Abstract
AbstractThis paper describes a newly observed phenomenon, a rare form of lamina protecting petroglyphs from weathering, and it attempts an explanation of such features. These laminae are not precipitates but represent the floors of the original cupules that have become more resistant to erosion through conversion to tectonite. The process involves crystallization of the syntaxial quartz overgrowths on quartz grains that constitute the cement component of quartzite and silica-rich schist. It is attributed to the cumulative application of kinetic energy that derives from the tens of thousands of hammerstone blows that produced the cupule. The tribological process results in products similar to those formed in ductile shear zones when sandstone has been subjected to great kinetic stresses. In the cupules reported here, the re-metamorphosed lamina preserves their original surface and prevents the erosion of the protolith (parent rock) concealed by the modified layer. The thickness of the layer is a function of the cumulative amount of energy applied to the rock's cement, and the process of alteration is defined as ‘kinetic energy metamorphosis’.
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