saprolites, in which original textures of rocks are presurved, although most minerals except quartz are converted into kaolinite, halloysite or gibbsite. Density of original rock (2.68g/cm3 in a fresh granodiorite) is reduced to 1.0g/cm3 in dry condition. Here almost 60% of materials are leached. The upper part of the layer is red coloured by ferric oxides, while lower part is white by halloysites. Because rocks are porous, lightened and weakened, the upper most part of the crust is often removed by wind and the crust is truncated by wide aeolian surfaces.In the second layer, rocks are moderately weathered manganese and ferrous hydroxides are deposited beside removed clay minerals, tuch as halloysite and vermiculite. Quartz and most feldspars are not decomposed, but micas are changed ingo vermiculite. The layer is 1.8-2.0g/cm3 in density. It is represented by the montmorillonite zone in another kind of weathering crust, containing dacite, dacite tuff and another volcanics.In the third layer, rocks are not changed chemically nor minerallogicaly, but they are fractured or fragmented by open clacks along grain boundaries or cleavages. Density of the rock is reduced to 2.4-2.0g/cm3 in this layer by leaching reaction of percolating water.Change in density by weathering is closely related to the geomorphology especially to the inclination of slopes along valleys.