Case hardening of rocks
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Case hardening is a weathering phenomenon of rock surface induration. It is observed commonly in: felsic alkaline rocks, such as nepheline syenite, phonolite and trachyte; pyroclastic rocks, as pyroclastic flow deposit, fine air-fall deposits and vent-filling pyroclastic deposits; sedimentary rocks, as sandstone and mudstone.
Chemical weathering alters the minerals constituent of rock surface. Decomposition of mafic and opaque minerals releases ions and colloids of iron, magnesium, calcium and sulphur. Alteration of feldspars and feldspathoids releases silica colloid. These materials are reached and transported by surface water. The remnant materials are highly aluminous and siliceous. They could have certain mechanical firmness of own minerals, however no cohesion. Therefore, physical disintegration of the rock takes place to form the surface.
In certain cases, the weathered surface obtains mechanical firmness higher than subsurface. The reached materials dissolved in the surface infiltrates in the weathered surface and cement the silica-aluminous remnant materials. The surface induration by means of this process is named case hardening.[1] The physical weakness of the subsurface in comparison with the surface is called core softening as exemplified by the tonalite at Catavina, Baja California.[2]
