Importance of porosity and transfer of matter in the rock weathering processes: two examples in central Spain

Abstract

Some physical properties (bulk and free porosity, pore size distribution), and the chemical compo- sition and mass balance of two deeply weathered profiles one developed on Hercynian granodiorite and the other on pre-Cambrian slates were studied. Hydric and mercury porosimetry, nitrogen adsorption techniques, chemical analyses and XRD techniques were used. On granodiorite, weathering has created increased porosity with a pore diameter \5 lm, whereas on slates the weathering has produced of ca. 1 lm in diameter. These pore sizes have played an important role in the weathering processes. Assuming that weathering preserves volumes, except in the uppermost part of the profiles, it brought about a loss of matter of more than 12% (*300 kg/m 3 ) on granodiorite and ca. 30% (*800 kg/m 3 ) on slates. These changes are related to shifts in the mineralogical evolution, with the appearance of new 2:1 and 1:1 phyllosilicates and Fe oxy- hydroxides as the main authigenic minerals. The release of matter, at least since the upper Neogene until the present, has led to the lowering of relief in a more or less homo- geneous way, giving rise to gentle hillsides and flat sur- faces below which the current river networks are incised. Porosity studies have the potential to explain several specific landforms as well as affecting landscape devel- opment in general.