Subsurface initiation of tafoni in granite terrains — Geophysical evidence from NE Spain: Geomorphological implications

Granite cavities in the north-east of Spain, which developed below the weathering front, are reported. These cavities, up to 3 m across and probably filled with water-saturated weathered granite, have been detected by electrical resistivity tomography (ERT) as low resistivity anomalies enclosed in high resistivity media corresponding to unweathered granite rock. The caverns appear as hyperbolic reflections in a radargram obtained by ground penetrating radar (GPR) and show similar shape and size to the tafoni found in the surrounding granite outcrops. Data gathered in this study support a subterranean origin of some of the tafoni. A new conceptual model for the initiation and growth of granite cavities under subsurface conditions is proposed. Groundwater flow through vertical and sheet fractures triggers preferential weathering at points where meter-sized spheroidal pockets of weathered granite develop. Erosion of the rock mass leads to the exposure of these pockets evolving under subaerial conditions as typical tafoni.